Human monomethylarsonic acid (MMA(V)) reductase is a member of the glutathione-S-transferase superfamily

Translocation, enzymatic reduction and toxicity of dimethylarsenate in rice

Dimethylarsenate [DMAs(V)] can be produced by some soil microorganisms through methylation of inorganic arsenic (As), especially in anoxic paddy soils. DMAs(V) is more phytotoxic than inorganic As and can cause the physiological disorder straighthead disease in rice. Rice cultivars vary widely in the resistance to DMAs(V), but the mechanism remains elusive. Here, we investigated the differences in DMAs(V) uptake, translocation, and reduction to dimethylarsenite [DMAs(III)], as well as the effects on the metabolome, between two rice cultivars Mars and Zhe733. We found that Mars was 11-times more resistant to DMAs(V) than Zhe733. Mars accumulated more DMAs(V) in the roots, whereas Zhe733 translocated more DMAs(V) to the shoots and reduced more DMAs(V) to DMAs(III). DMAs(III) was more toxic than DMAs(V). Using heterologous expression and in vitro enzyme assays, we showed that the glutathione-S-transferases OsGSTU17 and OsGSTU50 were able to reduce DMAs(V) to DMAs(III). The expression levels of OsGSTU17 and OsGSTU50 were higher in the shoot of Zhe733 compared to Mars. Metabolomic analysis in rice shoots showed that glutathione (GSH) metabolism was perturbed by DMAs(V) toxicity in Zhe733. Application of exogenous GSH significantly alleviated the toxicity of DMAs(V) in Zhe733. Taken together, the results suggest that Mars is more resistant to DMAs(V) than Zhe733 because of a lower root-to-shoot translocation and a smaller capacity to reduce DMAs(V) to DMAs(III).

Arsenic Contamination of Groundwater Is Determined by Complex Interactions between Various Chemical and Biological Processes

At a great many locations worldwide, the safety of drinking water is not assured due to pollution with arsenic. Arsenic toxicity is a matter of both systems chemistry and systems biology: it is determined by complex and intertwined networks of chemical reactions in the inanimate environment, in microbes in that environment, and in the human body. We here review what is known about these networks and their interconnections. We then discuss how consideration of the systems aspects of arsenic levels in groundwater may open up new avenues towards the realization of safer drinking water. Along such avenues, both geochemical and microbiological conditions can optimize groundwater microbial ecology vis-à-vis reduced arsenic toxicity.

Influence of genetic polymorphisms on arsenic methylation efficiency during pregnancy: Evidence from a Spanish birth cohort

BACKGROUND

Inorganic arsenic (iAs) is a widespread toxic metalloid. It is well-known that iAs metabolism and its toxicity are mediated by polymorphisms in AS3MT and other genes. However, studies during pregnancy are scarce. We aimed to examine the role of genetic polymorphisms in AS3MT, GSTO2, N6AMT1, MTHFR, MTR, FTCD, CBS, and FOLH1 in iAs methylation efficiency during pregnancy.

METHODS

The study included 541 pregnant participants from the INMA (Environment and Childhood) Spanish cohort. Using high-performance liquid chromatography coupled to inductively coupled plasma-tandem mass, we measured arsenic (iAs and the metabolites monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA)) in urine samples collected during the first trimester. iAs methylation efficiency was determined based on relative concentrations of the As metabolites in urine (%MMA, %DMA, and %iAs). Thirty-two single nucleotide polymorphisms (SNPs) in nine genes were determined in maternal DNA; AS3MT haplotypes were inferred. We assessed the association between genotypes/haplotypes and maternal As methylation efficiency using multivariate linear regression models.

RESULTS

The median %MMA and %DMA were 5.3 %, and 89 %, respectively. Ancestral alleles of AS3MT SNPs (rs3740393, rs3740390, rs11191453, and rs11191454) were significantly associated with higher %MMA, %iAs, and lower %DMA. Pregnant participants with zero copies of the GGCTTCAC AS3MT haplotype presented a higher %MMA. Statistically significant associations were also found for the FOLH1 SNP rs202676 (β 0.89 95%CI: 0.24, 1.55 for carriers of the G allele vs. the A allele).

CONCLUSIONS

Our study shows that ancestral alleles in AS3MT polymorphisms were associated with lower As methylation efficiency in early pregnancy and suggests that FOLH1 also plays a role in As methylation efficiency. These results support the hypothesis that As metabolism is multigenic, being a key element for identifying susceptible populations.

The Polymorphisms in GSTO Genes (GSTO1 rs4925, GSTO2 rs156697, and GSTO2 rs2297235) Affect the Risk for Testicular Germ Cell Tumor Development: A Pilot Study

Members of the omega class of glutathione transferases (GSTs), GSTO1, and GSTO2, catalyze a range of reduction reactions as a part of the antioxidant defense system. Polymorphisms of genes encoding antioxidant proteins and the resultant altered redox profile have already been associated with the increased risk for testicular germ cell cancer (GCT) development. The aim of this pilot study was to assess the individual, combined, haplotype, and cumulative effect of GSTO1rs4925, GSTO2rs156697, and GSTO2rs2297235 polymorphisms with the risk for testicular GCT development, in 88 patients and 96 matched controls, through logistic regression models. We found that carriers of the GSTO1*C/A*C/C genotype exhibited an increased risk for testicular GCT development. Significant association with increased risk of testicular GCT was observed in carriers of GSTO2rs2297235*A/G*G/G genotype, and in carriers of combined GSTO2rs156697*A/G*G/G and GSTO2rs2297235*A/G*G/G genotypes. Haplotype H7 (GSTO1rs4925*C/GSTO2rs2297235*G/GSTO2rs156697*G) exhibited higher risk of testicular GCT, however, without significant association (p > 0.05). Finally, 51% of testicular GCT patients were the carriers of all three risk-associated genotypes, with 2.5-fold increased cumulative risk. In conclusion, the results of this pilot study suggest that GSTO polymorphisms might affect the protective antioxidant activity of GSTO isoenzymes, therefore predisposing susceptible individuals toward higher risk for testicular GCT development.

Differential susceptibility to arsenic in glutathione S-transferase omega 2 (GST-O2)-targeted freshwater water flea Daphnia magna mutants

To examine the role of glutathione S-transferase omega class (GST-O2) genes in the biotransformation and detoxification in Daphnia magna, various responses such as in vivo endpoints, arsenic speciation, enzymatic activities, and gene expression pathways related to arsenic metabolism were investigated in wild-type (WT) and GST-O2-mutant-type (MT) fleas produced by CRISPR/Cas9. Sensitivity to arsenic in MT fleas was higher than in WT fleas. Also, the reduction rate of arsenate (As^V) to arsenite (As^III) in the MT group was significantly lower and led to accumulation of higher arsenic concentrations, resulting in decreased protection against arsenic toxicity. Relative mRNA expression of other GST genes in the GST-O2-targeted MT group generally increased but the enzymatic activity of GST decreased compared with the WT group. Oxidative stress on arsenic exposure was more strongly induced in the MT group compared with the WT group, resulting in a decrease in the ability to defend against toxicity in GST-O2-targeted mutant D. magna. Our results suggest that GST-O2 plays an important role in arsenic biotransformation and detoxification functions in D. magna.

Genome-Wide Identification, Evolution and Expression Analysis of the Glutathione S-Transferase Supergene Family in Euphorbiaceae

Euphorbiaceae, a family of plants mainly grown in the tropics and subtropics, is also widely distributed all over the world and is well known for being rich in rubber, oil, medicinal materials, starch, wood and other economically important plant products. Glutathione S-transferases (GSTs) constitute a family of proteins encoded by a large supergene family and are widely expressed in animals, bacteria, fungi and plants, but with few reports of them in Euphorbiaceae plants. These proteins participate in and regulate the detoxification and oxidative stress response of heterogeneous organisms, resistance to stress, growth and development, signal transduction and other related processes. In this study, we identified and analyzed the whole genomes of four species of Euphorbiaceae, namely Ricinus communis, Jatropha curcas, Hevea brasiliensis, and Manihot esculenta, which have high economic and practical value. A total of 244 GST genes were identified. Based on their sequence characteristics and conserved domain types, the GST supergene family in Euphorbiaceae was classified into 10 subfamilies. The GST supergene families of Euphorbiaceae and Arabidopsis have been found to be highly conserved in evolution, and tandem repeats and translocations in these genes have made the greatest contributions to gene amplification here and have experienced strong purification selection. An evolutionary analysis showed that Euphorbiaceae GST genes have also evolved into new subtribes (GSTO, EF1BG, MAPEG), which may play a specific role in Euphorbiaceae. An analysis of expression patterns of the GST supergene family in Euphorbiaceae revealed the functions of these GSTs in different tissues, including resistance to stress and participation in herbicide detoxification. In addition, an interaction analysis was performed to determine the GST gene regulatory mechanism. The results of this study have laid a foundation for further analysis of the functions of the GST supergene family in Euphorbiaceae, especially in stress and herbicide detoxification. The results have also provided new ideas for the study of the regulatory mechanism of the GST supergene family, and have provided a reference for follow-up genetics and breeding work.

Glutathione is involved in the reduction of methylarsenate to generate antibiotic methylarsenite in Enterobacter sp. CZ-1

Methylarsenate (MAs(V)) is a product of microbial arsenic (As) biomethylation and has also been widely used as an herbicide. Some microbes are able to reduce nontoxic MAs(V) to highly toxic methylarsenite (MAs(III)) possibly as an antibiotic. The mechanism of MAs(V) reduction in microbes has not been elucidated. Here, we found that the bacterium Enterobacter sp. CZ-1 isolated from an As-contaminated paddy soil has a strong ability to reduce MAs(V) to MAs(III). Using a MAs(III)-responsive biosensor to detect MAs(V) reduction in E. coli Trans5α transformants of a genomic library of Enterobacter sp. CZ-1, we identified gshA, encoding a glutamate-cysteine ligase, as a key gene involved in MAs(V) reduction. Heterologous expression of gshA increased the biosynthesis of glutathione (GSH) and MAs(V) reduction in E. coli Trans5α. Deletion of gshA in Enterobacter sp. CZ-1 abolished its ability to synthesize GSH and decreased its MAs(V) reduction ability markedly, which could be restored by supplementation of exogenous GSH. In the presence of MAs(V), Enterobacter sp. CZ-1 was able to inhibit the growth of Bacillus subtilis 168; this ability was lost in the gshA-deleted mutant. In addition, deletion of gshA greatly decreased the reduction of arsenate to arsenite. These results indicate that GSH plays an important role in MAs(V) reduction to generate MAs(III) as an antibiotic. IMPORTANCE Arsenic is a ubiquitous environmental toxin. Some microbes detoxify inorganic arsenic through biomethylation, generating relatively nontoxic pentavalent methylated arsenicals, such as methylarsenate. Methylarsenate has also been widely used as an herbicide. Surprisingly, some microbes reduce methylarsenate to highly toxic methylarsenite possibly to use the latter as an antibiotic. How microbes reduce methylarsenate to methylarsenite is unknown. Here, we show that gshA encoding a glutamate-cysteine ligase in the glutathione biosynthesis pathway is involved in methylarsenate reduction in Enterobacter sp. CZ-1. Our study provides new insights into the crucial role of glutathione in the transformation of a common arsenic compound to a natural antibiotic.

The freshwater water flea Daphnia magna NIES strain genome as a resource for CRISPR/Cas9 gene targeting: The glutathione S-transferase omega 2 gene

The water flea Daphnia magna is a small freshwater planktonic animal in the Cladocera. In this study, we assembled the genome of the D. magna NIES strain, which is widely used for gene targeting but has no reported genome. We used the long-read sequenced data of the Oxford nanopore sequencing tool for assembly. Using 3,231 genetic markers, the draft genome of the D. magna NIES strain was built into ten linkage groups (LGs) with 483 unanchored contigs, comprising a genome size of 173.47 Mb. The N50 value of the genome was 12.54 Mb and the benchmarking universal single-copy ortholog value was 98.8%. Repeat elements in the D. magna NIES genome were 40.8%, which was larger than other Daphnia spp. In the D. magna NIES genome, 15,684 genes were functionally annotated. To assess the genome of the D. magna NIES strain for CRISPR/Cas9 gene targeting, we selected glutathione S-transferase omega 2 (GST-O2), which is an important gene for the biotransformation of arsenic in aquatic organisms, and targeted it with an efficient make-up (25.0%) of mutant lines. In addition, we measured reactive oxygen species and antioxidant enzymatic activity between wild type and a mutant of the GST-O2 targeted D. magna NIES strain in response to arsenic. In this study, we present the genome of the D. magna NIES strain using GST-O2 as an example of gene targeting, which will contribute to the construction of deletion mutants by CRISPR/Cas9 technology.

Proposed mechanism for monomethylarsonate reductase activity of human omega-class glutathione transferase GSTO1-1

Contamination of drinking water with toxic inorganic arsenic is a major public health issue. The mechanisms of enzymes and transporters in arsenic elimination are therefore of interest. The human omega-class glutathione transferases have been previously shown to possess monomethylarsonate (V) reductase activity. To further understanding of this activity, molecular dynamics of human GSTO1-1 bound to glutathione with a monomethylarsonate isostere were simulated to reveal putative monomethylarsonate binding sites on the enzyme. The major binding site is in the active site, adjacent to the glutathione binding site. Based on this and previously reported biochemical data, a reaction mechanism for this enzyme is proposed. Further insights were gained from comparison of the human omega-class GSTs to homologs from a range of animals.

Glutathione S-Transferase Omega-2 and Transforming Growth Factor-β1 Polymorphisms in Iranian Glaucoma Patients

Background

To investigate the association of glutathione s-transferase omega 2 (GSTO2) (142N > D) and transforming growth factor-β1 (TGF-β1) (869T > C) gene polymorphisms on the pathogenesis of two common types of glaucoma (including primary open-angle glaucoma (POAG) and chronic angle-closure glaucoma (CACG)) in the Iranian population.

Methods

A total of 100 glaucoma patients (60% males and 40% females with an age mean ± SD of 34.66 ± 14.25 years; 56 cases of POAG and 44 cases of CACG) were enrolled in this study. GSTO2 (142N > D) and TGF-β1 (869T > C) polymorphisms were evaluated by PCR-based methods in patients and controls.

Results

At locus GSTO2 (142N > D), the odds of ND genotype with respect to DD and NN genotypes were 1.55 and 2.08 times higher in POAG and CACG patients compared to those of patients in the control group (95% CI₁: 0.80–2.98; 95% CI₂: 1.00–4.33) which was statistically significant in CACG patients. However, the odds of DD and NN genotypes against the reference genotype in two patients group were not statistically significant as compared to those of patients in the control group. There was a significant association between the ND genotype and male patients (OR = 2.28, 95% CI: 1.06–4.92). The analysis of TGF-β1 (869T > C) polymorphisms showed no significant difference between the genotypes of TGF-β1 (869T > C) polymorphisms in patients and control groups; however, the CT genotype of TGF-β1 significantly differed between female controls and patients (OR = 0.42, 95% CI: 0.18–0.96).

Conclusion

The presented results revealed that there was a significant association between the ND genotype of GSTO2 and the pathogenesis of glaucoma. Furthermore, this genotype can be considered as a sex-dependent genetic risk factor for the development of glaucoma. In contrast, the CT genotype of TGF-β1 is suggested to be a protective genetic factor against the pathogenesis of glaucoma.

Arsenic toxicokinetic modeling and risk analysis: Progress, needs and applications

Arsenic (As) poses unique challenges in PBTK model development and risk analysis applications. Arsenic metabolism is complex, adequate information to attribute specific metabolites to particular adverse effects in humans is sparse, and measurement of relevant metabolites in biological media can be difficult. Multiple As PBTK models have been published and used or adapted for use in various exposure and risk analysis applications. These applications illustrate the broad utility of PBTK models for exposure and dose-response analysis, particularly for arsenic where multi-pathway, multi-route exposures and multiple toxic effects are of concern. Arsenic PBTK models have been used together with exposure reconstruction and dose-response functions to estimate risk of specific adverse health effects due to drinking water exposure and consumption of specific foodstuffs (e.g. rice, seafood), as well as to derive safe exposure levels and develop consumption advisories. Future refinements to arsenic PBTK models can enhance the confidence in such analyses. Improved estimates for methylation biotransformation parameters based on in vitro to in vivo extrapolation (IVIVE) methods and estimation of interindividual variability in key model parameters for specific toxicologically relevant metabolites are two important areas for consideration.

Origins, fate, and actions of methylated trivalent metabolites of inorganic arsenic: progress and prospects

The toxic metalloid inorganic arsenic (iAs) is widely distributed in the environment. Chronic exposure to iAs from environmental sources has been linked to a variety of human diseases. Methylation of iAs is the primary pathway for metabolism of iAs. In humans, methylation of iAs is catalyzed by arsenic (+ 3 oxidation state) methyltransferase (AS3MT). Conversion of iAs to mono- and di-methylated species (MAs and DMAs) detoxifies iAs by increasing the rate of whole body clearance of arsenic. Interindividual differences in iAs metabolism play key roles in pathogenesis of and susceptibility to a range of disease outcomes associated with iAs exposure. These adverse health effects are in part associated with the production of methylated trivalent arsenic species, methylarsonous acid (MAs^III) and dimethylarsinous acid (DMAs^III), during AS3MT-catalyzed methylation of iAs. The formation of these metabolites activates iAs to unique forms that cause disease initiation and progression. Taken together, the current evidence suggests that methylation of iAs is a pathway for detoxification and for activation of the metalloid. Beyond this general understanding of the consequences of iAs methylation, many questions remain unanswered. Our knowledge of metabolic targets for MAs^III and DMAs^III in human cells and mechanisms for interactions between these arsenicals and targets is incomplete. Development of novel analytical methods for quantitation of MAs^III and DMAs^III in biological samples promises to address some of these gaps. Here, we summarize current knowledge of the enzymatic basis of MAs^III and DMAs^III formation, the toxic actions of these metabolites, and methods available for their detection and quantification in biomatrices. Major knowledge gaps and future research directions are also discussed.

Molecular characterization, immune and xenobiotic responses of glutathione S-transferase omega 1 from the big-belly seahorse: Novel insights into antiviral defense

Glutathione S-transferases (GSTs) are important enzymes involved in phase II detoxification and function by conjugating with the thiol group of glutathione. In this study, we isolated an omega class GST from the big-belly seahorse (Hippocampus abdominalis; HaGSTO1) to study the putative xenobiotic responses and defense ability against viral and bacterial infections in this animal. The isolated HaGSTO1 gene, with a cording sequence of 720 bp, encodes a peptide of 239 amino acids. The predicted molecular mass and theoretical isoelectric point of HaGSTO1 was 27.47 kDa and 8.13, respectively. In-silico analysis of HaGSTO1 revealed a characteristic N-terminal thioredoxin-like domain and a C-terminal domain. Unlike other GSTs, the C-terminal of HaGSTO1 reached up to the N-terminal, and the N-terminal functional group was cysteine rather than tyrosine or serine, as observed in other GSTs. Phylogenetic analysis showed the evolutionary proximity of HaGSTO1 with other identified vertebrate and invertebrate GST orthologs. For the first time, we demonstrated the viral defense capability of HaGSTO1 against viral hemorrhagic septicemia virus (VHSV) infection. All six nucleoproteins of VHSV were significantly downregulated in HaGSTO1-overexpressing FHM cells at 24 h after infection compared with those in the control. Moreover, arsenic toxicity was significantly reduced in HaGSTO1-overexpressing FHM cells, and cell viability increased. Real-time polymerase chain reaction analysis showed that HaGSTO1 transcripts were highly expressed in the pouch and gill when compared with those in other tissues. Blood HaGSTO1 transcripts were significantly upregulated after Edwardsiella tarda, Streptococcus iniae, lipopolysaccharide, and polyinosinic:polycytidylic acid challenge experiments. Collectively, these findings suggest the involvement of HaGSTO1 in the host defense mechanism of seahorses.

Metabolism of Inorganic Arsenic in Mice Lacking Genes Encoding GST-P, GST-M, and GST-T

To investigate the role of glutathione transferases (GSTs) in the metabolism of inorganic arsenic (iAs), we compared the disposition of iAs and its metabolites in wild-type mice and mice lacking genes encoding GST-P, -M and -T after exposure to 100 ppb iAs in drinking water. We found no differences between the two genotypes in the concentrations of total arsenic or arsenic species in urine, liver, and kidneys. No genotype-dependent differences were found in proportions of arsenicals in the tissues, and only small differences were observed in the urine. Thus, under these conditions, GST-P, -M and -T did not play a significant role in iAs metabolism in mice.

Effects of oral exposure to arsenite on arsenic metabolism and transport in rat kidney

Nephrotoxicity is within the recognized toxic effects of arsenic. In this study we assessed the effect of arsenite on the renal capacity to metabolize and handle arsenicals in rats exposed to drinking water with 0, 1, 5, or 10 ppm sodium arsenite for ten days. Arsenite treatment did not affect the gene expression of the main enzyme catalyzing methylation of arsenite, As3mt, while it reduced the expression of GSTO1 mRNA and protein. Arsenite decreased the expression of Aqp3, Mrp1, Mrp4, and Mdr1b (i.e., transporters and channels used by arsenic), but not that of Aqp7, Glut1, Mrp2, and Mdr1a. The protein abundance of AQP3 was also reduced by arsenite. Arsenite increased urinary NGAL and FABP3 and decreased Klotho plasma levels, without alteration of creatinine, which evidenced early tubular damage. Renal Klotho mRNA and protein expressions were also downregulated, which may exacerbate renal damage. No effect was observed in selected miRNAs putatively associated with renal injury. Plasma PTH and FGF23 were similar between groups, but arsenite decreased the renal expression of Fgfr1 mRNA. In conclusion, exposure to arsenite alters the gene expression of proteins involved in the cellular handling of arsenical species and elicits tubular damage.

Factors Affecting Arsenic Methylation in Contaminated Italian Areas

Chronic arsenic (As) exposure is a critical public health issue. The As metabolism can be influenced by many factors. The objective of this study is to verify if these factors influence As metabolism in four Italian areas affected by As pollution. Descriptive analyses were conducted on 271 subjects aged 20-49 in order to assess the effect of each factor considered on As methylation. Percentages of metabolites of As in urine, primary and secondary methylation indexes were calculated as indicators for metabolic capacity. The results indicate that women have a better methylation capacity (MC) than men, and drinking As-contaminated water from public aqueducts is associated with poorer MC, especially in areas with natural As pollution. In areas with anthropogenic As pollution occupational exposure is associated with a higher MC while smoking with a poorer MC. Dietary habits and genetic characteristics are probably implicated in As metabolism. BMI, alcohol consumption and polymorphism of the AS3MT gene seem not to influence As MC. Arsenic metabolism may be affected by various factors and in order to achieve a comprehensive risk assessment of As-associated disease, it is crucial to understand how these factors contribute to differences in As metabolism.

Glutathione S-Transferase Rescues Motor Neuronal Toxicity in Fly Model of Amyotrophic Lateral Sclerosis

Transactive response DNA-binding protein-43 (TDP-43) is involved in the pathology of familial and sporadic amyotrophic lateral sclerosis (ALS). TDP-43-mediated ALS models in mice, Drosophila melanogaster, and zebrafish exhibit dysfunction of locomotor function, defective neuromuscular junctions, and motor neuron defects. There is currently no effective cure for ALS, and the underlying mechanisms of TDP-43 in ALS remain poorly understood. In this study, a genetic screen was performed to identify modifiers of human TDP-43 (hTDP-43) in a Drosophila model, and glutathione S-transferase omega 2 (GstO2) was found to be involved in hTDP-43 neurotoxicity. GstO2 overexpressed on recovered defective phenotypes resulting from hTDP-43, including defective neuromuscular junction (NMJ) boutons, degenerated motor neuronal axons, and reduced larvae and adult fly locomotive activity, without modulating the levels of hTDP-43 protein expression. GstO2 modulated neurotoxicity by regulating reactive oxygen species (ROS) produced by hTDP-43 in the Drosophila model of ALS. Our results demonstrated that GstO2 was a key regulator in hTDP-43-related ALS pathogenesis and indicated its potential as a therapeutic target for ALS.

Biotransformation of arsenic and toxicological implication of arsenic metabolites

Arsenic is a well-known environmental carcinogen and chronic exposure to arsenic through drinking water has been reported to cause skin, bladder and lung cancers, with arsenic metabolites being implicated in the pathogenesis. In contrast, arsenic trioxide (As₂O₃) is an effective therapeutic agent for the treatment of acute promyelocytic leukemia, in which the binding of arsenite (iAs^III) to promyelocytic leukemia (PML) protein is the proposed initial step. These findings on the two-edged sword characteristics of arsenic suggest that after entry into cells, arsenic reaches the nucleus and triggers various nuclear events. Arsenic is reduced, conjugated with glutathione, and methylated in the cytosol. These biotransformations, including the production of reactive metabolic intermediates, appear to determine the intracellular dynamics, target organs, and biological functions of arsenic.

The mercapturic acid pathway

The mercapturic acid pathway is a major route for the biotransformation of xenobiotic and endobiotic electrophilic compounds and their metabolites. Mercapturic acids (N-acetyl-l-cysteine S-conjugates) are formed by the sequential action of the glutathione transferases, γ-glutamyltransferases, dipeptidases, and cysteine S-conjugate N-acetyltransferase to yield glutathione S-conjugates, l-cysteinylglycine S-conjugates, l-cysteine S-conjugates, and mercapturic acids; these metabolites constitute a "mercapturomic" profile. Aminoacylases catalyze the hydrolysis of mercapturic acids to form cysteine S-conjugates. Several renal transport systems facilitate the urinary elimination of mercapturic acids; urinary mercapturic acids may serve as biomarkers for exposure to chemicals. Although mercapturic acid formation and elimination is a detoxication reaction, l-cysteine S-conjugates may undergo bioactivation by cysteine S-conjugate β-lyase. Moreover, some l-cysteine S-conjugates, particularly l-cysteinyl-leukotrienes, exert significant pathophysiological effects. Finally, some enzymes of the mercapturic acid pathway are described as the so-called "moonlighting proteins," catalytic proteins that exert multiple biochemical or biophysical functions apart from catalysis.

Ex vivo and in vivo effects of arsenite on GST and ABCC2 activity and expression in the middle intestine of the rainbow trout Oncorhynchus mykiss

In fish of freshwaters environments, the accumulation and toxic effects of arsenite (AsIII) can be attenuated by detoxification proteins such as GST and ABCC transporters. We studied the effects of AsIII on the middle intestine of O. mykiss in ex-vivo and in vivo/ex vivo assays. For the ex vivo assays, we measured the transport rate of the ABCC substrate DNP-SG and GST activity in intestinal strips and everted sacs. AsIII inhibited DNP-SG transport in a concentration-dependent manner, specifically when we applied it on the basolateral side. GST activity increased when we applied a maximum concentration of AsIII. For the in vivo/ex vivo assays, we kept fish in water with or without 7.7 μmol L^-1 of AsIII for 48 h. Then, we measured DNP-SG transport rate, GST activity, and PP1 activity in intestine strips during one hour. For PP1 activity, we incubated the strips with or without microcystin-LR (MCLR), a toxin excreted through ABCC2 proteins. We also analyzed Abcc2 and Gst-π mRNA expression in intestine and liver tissue. In the group exposed in vivo to AsIII, DNP-SG transport rate and GST activity were higher and the effect of MCLR over PP1 activity was attenuated. AsIII significantly induced only Abcc2 mRNA expression in both middle intestine and liver. Our results suggest that, in the middle intestine of O. mykiss, AsIII is absorbed mainly at the basolateral side of the enterocytes, excreted to the lumen by ABCC2 transporters, and is capable of modulating Abcc2 mRNA expression by a transcriptional mechanism.

Functional, Structural and Biochemical Features of Plant Serinyl-Glutathione Transferases

Glutathione transferases (GSTs) belong to a ubiquitous multigenic family of enzymes involved in diverse biological processes including xenobiotic detoxification and secondary metabolism. A canonical GST is formed by two domains, the N-terminal one adopting a thioredoxin (TRX) fold and the C-terminal one an all-helical structure. The most recent genomic and phylogenetic analysis based on this domain organization allowed the classification of the GST family into 14 classes in terrestrial plants. These GSTs are further distinguished based on the presence of the ancestral cysteine (Cys-GSTs) present in TRX family proteins or on its substitution by a serine (Ser-GSTs). Cys-GSTs catalyze the reduction of dehydroascorbate and deglutathionylation reactions whereas Ser-GSTs catalyze glutathione conjugation reactions and eventually have peroxidase activity, both activities being important for stress tolerance or herbicide detoxification. Through non-catalytic, so-called ligandin properties, numerous plant GSTs also participate in the binding and transport of small heterocyclic ligands such as flavonoids including anthocyanins, and polyphenols. So far, this function has likely been underestimated compared to the other documented roles of GSTs. In this review, we compiled data concerning the known enzymatic and structural properties as well as the biochemical and physiological functions associated to plant GSTs having a conserved serine in their active site.

The role of GST omega in metabolism and detoxification of arsenic in clam Ruditapes philippinarum

The major hazard of arsenic in living organisms is increasingly being recognized. Marine mollusks are apt to accumulate high levels of arsenic, but knowledge related to arsenic detoxification in marine mollusks is still less than sufficient. In this study, arsenic bioaccumulation as well as the role of glutathione S-transferase omega (GSTΩ) in the process of detoxification were investigated in the Ruditapes philippinarum clam after waterborne exposure to As(III) or As(V) for 30 days. The results showed that the gills accumulated significantly higher arsenic levels than the digestive glands. Arsenobetaine (AsB) and dimethylarsenate (DMA) accounted for most of the arsenic found, and monomethylarsonate (MMA) can be quickly metabolized. A subcellular distribution analysis showed that most arsenic was in biologically detoxified metal fractions (including metal-rich granules and metallothionein-like proteins), indicating their important roles in protecting cells from arsenic toxicity. The relative mRNA expressions of two genes encoding GSTΩ were up-regulated after arsenic exposure, and the transcriptional responses were more sensitive to As(III) than As(V). The recombinant GSTΩs exhibited high activities at optimal conditions, especially at 37 °C and pH 4-5, with an As(V) concentration of 60 mM. Furthermore, the genes encoding GSTΩ significantly enhance the arsenite tolerance but not the arsenate tolerance of E. coli AW3110 (DE3) (ΔarsRBC). It can be deduced from these results that GSTΩs play an important role in arsenic detoxification in R. philippinarum.

Comparison of arsenic methylation capacity and polymorphisms of arsenic methylation genes between bladder cancer and upper tract urothelial carcinoma

Arsenic exposure is an environmental risk factor for urothelial carcinoma (UC). The natural history of upper tract urothelial carcinoma (UTUC) differs from that of bladder cancer (BC). However, the risk factors of BC and UTUC are not exactly the same and should be discussed separately. The aims of this study were to evaluate 1) the association between arsenic methylation capacity and UTUC and/or BC, separately, and 2) the association between polymorphisms of the arsenic metabolism-related genes AS3MT, GSTOs, and PNP against BC and/or UTUC, separately. We conducted a hospital-based study and collected 216 BC and 212 UTUC cases, and 813 healthy controls, from September 2007 to October 2011. Urinary arsenic profiles were measured using high-performance liquid chromatography-hydride generator-atomic absorption spectrometry. The polymorphisms of AS3MT, GSTO, and PNP were identified using the Sequenom MassARRAY platform with iPLEX Gold chemistry. We found that inefficient arsenic methylation capacity was associated with BC in a significant dose-response relationship, but only found that high urinary total arsenic concentration was related to the risk of UTUC, also in a significant dose-response manner. Those with a total urinary arsenic level of > 30.28 μg/L compared to ≤ 9.78 μg/L, had a odds ratio (OR), and 95% confidence interval (CI) of UTUC, of 4.80 (2.22-10.39). The polymorphisms of AS3MT rs11191438, AS3MT rs10748835, and AS3MT rs1046778 were related to the risk of BC and UTUC, while the polymorphisms of AS3MT rs3740393, AS3MT rs11191453, and AS3MT rs11191454 were associated with arsenic methylation capacity. The AS3MT gene polymorphisms and arsenic methylation capacity appear to independently affect the risk of BC and UTUC.

Reviewing Hit Discovery Literature for Difficult Targets: Glutathione Transferase Omega-1 as an Example

Early stage drug discovery reporting on relatively new or difficult targets is often associated with insufficient hit triage. Literature reviews of such targets seldom delve into the detail required to critically analyze the associated screening hits reported. Here we take the enzyme glutathione transferase omega-1 (GSTO1-1) as an example of a relatively difficult target and review the associated literature involving small-molecule inhibitors. As part of this process we deliberately pay closer-than-usual attention to assay interference and hit quality aspects. We believe this Perspective will be a useful guide for future development of GSTO1-1 inhibitors, as well serving as a template for future review formats of new or difficult targets.

Shielding effect of anethole against arsenic induced genotoxicity in cultured human peripheral blood lymphocytes and effect of GSTO1 polymorphism

Chronic exposure of inorganic arsenic compounds is responsible for the manifestation of various tumours as well as other diseases. The principal mechanism behind arsenic toxicity is the induction of a strong oxidative stress with production of free radicals in cells. The present study was aimed to explore the shielding effect of anethole against oxidative damage induced by arsenic in cultured human peripheral blood lymphocytes and the effect of GSTO1 polymorphism. Sister chromatid exchange (SCE) frequency, comet tail moment and lipid peroxidation levels were used as biomarkers to assess the oxidative damage. Heparinised venous blood was collected from healthy individuals and treated with sodium arsenite (50 µM) in the presence of anethole (25 and 50 µM) for the analysis of shielding effect of anethole. For the genotyping of GSTO1, PCR RFLP method was adopted. A significant dose-dependent increase in the frequency of SCEs, tail moment and lipid peroxidation levels, was observed when lymphocytes were treated with sodium arsenite. Anethole in combination with sodium arsenite has shown a dose-dependent significant decrease in the frequency of SCEs, tail moment and lipid peroxidation levels. Genetic polymorphism of GSTO1 was found to effect individual susceptibility towards arsenic-mediated genotoxicity and was found insignificant when antigenotoxic effect of anethole was considered. GSTO1 mutant genotypes were found to have significant higher genotoxicity of sodium arsenite as compared to wild-type genotype. The results of the present study suggest ameliorative effects of anethole against arsenic-mediated genotoxic damage in cultured human peripheral blood lymphocytes. A significant effect of GSTO1 polymorphism was observed on genotoxicity of sodium arsenite.

Role of Glutathione S-Transferase Omega Gene Polymorphisms in Breast-Cancer Risk

Background/Aims

Genetically influenced variations in the levels of activity and/or expression of some members of the glutathione S-transferase (GST) family have been identified as risk factors for cancer. One, GST omega (GSTO), has been found in a very limited number of studies. The aim of the present study was to investigate the influence of GSTO1 and GSTO2 polymorphisms on breast cancer risk.

Methods

DNA isolated from the blood of 101 patients with breast cancer and 151 healthy controls was investigated for GSTO1 and GSTO2 polymorphisms by polymerase chain reaction-restriction-fragment length polymorphism.

Results

Univariate and multivariate analyses showed no association between GSTO1 and GSTO2 genotypes and the risk of breast cancer. A higher prevalence of wild-type GSTO1 (A140/A140) was significantly correlated with advanced-stage breast cancer (OR = 0.1, 95% CI, 0.01-0.77), but the presence of the genotype did not correlate with patient age at diagnosis, menopausal status, tumor size, lymph node metastasis, or estrogen-receptor status. No association was found between GSTO2 genotype and clinicopathological features.

Conclusions

The results of the study suggest that GSTO1 and GSTO2 variants are not associated with breast cancer risk, but that wild-type GSTO1 (A140/A140) is likely among cases at an advanced stage.

Omega Class Glutathione S-Transferase: Antioxidant Enzyme in Pathogenesis of Neurodegenerative Diseases

The omega class glutathione S-transferases (GSTOs) are multifunctional enzymes involved in cellular defense and have distinct structural and functional characteristics, which differ from those of other GSTs. Previous studies provided evidence for the neuroprotective effects of GSTOs. However, the molecular mechanisms underpinning the neuroprotective functions of GSTOs have not been fully elucidated. Recently, our genetic and molecular studies using the Drosophila system have suggested that GstO1 has a protective function against H₂O₂-induced neurotoxicity by regulating the MAPK signaling pathway, and GstO2 is required for the activation of mitochondrial ATP synthase in the Drosophila neurodegenerative disease model. The comprehensive understanding of various neuroprotection mechanisms of Drosophila GstOs from our studies provides valuable insight into the neuroprotective functions of GstOs in vivo. In this review, we briefly introduce recent studies and summarize the novel biological functions and mechanisms underpinning neuroprotective effects of GstOs in Drosophila.

GSTO1-1 plays a pro-inflammatory role in models of inflammation, colitis and obesity

Glutathione transferase Omega 1 (GSTO1-1) is an atypical GST reported to play a pro-inflammatory role in response to LPS. Here we show that genetic knockout of Gsto1 alters the response of mice to three distinct inflammatory disease models. GSTO1-1 deficiency ameliorates the inflammatory response stimulated by LPS and attenuates the inflammatory impact of a high fat diet on glucose tolerance and insulin resistance. In contrast, GSTO1-1 deficient mice show a more severe inflammatory response and increased escape of bacteria from the colon into the lymphatic system in a dextran sodium sulfate mediated model of inflammatory bowel disease. These responses are similar to those of TLR4 and MyD88 deficient mice in these models and confirm that GSTO1-1 is critical for a TLR4-like pro-inflammatory response in vivo. In wild-type mice, we show that a small molecule inhibitor that covalently binds in the active site of GSTO1-1 can be used to ameliorate the inflammatory response to LPS. Our findings demonstrate the potential therapeutic utility of GSTO1-1 inhibitors in the modulation of inflammation and suggest their possible application in the treatment of a range of inflammatory conditions.

Microbial Processes Mediating the Evolution of Methylarsine Gases from Dimethylarsenate in Paddy Soils

Arsenic (As) biovolatilization is an important component of the global As biogeochemical cycle. Soils can emit various methylarsine gases, but the underlying microbial processes remain unclear. Here, we show that the addition of molybdate (Mo), an inhibitor of sulfate-reducing bacteria, greatly enhanced dimethylarsine evolution from dimethylarsenate [DMAs(V)] added to two paddy soils. Molybdate addition significantly affected the microbial community structure. The aerobic enrichment cultures from both soils volatilized substantial amounts of dimethylarsine from DMAs(V) in the presence of Mo, whereas the anaerobic enrichment cultures did not. A Bacillus strain (CZ-2) capable of reducing DMAs(V) to dimethylarsine was isolated from the aerobic enrichment culture, and its volatilization ability was enhanced by Mo. RNA-seq analysis identified 10 reductase genes upregulated by Mo. Addition of the reducing agent NADH increased dimethylarsine volatilization by strain CZ-2, suggesting that DMAs(V) reductase is an NADH-dependent enzyme. The strain could not methylate arsenite or convert monomethylarsenate and DMAs(V) to trimethylarsine. Our results show that dimethylarsine evolution from DMAs(V) is independent of the As methylation pathway and that Mo enhances dimethylarsine evolution from paddy soils by shifting the microbial community structure and enhancing the reduction of DMAs(V) to dimethylarsine, possibly through upregulating the expression of DMAs(V) reductase gene(s).

Epigenetic mechanisms underlying the toxic effects associated with arsenic exposure and the development of diabetes

BACKGROUND

Exposure to inorganic arsenic (iAs) is a major threat to the human health worldwide. The consumption of arsenic in drinking water and other food products is associated with the risk of development of type-2 diabetes mellitus (T2DM). The available experimental evidence indicates that epigenetic alterations may play an important role in the development of diseases that are linked with exposure to environmental toxicants. iAs seems to be associated with the epigenetic modifications such as alterations in DNA methylation, histone modifications, and micro RNA (miRNA) abundance.

OBJECTIVE

This article reviewed epigenetic mechanisms underlying the toxic effects associated with arsenic exposure and the development of diabetes.

METHOD

Electronic databases such as PubMed, Scopus and Google scholar were searched for published literature from 1980 to 2017. Searched MESH terms were "Arsenic", "Epigenetic mechanism", "DNA methylation", "Histone modifications" and "Diabetes".

RESULTS

There are various factors involved in the pathogenesis of T2DM but it is assumed that arsenic consumption causes the epigenetic alterations both at the gene-specific level and generalized genome level.

CONCLUSION

The research indicates that exposure from low to moderate concentrations of iAs is linked with the epigenetic effects. In addition, it is evident that, arsenic can change the components of the epigenome and hence induces diabetes through epigenetic mechanisms, such as alterations in glucose transport and/or metabolism and insulin expression/secretion.

Deficiency of long isoforms of Nfe2l1 sensitizes MIN6 pancreatic β cells to arsenite-induced cytotoxicity

Increasing evidence indicates that chronic inorganic arsenic exposure is associated with type 2 diabetes (T2D), a disease of growing prevalence. Pancreatic β-cells were targeted and damaged by oxidative stress induced by arsenite. We previously showed that nuclear factor erythroid 2 like 2 (Nfe2l2)-deficient pancreatic β-cells were vulnerable to cell damage induced by oxidative stressors including arsenite, due to a muted antioxidant response. Like nuclear factor erythroid 2 like 2 (NFE2L2), NFE2L1 also belongs to the cap 'n' collar (CNC) basic-region leucine zipper (bZIP) transcription factor family, and regulates antioxidant response element (ARE) related genes. Our prior work showed NFE2L1 regulates glucose-stimulated insulin secretion (GSIS) in pancreatic β-cells and isolated islets. In the current study, we demonstrated that MIN6 cells with a specific knockdown of long isoforms of Nfe2l1 (L-Nfe2l1) by lentiviral shRNA (Nfe2l1(L)-KD) were vulnerable to arsenite-induced apoptosis and cell damage. The expression levels of antioxidant genes, such as Gclc, Gclm and Ho-1, and intracellular reactive oxygen species (ROS) levels were not different in Scramble and Nfe2l1(L)-KD cells, while the expression of arsenic metabolism related-genes, such as Gsto1, Gstm1 and Nqo1, increased in Nfe2l1(L)-KD cells with or without arsenite treatment. The up-regulation of arsenic biotransformation genes was due to activated NFE2L2 in Nfe2l1(L)-KD MIN6 cells. Furthermore, the level of intracellular monomethylarsenic (MMA) was higher in Nfe2l1(L)-KD MIN6 cells than in Scramble cells. These results showed that deficiency of L-Nfe2l1 in pancreatic β-cells increased susceptibility to acute arsenite-induced cytotoxicity by promoting arsenic biotransformation and intracellular MMA levels.

Skin score correlates with global DNA methylation and GSTO1 A140D polymorphism in arsenic-affected population of Eastern India

Arsenic is a potent environmental toxicant causing serious public health concerns in India, Bangladesh and other parts of the world. Gene- and promoter-specific hypermethylation has been reported in different arsenic-exposed cell lines, whereas whole genome DNA methylation study suggested genomic hypo- and hypermethylation after arsenic exposure in in vitro and in vivo studies. Along with other characteristic biomarkers, arsenic toxicity leads to typical skin lesions. The present study demonstrates significant correlation between severities of skin manifestations with their whole genome DNA methylation status as well as with a particular polymorphism (Ala 140 Asp) status in arsenic metabolizing enzyme Glutathione S-transferase Omega-1 (GSTO1) in arsenic-exposed population of the district of Nadia, West Bengal, India.

Interactive Influence of N6AMT1 and As3MT Genetic Variations on Arsenic Metabolism in the Population of Inner Mongolia, China

Chronic arsenic exposure via drinking water has become a worldwide public health concern. In humans, inorganic arsenic (iAs) is metabolized to monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) mainly mediated by arsenic (+3 oxidation state) methyltransferase (As3MT). We reported recently that N-6 adenine-specific DNA methyltransferase 1 (N6AMT1) was involved in arsenic metabolism, and examined its interactive effect with As3MT on arsenic metabolism in vitro To further evaluate the interactive effect of N6AMT1 and As3MT on arsenic biomethylation in humans, we conducted a human population-based study including 289 subjects living in rural villages in Inner Mongolia, China, and assessed their urinary arsenic metabolites profiles in relation to genetic polymorphisms and haplotypes of N6AMT1 and As3MT Five N6AMT1 single nucleotide polymorphisms (SNPs; rs1003671, rs7282257, rs2065266, rs2738966, rs2248501) and the N6AMT1 haplotype 2_GGCCAT were significantly associated with the percentage of iAs (% iAs) in urine (e.g., for rs7282257, mean was 9.62% for TT, 6.73% for AA). Rs1003671 was also in a significant relationship with urinary MMA and DMA (the mean of %MMA was 24.95% for GA, 31.69% for GG; the mean of % DMA was 69.21% for GA, 59.82% for GG). The combined effect of N6AMT1 haplotype 2_GGCCAT and As3MT haplotype 2_GCAC showed consistence with the additive significance of each haplotype on % iAs: the mean was 5.47% and 9.36% for carriers with both and null haplotypes, respectively. Overall, we showed that N6AMT1 genetic polymorphisms were associated with arsenic biomethylation in the Chinese population, and its interaction with As3MT was observed in specific haplotype combinations.

[Relationship between Arsenic (+3 Oxidation State) Methyltransferase Genetic Polymorphisms and Methylation Capacity of Inorganic Arsenic]

Arsenic metabolism affects the susceptibility of humans to arsenic toxicity; therefore, clarification of the factors associated with individual variations in arsenic metabolism is an important task. Genetic polymorphisms such as single nucleotide polymorphisms (SNPs) in arsenic (+3 oxidation state) methyltransferase (AS3MT), which can methylate arsenic compounds using S-adenosyl-l-methionine (AdoMet), have been reported to modify arsenic methylation. In this review, we summarize studies conducted by us in Vietnam and by others on the association of AS3MT genetic polymorphisms with arsenic metabolism as well as human health effects. Most of the SNPs in AS3MT showed inconsistent results in terms of genotype-dependent differences in arsenic metabolism among the studies. However, AS3MT 12390 (rs3740393) and 14458 (rs11191439) were consistently related to arsenic methylation regardless of the study population: AS3MT 12390 (rs3740393) affected the second step of methylation of arsenic, whereas 14458 (rs11191439) affected the first methylation step.

Structure, function and disease relevance of Omega-class glutathione transferases

The Omega-class cytosolic glutathione transferases (GSTs) have distinct structural and functional attributes that allow them to perform novel roles unrelated to the functions of other GSTs. Mammalian GSTO1-1 has been found to play a previously unappreciated role in the glutathionylation cycle that is emerging as significant mechanism regulating protein function. GSTO1-1-catalyzed glutathionylation or deglutathionylation of a key signaling protein may explain the requirement for catalytically active GSTO1-1 in LPS-stimulated pro-inflammatory signaling through the TLR4 receptor. The observation that ML175 a specific GSTO1-1 inhibitor can block LPS-stimulated inflammatory signaling has opened a new avenue for the development of novel anti-inflammatory drugs that could be useful in the treatment of toxic shock and other inflammatory disorders. The role of GSTO2-2 remains unclear. As a dehydroascorbate reductase, it could contribute to the maintenance of cellular redox balance and it is interesting to note that the GSTO2 N142D polymorphism has been associated with multiple diseases including Alzheimer's disease, Parkinson's disease, familial amyotrophic lateral sclerosis, chronic obstructive pulmonary disease, age-related cataract and breast cancer.

Determinants and Consequences of Arsenic Metabolism Efficiency among 4,794 Individuals: Demographics, Lifestyle, Genetics, and Toxicity

BACKGROUND

Exposure to inorganic arsenic (iAs), a class I carcinogen, affects several hundred million people worldwide. Once absorbed, iAs is converted to monomethylated (MMA) and then dimethylated forms (DMA), with methylation facilitating urinary excretion. The abundance of each species in urine relative to their sum (iAs%, MMA%, and DMA%) varies across individuals, reflecting differences in arsenic metabolism capacity.

METHODS

The association of arsenic metabolism phenotypes with participant characteristics and arsenical skin lesions was characterized among 4,794 participants in the Health Effects of Arsenic Longitudinal Study (Araihazar, Bangladesh). Metabolism phenotypes include those obtained from principal component (PC) analysis of arsenic species.

RESULTS

Two independent PCs were identified: PC1 appears to represent capacity to produce DMA (second methylation step), and PC2 appears to represent capacity to convert iAs to MMA (first methylation step). PC1 was positively associated (P <0.05) with age, female sex, and BMI, while negatively associated with smoking, arsenic exposure, education, and land ownership. PC2 was positively associated with age and education but negatively associated with female sex and BMI. PC2 was positively associated with skin lesion status, while PC1 was not. 10q24.32/AS3MT region polymorphisms were strongly associated with PC1, but not PC2. Patterns of association for most variables were similar for PC1 and DMA%, and for PC2 and MMA% with the exception of arsenic exposure and SNP associations.

CONCLUSIONS

Two distinct arsenic metabolism phenotypes show unique associations with age, sex, BMI, 10q24.32 polymorphisms, and skin lesions.

IMPACT

This work enhances our understanding of arsenic metabolism kinetics and toxicity risk profiles.

Non-monotonic relationships between arsenic and selenium excretion and its implication on arsenic methylation pattern in a Bangladeshi population

The toxicity of arsenic differs markedly between individuals and populations, which might be related to the metabolism (methylation) of inorganic arsenic (As), as well as the selenium (Se) nutritional status. Urinary excretion of As (u-As) and Se (u-Se) was examined in an adult population (n=128) living in an As-contaminated area in Bangladesh. Although there was a significant negative correlation between u-Se and u-As (median 137; range 49-927 μg/g creatinine), closer examination revealed a non-monotonous relationship between them. A quadratic curve with an axis of As at 155 μg/g Cre gave a better fit, and u-As and u-Se were positively or negatively correlated depending on whether the As concentration was lower or higher than 155 μg As/g Cre, respectively. Likewise, the relationships between the As methylation pattern and glutathione-S-transferase (GST) polymorphism, body mass index (BMI), and u-Se differed depending on the u-As range; i.e., higher or lower than 155 μg/g Cre. Although we did not determine the causal mechanism for these observations, the non-monotonic relationship between As exposure and the variables examined suggested the existence of a threshold at which the handling of As by human body is qualitatively changed. The possible importance of Se nutrition for As toxicity is also discussed.

Metals exposure and risk of small-for-gestational age birth in a Canadian birth cohort: The MIREC study

BACKGROUND

Lead, mercury, cadmium and arsenic are some of the most common toxic metals to which Canadians are exposed. The effect of exposure to current low levels of toxic metals on fetal growth restriction is unknown.

OBJECTIVE

The aim of this study was to examine relationships between exposure to lead, mercury, cadmium and arsenic during pregnancy, and risk of small for gestational age (SGA) birth.

METHODS

Lead, mercury, cadmium and arsenic levels were measured in blood samples from the first and third trimesters in 1835 pregnant women from across Canada. Arsenic species in first trimester urine were also assessed. Relative risks and 95% confidence intervals were estimated using log binomial multivariate regression. Important covariates including maternal age, parity, pre-pregnancy BMI, and smoking, were considered in the analysis. An exploratory analysis was performed to examine potential effect modification of these relationships by single nucleotide polymorphisms (SNPs) in GSTP1 and GSTO1 genes.

RESULTS

No association was found between blood lead, cadmium or arsenic and risk for SGA. We observed an increased risk for SGA for the highest compared to the lowest tertile of exposure for mercury (>1.6 µg/L, RR=1.56.; 95% CI=1.04-2.58) and arsenobetaine (>2.25 µg/L, RR=1.65; 95% CI=1.10-2.47) after adjustment for the effects of parity and smoking. A statistically significant interaction was observed in the relationship between dimethylarsinic acid (DMA) levels in urinary arsenic and SGA between strata of GSTO1 A104A (p for interaction=0.02). A marginally significant interaction was observed in the relationship between blood lead and SGA between strata of GSTP1 A114V (p for interaction=0.06).

CONCLUSIONS

These results suggest a small increase in risk for SGA in infants born to women exposed to mercury and arsenic. Given the conflicting evidence in the literature this warrants further investigation in other pregnant populations.

An alternate pathway of arsenate resistance in E. coli mediated by the glutathione S-transferase GstB

Microbial arsenate resistance is known to be conferred by specialized oxidoreductase enzymes termed arsenate reductases. We carried out a genetic selection on media supplemented with sodium arsenate for multicopy genes that can confer growth to E. coli mutant cells lacking the gene for arsenate reductase (E. coli ΔarsC). We found that overexpression of glutathione S-transferase B (GstB) complemented the ΔarsC allele and conferred growth on media containing up to 5 mM sodium arsenate. Interestingly, unlike wild type E. coli arsenate reductase, arsenate resistance via GstB was not dependent on reducing equivalents provided by glutaredoxins or a catalytic cysteine residue. Instead, two arginine residues, which presumably coordinate the arsenate substrate within the electrophilic binding site of GstB, were found to be critical for transferase activity. We provide biochemical evidence that GstB acts to directly reduce arsenate to arsenite with reduced glutathione (GSH) as the electron donor. Our results reveal a pathway for the detoxification of arsenate in bacteria that hinges on a previously undescribed function of a bacterial glutathione S-transferase.

The still mysterious roles of cysteine-containing glutathione transferases in plants

Glutathione transferases (GSTs) represent a widespread multigenic enzyme family able to modify a broad range of molecules. These notably include secondary metabolites and exogenous substrates often referred to as xenobiotics, usually for their detoxification, subsequent transport or export. To achieve this, these enzymes can bind non-substrate ligands (ligandin function) and/or catalyze the conjugation of glutathione onto the targeted molecules, the latter activity being exhibited by GSTs having a serine or a tyrosine as catalytic residues. Besides, other GST members possess a catalytic cysteine residue, a substitution that radically changes enzyme properties. Instead of promoting GSH-conjugation reactions, cysteine-containing GSTs (Cys-GSTs) are able to perform deglutathionylation reactions similarly to glutaredoxins but the targets are usually different since glutaredoxin substrates are mostly oxidized proteins and Cys-GST substrates are metabolites. The Cys-GSTs are found in most organisms and form several classes. While Beta and Omega GSTs and chloride intracellular channel proteins (CLICs) are not found in plants, these organisms possess microsomal ProstaGlandin E-Synthase type 2, glutathionyl hydroquinone reductases, Lambda, Iota and Hemerythrin GSTs and dehydroascorbate reductases (DHARs); the four last classes being restricted to the green lineage. In plants, whereas the role of DHARs is clearly associated to the reduction of dehydroascorbate to ascorbate, the physiological roles of other Cys-GSTs remain largely unknown. In this context, a genomic and phylogenetic analysis of Cys-GSTs in photosynthetic organisms provides an updated classification that is discussed in the light of the recent literature about the functional and structural properties of Cys-GSTs. Considering the antioxidant potencies of phenolic compounds and more generally of secondary metabolites, the connection of GSTs with secondary metabolism may be interesting from a pharmacological perspective.

AS3MT, GSTO, and PNP polymorphisms: impact on arsenic methylation and implications for disease susceptibility

BACKGROUND

Oral exposure to inorganic arsenic (iAs) is associated with adverse health effects. Epidemiological studies suggest differences in susceptibility to these health effects, possibly due to genotypic variation. Genetic polymorphisms in iAs metabolism could lead to increased susceptibility by altering urinary iAs metabolite concentrations.

OBJECTIVE

To examine the impact of genotypic polymorphisms on iAs metabolism.

METHODS

We screened 360 publications from PubMed and Web of Science for data on urinary mono- and dimethylated arsenic (MMA and DMA) percentages and polymorphic genes encoding proteins that are hypothesized to play roles in arsenic metabolism. The genes we examined were arsenic (+3) methyltransferase (AS3MT), glutathione-s-transferase omega (GSTO), and purine nucleoside phosphorylase (PNP). Relevant data were pooled to determine which polymorphisms are associated across studies with changes in urinary metabolite concentration.

RESULTS

In our review, AS3MT polymorphisms rs3740390, rs11191439, and rs11191453 were associated with statistically significant changes in percent urinary MMA. Studies of GSTO polymorphisms did not indicate statistically significant associations with methylation, and there are insufficient data on PNP polymorphisms to evaluate their impact on metabolism.

DISCUSSION

Collectively, these data support the hypothesis that AS3MT polymorphisms alter in vivo metabolite concentrations. Preliminary evidence suggests that AS3MT genetic polymorphisms may impact disease susceptibility. GSTO polymorphisms were not associated with iAs-associated health outcomes. Additional data are needed to evaluate the association between PNP polymorphisms and iAs-associated health outcomes. Delineation of these relationships may inform iAs mode(s) of action and the approach for evaluating low-dose health effects for iAs.

CONCLUSIONS

Genotype impacts urinary iAs metabolite concentrations and may be a potential mechanism for iAs-related disease susceptibility.

Mathematical modeling of the effects of glutathione on arsenic methylation

Background

Arsenic is a major environmental toxin that is detoxified in the liver by biochemical mechanisms that are still under study. In the traditional metabolic pathway, arsenic undergoes two methylation reactions, each followed by a reduction, after which it is exported and released in the urine. Recent experiments show that glutathione plays an important role in arsenic detoxification and an alternative biochemical pathway has been proposed in which arsenic is first conjugated by glutathione after which the conjugates are methylated. In addition, in rats arsenic-glutathione conjugates can be exported into the plasma and removed by the liver in the bile.

Methods

We have developed a mathematical model for arsenic biochemistry that includes three mechanisms by which glutathione affects arsenic methylation: glutathione increases the speed of the reduction steps; glutathione affects the activity of arsenic methyltranferase; glutathione sequesters inorganic arsenic and its methylated downstream products. The model is based as much as possible on the known biochemistry of arsenic methylation derived from cellular and experimental studies.

Results

We show that the model predicts and helps explain recent experimental data on the effects of glutathione on arsenic methylation. We explain why the experimental data imply that monomethyl arsonic acid inhibits the second methylation step. The model predicts time course data from recent experimental studies. We explain why increasing glutathione when it is low increases arsenic methylation and that at very high concentrations increasing glutathione decreases methylation. We explain why the possible temporal variation of the glutathione concentration affects the interpretation of experimental studies that last hours.

Conclusions

The mathematical model aids in the interpretation of data from recent experimental studies and shows that the Challenger pathway of arsenic methylation, supplemented by the glutathione effects described above, is sufficient to understand and predict recent experimental data. More experimental studies are needed to explicate the detailed mechanisms of action of glutathione on arsenic methylation. Recent experimental work on the effects of glutathione on arsenic methylation and our modeling study suggest that supplements that increase hepatic glutathione production should be considered as strategies to reduce adverse health effects in affected populations.

Earth Abides Arsenic Biotransformations

Arsenic is the most prevalent environmental toxic element and causes health problems throughout the world. The toxicity, mobility, and fate of arsenic in the environment are largely determined by its speciation, and arsenic speciation changes are driven, at least to some extent, by biological processes. In this article, biotransformation of arsenic is reviewed from the perspective of the formation of Earth and the evolution of life, and the connection between arsenic geochemistry and biology is described. The article provides a comprehensive overview of molecular mechanisms of arsenic redox and methylation cycles as well as other arsenic biotransformations. It also discusses the implications of arsenic biotransformation in environmental remediation and food safety, with particular emphasis on groundwater arsenic contamination and arsenic accumulation in rice.

Isomerization of Δ5-androstene-3,17-dione into Δ4-androstene-3,17-dione catalyzed by human glutathione transferase A3-3: a computational study identifies a dual role for glutathione

Glutathione transferases (GSTs) are important enzymes in the metabolism of electrophilic xenobiotic and endobiotic toxic compounds. In addition, human GST A3-3 also catalyzes the double bond isomerization of Δ5-androstene-3,17-dione (Δ(5)-AD) and Δ(5)-pregnene-3,20-dione (Δ(5)-PD), which are the immediate precursors of testosterone and progesterone. In fact, GST A3-3 is the most efficient human enzyme known to exist in the catalysis of these reactions. In this work, we have used density functional theory (DFT) calculations to propose a refined mechanism for the isomerization of Δ(5)-AD catalyzed by GST A3-3. In this mechanism the glutathione (GSH) thiol and Tyr9 catalyze the proton transfer from the Δ(5)-AD C4 atom to the Δ(5)-AD C6 atom, with a rate limiting activation energy of 15.8 kcal · mol(-1). GSH has a dual function, because it is also responsible for stabilizing the negative charge that is formed in the O3 atom of the enolate intermediate. The catalytic role of Tyr9 depends on significant conformational rearrangements of its side chain. Neither of these contributions to catalysis has been observed before. Residues Phe10, Leu111, Ala 208, and Ala 216 complete the list of the important catalytic residues. The mechanism detailed here is based on the GST A3-3:GSH:Δ(4)-AD crystal structure and is consistent with all available experimental data.