Biotransformation Of Inorganic Arsenic Research Articles

Influence of humic acid and fluvic acid on the altered toxicities of arsenite and arsenate toward two freshwater algae.

Arsenic pollution in freshwater poses a serious threat to aquatic organisms. However, dissolved organic matter (DOM) in water can modulate arsenic environmental toxicity by either suppressing or promoting its bioaccumulation. In this study, we investigated the toxicity, bioaccumulation, and biotransformation of inorganic arsenic (arsenite AsIII and arsenate AsV) combined with two types of DOM, i.e., humic acid (HA) and fulvic acid (FA), in the algae Chlamydomonas reinhardtii and Ochromonas danica. C. reinhardtii has a cell wall and cannot bioaccumulate arsenic complexation, whereas O. danica has no cell wall. Without DOM, AsV was more toxic than AsIII for C. reinhardtii, and AsV was less toxic than AsIII for O. danica. HA and FA addition reduced AsV and AsIII toxicities; the larger molecular weight (Mw) of HA contributed to the reduction in toxicity to an even greater extent, and reduced arsenic accumulation while promoting the biotransformation ability of C. reinhardtii, which has a cell wall. However, HA and FA addition increased AsV and AsIII toxicities and arsenic accumulation while relatively enhancing the biotransformation ability of O. danica, which has no cell wall. Coupling toxicity, bioaccumulation, and biotransformation, DOM (HA and FA) contributed to the altered toxicity of freshwater algae to AsV and AsIII through reduced/increased arsenic accumulation and enhanced biotransformation. Overall, our study considered the combined toxicity of inorganic arsenic and DOM in phytoplankton, helping estimate the potential environmental risk of arsenic in aqueous environments.

Arsenic Biotransformation: It is a complex process

Arsenic Biotransformation: It is a complex process

Biotransformation of inorganic arsenic: Influence of gender, arsenic dose level, and creatinine formation

Biotransformation of inorganic arsenic: Influence of gender, arsenic dose level, and creatinine formation

Bioaccumulation and biotransformation of inorganic arsenic in zhikong scallop (Chlamys farreri) after waterborne exposure

Arsenic (As) and As speciation in marine bivalves have been widely investigated. However, little is known about the bioaccumulation and biotransformation of inorganic As in different tissues of scallops. Therefore, the tissue-specific accumulation, biotransformation and subcellular partitioning of As were investigated in Chlamys farreri following 12 d inorganic As [arsenite (AsⅢ) and arsenate (AsⅤ)] exposure and 30 d depuration. Total As levels were highest in the kidneys and lowest in the adductor muscle after 12 d exposure for both As (Ⅲ) and As (Ⅴ) treatment groups, and the bioavailability of As (Ⅲ) was significantly higher than that of As (Ⅴ) for C. farreri. After 30 d elimination, total As levels were significantly decreased to the control levels. The subcellular fate of As in five different tissues was similar for different inorganic As treatment groups. The greatest proportion of As was found in the metallothionein-like protein fraction (MTLP) and the second was the cellular debris (CD). A little part of As (Ⅲ) could be oxidized to As (Ⅴ) in the gill and digestive gland for As (Ⅲ) treatment groups, and the reduction of As (Ⅴ) to As (Ⅲ) happened in the gill and kidney under As (Ⅴ) exposure. Although a high methylation activity was found in C. farreri, it varied in different tissues with different inorganic As species exposure. The present results indicated that exposure to As (Ⅲ) and As (Ⅴ) could induce different responses in bioaccumulation and biotransformation in five tissues of C. farreri.

Trace elements and arsenic speciation in tissues of tube dwelling polychaetes from hydrothermal vent ecosystems (East Pacific Rise): An ecological role as antipredatory strategy?

Hydrothermal vent systems are inhabited by dense benthic communities adapted to extreme conditions such as high temperature, hydrogen sulphide (H2S) and elevated fluxes of metals. In the present work, a wide range of trace elements (Ag, Al, As, Ba, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Sb, Se, V and Zn) were measured in tissues of three tube dwelling annelids, Alvinella pompejana, Alvinella caudata and Riftia pachyptila, which colonize distinct habitats of the East Pacific Rise (EPR) at 2500 m depth. Metals concentrations in alvinellids were often 2–4 orders of magnitude higher than those commonly found in marine organisms, while much lower values were observed in the vestimentiferan polychaete. Mobility of trace elements was further characterized in tissues of A. pompejana where metals appeared mostly in insoluble forms, i.e. associated with hydrated oxides and sulphides. Arsenic was mainly present in a weakly insoluble form and with concentrations in the branchial tentacles of alvinellids, approximately 5–15 fold higher than those measured in the thorax. Chemical speciation of this element in tissues of the three polychaete species revealed a major contribution of methylated arsenic compounds, like dimethylarsinate (DMA) and, to a lower extent, monomethylarsonate (MMA) and trimethylarsine oxide (TMAO). Although the biotransformation of inorganic arsenic might represent a detoxification mechanism in polychaetes from hydrothermal vents, the elevated levels of methylated forms of arsenic in branchial tissues also suggest an ecological role of this element as an antipredatory strategy for more vulnerable tissues toward generalist consumers.

Structure of the ArsI C–As Lyase: Insights into the Mechanism of Degradation of Organoarsenical Herbicides and Growth Promoters

Arsenic is a ubiquitous and carcinogenic environmental element that enters the biosphere primarily from geochemical sources, but also through anthropogenic activities. Microorganisms play an important role in the arsenic biogeochemical cycle by biotransformation of inorganic arsenic into organic arsenicals and vice versa. ArsI is a microbial non-heme, ferrous-dependent dioxygenase that transforms toxic methylarsenite [MAs(III)] to less toxic and carcinogenic inorganic arsenite [As(III)] by C–As bond cleavage. An ArsI ortholog, TcArsI, from the thermophilic bacterium Thermomonospora curvata was expressed, purified, and crystallized. The structure was solved in both the apo form and with Ni(II), Co(II), or Fe(III). The MAs(III) binding site is a vicinal cysteine pair in a flexible loop. A structure with the loop occupied with β-mercaptoethanol mimics binding of MAs(III). The structure of a mutant protein (Y100H/V102F) was solved in two different crystal forms with two other orientations of the flexible loop. These results suggest that a loop-gating mechanism controls the catalytic reaction. In the ligand-free open state, the loop is exposed to solvent, where it can bind MAs(III). The loop moves toward the active site, where it forms a closed state that orients the C–As bond for dioxygen addition and cleavage. Elucidation of the enzymatic mechanism of this unprecedented C–As lyase reaction will enhance our understanding of recycling of environmental organoarsenicals.

Biotransformation of inorganic arsenic in a marine herbivorous fish Siganus fuscescens after dietborne exposure

Arsenic (As) is well known to be biodiminished along marine food chains. The marine herbivorous fish at a lower trophic level are expected to accumulate more As. However, little is known about how marine herbivorous fish biotransform the potential high As bioaccumulation. Therefore, the present study quantified the biotransformation of two inorganic As species (As(III) and As(V)) in a marine herbivorous fish Siganus fuscescens following dietborne exposure. The fish were fed on As contaminated artificial diets at nominal concentrations of 400 and 1500 μg As(III) or As(V) g−1 (dry weight) for 21 d and 42 d. After exposure, As concentrations in intestine, liver, and muscle tissues of rabbitfish increased significantly and were proportional to the inorganic As exposure concentrations. The present study demonstrated that both inorganic As(III) and As(V) in the dietborne phases were able to be biotransformed to the less toxic arsenobetaine (AsB) (63.3–91.3% in liver; 79.0%–95.2% in muscle). The processes of As biotransformation in rabbitfish could include oxidation of As(III) to As(V), reduction of As(V) to As(III), methylation to monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA), and subsequent conversion to AsB. These results also demonstrated that AsB synthesis processes were diverse facing different inorganic As species in different tissues. In summary, the present study elucidated that marine herbivorous fish had high ability to biotransform inorganic As to the organic forms (mainly AsB), resulting in high As bioaccumulation. Therefore, marine herbivorous fish could detoxify inorganic As in the natural environment.

Biotransformation and biomethylation of arsenic by Shewanella oneidensis MR-1.

The resistance of Shewanella oneidensis MR-1 to toxic arsenic was investigated by measuring the growth of the bacteria in the presence of As(III) and As(V) in different growth media. The bacteria were shown to biotransform arsenic through the partial methylation of inorganic arsenic into methylated metabolites. This biotransformation of inorganic arsenic by S. oneidensis MR-1 was affected by the methyl donor, the composition of the medium, and the presence of Fe(III). The relative content of methylated arsenic in the medium containing S-adenosyl methionine as the methyl donor was greater than that in the medium containing methylcobalamin. The biotransformation process driven by Fe-reducing bacteria, and occurred in combination with microbially mediated As-Fe reduction in the presence of Fe(III). The results demonstrate that S. oneidensis MR-1 methylates inorganic arsenic into less toxic organoarsenic compounds. This process has potential applications in the bioremediation of environmental arsenic, and the results provide new insights into the control of in situ arsenic pollution.

Toxicity of arsenic species to three freshwater organisms and biotransformation of inorganic arsenic by freshwater phytoplankton (Chlorella sp. CE-35)

In the environment, arsenic (As) exists in a number of chemical species, and arsenite (AsIII) and arsenate (AsV) dominate in freshwater systems. Toxicity of As species to aquatic organisms is complicated by their interaction with chemicals in water such as phosphate that can influence the bioavailability and uptake of AsV. In the present study, the toxicities of AsIII, AsV and dimethylarsinic acid (DMA) to three freshwater organisms representing three phylogenetic groups: a phytoplankton (Chlorella sp. strain CE-35), a floating macrophyte (Lemna disperma) and a cladoceran grazer (Ceriodaphnia cf. dubia), were determined using acute and growth inhibition bioassays (EC50) at a range of total phosphate (TP) concentrations in OECD medium. The EC50 values of AsIII, AsV and DMA were 27±10, 1.15±0.04 and 19±3mg L−1 for Chlorella sp. CE-35; 0.57±0.16, 2.3±0.2 and 56±15mgL−1 for L. disperma, and 1.58±0.05, 1.72±0.01 and 5.9±0.1mgL−1 for C. cf. dubia, respectively. The results showed that AsIII was more toxic than AsV to L. disperma; however, AsV was more toxic than AsIII to Chlorella sp. CE-35. The toxicities of AsIII and AsV to C. cf. dubia were statistically similar (p>0.05). DMA was less toxic than iAs species to L. disperma and C. cf. dubia, but more toxic than AsIII to Chlorella sp. CE-35. The toxicity of AsV to Chlorella sp. CE-35 and L. disperma decreased with increasing TP concentrations in the growth medium. Phosphate concentrations did not influence the toxicity of AsIII to either organism. Chlorella sp. CE-35 showed the ability to reduce AsV to AsIII, indicating a substantial influence of phytoplankton on As biogeochemistry in freshwater aquatic systems.

Crystallization and preliminary X-ray crystallographic studies of the ArsI C-As lyase from Thermomonospora curvata.

Arsenic is a ubiquitous and carcinogenic environmental element that enters the biosphere primarily from geochemical sources, but also through anthropogenic activities. Microorganisms play an important role in the arsenic biogeochemical cycle by biotransformation of inorganic arsenic into organic arsenicals and vice versa. ArsI is a microbial nonheme ferrous-dependent dioxygenase that transforms toxic methylarsonous acid to the less toxic inorganic arsenite by C-As bond cleavage. An ArsI ortholog from the thermophilic bacterium Thermomonospora curvata was expressed, purified and crystallized. The crystals diffracted to 1.46 Å resolution and belonged to space group P4₃2₁2 or its enantiomer P4₁2₁2, with unit-cell parameters a=b=42.2, c=118.5 Å.

Arsenic and the Epigenome: Interindividual Differences in Arsenic Metabolism Related to Distinct Patterns of DNA Methylation

Biotransformation of inorganic arsenic (iAs) is one of the factors that determines the character and magnitude of the diverse detrimental health effects associated with chronic iAs exposure, but it is unknown how iAs biotransformation may impact the epigenome. Here, we integrated analyses of genome-wide, gene-specific promoter DNA methylation levels of peripheral blood leukocytes with urinary arsenical concentrations of subjects from a region of Mexico with high levels of iAs in drinking water. These analyses revealed dramatic differences in DNA methylation profiles associated with concentrations of specific urinary metabolites of arsenic (As). The majority of individuals in this study had positive indicators of As-related disease, namely pre-diabetes mellitus or diabetes mellitus (DM). Methylation patterns of genes with known associations with DM were associated with urinary concentrations of specific iAs metabolites. Future studies will determine whether these DNA methylation profiles provide mechanistic insight into the development of iAs-associated disease, predict disease risk, and/or serve as biomarkers of iAs exposure in humans.

Gene polymorphisms of glutathione S-transferase omega 1 and 2, urinary arsenic methylation profile and urothelial carcinoma

Genetic polymorphisms in arsenic-metabolizing enzymes may be involved in the biotransformation of inorganic arsenic and may increase the risk of developing urothelial carcinoma (UC). The present study evaluated the roles of glutathione S-transferase omega 1 (GSTO1) and GSTO2 polymorphisms in UC carcinogenesis. A hospital-based case-control study was conducted. Questionnaire information and biological specimens were collected from 149 UC cases and 251 healthy controls in a non-obvious inorganic arsenic exposure area in Taipei, Taiwan. The urinary arsenic profile was determined using high-performance liquid chromatography and hydride generator-atomic absorption spectrometry. Genotyping for GSTO1 Ala140Asp and GSTO2 Asn142Asp was conducted using polymerase chain reaction–restriction fragment length polymerase. GSTO1 Glu208Lys genotyping was performed using high-throughput matrix-assisted laser desorption and ionization time-of-flight mass spectrometry. A significant positive association was found between total arsenic, inorganic arsenic percentage and monomethylarsonic acid percentage and UC, while dimethylarsinic acid percentage was significantly inversely associated with UC. The minor allele frequency of GSTO1 Ala140Asp, GSTO1 Glu208Lys and GSTO2 Asn142Asp was 18%, 1% and 26%, respectively. A significantly higher MMA% was found in people who carried the wild type of GSTO1 140 Ala/Ala compared to those who carried the GSTO1 140 Ala/Asp and Asp/Asp genotype ( p = 0.02). The homogenous variant genotype of GSTO2 142 Asp/Asp was inversely associated with UC risk (OR = 0.17; 95% CI, 0.03 - 0.88; p = 0.03). Large-scale studies will be required to verify the association between the single nucleotide polymorphisms of arsenic-metabolism-related enzymes and UC risk.

Arsenic (+ 3 Oxidation State) Methyltransferase and the Methylation of Arsenicals in the Invertebrate Chordate Ciona intestinalis

Biotransformation of inorganic arsenic (iAs) involves methylation catalyzed by arsenic (+ 3 oxidation state) methyltransferase (As3mt) yielding mono-, di-, and trimethylated arsenicals. To investigate the evolution of molecular mechanisms that mediate arsenic biotransformation, a comparative genomic approach focusing on the invertebrate chordate Ciona intestinalis was used. Bioinformatic analyses identified an As3mt gene in the C. intestinalis genome. Constitutive As3mt RNA expression was observed in heart, branchial sac, and gastrointestinal tract. Adult animals were exposed to 0 or 1 ppm of iAs for 1 or 5 days. Steady-state As3mt RNA expression in the gastrointestinal tract was not modulated significantly by 5 days of exposure to iAs. Tissue levels of iAs and its methylated metabolites were determined by hydride generation-cryotrapping-gas chromatography-atomic absorption spectrometry. At either time point, exposure to iAs significantly increased concentrations of iAs and its methylated metabolites in tissues. After 5 days of exposure, total speciated arsenic concentrations were highest in branchial sac (3705 ng/g), followed by heart (1019 ng/g) and gastrointestinal tract (835 ng/g). At this time point, the sum of the speciated arsenical concentrations in gastrointestinal tract and heart equaled or exceeded that of iAs; in branchial sac, iAs was the predominant species present. Ciona intestinalis metabolizes iAs to its methylated metabolites, which are retained in tissues. This metabolic pattern is consistent with the presence of an As3mt ortholog in its genome and constitutive expression of the gene in prominent organs, making this basal chordate a useful model to examine the evolution of arsenic detoxification.

Effects of different inorganic arsenic species in Cyprinus carpio (Cyprinidae) tissues after short-time exposure: Bioaccumulation, biotransformation and biological responses

Differences in the toxicological and metabolic pathway of inorganic arsenic compounds are largely unknown for aquatic species. In the present study the effects of short-time and acute exposure to As III and As V were investigated in gills and liver of the common carp, Cyprinus carpio (Cyprinidae), measuring accumulation and chemical speciation of arsenic, and the activity of glutathione-S-transferase omega (GST Ω), the rate limiting enzyme in biotransformation of inorganic arsenic. Oxidative biomarkers included antioxidant defenses (total glutathione-S-transferases, glutathione reductase, glutathione, and glucose-6-phosphate dehydrogenase), total scavenging capacity toward peroxyl radicals, reactive oxygen species (ROS) measurement and lipid peroxidation products. A marked accumulation of arsenic was observed only in gills of carps exposed to 1000 ppb As V. Also in gills, antioxidant responses were mostly modulated through a significant induction of glucose-6-phosphate dehydrogenase activity which probably contributed to reduce ROS formation; however this increase was not sufficient to prevent lipid peroxidation. No changes in metal content were measured in liver of exposed carps, characterized by lower activity of GST Ω compared to gills. On the other hand, glutathione metabolism was more sensitive in liver tissue, where a significant inhibition of glutathione reductase was concomitant with increased levels of glutathione and higher total antioxidant capacity toward peroxyl radicals, thus preventing lipid peroxidation and ROS production. The overall results of this study indicated that exposure of C. carpio to As III and As V can induce different responses in gills and liver of this aquatic organism.

Arsenic (+3 oxidation state) methyltransferase and the methylation of arsenicals in the invertebrate chordate Ciona intestinalis

The biotransformation of inorganic arsenic (iAs) involves methylation by an arsenic (+3 oxidation state) methyltransferase (AS3MT), yielding methyl arsenic (MA), dimethyl arsenic (DMA), and trimethylarsenic (TMA). To identify molecular mechanisms that coordinate arsenic biotransformation, we are using a comparative genomic approach focused on the invertebrate chordate, Ciona intestinalis. Bioinformatic analyses were used to annotate an AS3MT gene in Ciona. RT‐PCR analysis from replicate animals demonstrated constitutive expression of AS3MT in branchial sac (BS), gastrointestinal tract (GIT), and heart. Also, replicate animals were exposed to zero (control) or 1 ppm of iAs (as arsenite) in sea water during 24 h or 5 d. Hydride generation‐atomic absorption spectrometry was used to measure iAs, MA, DMA, and TMA concentrations in BS, GIT, and heart. Methylated metabolites were detected in each of the tissues at both 24 h and 5 d in Ciona specimens exposed to iAs compared with controls (P < 0.01). Average total speciated As concentrations were highest in the BS (3705 ng/g) followed by heart (1019 ng/g) and GIT (835 ng/g) after 5 d. Thus Ciona metabolizes iAs to its methylated metabolites, consistent with the presence of an AS3MT ortholog in its genome, making this basal chordate a useful model to examine the evolution of As detoxification. (This abstract does not reflect U.S. EPA policy.)

GSTO polymorphism analysis in thyroid nodules suggest that GSTO1 variants do not influence the risk for malignancy

A new class of glutathione S-transferase enzymes named omega (GSTO) has been recently identified and shown to be expressed in a wide range of human tissues. A genetic polymorphism of the GSTO1 gene causing an alanine-to-aspartate (A140D) substitution in amino acid 140 produces a variant with lowered enzyme activities in the biotransformation of inorganic arsenic, a common contaminant of drinking water in many regions of the world and a well-known carcinogen. In order to investigate the role of GSTO1 inheritance pattern on thyroid cancer risk we used a polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP)-sequencing approach to compare the genotypes of 173 (87 women, 86 men; 18-81 years old; 47+/-18 years old) healthy control individuals with those of 145 patients with thyroid nodules (84 women, 61 men; 17-81 years old; 49+/-14 years old) including 17 follicular carcinomas, 76 papillary carcinomas, 21 follicular adenomas and 31 multinodular goiters. The incidence of GSTO1 variants was similar in the control population and population with the benign and malignant nodules. There was no association between genotype and the patients' clinical features, tumour parameters of aggressiveness at diagnosis or behaviour during follow-up. We conclude that GSTO1 variants do not influence the risk for thyroid nodules or their pathologic and clinical characteristics.

Arsenic Speciation in Lichens and in Coarse and Fine Airborne Particulate Matter by HPLC–UV–HG–AFS

A three-step sequential extraction procedure with Milli-Q, CaCl2 and H3PO4 was applied for extraction of arsenic species in lichen transplants and airborne particulate matter (fine and coarse fractions). The samples used in this work were collected in 1994–1995 near coal-fired power plants. Both transplant lichens and airborne particulate matter were submitted to the same environment simultaneously. Arsenic species identification and quantification was performed by HPLC–UV–HG–AFS. Inorganic forms of arsenic (arsenite and arsenate) were present in significant amounts in most of the samples. Only in lichens also organic forms of arsenic (monomethyl arsonic acid and dimethyl arsinic acid) were identified which may indicate biotransformation of inorganic arsenic.

Genetic polymorphism in the biotransformation of inorganic arsenic and its role in toxicity

Arsenic is a recognized human carcinogen, but experimental cancer studies are negative. There is a variation in susceptibility among individuals, which probably is related to variation in metabolism. Inorganic arsenic is methylated to methylarsonic acid (MMA) and dimethylarsinic acid (DMA), which are less toxic and more readily excreted in urine than the inorganic arsenic. The rate of methylation of arsenic varies considerably between species. Most population groups studied so far have on average 10–30% inorganic, 10–20% MMA, and 60–70% DMA in urine, but there is a considerable inter-individual variation. Also, recent studies have identified groups with unusually low or high urinary excretion of MMA. Thus, there seems to be a genetic polymorphism in the biomethylation of arsenic. However, the methyltransferases involved in arsenic methylation have not been characterized.

Diversity of inorganic arsenite biotransformation.

Biotransformation of inorganic arsenic in mammals is catalyzed by three serial enzyme activities: arsenate reductase, arsenite methyltransferase, and monomethylarsonate methyltransferase. Our laboratory has purified and characterized these enzymes in order to understand the mechanisms and elucidate the variations of the responses to arsenate/arsenite challenge. Our results indicate a marked deficiency and diversity of these enzyme activities in various animal species.

Study of inorganic arsenic methylation by rat liver in vitro: relevance for the interpretation of observations in man.

The biotransformation of inorganic arsenic by rat liver in vitro leads to the production of a monomethylated and a dimethylated arsenic derivative, measured by flameless atomic absorption as monomethylarsonic (MMA) and dimethylarsinic (DMA) acids respectively. The methylating activity is localized in the cytosol and accepts only As3+ as substrate. Its optimum pH lies between 7.5 and 8.0, and reduced glutathione (10(-2)M) is required for full activity. S-Adenosylmethionine is the essential methyl group donor and corrinoïd derivatives act synergistically. An excess of substrate and the addition of mercuric ions prevent the formation of the dimethylated arsenic derivative without affecting that of the monomethylated compound. This indicates that two different enzymatic activities are involved in the methylation of inorganic arsenic in mammals. Previous observations in man (Buchet et al. 1981 b, 1984) and the results of the present study suggest that DMA production results from the subsequent methylation of the MMA precursor, although the possibility that metabolites are also produced by two completely independent pathways cannot yet be conclusively rejected. The kinetics of MMA and DMA production provide an explanation for the observations that in volunteers given increasing amounts of As3+, the urinary excretion of DMA levels off faster than that of MMA and in patients acutely intoxicated with As3+, several days may elapse before DMA becomes the preponderant metabolite. The results of the present study also suggest that the reduction of MMA production associated with an increased synthesis of DMA found in patients with liver diseases given a standard dose of As3+ might be due to a reduction of As3+ uptake by the liver cells.