Recently the Mg 2+ -dependent fluorescence of mag-fura 2 was used to measure the free Mg 2+  content of  S. typhimurium  cells in response to external Mg 2+ , which showed that CorA is the major uptake system for Mg 2+  in bacteria. [ 19 ]  The authors also showed for the first time that the changes in the electric potential (ΔΨ) across the plasma membrane of the cell affected both the rate of Mg 2+  uptake and the free Mg 2+  content of the cell; depolarisation suppressed transport, while hyperpolarisation increased transport. The kinetics of transport were defined only by the rate of change of free Mg 2+  inside the cells (250 μM s −1 ). Because no quantification of the amount of CorA protein in the membrane was made, this value cannot be compared with other experiments on Mg 2+  transporters. [ 18 ]


The efflux of Mg 2+  from bacterial cells was first observed by Lusk and Kennedy (1969) [ 21 ]  and is mediated by the CorA Mg 2+  transport system in the presence of high extracellular concentrations of Mg 2+ . [ 38 ]  The efflux can also be triggered by Co 2+ , Mn 2+  and Ni 2+ , although not to the same degree as Mg 2+ . [ 23 ]  No Co 2+  efflux through the CorA transport system was observed. The process of Mg 2+  efflux additionally requires one of the CorB, CorC or CorD genes. [ 39 ]  The mutation of any single one of these genes leads to a Co 2+  resistance a little less than half of that provided by a CorA mutant. This effect may be due to the inhibition of Mg 2+  loss that would otherwise occur in the presence of high levels of Co 2+ . It is currently unknown whether Mg 2+  is more toxic when the CorBCD genes are deleted.


It has been speculated that the Mg 2+  ion will initially interact with any transport protein through its hydration shell. [ 40 ]  Cobalt (III) hexaammine, Co(III)Hex, is a covalently bound (non-labile) analog for the first shell of hydration for several divalent cations, including Mg 2+ . The radius of the Co(III)Hex molecule is 244 pm, very similar to the 250 pm radius of the first hydration shell of Mg 2+ . This analog is a potent inhibitor of the CorA transport system, more so than Mg 2+ , Co 2+  or Ni 2+ . [ 41 ]  The additional strength of the Co(III)Hex inhibition might come from the blocking of the transport pore due to the inability of the protein to ‘dehydrate’ the substrate. It was also shown that Co(III)Hex was not transported into the cells, [ 41 ]  suggesting that at least partial dehydration would be required for the transport of the normal substrate (Mg 2+ ). Nickel (II) hexaammine, with a radius of 255 pm, did not inhibit the CorA transport system, suggesting a maximum size limit exists for the binding of the CorA substrate ion. [ 41 ]  These results suggest that the important property involved in the recognition of Mg 2+  by CorA is the size of the ion with its first shell of hydration. Hence, the volume change generally quoted for the bare to hydrated Mg 2+  ion of greater than 500-fold, including the second sphere of hydration, may not be biologically relevant, and may be a reason for the first sphere volume change of 56-fold to be more commonly used.


The presence of these two genes was first suspected when Nelson and Kennedy (1972) [ 25 ]  showed that there were Mg 2+ -repressible and non-repressible Mg 2+  uptake systems in  E. coli . The non-repressible uptake of Mg 2+  is mediated by the CorA protein. In  S. typhimurium  the repressible Mg 2+  uptake was eventually shown to be via the MgtA and MgtB proteins. [ 37 ]


Both MgtA and MgtB are regulated by the PhoPQ system and are actively transcribed during the process of infection of human patients by  S. typhimurium . [ 31 ] [ 42 ] [ 43 ]  Although neither gene is required for pathogenicity, the MgtB protein does enhance the long-term survival of the pathogen in the cell. [ 44 ]  The genes are also upregulated  in vitro  when the Mg 2+  concentration falls below 50 μM (Snavely  et al. , 1991a). Although the proteins have km values similar to CorA and transport rates approximately 10 times less, the genes may be part of a Mg 2+  scavenging system. Chamnongpol and Groisman (2002) presents evidence that the role of these proteins may be to compensate for the inactivation of the CorA protein by the PhoPQ regulon. [ 30 ]  The authors suggest that the CorA protein is inactivated to allow the avoidance of metal toxicity via the protein in the low Mg 2+  environments  S. typhimurium  is subjected to by cells after infection.


The proteins are both P-type ATPases [ 38 ]
 [ 45 ]  and neither gene shows any similarity to CorA. The MgtA and MgtB proteins are 75% similar (50% identical), although it seems that MgtB may have been acquired by  horizontal gene transfer  as part of Salmonella Pathogenicity Island 3. [ 45 ] [ 46 ]  The TM topology of the MgtB protein has been experimentally determined, showing that the protein has ten TM-spanning helices with the termini of the protein in the cytoplasm (see figure
). MgtA is present in widely divergent bacteria, but is not nearly as common as CorA, while MgtB appears to have a quite restricted distribution. [ 47 ]  No hypotheses for the unusual distribution have been suggested.


The figure, adapted from Smith  et al.  (1993b), [ 48 ]  shows the experimentally determined membrane topology of the MgtB protein in  S. typhimurium . The TM domains are shown in light blue and the orientation in the membrane and the positions of the N- and C-termini are indicated. The figure is not drawn to scale.


While the MgtA and MgtB proteins are very similar, they do show some minor differences in activity. MgtB is very sensitive to temperature, losing all activity (with regard to Mg 2+  transport) at a temperature of 20 °C. [ 38 ]  Additionally, MgtB and MgtA are inhibited by different ranges of cations (Table A10.1 [ 38 ] ).


The table lists cation transport characteristics of the MgtA and MgtB proteins in  S. typhimurium  as well as the kinetic data for the MgtA and MgtB transport proteins at 37 °C. [ 38 ]  The Vmax numbers listed in parentheses are those for uptake at 20 °C. The inhibition of Mg 2+  transport by Mn 2+  via MgtA showed unusual kinetics (see Figure 1 of Snavely  et al. , 1989b [ 38 ] )


The MgtA and MgtB proteins are ATPases, using one molecule of ATP per transport cycle, whereas the Mg 2+  uptake via CorA is simply electrochemically favourable. Chamnongpol and Groisman (2002) have suggested that the MgtA and MgtB proteins form part of a metal toxicity avoidance system. [ 30 ]  Alternatively, as most P-type ATPases function as efflux mediating transporters, it has been suggested that the MgtA and MgtB proteins act as efflux proteins for a currently unidentified cation, and Mg 2+  transport is either non-specific or exchanged to maintain the electro-neutrality of the transport process. [ 49 ]  Further experiments will be required to define the physiological function of these proteins.


Two papers describe MgtE, a fourth Mg 2+  uptake protein in bacteria unrelated to MgtA/B or CorA. [ 3 ] [ 4 ]  This gene has been sequenced and the protein, 312 amino acids in size, is predicted to contain either four or five TM spanning domains that are closely arranged in the C-terminal part of the protein (see figure). This region of the protein has been identified in the  Pfam  database as a conserved protein domain (PF01769) and species containing proteins that have this protein domain are roughly equally distributed throughout the Eubacteria and Archaea, although it is quite rare in comparison with the distribution of CorA. However, the diversity of the proteins containing the domain is significantly larger than that of the CorA domain. The Pfam database lists seven distinct groups of MgtE domain containing proteins, of which six contain an archaic or eubacterial member. The expression of MgtE is frequently controlled by a conserved RNA structure,  YkoK leader  or M-box. [ 51 ]


The figure (right), adapted from Smith  et al.  (1995) [ 4 ]  and the PFAM database entry, shows the computer-predicted membrane topology of the MgtE protein in  Bacillus firmus  OF4. The TM domains are shown in light blue. The  CBS domains , named for the protein they were identified in,  cystathionine-beta synthase , shown in orange, are identified in the Pfam database as regulatory domains, but the mechanism of action has not yet been described. They are found in several voltage-gated chloride channels. [ 52 ]  The orientation in the membrane and the positions of the N- and C-termini are indicated. This figure is not drawn to scale. This transporter has recently had its structure solved by x-ray crystallography. [ 53 ]


The MgtE gene was first identified by Smith  et al.  (1995) during a screen for CorA-like proteins in bacteria and complements the Mg 2+ -uptake-deficient  S. typhimurium  strain MM281 (corA mgtA mgtB), restoring wild type growth on standard media. [ 4 ]  The kinetics of Mg 2+  transport for the protein were not determined, as  28 Mg 2+  was unavailable. As a substitute, the uptake of  57 Co 2+  was measured and was shown to have a km of 82 μM and a Vmax of 354 pmol min −1  10 8  cells −1 . Mg 2+  was a competitive inhibitor with a Ki of 50 μM—the Ki of Mg 2+  inhibition of  60 Co 2+  uptake via CorA is 10 μM. [ 2 ]  A comparison of the available kinetic data for MgtA and CorA is shown in the table. Clearly, MgtE does not transport Co 2+  to the same degree as CorA, and the inhibition of transport by Mg 2+  is also less efficient, which suggests that the affinity of MgtE for Mg 2+  is lower than that of CorA. The strongest inhibitor of Co 2+  uptake was Zn 2+ , with a Ki of 20 μM. [ 4 ]  The transport of Zn 2+  by this protein may be as important as that of Mg 2+ .


The table shows a comparison of the transport kinetics of MgtE and CorA, and key kinetic parameter values for them are listed. As shown, the data has been generated at differing incubation temperatures. km and Ki are not significantly altered by the differing incubation temperature. Conversely, Vmax shows a strong positive correlation with temperature, hence the value of Co 2+  Vmax for MgtE is not directly comparable with the values for CorA.


The earliest research showing that yeast takes up Mg 2+  appears to be done by Schmidt  et al.  (1949). However, these authors only showed altered yeast Mg 2+  content in a table within the paper, and the report's conclusions dealt entirely with the metabolism of phosphate. A series of experiments by Rothstein [ 54 ] [ 55 ]  shifted the focus more towards the uptake of the metal cations, showing that yeast take up cations with the following affinity series; Mg 2+ , Co 2+ , Zn 2+  > Mn 2+  > Ni 2+  > Ca 2+  > Sr 2+ . Additionally, it was suggested that the transport of the different cations is mediated by the same transport system [ 55 ] [ 56 ] [ 57 ] [ 58 ]  — a situation very much like that in bacteria.


In 1998, MacDiarmid and Gardner finally identified the proteins responsible for the observed cation transport phenotype in  Saccharomyces cerevisiae . [ 5 ]  The genes involved in this system and a second mitochondrial Mg 2+  transport system, functionally identified significantly after the gene was cloned, are described in the sections below.


Two genes, ALR1 and ALR2, were isolated in a screen for Al 3+  tolerance (resistance) in yeast. [ 5 ]  Over-expression constructs containing yeast genomic DNA were introduced into wild type yeast and the transformants were screened for growth on toxic levels of Al 3+ . ALR1 and ALR2 containing plasmids allowed the growth of yeast in these conditions.


The Alr1p and Alr2p proteins consist of 859 and 858 amino acids respectively and are 70% identical. In a region in the C-terminal, half of these proteins are weakly similar to the full CorA protein. The computer-predicted TM topology of Alr1p is shown in the figure. The presence of a third TM domain was suggested by MacDiarmid and Gardner (1998), [ 5 ]  on the strength on sequence homology, and more recently by Lee and Gardner (2006), [ 59 ]  on the strength of mutagenesis studies, making the TM topology of these proteins more like that of CorA (see figure). Also, Alr1p contains the conserved GMN motif at the outside end of TM 2 (TM 2') and the mutation of the methionine (M) in this motif to a leucine (L) led to the loss of transport capability. [ 59 ]


The figure shows the two possible TM topologies of Alr1p. Part A of the figure shows the computer-predicted membrane topology of the Alr1p protein in yeast and part B shows the topology of Alr1p based on the experimental results of Lee and Gardner (2006). [ 59 ]  The GMN motif location is indicated in red and the TM domains in light blue. The orientation in the membrane and the positions of the N- and C-termini are indicated, the various sizes of the soluble domains are given in amino acids (AA), and TM domains are numbered by their similarity to CorA. Where any TM domain is missing, the remaining domains are numbered with primes. The figure is not drawn to scale.
A third ALR-like gene is present in  S. cerevisiae  and there are two homologous genes in both  Schizosaccharomyces pombe  and  Neurospora crassa . These proteins contain a GMN motif like that of CorA, with the exception of the second  N. crassa  gene. No ALR-like genes have been identified in species outside of the fungi.


Membrane fractionation and green fluorescent protein (GFP) fusion studies established that Alr1p is localised to the plasma membrane. [ 60 ] [ 61 ]  The localisation of the Alr1p was observed to be internalised and degraded in the vacuole in response to extracellular cations. Mg 2+ , at very low extracellular concentrations (100 μM; < 10% of the standard media Mg 2+  content), and Co 2+  and Mn 2+  at relatively high concentrations (> 20× standard media), induced the change in Alr1p protein localisation, and the effect was dependent on functional ubiquitination, endocytosis and vacuolar degradation. [ 60 ]  This mechanism was proposed to allow the regulation of Mg 2+  uptake by yeast.
However, a recent report  [ 61 ]  indicates that several of the observations made by Stadler et al. [ 60 ]  were not reproducible. [ 61 ]  For example, regulation of ALR1 mRNA accumulation by Mg 2+  supply was not observed, and the stability of the Alr1 protein was not reduced by exposure to excess Mg 2+ . The original observation of Mg-dependent accumulation of the Alr1 protein under steady-state low-Mg conditions was replicated, but this effect was shown to be an artifact caused by the addition of a small peptide (epitope) to the protein to allow its detection. Despite these problems, Alr1 activity was demonstrated to respond to Mg supply, [ 61 ]  suggesting that the activity of the protein is regulated directly, as was observed for some bacterial CorA proteins. [ 19 ]


A functional Alr1p (wild type) or Alr2p (overexpressed) is required for  S. cerevisiae  growth in standard conditions (4 mM Mg 2+ [ 5 ] ), and Alr1p can support normal growth at Mg 2+  concentrations as low as 30 μM. [ 60 ]   57 Co 2+  is taken up into yeast via the Alr1p protein with a km of 77 – 105 μM (; [ 56 ]  C. MacDiarmid and R. C. Gardner, unpublished data), but the Ki for Mg 2+  inhibition of this transport is currently unknown. The transport of other cations by the Alr1p protein was assayed by the inhibition of yeast growth. The overexpression of Alr1p led to increased sensitivity to Ca 2+ , Co 2+ , Cu 2+ , La 3+ , Mn 2+ , Ni 2+  and Zn 2+ , an array of cations similar to those shown to be transported into yeast by a CorA-like transport system. [ 5 ]  The increased toxicity of the cations in the presence of the transporter is assumed to be due to the increased accumulation of the cation inside the cell.


The evidence that Alr1p is primarily a Mg 2+  transporter is that the loss of Alr1p leads to a decreased total cell content of Mg 2+ , but not of other cations. Additionally, two electrophysiological studies where Alr1p was produced in yeast or  Xenopus  oocytes showed a Mg 2+ -dependent current in the presence of the protein; [ 62 ]  Salih  et al. , in prep.


The kinetics of Mg 2+  uptake by Alr1p have been investigated by electrophysiology techniques on whole yeast cells. [ 62 ]  The results suggested that Alr1p is very likely to act as an ion-selective channel. In the same paper, the authors reported that Mg 2+  transport by Alr1p varied from 200 pA to 1500 pA, with a mean current of 264 pA. No quantification of the amount of protein producing the current was presented, so the results lack comparability with the bacterial Mg 2+  transport proteins.


The alternative techniques of  28 Mg 2+  radiotracer analysis and mag-fura 2 to measure Mg 2+  uptake have not yet been used with Alr1p.  28 Mg 2+  is currently not available and the mag-fura 2 system is unlikely to provide simple uptake data in yeast. The yeast cell maintains a heterogeneous distribution of Mg 2+ [ 63 ]  suggesting that multiple systems inside the yeast are transporting Mg 2+  into storage compartments. This internal transport will very likely mask the uptake process. The expression of ALR1 in  S. typhimurium  without Mg 2+  uptake genes may be an alternative, but, as stated earlier, the effects of a heterologous expression system would need to be taken into account.


The MNR2 gene encodes a protein closely related to the Alr proteins, but includes conserved features that define a distinct subgroup of CorA proteins in fungal genomes, suggesting a distinct role in Mg 2+  homeostasis. Like an alr1 mutant, growth of an mnr2 mutant was sensitive to Mg 2+ -deficient conditions, but the mnr2 mutant was observed to accumulate more Mg 2+  than a wild-type strain under these conditions. [ 64 ]  These phenotypes suggested that Mnr2 may regulate Mg 2+  storage within an intracellular compartment. Consistent with this interpretation, the Mnr2 protein was localized to the membrane of the vacuole, an internal compartment implicated in the storage of excess mineral nutrients by yeast. A direct role of Mnr2 in Mg 2+  transport was suggested by the observation that increased Mnr2 expression, which redirected some Mnr2 protein to the cell surface, also suppressed the Mg 2+ -requirement of an alr1 alr2 double mutant strain. The mnr2 mutation also altered accumulation of other divalent cations, suggesting this mutation may increase Alr gene expression or protein activity. Recent work  [ 61 ]  supported this model, by showing that Alr1 activity was increased in an mnr2 mutant strain, and that the mutation was associated with induction of Alr1 activity at a higher external Mg concentration than was observed for an Mnr2 wild-type strain. These effects were observed without any change in Alr1 protein accumulation, again indicating that Alr1 activity may be regulated directly by the Mg concentration within the cell.


Like the ALR genes, the MRS2 gene was cloned and sequenced before it was identified as a Mg 2+  transporter. The MRS2 gene was identified in the nuclear genome of yeast in a screen for suppressors of a mitochondrial gene RNA splicing mutation, [ 65 ]  and was cloned and sequenced by Wiesenberger  et al.  (1992). [ 66 ]  Mrs2p was not identified as a putative Mg 2+  transporter until Bui  et al.  (1999). [ 6 ]  Gregan  et al.  (2001a) identified LPE10 by homology to MRS2 and showed that both LPE10 and MRS2 mutants altered the Mg 2+  content of yeast mitochondria and affected RNA splicing activity in the organelle. [ 67 ] [ 68 ]  Mg 2+  transport has been shown to be directly mediated by Mrs2p, [ 18 ]  but not for Lpe10p.


The Mrs2p and Lpe10p proteins are 470 and 413 amino acid residues in size, respectively, and a 250–300 amino acid region in the middle of the proteins shows a weak similarity to the full CorA protein. The TM topologies of the Mrs2p and Lpe10p proteins have been assessed using a protease protection assay [ 6 ] [ 67 ]  and are shown in the figure. TM 1 and 2 correspond to TM 2 and 3 in the CorA protein. The conserved GMN motif is at the outside end of the first TM domain, and when the glycine (G) in this motif was mutated to a cysteine (C) in Mrs2p, Mg 2+  transport was strongly reduced. [ 18 ]


The figure shows the experimentally determined topology of Mrs2p and Lpe10p as adapted from Bui  et al.  (1999) [ 6 ]  and Gregan  et al.  (2001a). [ 67 ]  The GMN motif location is indicated in red and the TM domains in light blue. The orientation in the membrane and the positions of the N- and C-termini are indicated. The various sizes of the soluble domains are given in amino acids (AA), TM domains are numbered, and the figure is not drawn to scale.


Mrs2p has been localised to the mitochondrial inner membrane by subcellular fractionation and immunodetection [ 6 ]  and Lpe10p to the mitochondria. [ 67 ]  Mitochondria lacking Mrs2p do not show a fast Mg 2+  uptake, only a slow ‘leak’, and overaccumulation of Mrs2p leads to an increase in the initial rate of uptake. [ 18 ]  Additionally, CorA, when fused to the mitochondrial leader sequence of Mrs2p, can partially complement the mitochondrial defect conferred by the loss of either Mrs2p or Lpe10p. Hence, Mrs2p and/or Lpe10p may be the major Mg 2+  uptake system for mitochondria. A possibility is that the proteins form heterodimers, as neither protein (when overexpressed) can fully complement the loss of the other. [ 67 ]


The characteristics of Mg 2+  uptake in isolated mitochondria by Mrs2p were quantified using mag-fura 2. [ 18 ]  The uptake of Mg 2+  by Mrs2p shared a number of attributes with CorA. First, Mg 2+  uptake was directly dependent on the electric potential (ΔΨ) across the boundary membrane. Second, the uptake is saturated far below that which the ΔΨ theoretically permits, so the transport of Mg 2+  by Mrs2p is likely to be regulated in a similar manner to CorA, possibly by the inactivation of the protein. Third, Mg 2+  efflux was observed via Mrs2p upon the artificial depolarisation of the mitochondrial membrane by valinomycin. Finally, the Mg 2+  fluxes through Mrs2p are inhibited by cobalt (III) hexaammine. [ 18 ]


The kinetics of Mg 2+  uptake by Mrs2p were determined in the Froschauer  et al.  (2004) paper on CorA in bacteria. [ 19 ]  The initial change in free Mg 2+  concentration was 150 μM s-1 for wild type and 750 μM s-1 for mitochondria from yeast overexpressing MRS2. No attempt was made to scale the observed transport to the amount of transporter present.


The transport of Mg 2+  into Paramecium has been characterised largely by R. R. Preston and his coworkers. Electrophysiological techniques on whole Paramecium were used to identify and characterise Mg 2+  currents in a series of papers [ 69 ] [ 70 ] [ 71 ] [ 72 ]  before the gene was cloned by Haynes  et al.  (2002). [ 7 ]


The open reading frame for the XNTA gene is 1707 bp in size, contains two introns and produces a predicted protein of 550 amino acids. [ 7 ]  The protein has been predicted to contain 11 TM domains and also contains the α1 and α2 motifs (see figure) of the SLC8 ( Na+/Ca 2+  exchanger [ 73 ] ) and SLC24 ( K+ dependent Na+/Ca 2+  exchanger [ 74 ] ) human solute transport proteins. The XntAp is equally similar to the SLC8 and SLC24 protein families by amino acid sequence, but the predicted TM topology is more like that of SLC24, but the similarity is at best weak and the relationship is very distant. [ 7 ]  The AtMHX protein from plants also shares a distant relationship with the SLC8 proteins.


The figure shows the predicted TM topology of XntAp. Adapted from Haynes  et al.  (2002), [ 7 ]  this figure shows the computer predicted membrane topology of XntAp in Paramecium. The orientation in the membrane was determined using HMMTOP. [ 75 ] [ 76 ]  The TM domains are shown in light blue, the α1 and α2 domains are shown in green. The orientation in the membrane and the positions of the N- and C-termini are indicated and the figure is not drawn to scale.


The Mg 2+ -dependent currents carried by XntAp are kinetically like that of a channel protein and have an ion selectivity order of Mg 2+  > Co 2+ , Mn 2+  > Ca 2+  — a series again very similar to that of CorA. [ 72 ]  Unlike the other transport proteins reported so far, XntAp is dependent on intracellular Ca 2+ . The transport is also dependent on ΔΨ, but again Mg 2+  is not transported to equilibrium, being limited to approximately 0.4 mM free Mg 2+  in the cytoplasm. The existence of an intracellular compartment with a much higher free concentration of Mg 2+  (8 mM) was supported by the results.


The investigation of Mg 2+  in animals, including humans, has lagged behind that in bacteria and yeast. This is largely because of the complexity of the systems involved, but also because of the impression within the field that Mg 2+  was maintained at high levels in all cells and was unchanged by external influences. Only in the last 25 years has a series of reports begun to challenge this view, with new methodologies finding that free Mg 2+  content is maintained at levels where changes might influence cellular metabolism. [ 77 ]


A bioinformatic search of the sequence databases identified one homologue of the MRS2 gene of yeast in a range of metazoans. [ 8 ]  The protein has a very similar sequence and predicted TM topology to the yeast protein, and the GMN motif is intact at the end of the first TM domain. The human protein, hsaMrs2p, has been localised to the mitochondrial membrane in mouse cells using a GFP fusion protein.


Very little is known about the Mg 2+  transport characteristics of the protein in mammals, but Zsurka  et al.  (2001) has shown that the human Mrs2p complements the mrs2 mutants in the yeast mitochondrial Mg 2+  uptake system. [ 8 ]


The identification of this gene family in the metazoa began with a signal sequence trap method for isolating secreted and membrane proteins. [ 9 ]  Much of the identification has come from bioinformatic analyses. Three genes were eventually identified in humans, another three in mouse and three in  Caenorhabditis elegans , with a single gene in  Anopheles gambiae . The pFAM database lists the MgtE domain as pFAM01769 and additionally identifies a MgtE domain-containing protein in  Drosophila melanogaster . The proteins containing the MgtE domain can be divided into seven classes, as defined by pFAM using the type and organisation of the identifiable domains in each protein. Metazoan proteins are present in three of the seven groups. All of the metazoa proteins contain two MgtE domains, but some of these have been predicted only by context recognition (Coin, Bateman and Durbin, unpublished. See the pFAM website for further details).


The human SLC41A1 protein contains two MgtE domains with 52% and 46% respective similarity to the PF01769 consensus sequence and is predicted to contain ten TM domains, five in each MgtE domain (see figure), which suggests that the MgtE protein of bacteria may work as a dimer.


Adapted from Wabakken  et al.  (2003) [ 9 ]  and the pFAM database, the figure shows the computer predicted membrane topology of MgtE in  H. sapiens . The TM domains are shown in light blue, the orientation in the membrane and the positions of the N- and C-termini are indicated, and the figure is not drawn to scale.


Wabakken  et al.  (2003) [ 9 ]  found that the transcript of the SLC41A1 gene was expressed in all human tissues tested, but at varying levels, with the heart and testis having the highest expression of the gene. No explanation of the expression pattern has been suggested with regard to Mg 2+ -related physiology.


It has not been shown whether the SLC41 proteins transport Mg 2+  or complement a Mg 2+  transport mutation in any experimental system. However, it has been suggested that as MgtE proteins have no other known function, they are likely to be Mg 2+  transporters in the metazoa as they are in the bacteria. [ 9 ]  This will need to be verified using one of the now standard experiment systems for examining Mg 2+  transport.


The investigation of the TRPM genes and proteins in human cells is an area of intense recent study and, at times, debate. Montell  et al.  (2002) [ 78 ]  have reviewed the research into the TRP genes, and a second review by Montell (2003) [ 79 ]  has reviewed the research into the TRPM genes.


The TRPM family of ion channels has members throughout the metazoa. The TRPM6 and TRPM7 proteins are highly unusual, containing both an ion channel domain and a kinase domain (Figure 1.7), the role of which brings about the most heated debate. [ 79 ]


The activity of these two proteins has been very difficult to quantify. TRPM7 by itself appears to be a Ca 2+  channel [ 80 ]  but in the presence of TRPM6 the affinity series of transported cations places Mg 2+  above Ca 2+ . [ 10 ] [ 81 ]  The differences in reported conductance were caused by the expression patterns of these genes. TRPM7 is expressed in all cell types tested so far, while TRPM6 shows a more restricted pattern of expression. [ 82 ]  An unfortunate choice of experimental system by Voets  et al. , (2004) [ 83 ]  led to the conclusion that TRPM6 is a functional Mg 2+  transporter. However, later work by Chubanov  et al.  (2004) [ 82 ]  clearly showed that TRPM7 is required for TRPM6 activity and that the results of Voets  et al.  are explained by the expression of TRPM7 in the experimental cell line used by Voets  et al.  in their experiments. Whether TRPM6 is functional by itself is yet to be determined.


The predicted TM topology of the TPRM6 and TRPM7 proteins has been adapted from Nadler  et al.  (2001), [ 10 ]  Runnels  et al.  (2001) [ 84 ]  and Montell  et al.  (2002), [ 78 ]  this figure shows the computer predicted membrane topology of the TRPM6 and TRPM7 proteins in  Homo sapiens . At this time, the topology shown should be considered a tentative hypothesis. The TM domains are shown in light blue, the pore loop in purple, the TRP motif in red and the kinase domain in green. The orientation in the membrane and the positions of the N- and C-termini are indicated and the figure is not drawn to scale.


The conclusions of the Voets  et al.  (2004) [ 83 ]  paper are probably incorrect in attributing the Mg 2+  dependent currents to TRPM7 alone, and their kinetic data are likely to reflect the combined TRPM7/ TRPM6 channel. The report presents a robust collection of data consistent with a channel-like activity passing Mg 2+ , based on both electrophysiological techniques and also mag-fura 2 to determine changes in cytoplasmic free Mg 2+ .


Claudins  allow for Mg 2+  transport via the  paracellular  pathway; that is, it mediates the transport of the ion through the tight junctions between cells that form an epithelial cell layer. In particular, Claudin-16 allows the selective reuptake of Mg 2+  in the human kidney. Some patients with mutations in the  CLDN19  gene also have altered magnesium transport. [ 85 ] [ 86 ]


The gene  Claudin-16  was cloned by Simon  et al.  (1999), [ 12 ]  but only after a series of reports described the Mg 2+  flux itself with no gene or protein. [ 87 ] [ 88 ] [ 89 ]  The expression pattern of the gene was determined by RT-PCR, and was shown to be very tightly confined to a continuous region of the kidney tubule running from the medullary thick descending limb to the distal convoluted tubule. [ 12 ]  This localisation was consistent with the earlier reports for the location of Mg 2+  re-uptake by the kidney. Following the cloning, mutations in the gene were identified in patients with familial hypomagnesaemia with hypercalciuria and nephrocalcinosis, [ 90 ] [ 91 ]  strengthening the links between the gene and the uptake of Mg 2+ .


The current knowledge of the molecular mechanisms for Mg 2+  transport in plants is very limited, with only three publications reporting a molecular basis for Mg 2+  transport in plants. [ 13 ] [ 14 ] [ 15 ]  However, the importance of Mg 2+  to plants has been well described, and physiological and ecophysiological studies about the effects of Mg 2+  are numerous. This section will summarise the knowledge of a gene family identified in plants that is distantly related to CorA. Another gene, a Mg 2+ /H +  exchanger (AtMHX [ 15 ] ), unrelated to this gene family and to CorA has also been identified, is localised to the vacuolar membrane, and will be described last.


Schock  et al.  (2000) identified and named the family AtMRS2 based on the similarity of the genes to the MRS2 gene of yeast. [ 13 ]  The authors also showed that the AtMRS2-1 gene could complement a Δmrs2 yeast mutant phenotype. Independently, Li  et al.  (2001) [ 14 ]  published a report identifying the family and showing that two additional members could complement Mg 2+  transport deficient mutants, one in  S. typhimurium  and the other in  S. cerevisiae .


The three genes that have been shown to transport Mg 2+  are AtMRS2-1, AtMRS2-10 and AtMRS2-11, and these genes produce proteins 442, 443 and 459 amino acids in size, respectively. Each of the proteins shows significant similarity to Mrs2p of yeast and a weak similarity to CorA of bacteria, contains the conserved GMN amino acid motif at the outside end of the first TM domain, and is predicted to have two TM domains.


The AtMRS2-1 gene, when expressed in yeast from the MRS2 promoter and being fused C-terminally to the first 95 amino acids of the Mrs2p protein, was directed to the mitochondria, where it complemented a Δmrs2 mutant both phenotypically (mitochondrial RNA splicing was restored) and with respect to the Mg 2+  content of the organelle. [ 13 ]  No data on the kinetics of the transport was presented. The AtMRS2-11 gene was analysed in yeast (in the alr1 alr2 strain), where it was shown that expression of the gene significantly increased the rate of Mg 2+  uptake into starved cells over the control, as measured using flame atomic absorption spectroscopy of total cellular Mg 2+  content. However, Alr1p was shown to be significantly more effective at transporting Mg 2+  at low extracellular concentrations, suggesting that the affinity of AtMRS2-11 for Mg 2+  is lower than that of Alr1p. [ 14 ]  An electrophysiological (voltage clamp) analysis of the AtMRS2-11 protein in Xenopus oocytes also showed a Mg 2+ -dependent current at membrane potentials (ΔΨ) of –100 – –150 mV inside. [ 92 ]  These values are physiologically significant, as several membranes in plants maintain ΔΨ in this range. However, the author had difficulty reproducing these results due to an apparent "death" of oocytes containing the AtMRS2-11 protein, and therefore these results should be viewed with caution.


The AtMRS2-10 transporter has been analysed using radioactive tracer uptake analysis. [ 14 ]  63Ni 2+  was used as the substitute ion and Mg 2+  was shown to inhibit the uptake of 63Ni 2+  with a Ki of 20 μM. Uptake was also inhibited by Co(III)Hex and by other divalent cations. Only Co 2+  and Cu 2+  inhibited transport with Ki values less than 1 mM.


The AtMRS2-10 protein was fused to GFP, and was shown to be localised to the plasma membrane. [ 14 ]  A similar experiment was attempted in the Schock  et al.  (2000) paper, [ 13 ]  but the observed localisation was not significantly different from that seen with unfused GFP. The most likely reason for the lack of a definitive localisation of AtMRS2-1 in the Schock  et al.  paper is that the authors removed the TM domains from the protein, thereby precluding its insertion into a membrane.


The exact physiological significance of the AtMRS2-1 and AtMRS2-10 proteins in plants has yet to be clarified. The AtMRS2-11 gene has been overexpressed (from the CaMV 35S promoter) in A. thaliana. [ 92 ]  The transgenic line has been shown to accumulate high levels of the AtMRS2-11 transcript. A strong Mg 2+  deficiency phenotype (necrotic spots on the leaves, see Chapter 1.5 below) was recorded during the screening process (in both the T1 and T2 generations) for a homozygote line, but this phenotype was lost in the T3 generation and could not be reproduced when the earlier generations were screened a second time. The author suggested that environmental effects were the most likely cause of the inconsistent phenotype.


The first magnesium transporter isolated in any multicellular organism, AtMHX shows no similarity to any previously isolated Mg 2+  transport protein. [ 15 ]  The gene was initially identified in the A. thaliana genomic DNA sequence database, by its similarity to the SLC8 family of Na+/Ca 2+  exchanger genes in humans.


The cDNA sequence of 1990 bp is predicted to produce a 539-amino acid protein. AtMHX is quite closely related to the SLC8 family at the amino acid level and shares a topology with eleven predicted TM domains (Figure A10.5). There is one major difference in the sequence, in that the long non-membranal loop (see Figure A10.5) is 148 amino acids in the AtMHX protein but 500 amino acids in the SLC8 proteins. However, this loop is not well conserved and is not required for transport function in the SLC8 family. [ 15 ]


The AtMHX gene is expressed throughout the plant but most strongly in the vascular tissue. [ 15 ]  The authors suggest that the physiological role of the protein is to store Mg 2+  in these tissues for later release when needed. The protein localisation to the vacuolar membrane supports this suggestion (see also Chapter 1.5).


The protein transports Mg 2+  into the vacuolar space and H +  out, as demonstrated by electrophysiological techniques. [ 15 ]  The transport is driven by the ΔpH maintained between the vacuolar space (pH 4.5 – 5.9) and the cytoplasm (pH 7.3 – 7.6) by an H + -ATPase. [ 93 ] [ 94 ]  How the transport of Mg 2+  by the protein is regulated was not determined. Currents were observed to pass through the protein in both directions, but the Mg 2+  out current required a ‘cytoplasmic’ pH of 5.5, a condition not found in plant cells under normal circumstances. In addition to the transport of Mg 2+ , Shaul  et al.  (1999) [ 15 ]  also showed that the protein could transport Zn 2+  and Fe 2+ , but did not report on the capacity of the protein to transport other divalent cations (e.g. Co 2+  and Ni 2+ ) or its susceptibility to inhibition by cobalt (III) hexaammine.


The detailed kinetics of Mg 2+  transport have not been determined for AtMHX. However, physiological effects have been demonstrated. When A. thaliana plants were transformed with overexpression constructs of the AtMHX gene driven by the CaMV 35S promoter, the plants over-accumulated the protein and showed a phenotype of necrotic lesions in the leaves, which the authors suggest is caused by a disruption in the normal function of the vacuole, given their observation that the total Mg 2+  (or Zn 2+ ) content of the plants was not altered in the transgenic plants.


The image has been adapted from Shaul  et al.  (1999) [ 15 ]  and Quednau et al. (2004), [ 73 ]  and combined with an analysis using HMMTOP, this figure shows the computer predicted membrane topology of the AtMHX protein in  Arabidopsis thaliana . At this time the topology shown should be considered a tentative hypothesis. The TM domains are shown in light blue, the orientation in the membrane and the positions of the N- and C-termini are indicated, and the figure is not drawn to scale. The α1 and α2 domains, shown in green, are both quite hydrophobic and may both be inserted into the membrane.


Manganese  is an essential biological element in all organisms. [ 1 ]  It is used in many  enzymes  and proteins. [ 2 ] [ 3 ]  It is essential in  plants . [ 4 ]


The classes of  enzymes  that have manganese  cofactors  include  oxidoreductases ,  transferases ,  hydrolases ,  lyases ,  isomerases  and  ligases . Other enzymes containing manganese are  arginase  and  Mn-containing  superoxide dismutase  ( Mn-SOD ).  Also the enzyme class of  reverse transcriptases  of many  retroviruses  (though not  lentiviruses  such as  HIV ) contains manganese. Manganese-containing  polypeptides  are the  diphtheria toxin ,   lectins  and  integrins . [ 2 ]


Manganese is an essential human dietary element. It is present as a  coenzyme  in several biological processes, which include macronutrient metabolism, bone formation, and  free radical  defense systems. It is a critical component in dozens of proteins and enzymes. [ 3 ]  The human body contains about 12 mg of manganese, mostly in the bones. The soft tissue remainder is concentrated in the liver and kidneys. [ 5 ]  In the human brain, the manganese is bound to manganese  metalloproteins , most notably  glutamine synthetase  in  astrocytes . [ 6 ]


The  U.S. Institute of Medicine  (IOM) updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for minerals in 2001. For manganese, there was not sufficient information to set EARs and RDAs, so needs are described as estimates for  Adequate Intakes  (AIs). As for safety, the IOM sets  Tolerable upper intake levels  (ULs) for vitamins and minerals when evidence is sufficient. In the case of manganese, the adult UL is set at 11 mg/day. Collectively the EARs, RDAs, AIs and ULs are referred to as  Dietary Reference Intakes  (DRIs). [ 7 ]  Manganese deficiency is rare. [ 8 ]


The  European Food Safety Authority  (EFSA) refers to the collective set of information as Dietary Reference Values, with Population Reference Intake (PRI) instead of RDA, and Average Requirement instead of EAR. AI and UL are defined the same as in the United States. For people ages 15 and older, the AI is set at 3.0 mg/day. AIs for pregnancy and lactation is 3.0 mg/day. For children ages 1–14 years, the AIs increase with age from 0.5 to 2.0 mg/day. The adult AIs are higher than the U.S. RDAs. [ 9 ]  The EFSA reviewed the same safety question and decided that there was insufficient information to set a UL. [ 10 ]


For U.S. food and dietary supplement labeling purposes, the amount in a serving is expressed as a percent of Daily Value (%DV). For manganese labeling purposes, 100% of the Daily Value was 2.0 mg, but as of 27 May 2016 it was revised to 2.3 mg to bring it into agreement with the RDA. [ 11 ] [ 12 ]  A table of the old and new adult daily values is provided at  Reference Daily Intake .


Excessive exposure or intake may lead to a condition known as  manganism , a  neurodegenerative  disorder that causes  dopaminergic  neuronal death and symptoms similar to  Parkinson's disease . [ 5 ] [ 13 ]


Manganese deficiency in humans, which is rare, results in a number of medical problems. Many common vitamin and mineral supplement products fail to include manganese in their compositions. Relatively high dietary intake of other minerals such as iron, magnesium, and calcium may inhibit the proper intake of manganese. A deficiency of manganese causes skeletal deformation in animals and inhibits the production of collagen in wound healing.


Many enzymatic systems need Mn to function, but in high levels, Mn can become toxic. One environmental reason Mn levels can increase in seawater is when hypoxic periods occur. [ 14 ]  Since 1990 there have been reports of Mn accumulation in marine organisms including fish, crustaceans, mollusks, and echinoderms. Specific tissues are targets in different species, including the gills, brain, blood, kidney, and liver/hepatopancreas. Physiological effects have been reported in these species. Mn can affect the renewal of immunocytes and their functionality, such as phagocytosis and activation of pro-phenoloxidase, suppressing the organisms' immune systems. This causes the organisms to be more susceptible to infections. As climate change occurs, pathogen distributions increase, and in order for organisms to survive and defend themselves against these pathogens, they need a healthy, strong immune system. If their systems are compromised from high Mn levels, they will not be able to fight off these pathogens and die. [ 15 ]


Mn-SOD is the type of SOD present in  eukaryotic   mitochondria , and also in most bacteria (this fact is in keeping with the bacterial-origin theory of mitochondria). The Mn-SOD enzyme is probably one of the most ancient, for nearly all organisms living in the presence of oxygen use it to deal with the toxic effects of  superoxide  ( O − 2 ), formed from the 1-electron reduction of dioxygen. The exceptions, which are all bacteria, include  Lactobacillus plantarum  and related  lactobacilli , which use a different nonenzymatic mechanism with manganese (Mn 2+ ) ions complexed with polyphosphate, suggesting a path of evolution for this function in aerobic life.


Manganese is also important in photosynthetic  oxygen evolution  in  chloroplasts  in plants. The  oxygen-evolving complex  (OEC) is a part of  photosystem II  contained in the  thylakoid membranes  of chloroplasts; it is responsible for the terminal  photooxidation of water  during the  light reactions  of  photosynthesis , and has a metalloenzyme core containing four atoms of manganese. [ 16 ] [ 17 ]  To fulfill this requirement, most broad-spectrum plant fertilizers contain manganese.


Molybdenum  is an essential element in most organisms. [ 1 ]  It is most notably present in  nitrogenase [ 2 ]  which is an essential part of  nitrogen fixation . [ 3 ] [ 4 ]


Molybdenum is an essential element in most organisms; a 2008 research paper speculated that a scarcity of molybdenum in the Earth's early oceans may have strongly influenced the evolution of  eukaryotic life  (which includes all plants and animals). [ 1 ]


At least 50 molybdenum-containing enzymes have been identified, mostly in bacteria. [ 5 ] [ 6 ]  Those enzymes include  aldehyde oxidase ,  sulfite oxidase  and  xanthine oxidase . [ 7 ]  With one exception, Mo in proteins is bound by  molybdopterin  to give the molybdenum cofactor. The only known exception is  nitrogenase , which uses the  FeMoco  cofactor, which has the formula Fe 7 MoS 9 C. [ 8 ]


In terms of function, molybdoenzymes catalyze the oxidation and sometimes reduction of certain small molecules in the process of regulating  nitrogen ,  sulfur , and  carbon . [ 9 ]  In some animals, and in humans, the oxidation of  xanthine  to  uric acid , a process of  purine   catabolism , is catalyzed by  xanthine oxidase , a molybdenum-containing enzyme. The activity of xanthine oxidase is directly proportional to the amount of molybdenum in the body. An extremely high concentration of molybdenum reverses the trend and can inhibit purine catabolism and other processes. Molybdenum concentration also affects  protein synthesis ,  metabolism , and growth. [ 10 ]


Mo is a component in most  nitrogenases . Among molybdoenzymes, nitrogenases are unique in lacking the molybdopterin. [ 11 ] [ 12 ]  Nitrogenases catalyze the production of ammonia from atmospheric nitrogen:


The  biosynthesis  of the  FeMoco   active site  is highly complex. [ 13 ]


Molybdate is transported in the body as MoO 4 2− . [ 10 ]


Molybdenum is an essential trace  dietary element . [ 14 ]  Four mammalian Mo-dependent enzymes are known, all of them harboring a  pterin -based  molybdenum cofactor  (Moco) in their active site:  sulfite oxidase ,  xanthine oxidoreductase ,  aldehyde oxidase , and  mitochondrial amidoxime reductase . [ 15 ]  People severely deficient in molybdenum have poorly functioning sulfite oxidase and are prone to toxic reactions to sulfites in foods. [ 16 ] [ 17 ]  The human body contains about 0.07 mg of molybdenum per kilogram of body weight, [ 18 ]  with higher concentrations in the liver and kidneys and lower in the vertebrae. [ 19 ]  Molybdenum is also present within human  tooth enamel  and may help prevent its decay. [ 20 ]


Acute toxicity has not been seen in humans, and the toxicity depends strongly on the chemical state. Studies on rats show a  median lethal dose  (LD 50 ) as low as 180 mg/kg for some Mo compounds. [ 21 ]  Although human toxicity data is unavailable, animal studies have shown that chronic ingestion of more than 10 mg/day of molybdenum can cause diarrhea, growth retardation,  infertility , low birth weight, and  gout ; it can also affect the lungs, kidneys, and liver. [ 22 ] [ 23 ]   Sodium tungstate  is a  competitive inhibitor  of molybdenum. Dietary tungsten reduces the concentration of molybdenum in tissues. [ 19 ]


Low soil concentration of molybdenum in a geographical band from northern China to Iran results in a general dietary  molybdenum deficiency , and is associated with increased rates of  esophageal cancer . [ 24 ] [ 25 ] [ 26 ]  Compared to the United States, which has a greater supply of molybdenum in the soil, people living in those areas have about 16 times greater risk for  esophageal   squamous cell carcinoma . [ 27 ]


Molybdenum deficiency has also been reported as a consequence of non-molybdenum supplemented  total parenteral nutrition  (complete intravenous feeding) for long periods of time. It results in high blood levels of  sulfite  and  urate , in much the same way as  molybdenum cofactor deficiency . Since pure molybdenum deficiency from this cause occurs primarily in adults, the neurological consequences are not as marked as in cases of congenital cofactor deficiency. [ 28 ]


A congenital  molybdenum cofactor deficiency  disease, seen in infants, is an inability to synthesize  molybdenum cofactor , the heterocyclic molecule discussed above that binds molybdenum at the active site in all known human enzymes that use molybdenum. The resulting deficiency results in high levels of  sulfite  and  urate , and neurological damage. [ 29 ] [ 30 ]


Most molybdenum is excreted from the human body as molybdate in the urine. Furthermore, urinary excretion of molybdenum increases as dietary molybdenum intake increases. Small amounts of molybdenum are excreted from the body in the feces by way of the bile; small amounts also can be lost in sweat and in hair. [ 31 ] [ 32 ]


High levels of molybdenum can interfere with the body's uptake of  copper , producing  copper deficiency . Molybdenum prevents plasma proteins from binding to copper, and it also increases the amount of copper that is excreted in  urine .  Ruminants  that consume high levels of molybdenum suffer from  diarrhea , stunted growth,  anemia , and  achromotrichia  (loss of fur pigment). These symptoms can be alleviated by copper supplements, either dietary or injection. [ 33 ]  The effective copper deficiency can be aggravated by excess  sulfur . [ 19 ] [ 34 ]


Copper reduction or deficiency can also be deliberately induced for therapeutic purposes by the compound  ammonium tetrathiomolybdate , in which the bright red anion  tetrathiomolybdate  is the copper-chelating agent. Tetrathiomolybdate was first used therapeutically in the treatment of  copper toxicosis  in animals. It was then introduced as a treatment in  Wilson's disease , a hereditary copper metabolism disorder in humans; it acts both by competing with copper absorption in the bowel and by increasing excretion. It has also been found to have an inhibitory effect on  angiogenesis , potentially by inhibiting the membrane translocation process that is dependent on copper ions. [ 35 ]  This is a promising avenue for investigation of treatments for  cancer ,  age-related macular degeneration , and other diseases that involve a pathologic proliferation of blood vessels. [ 36 ] [ 37 ]


In some grazing livestock, most strongly in cattle, molybdenum excess in the soil of pasturage can produce scours ( diarrhea ) if the pH of the soil is neutral to alkaline; see  teartness .


Nickel allergy  is any of several  allergic  conditions provoked by exposure to the chemical element  nickel . Nickel allergy often takes the form of  nickel allergic contact dermatitis  ( Ni-ACD ), a form of  allergic contact dermatitis  (ACD). Ni-ACD typically causes a rash that is red and itchy and that may be bumpy or scaly.  The main treatment for it is avoiding contact with nickel-releasing metals, such as inexpensive jewelry. Another form of nickel allergy is a  systemic  form:  systemic nickel allergy syndrome  ( SNAS ) can mimic some of the symptoms of  irritable bowel syndrome  (IBS) and also has a dermatologic component. [ 1 ]


The most common sign of nickel allergy is inflammation of the skin at an area that comes into regular contact with nickel. [ 2 ]  This often takes the form of a  reddened  patch of skin with raised bumps ( papules ) or small blisters ( vesicles ), and  edema . [ 2 ]  People with chronic dermatitis tend to have dry, scaly, and cracked skin at the site of contact. [ 2 ]  These sites of inflammation (called "primary eruptions") can occur anywhere on the skin that contacts nickel, but are most common on the hands, face, or anywhere that contacts metal objects such as jewelry or metal clothes buttons. [ 2 ]  Particularly high levels of nickel exposure can cause irritated patches of skin to appear at other sites on the body (called "secondary eruptions"). These typically occur as blistering rashes on the hands, eyelids, and at the inside of flexing joints (inside the elbow, back of the knee, etc.). [ 2 ]


Ingestion of nickel may cause a systemic reaction, which can result in generalized inflammation of the skin across the body, small blisters in the hands, irritation inside the flexing joints (flexural eczema), and  redness and irritation of both buttocks . [ 2 ]


Systemic contact dermatitis  (SCD) is defined as a  dermatitis  occurring in an epi-cutaneously contact-sensitized person when exposed to  haptens  systemically such as  orally ,  per rectum ,  intravesically ,  transcutaneously ,  intrauterinely ,  intravenously , or by  inhalation . [ 3 ]


The  pathophysiology  of systemic nickel allergy syndrome (SNAS) is not well understood. The  clinical course  is determined by an immunological interplay between two types of  T cells  (Th1 and  Th2  responses). SCD is often considered a subset of SNAS, but with only skin manifestations. [ 4 ]  SNAS presents with an array of symptoms ranging from respiratory to generalized skin rash to gastrointestinal symptoms. [ 5 ]  The gastrointestinal symptoms may mimic those of  irritable bowel syndrome . [ 1 ]  A meta review evaluating SNAS found that 1% of patients sensitized to nickel reacted to the nickel content of a 'normal' diet, and with increasing doses of nickel more individuals reacted [ 6 ]  SNAS is a multilayered immunological response demonstrating variance between individuals and doses of nickel exposure.


Nickel is both naturally abundant – it is the fifth most common element on earth – and widely used in industry and commercial goods. [ 2 ]  Workplace nickel exposure is common in many industries, and the performance of normal work tasks can result in nickel skin levels sufficient to elicit dermatitis. [ 2 ]  Within the workplace, individuals may be exposed to significant amounts of nickel, airborne from the combustion of  fossil fuels  or from contact with tools that are  nickel plated . [ 7 ]  Historically, workplaces where prolonged contact with soluble nickel has been high have shown high risks for allergic contact nickel dermatitis. For example, nickel dermatitis was common in the past among nickel platers. [ 8 ]  Outbreaks of nickel allergy from consumer goods have been documented throughout the 20th century, with jewelry, stocking suspenders, and metallic buttons on blue jeans each resulting in dermatitis at the point of contact. [ 2 ]  Nickel can also be present in food and drinking water; ingestion of increased nickel is not associated with systemic allergic disease, but is associated with flare-ups of dermatitis or aggravation of vesicular hand eczema. [ 2 ]  Similarly, aggravation of dermatitis has been reported in response to nickel-containing surgical implants or dental gear. [ 2 ]


The risk of an object eliciting nickel allergy is linked to the amount of nickel released by its surface (and not to its total nickel content). [ 2 ]  Suspected objects can be screened by wiping the surface with a 1%  dimethylglyoxime  solution that turns pink if more than 0.5 μg/cm 2  per week is released by the surface. [ 2 ]  Various methods exist to test the skin or nails for nickel exposure, typically relying on wiping the skin, then quantifying the nickel on the wipe via  mass spectrometry . [ 2 ]


Dietary nickel exposure may come from high-nickel foods, possibly canned food (via the packaging), possibly stainless steel cookware (whereas some grades of stainless steel contain more nickel than others), or plumbing (especially the first water run from the tap in the morning). [ 1 ]


Nickel allergy results in a skin response after the skin comes in contact with an item that releases a large amount of nickel from its surface. It is commonly associated with nickel-containing belt buckles coming into prolonged contact with the skin. [ 9 ] [ 10 ] [ 11 ]  The skin reaction can occur at the site of contact, or sometimes spread beyond to the rest of the body. Free (released) nickel that is able to penetrate the  skin  is taken up by scavenger ( dendritic ) cells and then presented to the  immune system  T-Cells. With each subsequent exposure to nickel these  T cells  become stimulated and  duplicate  themselves. With enough exposure to nickel, the amassing clones of T-cells reach "threshold" and the skin develops a  rash . The rash can appear as acute, subacute, or chronic  eczema-like  skin patches, primarily at the site of contact with the nickel (e.g., earlobe from nickel earrings). From the time of exposure, the rash usually appears within 12–120 hours and can last for 3–4 weeks or for the continued duration of nickel contact/exposure. [ 9 ]


Three simultaneous conditions must occur to trigger Ni-ACD:


The  pathophysiology  is divided into  induction elicitation phases . Induction is the critical phase (immunological event) when skin contact to nickel results in antigen presentation to the T cells, and T cell duplication (cloning) occurs. The metal cation Ni ++  is a low molecular weight  hapten  that easily penetrates the  stratum corneum  (top layer of skin). Nickel then binds to skin protein carriers creating an antigenic  epitope . [ 13 ]  The determining factor in  sensitization  is exposure of significant amounts of "free nickel". [ 14 ]  This is important because different metal  alloys  release different amounts of free nickel. The antigenic epitope is collected by dermal dendritic cells and  Langerhans cells , the  antigen-presenting cells  (APC) of the skin, and undergo maturation and migration to regional  lymph nodes . The complex is predominantly expressed on  major histocompatibility complex  ( MHC) II , which activates and clonally expands naive  CD4+ T cells . [ 15 ]  Upon re-exposure these now primed T cells will be activated and massively recruited to the skin, resulting in the elicitation phase and the clinical presentation of Ni-ACD.


Although ACD has been considered a  Th1  predominate process, recent studies highlight a more complex picture. In Ni-ACD other cells are involved including:  Th17 , Th22, Th1/ IFN  and the  innate immune  responses consistent with  toll-like receptor 4 . [ 16 ]
 [ 17 ]


Nickel has a wide utility of application in manufactured metals because it is both strong and malleable, leading to ubiquitous presence and the potential for consumers to be in contact with it daily. However, for those who have the rash of allergic contact dermatitis (ACD) due to a nickel allergy, it can be a challenge to avoid. Foods, common kitchen utensils, cell phones, jewelry, and many other items may contain nickel and be a source of irritation due to the allergic reaction caused by the absorption of free released nickel through direct and prolonged contact. The most appropriate measure for nickel-allergic persons is to prevent contact with the allergen.


In 2011, researchers showed that applying a thin layer of  glycerine   emollient  containing  nanoparticles  of either  calcium carbonate  or  calcium phosphate  on an isolated piece of pig skin ( in vitro ) and on the skin of mice ( in vivo ) prevents the penetration of nickel ions into the skin. The  nanoparticles  capture nickel ions by cation exchange, and remain on the surface of the skin, allowing them to be removed by simple washing with water. Approximately 11-fold fewer nanoparticles by mass are required to achieve the same efficacy as the  chelating  agent  ethylenediamine tetraacetic acid . Using nanoparticles with diameters smaller than 500 nm in topical creams may be an effective way to limit the exposure to  metal ions  that can cause skin irritation'. [ 18 ]


Pre-emptive avoidance strategies (PEAS) might ultimately lower the sensitization rates of children who would develop ACD [ 19 ]  It is theorized that prevention of exposure to nickel early on could reduce the number of those that are sensitive to nickel by one-quarter to one-third. Identification of the many sources of nickel is vital to understanding the nickel sensitization story, food like chocolate and fish, zippers, buttons, cell phones and even orthodontic braces and eyeglass frames might contain nickel.  Items that contain sentimental value (heirlooms, wedding rings) could be treated with an  enamel  or  rhodium plating . [ 20 ]


The Dermatitis Academy has created an educational website to provide more information about nickel, including information about prevention, exposure, sources, and general information about nickel allergy. These resources provide guidance in a prevention initiative for children worldwide.


Prevention of SNAS includes modifying dietary choices to avoid certain foods that are higher in nickel than others. [ 1 ]


Nickel allergy is typically diagnosed by  patch testing  – applying a patch with 2.5% (in North America) or 5% (in Europe)  nickel sulfate  to the upper back and looking for irritation on the skin. [ 2 ]  As with other causes of allergic contact dermatitis, patches containing several common allergens are typically applied to the back for 48 hours, removed, then the spots examined for allergic reactions 2 to 5 days later. [ 21 ]


SNAS can often mimic IBS [ 1 ]  and may be more common than is widely appreciated. [ 1 ]  It therefore should be considered as a  differential diagnosis  item when a doctor is considering a diagnosis of IBS, [ 1 ]  and nickel allergy testing is advisable as a means to exclude or confirm SNAS. [ 1 ]  Even before such testing, some differentiating factors in the  medical history  are if certain foods prompt the symptoms (for example, peanuts or shellfish), [ 1 ]  whereas IBS is not specific to those foods. [ 1 ]


Once a nickel allergy is detected, the best treatment is avoidance of nickel-releasing items. The top 13 categories that contain nickel include beauty accessories, eyeglasses, money, cigarettes, clothes, kitchen and household, electronics and office equipment, metal utensils, aliment, jewelry, batteries, orthodontic and dental appliances, and medical equipment. [ 22 ]  Other than strict avoidance of items that release free nickel, there are other treatment options for reduction of exposure. The first step is to limit friction between skin and metallic items. Susceptible people may try to limit sweating while wearing nickel items, to reduce nickel release and thus decrease chances for developing sensitization or allergy. Another option is to shield electronics, metal devices, and tools with fabric, plastic, or acrylic coverings. [ 22 ]


There are  dimethylglyoxime  test kits that can be very helpful to check for nickel release from items prior to purchasing. [ 23 ]  The American Contact Dermatitis Society 'find a provider' resource can help identify clinicians with training in providing guidance lists of safe items. [ 24 ]   In addition to avoidance, healthcare providers may prescribe additional creams or medications to help relieve the skin reaction.


Nickel allergy is the most common  contact allergy  in industrialized countries, affecting around 8% to 19% of adults and 8% to 10% of children. [ 2 ]  Women are affected 4–10 times as frequently as men. [ 2 ]


As nickel can be harmful to skin, its use in daily products must be regulated. A safety directive has been in place in Europe since 2004.  Denmark  in 1980, and then shortly after the  European Union  (EU), enacted legislation that limited the amount of free nickel in consumer products that come in contact with the skin. This resulted in significantly decreased rates of sensitization among Danish children 0 to 18 years of age from 24.8% to 9.2% between 1985 and 1998, with similar reductions in sensitization throughout the EU. [ 25 ] [ 26 ]


No such directive exists in the United States, but efforts are under way to mandate safe use guidelines for nickel. In August 2015, the  American Academy of Dermatology  (AAD) adopted a nickel safety position paper. [ 27 ]  The exact prevalence of Ni-ACD in the general population in the US is largely unknown. However, current estimates gauge that roughly 2.5 million US adults and 250,000 children have a nickel allergy, which costs an estimated $5.7 billion per year for treatment of symptoms. [ 28 ]   Loma Linda University , Nickel Allergy Alliance, and Dermatitis Academy created the first open access self-reported patient registry to record nickel allergy prevalence data in the US.[ref 23] [ full citation needed ]


In the 17th century, copper miners in  Saxony , Germany, began to experience irritation caused by a "dark red ore". Since the substance, which would later be called nickel, led to many ailments, they believed it to be protected by " goblins ", and called it "Goblin's Copper". [ 29 ]   Josef Jadassohn  described the first case of metal  contact dermatitis  in 1895, to a  mercury -based therapeutic cream, and confirmed the cause by  epi-cutaneous  patch testing. [ 30 ]  In the next century nickel began to be mass-produced for jewelry worldwide due to its cheap cost, resistance to corrosion and high supply.


In 1979 a large comprehensive study of healthy US volunteers found that 9% had been unknowingly sensitized to nickel. [ 31 ]  As of 2008 [update] , that number has tripled. [ 32 ]  Most importantly, nickel allergy among children is increasing, with an estimated 250,000 children sensitized to nickel. [ 33 ]


Published literature shows an exponential increase in reported nickel allergy cases. [ 34 ]  The North American Contact Dermatitis Group (NACDG) patch tested 5,085 adults, presenting with eczema-like symptoms, showing 19.5% had a positive reaction to nickel. [ 35 ]  Nickel allergy is also more prevalent in women (17.1%) than men (3%), possibly due to cultural norms related to jewelry and ear piercings and therefore increased exposure to nickel. [ 36 ]  In order to investigate the current prevalence of nickel,  Loma Linda University , Nickel Allergy Alliance, and Dermatitis Academy, [ 37 ]  are conducting a self-reporting nickel allergy-dermatitis survey. [ 38 ]


A  nutrient  is a  substance  used by an organism to survive, grow and reproduce. The requirement for dietary nutrient intake applies to  animals ,  plants ,  fungi  and  protists . Nutrients can be incorporated into cells for  metabolic purposes  or  excreted  by cells to create non-cellular structures such as  hair ,  scales ,  feathers , or  exoskeletons . Some nutrients can be metabolically converted into smaller molecules in the process of releasing energy such as for  carbohydrates ,  lipids ,  proteins  and  fermentation  products ( ethanol  or  vinegar ) leading to end-products of water and  carbon dioxide . All organisms require water. Essential nutrients for animals are the energy sources, some of the  amino acids  that are combined to create  proteins , a subset of  fatty acids ,  vitamins  and certain  minerals . Plants require more diverse minerals absorbed through roots, plus carbon dioxide and oxygen absorbed through leaves.  Fungi  live on dead or living organic matter and meet nutrient needs from their host.


Different types of organisms have different essential nutrients. Ascorbic acid ( vitamin C ) is essential to humans and some animal species but most other animals and many plants are able to synthesize it. Nutrients may be  organic  or inorganic: organic compounds include most compounds containing carbon, while all other chemicals are inorganic. Inorganic nutrients include nutrients such as  iron ,  selenium , and  zinc , while organic nutrients include, protein, fats, sugars and vitamins.


A classification used primarily to describe nutrient needs of animals divides nutrients into  macronutrients  and  micronutrients . Consumed in relatively large amounts ( grams  or  ounces ), macronutrients (carbohydrates,  fats , proteins, water) are primarily used to generate energy or to incorporate into tissues for growth and repair. Micronutrients are needed in smaller amounts ( milligrams  or  micrograms ); they have subtle  biochemical  and  physiological  roles in cellular processes, like  vascular functions  or  nerve conduction . Inadequate amounts of essential nutrients or diseases that interfere with absorption, result in a deficiency state that compromises growth, survival and reproduction. Consumer advisories for dietary nutrient intakes such as the United States  Dietary Reference Intake , are based on the amount required to prevent deficiency and provide macronutrient and micronutrient guides for both lower and  upper limits  of intake. In many countries, regulations require that food product labels display information about the amount of any macronutrients and micronutrients present in the food in significant quantities. Nutrients in larger quantities than the body needs may have harmful effects. [ 1 ]  Edible plants also contain thousands of compounds generally called  phytochemicals  which have unknown effects on disease or health including a diverse class with non-nutrient status called  polyphenols  which remain poorly understood as of 2024.


Macronutrients are defined in several ways. [ 2 ]


Macronutrients provide energy:


Micronutrients  are essential dietary elements required in varying quantities throughout life to serve  metabolic  and  physiological functions . [ 6 ] [ 7 ]


An  essential nutrient  is a nutrient required for normal physiological function that cannot be synthesized in the body – either at all or in sufficient quantities – and thus must be obtained from a  dietary  source. [ 10 ] [ 11 ]  Apart from  water , which is universally required for the maintenance of  homeostasis  in mammals, [ 12 ]  essential nutrients are indispensable for various cellular  metabolic processes  and for the maintenance and function of tissues and organs. [ 13 ]  The nutrients considered essential for humans comprise nine amino acids, two fatty acids, thirteen  vitamins , fifteen  minerals  and  choline . [ 13 ]  In addition, there are several molecules that are considered conditionally essential nutrients since they are indispensable in certain developmental and pathological states. [ 13 ] [ 14 ] [ 15 ]


An essential amino acid is an amino acid that is required by an organism but cannot be synthesized  de novo  by it, and therefore must be supplied in its diet. Out of the twenty standard protein-producing amino acids, nine cannot be  endogenously  synthesized by humans:  phenylalanine ,  valine ,  threonine ,  tryptophan ,  methionine ,  leucine ,  isoleucine ,  lysine , and  histidine . [ 16 ] [ 17 ]


Essential fatty acids (EFAs) are  fatty acids  that humans and other animals must ingest because the body requires them for good health but cannot  synthesize  them. [ 18 ]  Only two fatty acids are known to be essential for humans:  alpha-linolenic acid  (an  omega-3 fatty acid ) and  linoleic acid  (an  omega-6 fatty acid ). [ 19 ]


Vitamins occur in a variety of related forms known as  vitamers . The vitamers of a given vitamin perform the functions of that vitamin and prevent symptoms of deficiency of that vitamin. Vitamins are those essential organic molecules that are not classified as amino acids or fatty acids. They commonly function as  enzymatic cofactors , metabolic regulators or  antioxidants . Humans require thirteen vitamins in their diet, most of which are actually groups of related molecules (e.g.  vitamin E  includes  tocopherols  and  tocotrienols ): [ 20 ]  vitamins A, C, D, E, K,  thiamine  (B 1 ),  riboflavin  (B 2 ),  niacin  (B 3 ),  pantothenic acid  (B 5 ),  pyridoxine  (B 6 ),  biotin  (B 7 ),  folate  (B 9 ), and  cobalamin  (B 12 ). The requirement for vitamin D is conditional, as people who get sufficient exposure to ultraviolet light, either from the sun or an artificial source, synthesize vitamin D in the skin. [ 21 ]


Minerals are the  exogenous   chemical elements  indispensable for life. Although the four elements:  carbon ,  hydrogen ,  oxygen , and  nitrogen  ( CHON ) are essential for life, they are so plentiful in food and drink that these are not considered nutrients and there are no recommended intakes for these as minerals. The need for nitrogen is addressed by requirements set for protein, which is composed of nitrogen-containing amino acids.  Sulfur  is essential, but again does not have a recommended intake. Instead, recommended intakes are identified for the sulfur-containing amino acids  methionine  and  cysteine .


The essential nutrient  trace elements  for humans, listed in order of  Recommended Dietary Allowance  (expressed as a mass), are  potassium ,  chloride ,  sodium ,  calcium ,  phosphorus ,  magnesium ,  iron ,  zinc ,  manganese ,  copper ,  iodine ,  chromium ,  molybdenum , and  selenium . Additionally,  cobalt  is a component of Vitamin B 12  which is essential. There are other minerals which are essential for some plants and animals, but may or may not be essential for humans, such as  boron  and  silicon .


Choline is an essential nutrient. [ 22 ] [ 23 ] [ 24 ]  The cholines are a family of water-soluble  quaternary ammonium compounds . [ 25 ] [ 26 ]  Choline is the parent compound of the cholines class, consisting of ethanolamine having three methyl substituents attached to the amino function. [ 27 ]  Healthy humans fed artificially composed diets that are deficient in choline develop fatty liver, liver damage, and muscle damage. Choline was not initially classified as essential because the human body can produce choline in small amounts through phosphatidylcholine metabolism. [ 28 ]


Conditionally essential nutrients are certain organic molecules that can normally be synthesized by an organism, but under certain conditions in insufficient quantities. In humans, such conditions include  premature birth , limited nutrient intake, rapid growth, and certain disease states. [ 14 ]   Inositol ,  taurine ,  arginine ,  glutamine  and  nucleotides  are classified as conditionally essential and are particularly important in neonatal diet and metabolism. [ 14 ]


Non-essential nutrients are substances within foods that can have a significant impact on health.  Dietary fiber  is not absorbed in the human digestive tract. [ 29 ]   Soluble fiber  is metabolized to butyrate and other short-chain fatty acids by bacteria residing in the large intestine. [ 30 ] [ 31 ] [ 32 ]  Soluble fiber is marketed as serving a  prebiotic  function with claims for promoting "healthy" intestinal bacteria. [ 33 ]


Ethanol  (C 2 H 5 OH) is not an essential nutrient, but it does supply approximately 29 kilojoules (7 kilocalories) of food energy per gram. [ 34 ]  For spirits (vodka, gin, rum, etc.) a standard serving in the United States is  44 millilitres ( 1 + 1 ⁄ 2  US fluid ounces), which at 40%   ethanol (80   proof) would be 14 grams and 410 kJ (98 kcal). At 50%   alcohol, 17.5 g and 513 kJ (122.5 kcal). Wine and beer contain a similar amount of ethanol in servings of 150 and 350 mL (5 and 12 US fl oz), respectively, but these beverages also contribute to food energy intake from components other than ethanol. A 150 mL (5 US fl oz) serving of wine contains 420 to 540 kJ (100 to 130 kcal). A 350 mL (12 US fl oz) serving of beer contains 400 to 840 kJ (95 to 200 kcal). [ citation needed ]


According to the U.S. Department of Agriculture, based on  NHANES  2013–2014 surveys, women ages 20 and up consume on average 6.8   grams of alcohol per day and men consume on average 15.5 grams per day. [ 35 ]  Ignoring the non-alcohol contribution of those beverages, the average ethanol contributions to daily food energy intake are 200 and 450 kJ (48 and 108 kcal), respectively. Alcoholic beverages are considered  empty calorie  foods because, while providing energy, they contribute no essential nutrients. [ 34 ]


By definition,  phytochemicals  include all nutritional and non-nutritional components of edible plants. [ 36 ]  Included as nutritional constituents are  provitamin  A  carotenoids , [ 37 ]  whereas those without nutrient status are diverse  polyphenols ,  flavonoids ,  resveratrol , and  lignans  that are present in numerous plant foods. [ 38 ]  Some phytochemical compounds are under preliminary research for their potential effects on human diseases and health. [ 36 ] [ 37 ] [ 38 ]  However, the qualification for nutrient status of compounds with poorly defined properties  in vivo  is that they must first be defined with a  Dietary Reference Intake  level to enable accurate food labeling, [ 39 ]  a condition not established for most phytochemicals that are claimed to provide antioxidant benefits. [ 40 ]


See  Vitamin ,  Mineral (nutrient) ,  Protein (nutrient)


An inadequate amount of a nutrient is a deficiency. Deficiencies can be due to several causes, including an inadequacy in nutrient intake, called a dietary deficiency, or any of several conditions that interfere with the utilization of a nutrient within an organism. [ 1 ]  Some of the conditions that can interfere with nutrient utilization include problems with nutrient absorption, substances that cause a greater-than-normal need for a nutrient, conditions that cause nutrient destruction, and conditions that cause greater nutrient excretion. [ 1 ]  Nutrient toxicity occurs when excess consumption of a nutrient does harm to an organism. [ 41 ]


In the United States and Canada, recommended dietary intake levels of essential nutrients are based on the minimum level that "will maintain a defined level of nutriture in an individual", a definition somewhat different from that used by the  World Health Organization  and  Food and Agriculture Organization  of a "basal requirement to indicate the level of intake needed to prevent pathologically relevant and clinically detectable signs of a dietary inadequacy". [ 42 ]


In setting human nutrient guidelines, government organizations do not necessarily agree on amounts needed to avoid deficiency or maximum amounts to avoid the risk of toxicity. [ 43 ] [ 44 ] [ 45 ]  For example, for  vitamin C , recommended intakes range from 40 mg/day in India [ 46 ]  to 155 mg/day for the European Union. [ 47 ]  The table below shows U.S. Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for vitamins and minerals, PRIs for the European Union (same concept as RDAs), followed by what three government organizations deem to be the safe upper intake. RDAs are set higher than EARs to cover people with higher-than-average needs. Adequate Intakes (AIs) are set when there is insufficient information to establish EARs and RDAs. Countries establish  tolerable upper intake levels , also referred to as upper limits (ULs), based on amounts that cause adverse effects. Governments are slow to revise information of this nature. For the U.S. values, except calcium and vitamin D, all data date from 1997 to 2004. [ 17 ]


* The daily recommended amounts of niacin and magnesium are higher than the tolerable upper limit because, for both nutrients, the ULs identify the amounts which will not increase risk of adverse effects when the nutrients are consumed as a serving of a dietary supplement. Magnesium supplementation above the UL may cause diarrhea. Supplementation with niacin above the UL may cause flushing of the face and a sensation of body warmth. Each country or regional regulatory agency decides on a safety margin below when symptoms may occur, so the ULs may differ based on source. [ 43 ] [ 44 ]


EAR  U.S. Estimated Average Requirements.


RDA  U.S. Recommended Dietary Allowances; higher for adults than for children, and may be even higher for women who are pregnant or lactating.


AI   U.S. Adequate Intake; AIs established when there is not sufficient information to set EARs and RDAs.


PRI  Population Reference Intake is European Union equivalent of RDA; higher for adults than for children, and may be even higher for women who are pregnant or lactating. For Thiamin and Niacin, the PRIs are expressed as amounts per megajoule (239 kilocalories) of food energy consumed.


Upper Limit   Tolerable upper intake levels.


ND   ULs have not been determined.


NE   EARs, PRIs or AIs have not yet been established or will not be (EU does not consider chromium an essential nutrient).


Plants absorb carbon, hydrogen, and oxygen from air and soil as  carbon dioxide  and water. [ 48 ]  Other nutrients are absorbed from soil (exceptions include some parasitic or carnivorous plants). Counting these, there are 17 important nutrients for plants: [ 49 ]  these are macronutrients; nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), sulfur (S), magnesium (Mg), carbon (C), oxygen(O) and hydrogen (H), and the micronutrients; iron (Fe), boron (B), chlorine (Cl), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo) and nickel (Ni). In addition to carbon, hydrogen, and oxygen,  nitrogen ,  phosphorus , and  sulfur  are also needed in relatively large quantities. Together, these  six  are the elemental macronutrients for all  organisms . [ 50 ]
They are sourced from inorganic matter (for example, carbon dioxide, water,  nitrates ,  phosphates ,  sulfates , and  diatomic molecules  of nitrogen and, especially, oxygen) and organic compounds such as carbohydrates, lipids, proteins.


Octasulfur  is an  inorganic   substance  with the chemical formula  S 8 . It is an odourless and tasteless yellow solid, and is a major industrial chemical. It is the most common  allotrope  of  sulfur  and occurs widely in nature. [ 4 ]


The name  octasulfur  is the most commonly used for this chemical. It is systematically named  cyclo -octasulfur  (which is the preferred IUPAC name) and  cyclooctasulfane . It is also the final member of the  thiocane heterocylic series , where every carbon atom is substituted with a sulfur atom, thus this sulfur allotrope is systematically named  octathiocane  as well.


The chemical consists of rings of 8 sulfur atoms. It adopts a crown conformation with D 4d   point group symmetry . The S–S bond lengths are equal, at about 2.05  Å . Octasulfur crystallizes in three distinct  polymorphs : rhombohedral, and two monoclinic forms, of which only two are stable at standard conditions. The rhombohedral crystal form is the accepted  standard state . The remaining polymorph is only stable between 96 and 115 °C at 100 kPa. Octasulfur forms several allotropes: α-sulfur, β-sulfur, γ-sulfur, and λ-sulfur.


λ-Sulfur is the liquid form of octasulfur, from which γ-sulfur can be crystallised by quenching. If λ-sulfur is crystallised slowly, it will revert to β-sulfur. Since it must have been heated over 115 °C, neither crystallised β-sulfur or γ-sulfur will be pure. The only known method of obtaining pure γ-sulfur is by crystallising from solution.


Octasulfur easily forms large crystals, which are typically yellow and are somewhat translucent.


Octasulfur is not typically produced as  S 8  per se. It is the main (99%) component of elemental sulfur, which is recovered from volcanic sources and is a major product of the  Claus process , associated with petroleum refineries.


The  oligodynamic effect  (from  Greek   oligos , "few", and  dynamis , "force") is a  biocidal  effect of  metals , especially  heavy metals , that occurs even in low concentrations. This effect is attributed to the antibacterial behavior of metal ions, which are absorbed by bacteria upon contact and damage their  cell membranes . [ 1 ]


In modern times, the effect was observed by  Carl Nägeli , although he did not identify the cause. [ 2 ]  Brass  doorknobs , brass  handrails , and  silverware  all exhibit this effect to an extent.


The metals react with  thiol  (-SH) or  amine  (-NH (1,2,3) ) groups of proteins, a mode of action to which  microorganisms  may develop  resistance . Such resistance may be transmitted by  plasmids . [ 3 ]


Aluminium has been found to compete with iron and magnesium and bind to DNA, membranes, or cell walls, leading to its toxic effect on microbes, such as cyanobacteria, soil bacteria and mycorrhizal fungi. [ 4 ]


Aluminium triacetate  ( Burow's solution ) is used as an  astringent  mild  antiseptic . [ 5 ]


Orthoesters  of  diarylstibinic acids  are  fungicides  and  bactericides , used in  paints ,  plastics , and  fibers . [ 6 ]   Trivalent  organic antimony was used in therapy for  schistosomiasis . [ 7 ]


For many decades,  arsenic  was used medicinally to treat  syphilis . It is still used in  sheep dips ,  rat poisons ,  wood preservatives ,  weed killers , and other  pesticides . Arsenic is  poisonous  if it enters the human body. [ 8 ]


Barium polysulfide  is a  fungicide  and  acaricide  used in fruit and grape growing. [ 9 ]


Bismuth compounds have been used because of their  astringent ,  antiphlogistic ,  bacteriostatic , and  disinfecting  actions. In dermatology  bismuth subgallate  is still used in vulnerary salves and powders as well as in antimycotics. [ 10 ]  In the past, bismuth has also been used to treat  syphilis  and  malaria . [ 11 ]


Boric acid  esters derived from  glycols  (example, organo-borate formulation,  Biobor JF ) are being used for the control of microorganisms in fuel systems containing water. [ 12 ]


Brass  vessels release a small amount of  copper  ions into stored water, thus killing  fecal  bacterial counts as high as 1 million bacteria per milliliter. [ 13 ]


Copper sulfate  mixed with  lime  ( Bordeaux mixture ) is used as a  fungicide  and  antihelminthic . [ 14 ]  Copper sulfate is used chiefly to destroy green algae ( algicide ) that grow in reservoirs, stock ponds, swimming pools, and fish tanks.  Copper 8-hydroxyquinoline  is sometimes included in paint to prevent  mildew . [ 15 ]


Paint containing copper is  used on boat bottoms  to prevent  barnacle  growth ( biofouling ).


Copper also has the ability to destroy viruses, such as influenza viruses, noroviruses or human immunodeficiency virus (HIV). [ 16 ]


Gold is used in  dental inlays  and inhibits the growth of bacteria. [ 17 ]


Physicians prescribed various forms of lead to heal ailments ranging from  constipation  to infectious diseases such as the  plague . Lead was also used to preserve or sweeten wine. [ 18 ]   Lead arsenate  is used in insecticides and herbicides. [ 19 ]  Some organic lead compounds are used as industrial biocides:  thiomethyl triphenyllead  is used as an antifungal agent, cotton preservative, and lubricant additive;  thiopropyl triphenyllead  as a rodent repellant;  tributyllead acetate  as a wood and cotton preservative;  tributyllead imidazole  as a lubricant additive and cotton preservative. [ 20 ]


Phenylmercuric borate  and  acetate  were used for disinfecting mucous membranes at an effective concentration of 0.07% in aqueous solutions. Due to toxicological and ecotoxicological reasons phenylmercury salts are no longer in use. However, some surgeons use  mercurochrome  despite toxicological objections. [ 3 ]  Mercurochrome is still available to purchase in Australia to use on minor wounds.  Dental amalgam  used in fillings inhibits bacterial reproduction. [ 13 ]


Organic mercury compounds have been used as topical disinfectants ( thimerosal ,  nitromersol , and  merbromin ) and preservatives in medical preparations ( thimerosal ) and grain products (both  methyl  and  ethyl mercurials ). Mercury was used in the treatment of  syphilis .  Calomel  was commonly used in infant teething powders in the 1930s and 1940s. Mercurials are also used agriculturally as  insecticides  and  fungicides . [ 21 ]


The toxicity of  nickel  to bacteria, yeasts, and fungi differs considerably. [ 22 ]


The metabolism of bacteria is adversely affected by silver ions at concentrations of 0.01–0.1 mg/L. Therefore, even less soluble silver compounds, such as  silver chloride , also act as bactericides or germicides, but not the much less soluble  silver sulfide . In the presence of atmospheric oxygen, metallic silver also has a bactericidal effect due to the formation of  silver oxide , which is soluble enough to cause it. Even objects with a solid silver surface (e.g., table silver, silver coins, or silver foil) have a bactericidal effect. Silver drinking vessels were carried by military commanders on expeditions for protection against disease. It was once common to place silver foil or even silver coins on wounds for the same reason. [ 23 ]


Silver sulfadiazine  is used as an antiseptic ointment for extensive burns. An equilibrium dispersion of colloidal silver with dissolved silver ions can be used to purify drinking water at sea. [ 3 ]  Silver is incorporated into medical implants and devices such as  catheters .  Surfacine  ( silver iodide ) is a relatively new antimicrobial for application to surfaces. Silver-impregnated wound dressings have proven especially useful against antibiotic-resistant bacteria.  Silver nitrate  is used as a hemostatic, antiseptic and astringent. At one time, many states [ clarification needed ]  required that the eyes of newborns be treated with a few drops of silver nitrate to guard against an infection of the eyes called  gonorrheal neonatal ophthalmia , which the infants might have contracted as they passed through the birth canal. Silver ions are increasingly incorporated into many hard surfaces, such as plastics and steel, as a way to control microbial growth on items such as toilet seats, stethoscopes, and even refrigerator doors. Among the newer products being sold are plastic food containers infused with silver  nanoparticles , which are intended to keep food fresher, and silver-infused athletic shirts and socks, which claim to minimize odors. [ 15 ] [ 17 ]


Thallium compounds such as  thallium sulfate  have been used for impregnating wood and leather to kill fungal spores and bacteria, and for the protection of textiles from attack by moths. [ 24 ]  Thallium sulfate has been used as a depilatory and in the treatment of venereal disease, skin fungal infections, and tuberculosis. [ 25 ]


Tetrabutyltin  is used as an antifouling paint for ships, for the prevention of slimes in industrial recirculating water systems, for combating freshwater snails that cause  bilharzia , as a wood and textile preservative, and as a disinfectant.  Tricyclohexyltin hydroxide  is used as an acaricide.  Triphenyltin hydroxide  and  triphenyltin acetate  are used as fungicides. [ 26 ]


Zinc oxide  is used as a weak antiseptic and in paints as a white pigment and mold-growth inhibitor. [ 27 ]   Zinc chloride  is a common ingredient in mouthwashes and deodorants, and  zinc pyrithione  is an ingredient in antidandruff shampoos. Galvanized (zinc-coated) fittings on roofs impede the growth of algae. Copper- and zinc-treated shingles are available. [ 15 ]   Zinc iodide  and  zinc sulfate  are used as topical antiseptics. [ 28 ]


Besides the individual toxic effects of each metal, a wide range of metals are  nephrotoxic  in humans and/or in animals. [ 29 ]  Some metals and their compounds are carcinogenic to humans. [ citation needed ]  A few metals, such as lead and mercury, can cross the  placental barrier  and adversely affect  fetal development . [ 30 ]  Several (cadmium, zinc, copper, and mercury) can induce special protein complexes called  metallothioneins . [ 31 ]


In  chemistry , a  phosphodiester bond  occurs when exactly two of the  hydroxyl groups  ( −OH ) in  phosphoric acid  react with hydroxyl groups on other molecules to form two  ester  bonds. The "bond" involves this linkage  C−O−PO − 2 O−C . [ 1 ]   Discussion of phosphodiesters is dominated by their prevalence in  DNA  and  RNA , but phosphodiesters occur in other biomolecules, e.g.  acyl carrier proteins ,  phospholipids  and the cyclic forms of GMP and AMP ( cGMP  and  cAMP ). [ 2 ]


Phosphodiester bonds make up the  backbones  of  DNA  and  RNA . In the phosphodiester bonds of nucleic acids, a phosphate is attached to the 5' carbon of one nucleoside and to the 3' carbon of the adjacent nucleoside. Specifically, it is the phosphodiester bonds that link the  3' carbon atom  of one sugar molecule and the 5' carbon atom of another (hence the name 3', 5' phosphodiester linkage used with reference to this kind of bond in DNA and RNA chains). [ 3 ]  The involved saccharide groups are  deoxyribose  in DNA and  ribose  in RNA. In order for the phosphodiester bond to form ,  joining the  nucleosides , the tri-phosphate or di-phosphate forms of the nucleotide building blocks are broken apart to give off energy required to drive the  enzyme -catalyzed reaction. [ 4 ]  In DNA replication, for example, formation of the phosphodiester bonds is catalyzed by a  DNA polymerase   enzyme , using a pair of  magnesium   cations  and other supporting structures. [ 3 ]  Formation of the bond occurs not only in DNA and RNA replication, but also in the repair and recombination of nucleic acids, and may require the involvement of various polymerases, primers, and/or ligases. During the replication of DNA, for example, the  DNA polymerase I  leaves behind a hole between the phosphates in the newly formed backbone.  DNA ligase  is able to form a phosphodiester bond between the nucleotides on each side of the gap. [ 2 ]


Phosphodiesters are negatively charged at  pH  7. [ 5 ]  The negative charge attracts  histones , metal cations such as  magnesium , and  polyamines  [needs citation]. Repulsion between these negative charges influences the conformation of the polynucleic acids.


Hydrolysis  (breaking) of phosphodiester bonds can be promoted in several ways.  Phosphodiesterases  are enzymes that catalyze the hydrolysis of the phosphodiester bond. These enzymes are involved in repairing DNA and RNA sequences, nucleotide salvage, and in the conversion of cGMP and cAMP to GMP and AMP, respectively. [ 2 ]   Hydrolysis of the phosphodiester bond also occurs chemically and spontaneously, without the aid of enzymes. For example, simple  ribose  (in RNA) has one more  hydroxyl group  than  deoxyribose  (in DNA), making the former less stable and more susceptible to  alkaline hydrolysis , wherein relatively high pH conditions induce the breaking of the phosphodiester linkage between two  ribonucleotides . The relative instability of RNA under hydroxyl attack of its phosphodiester bonds makes it inadequate for the storage of genomic information, but contributes to its usefulness in  transcription  and  translation .  [ 2 ]


Plant nutrition  is the study of the  chemical elements  and  compounds  necessary for plant growth and reproduction, plant metabolism and their external supply. In its absence the plant is unable to complete a normal life cycle, or that the element is part of some essential plant constituent or  metabolite . This is in accordance with  Justus von Liebig's   law of the minimum . [ 1 ]  The total essential plant nutrients include seventeen different elements:  carbon ,  oxygen  and  hydrogen  which are absorbed from the air, whereas other nutrients including  nitrogen  are typically obtained from the soil (exceptions include some  parasitic  or  carnivorous plants ).


Plants must obtain the following mineral nutrients from their growing medium: [ 2 ]


These elements stay beneath soil as  salts , so plants absorb these elements as  ions . The macronutrients are taken-up in larger quantities; hydrogen, oxygen, nitrogen and carbon contribute to over 95% of a plant's entire biomass on a dry matter weight basis. Micronutrients are present in plant tissue in quantities measured in parts per million, ranging from 0.1 [ 3 ]  to 200 ppm, or less than 0.02% dry weight. [ 4 ]


Most  soil  conditions across the world can provide plants adapted to that climate and soil with sufficient nutrition for a complete life cycle, without the addition of nutrients as  fertilizer . However, if the soil is cropped it is necessary to artificially modify  soil fertility  through the addition of  fertilizer  to promote vigorous growth and increase or sustain yield. This is done because, even with adequate water and light,  nutrient deficiency  can limit growth and crop yield.


Carbon ,  hydrogen  and  oxygen  are the basic nutrients plants receive from air and water.  Justus von Liebig  proved in 1840 that plants needed  nitrogen ,  potassium  and  phosphorus .  Liebig's law of the minimum  states that a plant's growth is limited by nutrient deficiency. [ 5 ]  Plant cultivation in media other than soil was used by Arnon and Stout in 1939 to show that  molybdenum  was essential to  tomato  growth. [ citation needed ]


Plants take up  essential elements  from the soil through their roots and from the air through their leaves. Nutrient uptake in the soil is achieved by  cation exchange , wherein  root hairs  pump hydrogen ions (H + ) into the soil through  proton pumps . These hydrogen ions displace cations attached to negatively charged soil particles so that the cations are available for uptake by the root. In the leaves,  stomata  open to take in carbon dioxide and expel oxygen. The carbon dioxide molecules are used as the carbon source in  photosynthesis .


The  root , especially the root hair, a unique cell, is the essential organ for the uptake of nutrients. The structure and architecture of the root can alter the rate of nutrient uptake. Nutrient ions are transported to the center of the root, the  stele , in order for the nutrients to reach the conducting tissues, xylem and phloem. [ 6 ]  The  Casparian strip , a cell wall outside the stele but in the root, prevents passive flow of water and nutrients, helping to regulate the uptake of nutrients and water.  Xylem  moves water and mineral ions in the plant and  phloem  accounts for organic molecule transportation.  Water potential  plays a key role in a plant's nutrient uptake. If the water potential is more negative in the plant than the surrounding soils, the nutrients will move from the region of higher solute concentration—in the soil—to the area of lower solute concentration - in the plant.


There are three fundamental ways plants uptake nutrients through the root:


Nutrients can be moved in plants to where they are most needed. For example, a plant will try to supply more nutrients to its younger leaves than to its older ones. When nutrients are mobile in the plant, symptoms of any deficiency become apparent first on the older leaves. However, not all nutrients are equally mobile. Nitrogen, phosphorus, and potassium are mobile nutrients while the others have varying degrees of mobility. When a less-mobile nutrient is deficient, the younger leaves suffer because the nutrient does not move up to them but stays in the older leaves.  This phenomenon is helpful in determining which nutrients a plant may be lacking.