Time dependence of accumulation and binding of inorganic and organic arsenic species in rabbit erythrocytes

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 (MAsIII) and dimethylarsinous acid (DMAsIII), 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 MAsIII and DMAsIII in human cells and mechanisms for interactions between these arsenicals and targets is incomplete. Development of novel analytical methods for quantitation of MAsIII and DMAsIII in biological samples promises to address some of these gaps. Here, we summarize current knowledge of the enzymatic basis of MAsIII and DMAsIII 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.

Multidrug Resistance Protein 1 (MRP1/ABCC1)-Mediated Cellular Protection and Transport of Methylated Arsenic Metabolites Differs between Human Cell Lines

The ATP-binding cassette (ABC) transporter multidrug resistance protein 1 (MRP1/ABCC1) protects cells from arsenic (a proven human carcinogen) through the cellular efflux of arsenic triglutathione [As(GS)3] and the diglutathione conjugate of monomethylarsonous acid [MMA(GS)2]. Previously, differences in MRP1 phosphorylation (at Y920/S921) and N-glycosylation (at N19/N23) were associated with marked differences in As(GS)3 transport kinetics between HEK293 and HeLa cell lines. In the current study, cell line differences in MRP1-mediated cellular protection and transport of other arsenic metabolites were explored. MRP1 expressed in HEK293 cells reduced the toxicity of the major urinary arsenic metabolite dimethylarsinic acid (DMAV), and HEK-WT-MRP1-enriched vesicles transported DMAV with high apparent affinity and capacity (Km 0.19 µM, Vmax 342 pmol⋅mg-1protein⋅min-1). This is the first report that MRP1 is capable of exporting DMAV, critical for preventing highly toxic dimethylarsinous acid formation. In contrast, DMAV transport was not detected using HeLa-WT-MRP1 membrane vesicles. MMA(GS)2 transport by HeLa-WT-MRP1 vesicles had a greater than threefold higher Vmax compared with HEK-WT-MRP1 vesicles. Cell line differences in DMAV and MMA(GS)2 transport were not explained by differences in phosphorylation at Y920/S921. DMAV did not inhibit, whereas MMA(GS)2 was an uncompetitive inhibitor of As(GS)3 transport, suggesting that DMAV and MMA(GS)2 have nonidentical binding sites to As(GS)3 on MRP1. Efflux of different arsenic metabolites by MRP1 is likely influenced by multiple factors, including cell and tissue type. This could have implications for the impact of MRP1 on both tissue-specific susceptibility to arsenic-induced disease and tumor sensitivity to arsenic-based therapeutics.

Arsenic-induced abnormalities in glucose metabolism: Biochemical basis and potential therapeutic and nutritional interventions

Health hazards due to the consumption of heavy metals such as arsenic have become a worldwide problem. Metabolism of arsenic produces various intermediates which are more toxic and cause toxicity. Arsenic exposure results in impairment of glucose metabolism, insulin secretion in pancreatic β-cells, altered gene expressions and signal transduction, and affects insulin-stimulated glucose uptake in adipocytes or skeletal muscle cells. Arsenic toxicity causes abnormalities in glucose metabolism through an increase in oxidative stress. Arsenic interferes with the sulfhydryl groups and phosphate groups present in various enzymes involved in glucose metabolism including pyruvate dehydrogenase and α-ketoglutarate dehydrogenase, and contributes to their impairment. Arsenic inhibits glucose transporters present in the cell membrane, alters expression of genes involved in glucose metabolism, transcription factors and inflammatory cytokines which stimulate oxidative stress. Some theories suggest that arsenic exposure under diabetic conditions inhibits hyperglycemia. However, the exact mechanism behind the behavior of arsenic as an antagonist or synergist on glucose homeostasis and insulin secretion is not yet fully understood. The present review delineates the relationship between arsenic and the biochemical basis of its relationship to glucose metabolism. This review also addresses potential therapeutic and nutritional interventions for attenuating arsenic toxicity. Several other potential nutritional supplements are highlighted in the review that could be used to combat arsenic toxicity.

Poly(lactic-co-glycolic) acid loaded nano-insulin has greater potentials of combating arsenic induced hyperglycemia in mice: some novel findings

Diabetes is a menacing problem, particularly to inhabitants of groundwater arsenic contaminated areas needing new medical approaches. This study examines if PLGA loaded nano-insulin (NIn), administered either intraperitoneally (i.p.) or through oral route, has a greater cost-effective anti-hyperglycemic potential than that of insulin in chronically arsenite-fed hyperglycemic mice. The particle size, morphology and zeta potential of nano-insulin were determined using dynamic light scattering method, scanning electronic and atomic force microscopies. The ability of the nano-insulin (NIn) to cross the blood-brain barrier (BBB) was also checked. Circular dichroic spectroscopic (CD) data of insulin and nano-insulin in presence or absence of arsenic were compared. Several diabetic markers in different groups of experimental and control mice were assessed. The mitochondrial functioning through indices like cytochrome c, pyruvate-kinase, glucokinase, ATP/ADP ratio, mitochondrial membrane potential, cell membrane potential and calcium-ion level was also evaluated. Expressions of the relevant marker proteins and mRNAs like insulin, GLUT2, GLUT4, IRS1, IRS2, UCP2, PI3, PPARγ, CYP1A1, Bcl2, caspase3 and p38 for tracking-down the signaling cascade were also analyzed. Results revealed that i.p.-injected nano-encapsulated-insulin showed better results; NIn, due to its smaller size, faster mobility, site-specific release, could cross BBB and showed positive modulation in mitochondrial signaling cascades and other downstream signaling molecules in reducing arsenic-induced-hyperglycemia. CD data indicated that nano-insulin had less distorted secondary structure as compared with that of insulin in presence of arsenic. Thus, overall analyses revealed that PLGA nano-insulin showed better efficacy in combating arsenite-induced-hyperglycemia than that of insulin and therefore, has greater potentials for use in nano-encapsulated form.

Methylated Arsenicals: The Implications of Metabolism and Carcinogenicity Studies in Rodents to Human Risk Assessment

Monomethylarsonic acid (MMA(V)) and dimethylarsinic acid (DMA(V)) are active ingredients in pesticidal products used mainly for weed control. MMA(V) and DMA(V) are also metabolites of inorganic arsenic, formed intracellularly, primarily in liver cells in a metabolic process of repeated reductions and oxidative methylations. Inorganic arsenic is a known human carcinogen, inducing tumors of the skin, urinary bladder, and lung. However, a good animal model has not yet been found. Although the metabolic process of inorganic arsenic appears to enhance the excretion of arsenic from the body, it also involves formation of methylated compounds of trivalent arsenic as intermediates. Trivalent arsenicals (whether inorganic or organic) are highly reactive compounds that can cause cytotoxicity and indirect genotoxicity in vitro. DMA(V) was found to be a bladder carcinogen only in rats and only when administered in the diet or drinking water at high doses. It was negative in a two-year bioassay in mice. MMA(V) was negative in 2-year bioassays in rats and mice. The mode of action for DMA(V)-induced bladder cancer in rats appears to not involve DNA reactivity, but rather involves cytotoxicity with consequent regenerative proliferation, ultimately leading to the formation of carcinoma. This critical review responds to the question of whether DMA(V)-induced bladder cancer in rats can be extrapolated to humans, based on detailed comparisons between inorganic and organic arsenicals, including their metabolism and disposition in various animal species. The further metabolism and disposition of MMA(V) and DMA(V) formed endogenously during the metabolism of inorganic arsenic is different from the metabolism and disposition of MMA(V) and DMA(V) from exogenous exposure. The trivalent arsenicals that are cytotoxic and indirectly genotoxic in vitro are hardly formed in an organism exposed to MMA(V) or DMA(V) because of poor cellular uptake and limited metabolism of the ingested compounds. Furthermore, the evidence strongly supports a nonlinear dose-response relationship for the biologic processes involved in the carcinogenicity of arsenicals. Based on an overall review of the evidence, using a margin-of-exposure approach for MMA(V) and DMA(V) risk assessment is appropriate. At anticipated environmental exposures to MMA(V) and DMA(V), there is not likely to be a carcinogenic risk to humans.

Sodium arsenite affects Na+ transport in the isolated skin of the toad Pleurodema thaul

Arsenic, applied as sodium arsenite (As(III)) to either inner or outer surfaces of the isolated toad skin, dose-dependently decreased the short-circuit current (Isc), potential difference (PD) and sodium conductance (G(Na)) in the concentration range 1-1000 microM, with effects often lasting over 3 h. Maximal inhibitory effect was over 90% with an IC(50) of about 34 microM. Applied during amiloride block, As(III) did not change this effect. However, an increase in electric parameters was noted during the initial 30 min in 22 experiments, indicating a possible translocation of cytosolic protein kinase C (PKC) to the membrane within 15 min, thus stimulating sodium transport; this is followed by a progressive inhibition of kinase activity. Comparative effects of amiloride (8 microM), As(III) (100 microM, outer surface) and noradrenaline (NA, 10 microM, inner surface) showed a significant increase in the stimulatory effect of NA on the electric parameters, which could be the result of arsenite clustering of cell surface receptors and activation of ensuing cellular signal transduction pathways. Ouabain 5 microM, followed by As(III) 100 microM, also stimulated the skin response to NA (10 microM), although the duration of the two phases of the response was markedly shortened. The exact mechanism is still in doubt: however, As(III) increases cerebral metabolites of NA and ouabain can increase NA efflux from tissue slices. The amiloride test, performed with As(III) in the outer surface, confirmed significant decrease in all the parameters: the driving force (E(Na)), sodium conductance (G(Na)), and importantly, shunt conductance (G(sh)), due to the known fact that arsenic inhibits gap junctional intercellular communication.

Molecular processes in cellular arsenic metabolism

Elucidating molecular processes that underlie accumulation, metabolism and binding of iAs and its methylated metabolites provides a basis for understanding the modes of action by which iAs acts as a toxin and a carcinogen. One approach to this problem is to construct a conceptual model that incorporates available information on molecular processes involved in the influx, metabolism, binding and efflux of arsenicals in cells. This conceptual model is initially conceived as a non-quantitative representation of critical molecular processes that can be used as a framework for experimental design and prediction. However, with refinement and incorporation of additional data, the conceptual model can be expressed in mathematical terms and should be useful for quantitative estimates of the kinetic and dynamic behavior of iAs and its methylated metabolites in cells. Development of a quantitative model will be facilitated by the availability of tools and techniques to manipulate molecular processes underlying transport of arsenicals across cell membranes or expression and activity of enzymes involved in methylation of arsenicals. This model of cellular metabolism might be integrated into more complex pharmacokinetic models for systemic metabolism of iAs and its methylated metabolites. It may also be useful in development of biologically based dose-response models describing the toxic and carcinogenic actions of arsenicals.

Gender-specific protective effect of hemoglobin on arsenic-induced skin lesions

Chronic arsenic poisoning remains a public health crisis in Bangladesh. As arsenic has been shown to bind to human hemoglobin (Hb), hematologic mechanisms may play a role in the pathway through which arsenic exerts its toxicity. Two separate studies, a case-control and a cohort, were conducted to investigate the role of Hb in the development of arsenic-induced skin lesions. In the first, conditional logistic regression was used to investigate the effect of Hb on skin lesions among 900 case-control pairs from Pabna, Bangladesh, in which individuals were matched on gender, age, and location. In the second, mixed linear regression models were used to examine the association between toenail arsenic, urinary arsenic, and Hb within a cohort of 184 individuals from 50 families in the same region who did not have arsenic-induced skin lesions. Hb was significantly associated with skin lesions but this association was gender specific. In males, a 40% reduction in the odds of skin lesions occurred for every 1 g/dL increase in Hb (odds ratio, 0.60; 95% confidence interval, 0.49-0.73). No effect was observed for females (odds ratio, 1.16; 95% confidence interval, 0.92-1.46). In the cohort of 184 individuals, no associations between toenail arsenic or urinary arsenic species and Hb levels were observed. Low Hb levels may exacerbate the detrimental health effects of chronic arsenic poisoning. Whereas providing clean water remains the optimal solution to Bangladesh's problem of arsenic poisoning, improving nutrition and reducing iron-deficiency anemia may ameliorate negative health effects, such as skin lesions in individuals who have been exposed.

The seleno bis(S-glutathionyl) arsinium ion is assembled in erythrocyte lysate

Approximately 75 million people are currently exposed to arsenic concentrations in drinking water, which is associated with the development of internal cancers. One way to ameliorate this undesirable situation is to remove arsenic (arsenite and arsenate) from drinking water. An alternative approach is the development of an inexpensive palliative dietary supplement that promotes the excretion of intestinally absorbed arsenite from the body. To this end, the simultaneous administration of New Zealand white rabbits with arsenite and selenite resulted in the biliary excretion of the seleno-bis (S-glutathionyl) arsinium ion, [(GS)2AsSe]-. This apparent detoxification mechanism has been recently extended to environmentally relevant doses [Gailer, J., Ruprecht, L., Reitmeir, P., Benker, B., and Schramel, P. (2004) Appl. Organometal. Chem. 18, 670-675]. The site of formation of this excretory product in the organism, however, is unknown. To investigate if [(GS)2AsSe]- is formed in rabbit blood, we added arsenite and selenite and analyzed blood aliquots using arsenic and selenium X-ray absorption spectroscopy. The characteristic arsenic and selenium X-ray absorption spectra of [(GS)2AsSe]- were detected within 2 min after addition and comprised 95% of the blood selenium 30 min after addition. To elucidate if erythrocytes are involved in the biosynthesis of [(GS)2AsSe]- in blood, arsenite and 77Se-selenite were added to rabbit erythrocyte lysate and the obtained solution was analyzed by 77Se NMR spectroscopy (273 K). This resulted in a 77Se NMR signal with a chemical shift identical to that of synthetic [(GS)2AsSe]- added to lysate. Combined, these results demonstrate that [(GS)2AsSe]- is rapidly formed in blood and that erythrocytes are an important site for the in vivo formation of this toxicologically important metabolite.

Arsenic in the aetiology of cancer

Arsenic, one of the most significant hazards in the environment affecting millions of people around the world, is associated with several diseases including cancers of skin, lung, urinary bladder, kidney and liver. Groundwater contamination by arsenic is the main route of exposure. Inhalation of airborne arsenic or arsenic-contaminated dust is a common health problem in many ore mines. This review deals with the questions raised in the epidemiological studies such as the dose-response relationship, putative confounders and synergistic effects, and methods evaluating arsenic exposure. Furthermore, it describes the metabolic pathways of arsenic, and its biological modes of action. The role of arsenic in the development of cancer is elucidated in the context of combined epidemiological and biological studies. However, further analyses by means of molecular epidemiology are needed to improve the understanding of cancer aetiology induced by arsenic.

Tissue dosimetry, metabolism and excretion of pentavalent and trivalent monomethylated arsenic in mice after oral administration

Exposure to monomethylarsonic acid (MMA(V)) and monomethylarsonous acid (MMA(III)) can result from their formation as metabolites of inorganic arsenic and by the use of the sodium salts of MMA(V) as herbicides. This study compared the disposition of MMA(V) and MMA(III) in adult female B6C3F1 mice. Mice were gavaged p.o. with MMA(V), either unlabeled or labeled with 14C at two dose levels (0.4 or 40 mg As/kg). Other mice were dosed p.o. with unlabeled MMA(III) at one dose level (0.4 mg As/kg). Mice were housed in metabolism cages for collection of excreta and sacrificed serially over 24 h for collection of tissues. MMA(V)-derived radioactivity was rapidly absorbed, distributed and excreted. By 8 h post-exposure, 80% of both doses of MMA(V) were eliminated in urine and feces. Absorption of MMA(V) was dose dependent; that is, there was less than a 100-fold difference between the two dose levels in the area under the curves for the concentration-time profiles of arsenic in blood and major organs. In addition, urinary excretion of MMA(V)-derived radioactivity in the low dose group was significantly greater (P < 0.05) than in the high dose group. Conversely, fecal excretion of MMA(V)-derived radioactivity was significantly greater (P < 0.05) in the high dose group than in the low dose group. Speciation of arsenic by hydride generation-atomic absorption spectrometry in urine and tissues of mice administered MMA(V) or MMA(III) found that methylation of MMA(V) was limited while the methylation of MMA(III) was extensive. Less than 10% of the dose excreted in urine of MMA(V)-treated mice was in the form of methylated products, whereas it was greater than 90% for MMA(III)-treated mice. In MMA(V)-treated mice, 25% or less of the tissue arsenic was in the form of dimethylarsenic, whereas in MMA(III)-treated mice, 75% or more of the tissue arsenic was in the form of dimethylarsenic. Based on urinary analysis, administered dose of MMA(V) did not affect the level of its metabolites excreted. In the tested range, dose affects the absorption, distribution and route of excretion of MMA(V) but not its metabolism.

Acute arsenic poisoning in two siblings

We report a case series of acute arsenic poisoning of 2 siblings, a 4-month-old male infant and his 2-year-old sister. Each child ingested solubilized inorganic arsenic from an outdated pesticide that was misidentified as spring water. The 4-month-old child ingested a dose of arsenic that was lethal despite extraordinary attempts at arsenic removal, including chelation therapy, extracorporeal membrane oxygenation, exchange transfusion, and hemodialysis. The 2-year-old fared well with conventional therapy.

The potential biological mechanisms of arsenic-induced diabetes mellitus

Although epidemiologic studies carried out in Taiwan, Bangladesh, and Sweden have demonstrated a diabetogenic effect of arsenic, the mechanisms remain unclear and require further investigation. This paper reviewed the potential biological mechanisms of arsenic-induced diabetes mellitus based on the current knowledge of the biochemical properties of arsenic. Arsenate can substitute phosphate in the formation of adenosine triphosphate (ATP) and other phosphate intermediates involved in glucose metabolism, which could theoretically slow down the normal metabolism of glucose, interrupt the production of energy, and interfere with the ATP-dependent insulin secretion. However, the concentration of arsenate required for such reaction is high and not physiologically relevant, and these effects may only happen in acute intoxication and may not be effective in subjects chronically exposed to low-dose arsenic. On the other hand, arsenite has high affinity for sulfhydryl groups and thus can form covalent bonds with the disulfide bridges in the molecules of insulin, insulin receptors, glucose transporters (GLUTs), and enzymes involved in glucose metabolism (e.g., pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase). As a result, the normal functions of these molecules can be hampered. However, a direct effect on these molecules caused by arsenite at physiologically relevant concentrations seems unlikely. Recent evidence has shown that treatment of arsenite at lower and physiologically relevant concentrations can stimulate glucose transport, in contrary to an inhibitory effect exerted by phenylarsine oxide (PAO) or by higher doses of arsenite. Induction of oxidative stress and interferences in signal transduction or gene expression by arsenic or by its methylated metabolites are the most possible causes to arsenic-induced diabetes mellitus through mechanisms of induction of insulin resistance and beta cell dysfunction. Recent studies have shown that, in subjects with chronic arsenic exposure, oxidative stress is increased and the expression of tumor necrosis factor alpha (TNFalpha) and interleukin-6 (IL-6) is upregulated. Both of these two cytokines have been well known for their effect on the induction of insulin resistance. Arsenite at physiologically relevant concentration also shows inhibitory effect on the expression of peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear hormone receptor important for activating insulin action. Oxidative stress has been suggested as a major pathogenic link to both insulin resistance and beta cell dysfunction through mechanisms involving activation of nuclear factor-kappaB (NF-kappaB), which is also activated by low levels of arsenic. Although without supportive data, superoxide production induced by arsenic exposure can theoretically impair insulin secretion by interaction with uncoupling protein 2 (UCP2), and oxidative stress can also cause amyloid formation in the pancreas, which could progressively destroy the insulin-secreting beta cells. Individual susceptibility with respect to genetics, nutritional status, health status, detoxification capability, interactions with other trace elements, and the existence of other well-recognized risk factors of diabetes mellitus can influence the toxicity of arsenic on organs involved in glucose metabolism and determine the progression of insulin resistance and impaired insulin secretion to a status of persistent hyperglycemia or diabetes mellitus. In conclusions, insulin resistance and beta cell dysfunction can be induced by chronic arsenic exposure. These defects may be responsible for arsenic-induced diabetes mellitus, but investigations are required to test this hypothesis.

Arsenite induces delayed mutagenesis and transformation in human osteosarcoma cells at extremely low concentrations

Arsenite is a human multisite carcinogen, but its mechanism of action is not known. We recently found that extremely low concentrations (</=0.1 microM) of arsenite transform human osteosarcoma TE85 (HOS) cells to anchorage-independence. In contrast to other carcinogens which transform these cells within days of exposure, almost 8 weeks of arsenite exposure are required for transformation. We decided to reexamine the question of arsenite mutagenicity using chronic exposure in a spontaneous mutagenesis assay we previously developed. Arsenite was able to cause a delayed increase in mutagenesis at extremely low concentrations (</=0.1 microM) in a dose-dependent manner. The increase in mutant frequency occurred after almost 20 generations of growth in arsenite. Transformation required more than 30 generations of continuous exposure. We also found that arsenite induced gene amplification of the dihydrofolate reductase (DHFR) gene in a dose-dependent manner. Since HOS cells are able to methylate arsenite at a very low rate, it was possible that active metabolites such as monomethylarsonous acid (MMA(III)) contributed to the delayed mutagenesis and transformation in these cells. However, when the assay was repeated with MMA(III), we found no significant increase in mutagenesis or transformation, suggesting that arsenite-induced delayed mutagenesis and transformation are not caused by arsenite's metabolites, but by arsenite itself. Our results suggest that long-term exposure to low concentrations of arsenite may affect signaling pathways that result in a progressive genomic instability.

The cellular metabolism and systemic toxicity of arsenic

Although it has been known for decades that humans and many other species convert inorganic arsenic to mono- and dimethylated metabolites, relatively little attention has been given to the biological effects of these methylated products. It has been widely held that inorganic arsenicals were the species that accounted for the toxic and carcinogenic effects of this metalloid and that methylation was properly regarded as a mechanism for detoxification of arsenic. Elucidation of the metabolic pathway for arsenic has changed our understanding of the significance of methylation. Both methylated and dimethylated arsenicals that contain arsenic in the trivalent oxidation state have been identified as intermediates in the metabolic pathway. These compounds have been detected in human cells cultured in the presence of inorganic arsenic and in urine of individuals who were chronically exposed to inorganic arsenic. Methylated and dimethylated arsenicals that contain arsenic in the trivalent oxidation state are more cytotoxic, more genotoxic, and more potent inhibitors of the activities of some enzymes than are inorganic arsenicals that contain arsenic in the trivalent oxidation state. Hence, it is reasonable to describe the methylation of arsenic as a pathway for its activation, not as a mode of detoxification. This review summarizes the current knowledge of the processes that control the formation and fate of the methylated metabolites of arsenic and of the biological effects of these compounds. Given the considerable interest in the dose-response relationships for arsenic as a toxin and a carcinogen, understanding the metabolism of arsenic may be critical to assessing the risk associated with chronic exposure to this element.

Arsenicals inhibit thioredoxin reductase in cultured rat hepatocytes

Thioredoxin reductase (TR), an NADPH-dependent flavoenzyme that catalyzes the reduction of many disulfide-containing substrates, plays an important role in the cellular response to oxidative stress. Trivalent arsenicals, especially methyl As that contains trivalent arsenic (MAs(III)), are potent noncompetitive inhibitors of TR purified from mouse liver. Because MAs(III) is produced in the biomethylation of As, it was postulated that the extent of inhibition of TR in cultured rat hepatocytes would correlate with the intracellular concentration of methyl As. Exposure of cultured hepatocytes to inorganic As(III) (iAs(III)), MAs(III), or aurothioglucose (ATG, a competitive inhibitor of TR activity) for 30 min caused a concentration-dependent reduction in TR activity. The estimated IC(50) was >>100 microM for iAs(III), approximately 10 microM for ATG, and approximately 3 microM for MAs(III). In hepatocytes exposed to 1 microM MAs(III) for up to 24 h, the inhibition of TR activity was maximal ( approximately 40%) after exposure for 15 min. After exposure for 3 h [when most MAs(III) has been converted to dimethyl As (DMAs)], TR activity in these cells had returned to control levels. Notably, exposure of the cell to 50 microM DMAs(III) did not affect TR activity. In hepatocytes exposed to 10 microM iAs(III) for up to 24 h, the inhibition of TR activity was progressive; at 24 h, activity was reduced approximately 35%. Following exposure to iAs(III) or MAs(III), the extent of inhibition of TR activity correlated strongly with the intracellular concentration of MAs. Taken together, these results suggest that arsenicals formed in the course of cellular metabolism of As are potent inhibitors of TR activity. In particular, MAs(III), an intermediate in the metabolic pathway, is an especially potent inhibitor of TR. Hence, the capacity of cells to produce or consume the intermediates in the pathway for As methylation may be an important determinant of susceptibility to the toxic effects of As.

Arsenite, arsenate and vanadate affect human erythrocyte membrane

Effects of arsenite, arsenate and vanadate on human erythrocyte membrane have been assessed according to their routes passing through the membrane, their binding modes to the membrane and their influences on membrane proteins and lipids. The uptake of arsenate (1.0 mM) by cells approached a limit with intracellular arsenic of about 0.2 mM in 5 h, and was strongly inhibited (approximately 95%) by 4,4'-diisothiocyano-2,2'-disulfonic stilbene (DIDS), indicating that arsenate, similar to vanadate, passed across the membrane through the anion exchange protein, band 3. Arsenite (1.0 mM) influx reached a maximum of about 0.4 mM in 30 min, and was not inhibited by DIDS. The transformed species of arsenite bound to the membrane from cytosol. In contrast, arsenate bound rapidly from the outside, followed by releasing and re-binding. The binding to the membrane via sulfhydryl was indicated by the decrease of the sulfhydryl level of membrane proteins. Polyacrylamide gel electrophoresis in sodium dodecyl sulfate (SDS-PAGE) analysis revealed that the proteins, bands 1-3, were among the targets of arsenite, arsenate and vanadate. Their binding to the membrane also induced changes in the fluidity of membrane lipids and in the negative charge density in the outer surface of the membrane.

Methylarsenicals and arsinothiols are potent inhibitors of mouse liver thioredoxin reductase

Thioredoxin reductase (TR, EC 1.6.4.5) was purified 5800-fold from the livers of adult male B6C3F1 mice. The estimated molecular mass of the purified protein was about 57 kDa. The activity of the purified enzyme was monitored by the NADPH-dependent reduction of 5, 5'-dithiobis(2-nitrobenzoic acid) (DTNB); this activity was fully inhibited by 1 microM aurothioglucose. Arsenicals and arsinothiols, complexes of As(III)-containing compounds with L-cysteine or glutathione, were tested as inhibitors of the DTNB reductase activity of the purified enzyme. Pentavalent arsenicals were much less potent inhibitors than trivalent arsenicals. Among all the arsenicals, CH(3)As(III) was the most potent inhibitor of TR. CH(3)As(III) was found to be a competitive inhibitor of the reduction of DTNB (K(i) approximately 100 nM) and a noncompetitive inhibitor of the oxidation of NADPH. The inhibition of TR by CH(3)As(III) was time-dependent and could not be reversed by the addition of a dithiol-containing molecule, 2,3-dimercaptosuccinic acid, to the reaction mixture. The inhibition of TR by CH(3)As(III) required the simultaneous presence of NADPH in the reaction mixture. However, unlike other pyridine nucleotide disulfide oxidoreductases, there was no evidence that mouse liver TR was inactivated by exposure to NADPH. Treatment with CH(3)As(III) did not increase the NADPH oxidase activity of the purified enzyme. Thus, CH(3)As(III), a putative intermediate in the pathway for the biomethylation of As, is a potent and irreversible inhibitor of an enzyme involved in the response of the cell to oxidative stress.

Dimethylarsinic acid effects on DNA damage and oxidative stress related biochemical parameters in B6C3F1 mice

Adult female B6C3F1 mice were given 720 mg/kg of DMA by oral gavage at one of three times (2 h, 15 h, or at both 21 and 4 h) before sacrifice. Significant (P < 0.05) decreases in liver GSH and GSSG contents (15-37%) were observed. Some evidence of DMA-induced hepatic DNA damage (at the P < 0.10 level only) was observed. Pulmonary and hepatic ODC activities were reduced (19-59%) by DMA treatment. Overall, these biochemical studies show that mice are much less responsive to DMA than rats.

Inorganic and methylated arsenic compounds induce cell death in murine macrophages via different mechanisms

We demonstrate in this study the cytotoxic effects of inorganic arsenicals, arsenite and arsenate, and organic arsenic compounds, monomethylarsonic acid (MAA), dimethylarsinic acid (DMAA), and trimethylarsine oxide (TMAO), which are metabolites of inorganic arsenicals in human bodies, using murine macrophages in vitro. Inorganic arsenicals, both arsenite and arsenate, are strongly toxic to macrophages, and the concentration that decreased the number of surviving cells to 50% of that in untreated controls (IC50) was 5 or 500 microM, respectively. These inorganic arsenicals mainly caused necrotic cell death with partially apoptotic cell death; about 80% of dead cells were necrotic, and 20% were apoptotic. The inorganic arsenicals also induced marked release of an inflammatory cytokine, tumor necrosis factor alpha (TNF alpha), at cytotoxic doses. This strong cytotoxicity of an inorganic arsenical, arsenite, might be mediated via active oxygen and protease activation because it was inhibited by the addition of some antioxidant reagents, such as superoxide dismutase (SOD), catalase, and GSH, or by a peptide inhibitor of interleukin-1 beta-converting enzyme (ICE). It is likely that these immunotoxic effects of inorganic arsenicals may evoke both immunosuppression and inflammation, and they may be central factors causing carcinogenesis and severe inflammatory responses, such as hepatomegaly and splenomegaly, in chronic arsenicosis patients who daily ingested arsenic-contaminated well water. In contrast, the cytotoxic effects of methylated arsenic compounds were lower than those of inorganic arsenicals. The IC50 value of DMAA was about 5 mM, and MAA and TMAO had no toxicity even at concentrations over 10 mM. Additionally, these methylated chemicals suppressed the TNFalpha release from macrophages. DMAA induced mainly apoptotic cell death in macrophages as indicated by cellular morphological changes, condensed nuclei, terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL), and DNA fragmentation. However, the cytotoxicity of DMAA might be induced via a different mechanism from that of inorganic arsenicals because it was not abolished by the additions of SOD, catalase, or ICE inhibitor. Conversely, GSH enhanced the toxicity of DMAA. These data suggest that methylation of inorganic arsenicals in mammals plays an important role in suppression of both severe immunosuppression and inflammatory responses caused by inorganic arsenicals.

Interactions between selenium and group Va-metalloids (arsenic, antimony and bismuth) in the biliary excretion

The interrelationship between the biliary excretion of exogenous group Va-metalloids (arsenic, antimony and bismuth) and selenium, as well as endogenous glutathione has been studied in rats injected intravenously with sodium selenite and one of the group Va-metalloids. Arsenic, antimony and bismuth appeared in the bile of rats together with large amounts of non-protein thiols (NPSH, representing glutathione and its SH-containing degradation products) and, with the exception of bismuth, they caused choleresis. Significant interactions were observed in the hepatobiliary disposition between selenium and each of the group Va-metalloids, however, their outcomes were not uniform. When coadministered with sodium arsenite or arsenate, selenite enhanced the initial biliary excretion of arsenic 2- and 8-fold, respectively, without further increasing the concomitant excretion of NPSH or the choleretic effect of arsenicals. However, selenite augmented neither the excretion of antimony or bismuth, nor the simultaneous biliary release of NPSH. In turn, arsenite, arsenate and antimony potassium tartrate increased the initial biliary excretion of selenium more than 10-fold and enhanced the accumulation of selenium in blood (exclusively in the erythrocytes). In contrast, administration of bismuth ammonium citrate diminished both the biliary excretion and the erythrocytic accumulation of selenium, while causing retention of selenium in the blood plasma. In rats receiving arsenic or antimony with selenite, the time courses of the biliary excretion of these group Va-metalloids, selenium and NPSH were similar. It is hypothesised that incorporation of selenol metabolites of selenite into the glutathione complexes of arsenic and antimony, resulting in cholephilic ternary complexes, accounts for the arsenic- and antimony-induced augmentation of the hepatobiliary transport of selenium. However, additional chemical and/or dispositional mechanisms are thought to be responsible for the selenite-induced increase in biliary excretion of arsenic.

Binding of arsenicals to proteins in an in vitro methylation system

The dynamics of interactions between rat liver cytosolic proteins and arsenicals were examined in an in vitro methylation system that contained cytosol, glutathione, S-adenosylmethionine, and 1 microM -73As-arsenite. After incubation at 37 degrees C for up to 90 min, low-molecular-weight components of the assay system (<10 kDa) were removed by ultrafiltration and cytosolic proteins were separated by size-exclusion chromatography on Sephacryl S-300 gel. Five 73As-labeled protein peaks were found in chromatographic profiles. The estimated molecular masses of 73As-labeled proteins eluting in the three earliest peaks were as follows: Vo, >/=1000 kDa; A, 135 kDa; and B, 38 kDa. Peak C eluted immediately before the total volume (VT) of the chromatographic column; peak D eluted after the VT. 73As bound to proteins was released by CuCl treatment and speciated by thin-layer chromatography. Amounts and ratios of inorganic As, methyl As, and dimethyl As associated with cytosolic proteins depended upon the incubation interval. Inorganic As was present in all protein peaks. Methyl As was primarily associated with peaks A and C; dimethyl As was associated with peaks B and C. To examine the effect of valence on the binding of methylarsenicals to cytosolic proteins, trivalent or pentavalent 14C-labeled methyl As or dimethyl As was incubated in an in vitro system designed to minimize the enzymatically catalyzed production of methylated arsenicals. Proteins in peaks A, B, and C bound preferentially trivalent methyl and dimethyl As. Peak D bound either trivalent or pentavalent methyl and dimethyl As. Protein-bound inorganic and methyl As were substrates for the production of dimethyl As in an in vitro methylation system, suggesting a role for protein-bound arsenicals in the biomethylation of this metalloid.

Comparative inhibition of yeast glutathione reductase by arsenicals and arsenothiols

Tri(gamma-glutamylcysteinylglycinyl)trithioarsenite (AsIII(GS)3) is formed in cells and is a more potent mixed-type inhibitor of the reduction of glutathione disulfide (GSSG) by yeast glutathione (GSH) reductase than either arsenite (AsIII) or GSH. The present work examines the effects of valence and complexation of arsenicals with GSH or L-cysteine (Cys) upon potency as competitive inhibitors of the reduction of GSH disulfide (GSSG) by yeast GSH reductase. Trivalent arsenicals were more potent inhibitors than their pentavalent analogs, and methylated trivalent arsenicals were more potent inhibitors than was inorganic trivalent As. Complexation of either inorganic trivalent As or methylarsonous diiodide (CH3As(III)I2) with Cys or GSH produced inhibitors of GSH reductase that were severalfold more potent than the parent arsenicals. In contrast, dimethylarsinous iodide ((CH3)2As(III)I) was a more potent inhibitor than its complexes with either GSH or Cys. Complexes of CH3AsIII with GSH (CH3-AsIII(GS)2) or with Cys (CH3AsIII(Cys)2) were the most potent inhibitors, with Ki's of 0.009 and 0.018 mM, respectively. Inhibition of GSH reductase by arsenicals or arsenothiols was prevented by addition of meso-2,3-dimercaptosuccinic acid (DMSA) to a mixture of enzyme, GSSG, and inhibitor before addition of NADPH. DMSA added to the reaction mixture after NADPH reversed inhibition by (CH3)2As(III)I but had little effect on inhibition by CH3As(III)I2, Ch3AsIII(GS)2, CH3AsIII(Cys)2, or AsIII(GS)3. Partial redox inactivation of the enzyme with NADPH increased the inhibitory potency of CH3As(III)I2 and (CH3)2As(III)I and changed the mode of inhibition for CH3As(III)I2 from competitive to noncompetitive. The greater potency of methylated trivalent arsenicals and arsenothiols than of inorganic trivalent As suggests that biomethylation of As could yield species that inhibit reduction of GSSG and alter the redox status of cells.

Mono- and dimethylation of arsenic in rat liver cytosol in vitro

Production of methylarsonate and dimethylarsinate from radiolabelled [73 As]arsenite and [73 As]arsenate was examined in an assay system that contained cytosol prepared from a 20% homogenate (w/v) of livers from 8- 10-week-old male Fischer 344 rats. After a 60-min incubation at 37 degrees C with added S-adenosylmethionine and glutathione, up to 50% of carrier-free [73As]arsenite and about 15% of carrier-free [73As]arsenate were methylated. Incubation of cytosol at 100% degrees C for 1 min before addition to the assay system completely abolished methylation of arsenite. Production of methylarsonate increased in proportion to the arsenite concentration in the assay system; however, 50 microM arsenite inhibited production of dimethylarsinate. Methylarsonate production from carrier-free [73-As]arsenite was not dependent on addition of exogenous S-adenosylmethionine to the assay system. Addition of 0.1 mM S-adenosylmethionine maximized dimethylarsinate production. Addition of 0.1 or 1.0 mM S-adenosylhomocysteine decreased methylation of arsenite, especially dimethylarsinate production. Omission of glutathione from the assay system nearly abolished the methylation of arsenite. Addition of exogenous glutathione to the assay system (up to 20 mM) decreased protein binding of arsenic and increased the production of methylarsonate and dimethylarsinate. The effects of sodium selenite, mercuric chloride, EDTA, p-anisic acid and 2,3-dichloro-alpha-methylbenzylamine on the methylation of arsenite were determined. Addition of 10 microM selenite to the assay system nearly abolished the formation of either methylated species. Addition of 1 or 10 microM mercuric chloride inhibited dimethylarsinate production in a concentration-dependent manner but had little effect on methylarsonate yield. Addition of 10 mM EDTA to the assay system inhibited formation of both methylated metabolites, suggesting that an endogenous divalent cation might be involved in enzymatic methylation of arsenic. Neither p-anisic acid, an inhibitor of cytosolic methyltransferases, nor 2,3-dichloro-alpha-methylbenzylamine, an inhibitor of microsomal methyltransferases, inhibited the conversion of inorganic arsenic to mono- or dimethylated metabolites.