Interactions of rat red blood cell sulfhydryls with arsenate and arsenite

Extracellular polymeric substances and mineral interfacial reactions control the simultaneous immobilization and reduction of arsenic (As(V))

Extracellular polymeric substances (EPS) play a crucial role in controlling the mobility and bioavailability of heavy metal(loid)s in water, soils, and sediments. The formation of EPS-mineral complex changes the reactivity of the end-member materials. However, little is known about the adsorption and redox mechanisms of arsenate (As(V)) in EPS and EPS-mineral complexes. Here we examined the reaction sites, valence state, thermodynamic parameters and distribution of As in the complexes using potentiometric titration, isothermal titration calorimetry (ITC), FTIR, XPS, and SEM-EDS. The results showed that ∼54% of As(V) was reduced to As(III) by EPS, potentially driven by an enthalpy change (ΔH) of - 24.95 kJ/mol. The EPS coating on minerals clearly affected the reactivity to As(V). The strong masking of functional sites between EPS and goethite inhibited both the adsorption and reduction of As. In contrast, the weak binding of EPS onto montmorillonite retained more reactive sites for the reaction with As. Meanwhile, montmorillonite facilitated the immobilization of As to EPS through the formation of As-organic bounds. Our findings deepen the understanding of EPS-mineral interfacial reactions in controlling the redox and mobility of As, and the knowledge is important for predicting the behavior of As in natural environments.

Effects of Arsenic on Trichloroethene-Dechlorination Activities of Dehalococcoides mccartyi 195

Arsenic and trichloroethene (TCE) are among the most prevalent groundwater contaminants in the United States. Co-contamination of these two compounds has been detected at 63% of current TCE-contaminated National Priorities List sites. When in situ TCE reductive dechlorination is stimulated by the addition of fermentable substrates to generate a reducing environment, the presence of arsenic can be problematic because of the potential for increased mobilization and toxicity caused by the reduction of arsenate [As(V)] to arsenite [As(III)]. This study assesses the effects of arsenic exposure on the TCE-dechlorinating activities of Dehalococcoides mccartyi strain 195. Our results indicate that 9.1 μM As(III) caused a 50% decrease in D. mccartyi cell growth. While As(V) concentrations up to 200 μM did not initially impact TCE dechlorination, inhibition was observed in cultures amended with 200 μM As(V) and 100 μM As(V) in 12 and 17 days, respectively, corresponding with the accumulation of As(III). Transcriptomic and metabolomic analyses were performed to evaluate cellular responses to both As(V) and As(III) stress. Amendment of amino acids enhanced arsenic tolerance of D. mccartyi. Results from this study improve our understanding of potential inhibitions of D. mccartyi metabolism caused by arsenic and can inform the design of bioremediation strategies at co-contaminated sites.

Serum folate levels and urinary arsenic methylation profiles in the US population: NHANES, 2003-2012

Arsenic is a prevalent environmental contaminant, and its folate-dependent methylation is important for detoxification in the body. In this study, we investigated the association between serum folate levels and methylation using data from the US National Health and Nutrition Examination Survey (NHANES) (2003-2012) (N = 11,016). Multivariate linear regression and penalized spline regression models were used to examine the association and possible upper limit of folate level regarding its impact on methylation in children (≤18 years) and adults (>18 years), respectively. Serum folate levels, methylation metabolites including urinary monomethylarsonic acid (MMA(V)) and dimethylarsinic acid (DMA(V)), and demographic variables were extracted from NHANES. Results showed that urinary percentage of DMA(V) (%DMA(V)) was positively associated with log(serum folate levels) after adjustment in children (β = 1.93, p < 0.01); urinary percentage of MMA(V) (%MMA(V)) was positively associated with log (serum folate levels) after adjustment in adults (β = 0.40, p < 0.01). No upper limit of folate level regarding its impact on arsenic methylation was identified. More than 50% of Non-Hispanic black and smokers with high total urinary arsenic levels had low serum folate levels. Our results indicate that folate promotes arsenic methylation, but the patterns are different in children versus in adults. Future interventions may be needed for the population exposed to high level of arsenic but with low serum folate to protect against the potential adverse health effects of arsenic.

Benefits of Alcohol on Arsenic Toxicity in Rats

INTRODUCTION

It has been demonstrated earlier that exposure to ethanol and/or arsenic compounds (such as sodium arsenite) produces toxic effects as shown by both in vitro and in vivo experiments. Chronic exposure of humans to arsenic through drinking water, pesticides or consumption of alcoholic beverages has produced major health problem and concern in recent years. Water being one of the main ingredients for alcohol formation (beer fermentation process) can lead to contamination with arsenic. Thus, people consuming such alcohol are getting continuously exposed to arsenic compounds as well along with alcohol.

AIM

The present study was undertaken to investigate the effect of alcohol co-administration on arsenic induced changes in carbohydrate metabolic status in adult male albino rats.

MATERIALS AND METHODS

Adult male albino rats of Wistar strain (weighing~100g) were divided into three groups (n=8 rats/group) including Control or vehicle treated (C), Arsenic treated (As) and Arsenic treated alcohol co-exposed (As+Alc). Treatment with Sodium-arsenite included intra-peritoneal injection consecutively for 14 days at a dose of 5.55 mg/kg (equivalent to 35% of LD50) per day. Absolute alcohol (15% v/v) was fed at a dose of 0.5 ml/100 g body weight per day for five consecutive days from start of the treatment schedule. Distilled water (D/W) was used as vehicle. Blood Glucose (BG) level, levels of glycogen, Pyruvic Acid (PA), Free Amino Acid Nitrogen (FAAN), total protein, Glutamate Oxalate transaminase (GOT) and Glutamate Pyruvate Transaminase (GPT) activity, and glucose-6-phosphatase (G6Pase) activity were measured in tissues including liver, kidney and muscle.

RESULTS

Treatment with arsenic decreased the levels of BG, liver glycogen and PA, tissue protein and G6Pase activity, GOT activity in liver and muscle, and increased free amino acid content in kidney and muscle, GPT activity in liver and kidney. Alcohol administration to rats co-exposed to arsenic treatment reversed these changes.

CONCLUSION

Thus, it is suggested that combined administration of alcohol with arsenic can result in the suppression of the down-regulating action of arsenic on glucose homeostasis as evidenced by its hypoglycaemic effect and increased gluconeogenesis and transamination in liver.

Antagonistic effect of N-ethylmaleimide on arsenic-mediated oxidative stress-induced poly(ADP-ribosyl)ation and cytotoxicity

Long-term exposure to arsenic has been known to induce neoplastic initiation and progression in several organs; however, the role of arsenic (As₂ O₃ ) in oxidative stress-mediated DNA damage remains elusive. One of the immediate cellular responses to DNA damage is poly(ADP-ribosyl)ation (PARylation), which mediates DNA repair and enhances cell survival. In this study, we found that oxidative stress (H₂ O₂ )-induced PARylation was suppressed by As₂ O₃ exposure in different human cancer cells. Moreover, As₂ O₃ treatment promoted H₂ O₂ -induced DNA damage and apoptosis, leading to increased cell death. We found that N-ethylmaleimide (NEM), an organic compound derived from maleic acid, could reverse As₂ O₃ -mediated effects, thus enhancing PARylation with attenuated cell death and increased cell survival. Pharmacologic inhibition of glutathione with l-buthionine-sulfoximine blocked the antagonistic effect of NEM on As₂ O₃ , thereby continuing As₂ O₃ -mediated suppression of PARylation and causing DNA damage. Our findings identify NEM as a potential antidote against As₂ O₃ -mediated DNA damage in a glutathione-dependent manner. Copyright © 2016 John Wiley & Sons, Ltd.

Ameliorative Effects of Acacia Honey against Sodium Arsenite-Induced Oxidative Stress in Some Viscera of Male Wistar Albino Rats

Cancer is a leading cause of death worldwide and its development is frequently associated with oxidative stress-induced by carcinogens such as arsenicals. Most foods are basically health-promoting or disease-preventing and a typical example of such type is honey. This study was undertaken to investigate the ameliorative effects of Acacia honey on sodium arsenite-induced oxidative stress in the heart, lung and kidney tissues of male Wistar rats. Male Wistar albino rats divided into four groups of five rats each were administered distilled water, Acacia honey (20%), sodium arsenite (5 mg/kg body weight), Acacia honey, and sodium arsenite daily for one week. They were sacrificed anesthetically using 60 mg/kg sodium pentothal. The tissues were used for the assessment of glutathione peroxidase, catalase, and superoxide dismutase activities, protein content and lipid peroxidation. Sodium arsenite significantly (P < 0.05) suppressed the glutathione peroxidase, catalase, superoxide dismutase activities with simultaneous induction of lipid peroxidation. Administration of Acacia honey significantly increased (P < 0.05) glutathione peroxidase, catalase, and superoxide dismutase activities with concomitant suppression of lipid peroxidation as evident by the decrease in malondialdehyde level. From the results obtained, Acacia honey mitigates sodium arsenite induced-oxidative stress in male Wistar albino rats, which suggest that it may attenuate oxidative stress implicated in chemical carcinogenesis.

Efflux of glutathione and glutathione complexes from human erythrocytes in response to inorganic arsenic exposure

The objective of the present study was to investigate if arsenic exposure results in glutathione efflux from human erythrocytes. Arsenite significantly depleted intracellular nonprotein thiol level in a time- and concentration-dependent manner. The intracellular nonprotein thiol level was decreased to 0.767 ± 0.0017 μmol/ml erythrocyte following exposure to 10 mM of arsenite for 4 h. Extracellular nonprotein thiol level was increased concomitantly with the intracellular decrease and reached to 0.481 ± 0.0005 μmol/ml erythrocyte in 4 h. In parallel with the change in extracellular nonprotein thiol levels, significant increases in extracellular glutathione levels were detected. Extracellular glutathione levels reached to 0.122 ± 0.0013, 0.226 ± 0.003, and 0.274 ± 0.004 μmol/ml erythrocyte with 1, 5, and 10 mM of arsenite, respectively. Dimercaptosuccinic acid treatment of supernatants significantly increased the glutathione levels measured in the extracellular media. Utilization of MK571 and verapamil, multidrug resistance-associated protein 1 and Pgp inhibitors, decreased the rate of glutathione efflux from erythrocytes suggesting a role for these membrane transporters in the process. The results of the present study indicate that human erythrocytes efflux glutathione in reduced free form and in conjugated form or forms that can be recovered with dimercaptosuccinic acid when exposed to arsenite.

Possible mechanisms for induction of oxidative stress and suppression of systemic nitric oxide production caused by exposure to environmental chemicals

The cytotoxic effects evoked by exposure to environmental chemicals having electrophilic properties are often attributable to covalent attachment to intracellular macromolecules through sulfhydryl groups or enzyme-mediated redox cycling, leading to the generation of reactive oxygen species (ROS). When huge amounts of ROS form they overwhelm antioxidant defenses resulting in the induction of oxidative stress. Nitric oxide (NO) which plays a crucial role in vascular tone, is formed by endothelial NO synthase (eNOS). Since a decrease in systemic NO production is implicated in the pathophysiological actions of vascular diseases, dysfunction of eNOS by environmental chemicals is associated with cardiopulmonary-related diseases and mortality. In this review, we introduce the mechanism-based toxicities (covalent attachment and redox cycling) of electrophiles. Therefore, this review will focus on the possible mechanisms for the induction of oxidative stress and impairment of NO production caused by environmental chemicals.

Arsenate V induced glutathione efflux from human erythrocytes

OBJECTIVE

The objective of the present study was to investigate if arsenate V exposure results in glutathione efflux from human erythrocytes.

PROCEDURE

The changes in intracellular and extracellular nonprotein sulfhydryl and glutathione levels were determined in arsenate (V) exposed erythrocytes. Presence of any cellular membrane damage was assessed by lactate dehydrogenase activity measurement in the supernatant.

RESULTS

When erythrocytes were exposed to 10 mM of arsenate (V) for 4 h, the intracellular NPSH level decreased to 0.28±0025 μmol/ml erythrocyte. In contrast, extracellular nonprotein thiol level was increased to 0.180±0.010 μmol/ml erythrocyte in 4 h. Extracellular glutathione levels reached to 0.028±0.001, 0.052±0.002, and 0.054±0.004 μmol/ml erythrocyte with 1, 5, and 10 mM of arsenate (V), respectively. Utilization of MK571 a multi drug resistance-associated protein 1 inhibitor decreased the rate of glutathione efflux from erythrocytes suggesting a role for this membrane transporter in the process.

CONCLUSION

The results of the present study indicate that erythrocytes efflux glutathione when exposed to arsenate (V).

Mushroom lectin protects arsenic induced apoptosis in hepatocytes of rodents

Acute and chronic arsenic exposure result in toxicity both in human and animal beings and cause many hepatic and renal manifestations. The present study stated that mushroom lectin prevents arsenic-induced apoptosis. Apoptosis was measured by morphological alterations, cell proliferation index (CPI), phagocytic activity (nitro blue tetrazolium index; NBT), nitric oxide (NO) production, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay, DNA fragmentation and caspase-3 activity. Arsenic exposure at 5 μM in the form of sodium arsenite resulted in significant elevation of deformed cells, NO production, TUNEL stained nuclei of hepatocytes, DNA fragmentation and caspase-3 activity. But the CPI and NBT index were significantly declined in arsenic-treated hepatocytes. The beneficial effect of mushroom lectin at 10 μg/mL, 20 μg/mL and 50 μg/mL) showed increased CPI and phagocytic activity. Mushroom lectin at those concentrations reduced deformed cells, NO production, DNA fragmentation and caspase-3 activity of hepatocytes. But significant better protection was observed in 50 μg/mL mushroom lectin-treated hepatocytes. This finding may be of therapeutic benefit in people suffering from chronic arsenic exposure.

Reduced arsenic clearance and increased toxicity in aquaglyceroporin-9-null mice

Expressed in liver, aquaglyceroporin-9 (AQP9) is permeated by glycerol, arsenite, and other small, neutral solutes. To evaluate a possible protective role, AQP9-null mice were evaluated for in vivo arsenic toxicity. After injection with NaAsO(2), AQP9-null mice suffer reduced survival rates (LD(50), 12 mg/kg) compared with WT mice (LD(50), 15 mg/kg). The highest tissue level of arsenic is in heart, with AQP9-null mice accumulating 10-20 times more arsenic than WT mice. Within hours after NaAsO(2) injection, AQP9-null mice sustain profound bradycardia, despite normal serum electrolytes. Increased arsenic levels are also present in liver, lung, spleen, and testis of AQP9-null mice. Arsenic levels in the feces and urine of AQP9-null mice are only approximately 10% of the WT levels, and reduced clearance of multiple arsenic species by the AQP9-null mice suggests that AQP9 is involved in the export of multiple forms of arsenic. Immunohistochemical staining of liver sections revealed that AQP9 is most abundant in basolateral membrane of hepatocytes adjacent to the sinusoids. AQP9 is not detected in heart or kidney by PCR or immunohistochemistry. We propose that AQP9 provides a route for excretion of arsenic by the liver, thereby providing partial protection of the whole animal from arsenic toxicity.

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.

Beneficial effects of Centella asiatica aqueous extract against arsenic-induced oxidative stress and essential metal status in rats

The efficacy of an aqueous extract of Centella asiatica (100, 200 and 500 mg/kg for 5 consecutive days) was studied in the depletion of arsenic and in the recovery of a few altered biochemical variables in arsenic pre-exposed rats (20 ppm in drinking water for 5 weeks). Exposure to arsenic significantly depleted delta-aminolevulinic acid dehydratase (ALAD) activity, reduced glutathione (GSH) level, superoxide dismutase (SOD) and increased thiobarbituric acid reactive substance (TBARS) activity in red blood cells. Significant depletion of ALAD activity, GSH level, glutathione peroxidase (GPx), SOD and catalase (CAT) activities and an increase in TBARS levels in liver tissues was also noted. There was a significant depletion of SOD, CAT and GPx activities in kidneys and an increased TBARS levels in kidney and brain accompanied by increased arsenic concentration in blood and soft tissues. Treatment with aqueous extract of Centella asiatica provided significant protection against ALAD, GSH and TBARS levels, particularly at doses of 200 and 500 mg. Centella asiatica also provided significant recovery in the inhibited liver ALAD and G6PD activities. Arsenic concentration in blood and soft tissues remained uninfluenced after Centella asiatica administration. The present study thus suggests a beneficial effect of Centella asiatica against arsenic-induced oxidative stress but possesses no chelating property.

Identification of arsenic-binding proteins in human breast cancer cells

As a cancer chemotherapeutic drug, arsenic acts on numerous intracellular signal transduction pathways in cancer cells. However, its mechanism of actions is still not fully understood. Previous studies suggest that arsenic reacts with closely spaced cysteine (Cys) residues of proteins with high Cys content and accessible sulfhydryl (SH) groups. In this study, human breast cancer cell line MCF-7 was examined as a cellular model to explore arsenic-binding proteins and the mechanism of binding. An arsenic-biotin conjugate was synthesized by coupling the pentafluorophenol ester of biotin with p-aminophenylarsenoxide. Arsenic-binding proteins were eluted with streptavidin resin from arsenic-biotin treated MCF-7 cells, separated by polyacrylamide gel electrophoresis, and identified by matrix assisted laser desorption ionization mass spectrometry (MALDI-MS). Arsenic-binding properties of two of these proteins, beta-tubulin and pyruvate kinase M2 (PKM2), were studied further in vitro and the biological consequences of this binding was evaluated. Binding assay with Western blotting confirmed binding of beta-tubulin and PKM2 by arsenic in a concentration-dependent manner. Arsenic binding inhibited tubulin polymerization, but surprisingly had no effect on PKM2 activity. Molecular modeling showed that binding of Cys(12) alone or vicinal Cys residues (Cys(12) and Cys(213)) of beta-tubulin by arsenic blocked the active site for access of GTP, which is necessary for tubulin polymerization. On the contrary, all Cys residues of PKM2 were far away from the active site of the enzyme. In summary, this study confirmed beta-tubulin and PKM2 as arsenic-binding proteins in MCF-7 cells. Functional consequence of such binding may depend on whether arsenic binding causes conformational changes or blocks active sites of target proteins.

Combined administration of iron and monoisoamyl-DMSA in the treatment of chronic arsenic intoxication in mice

Co-administration of iron in combination with monoisoamyl dimercaptosuccinic acid (MiADMSA) against chronic arsenic poisoning in mice was studied. Mice preexposed to arsenic (25 ppm in drinking water for 6 months) mice were treated with MiADMSA (50 mg/kg, intraperitoneally) either alone or in combination with iron (75 or 150 mg/kg, orally) once daily for 5 days. Arsenic exposure led to a significant depletion of blood delta-aminolevulinic acid dehydratase (ALAD) activity, hematocrit, and white blood cell (WBC) counts accompanied by small decline in blood hemoglobin level. Hepatic reduced glutathione (GSH) level, catalase and superoxide dismutase (SOD) activities showed a significant decrease while, oxidized glutathione (GSSG) and thiobarbituric acid-reactive substances (TBARS) levels increased on arsenic exposure, indicating arsenic-induced hepatic oxidative stress. Liver aspartate and alanine transaminases (AST and ALT) activities also decreased significantly on arsenic exposure. Kidney GSH, GSSG, catalase level and SOD activities remained unchanged, while, TBARS level increased significantly following arsenic exposure. Brain GSH, glutathione peroxidase (GPx), and SOD activities decreased, accompanied by a significant elevation of TBARS level after chronic arsenic exposure. Treatment with MiADMSA was marginally effective in reducing ALAD activity, while administration of iron was ineffective when given alone. Iron when co-administered with MiADMSA restored blood ALAD activity. Administration of iron alone had no beneficial effects on hepatic oxidative stress, while in combination with MiADMSA it produced significant decline in hepatic TBARS level compared to the individual effect of MiADMSA. Renal biochemical variables were insensitive to any of the treatments. Combined administration of iron with MiADMSA also had no additional beneficial effect over the individual protective effect of MiADMSA on brain oxidative stress. Interestingly, combined administration of iron with MiADMSA provided more pronounced depletion of blood arsenic, while no additional beneficial effects on tissue arsenic level over the individual effect of MiADMSA were noted. The results lead us to conclude that iron supplementation during chelation has some beneficial effects particularly on heme synthesis pathway and blood arsenic concentration.

Binding of dimethylarsinous acid to cys-13alpha of rat hemoglobin is responsible for the retention of arsenic in rat blood

The metabolism, disposition, and carcinogenicity of arsenic differ dramatically between humans and rats. To understand the molecular basis of these differences, we have characterized arsenic species in rats that were treated with inorganic arsenate (iAsV), monomethylarsonic acid (MMAV), or dimethylarsinic acid (DMAV) for up to 15 weeks. Arsenic significantly accumulated in the red blood cells (RBCs) of rats in the form of hemoglobin (Hb) complexed with dimethylarsinous acid (DMAIII), regardless of whether the rats were treated with iAsV, MMAV, or DMAV, suggesting rapid methylation of arsenic species followed by strong binding of DMAIII to rat Hb. The binding site for DMAIII was identified to be cysteine 13 in the alpha-chain of rat Hb with a stoichiometry of 1:1. Over 99% of the total arsenic (maximum 2.5-3.5 mM) in rat RBCs was bound to Hb for all rats examined (n = 138). In contrast, only 40-49% of the total arsenic (maximum approximately 10 muM) in rat plasma was bound to proteins. The ratios of the total arsenic in RBCs to that in plasma ranged from 88-423 for rats that were fed iAsV, 100-680 for rats that were fed MMAV, and 185-1393 for rats that were fed DMAV, when samples were obtained over the 15-week exposure duration. Previous studies have shown an increase in urothelial hyperplasia in rats fed DMAV. This is the first article reporting that treatment with iAsV in the drinking water also produces urothelial hyperplasia and at an even earlier time point than dietary DMAV. Dietary MMAV produced only a slight urothelial response. A correlation between the Hb-DMAIII complex and urothelial lesion severity in rats was observed. The lack of cysteine 13alpha in human Hb may be responsible for the shorter retention of arsenic in human blood. These differences in the disposition of arsenicals may contribute to the observed differences between humans and rats in susceptibility to arsenic carcinogenicity.

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.

Dissociation of arsenite-peptide complexes: Triphasic nature, rate constants, half-lives, and biological importance

We determined the number and the dissociation rate constants of different complexes formed from arsenite and two peptides containing either one (RVCAVGNDYASGYHYGV for peptide 20) or three cysteines (LECAWQGK CVEGTEHLYSMKCK for peptide 10) via radioactive 73As-labeled arsenite and vacuum filtration methodology. Nonlinear regression analysis of the dissociation of both arsenite-peptide complexes showed that triphasic fits gave excellent r2 values (0.9859 for peptide 20 and 0.9890 for peptide 10). The first phase of arsenite-peptide dissociation had the largest span (decrease in binding), and the rate was too fast to be measured using vacuum filtration methods. The dissociation rate constants of arsenite-peptide complexes for the second phase were 0.35 and 0.54 min(-1) and for the third phase were 0.0071 and 0.0045 min(-1) for peptides 20 and 10, respectively. For peptide 20, the three spans of triphasic decay were 85%, 9%, and 7% of the total binding of 16.1 nmol/mg protein. For peptide 10, which can bind in both an intermolecular and intramolecular manner, the three spans of triphasic decay were 59%, 16%, and 25% of the total binding of 43.7 nmol/mg protein. Binding of trivalent arsenicals to peptides and proteins can alter their structure and function and contribute to adverse health outcomes such as toxicity and carcinogenicity.

Toenail arsenic concentrations, GSTT1 gene polymorphisms, and arsenic exposure from drinking water

Toenail arsenic (As) concentrations were evaluated as a biomarker of inorganic As (As(in)) exposure in a population residing in an As-endemic region of Bangladesh. Drinking water and toenail samples were collected from 48 families (n = 223) every 3 months for 2 years and analyzed for As using inductively coupled plasma-mass spectrometry. Drinking water collected 3, 6, and 9 months before each toenail sample collection was combined into a weighted lagged exposure variable. The contribution of each water sample to the measured toenail As concentration was estimated using maximum likelihood that accounted for fluctuations in drinking water exposure and toenail growth. The best model attributed 69%, 14%, and 17% of the toenail As content to drinking water exposures that occurred 3, 6, and 9 months before toenail collection [95% confidence intervals (95% CI), 0.46-0.97, 0.00-0.31, and 0.03-0.35, respectively]. Generalized additive mixed models using penalized regression splines were employed to model the data. Below a drinking water concentration of 2 mug As/L, no relationship between drinking water As and toenail As concentrations was observed. Above this concentration, toenail As content increased in a dose-dependent fashion as drinking water As increased. Age was a significant effect modifier of drinking water As exposure on toenail As (beta = 0.01; 95% CI, 0.002-0.02). Individuals possessing GSTT1-null genotypes had significantly more As in their toenails in contrast to GSTT1 wild-type individuals (beta = 0.11; 95% CI, 0.06-0.2). Therefore, it seems that GSTT1 modifies the relationship between As(in) exposure and toenail As(in) content.

Arsenite binding to synthetic peptides based on the Zn finger region and the estrogen binding region of the human estrogen receptor-alpha

We selected the estrogen receptor protein for study because of prior results indicating that arsenite is a "potential nonsteroidal environmental estrogen". We utilized radioactive (73)As-labeled arsenite and vacuum filtration methodology to determine the binding affinity of arsenite to synthetic peptides. A zinc finger region containing four free sulfhydryls and the hormone binding region containing three free sulfhydryls based on the human estrogen receptor-alpha were studied. Peptide 15 (RYCAVCNDYASGYHYGVWSCEGCKA) bound arsenite with a K(d) of 2.2 microM and B(max) (maximal binding capacity) of 89 nmol/mg protein. Peptide 10 (LECAWQGKCVEGTEHLYSMKCKNV) had a K(d) of 1.3 microM and B(max) of 59 nmol/mg protein. In contrast, the monothiol peptide 19 (LEGAWQGKGVEGTEHLYSMKCKNV) bound arsenite with a higher K(d) of 124 microM and a B(max) of 26 nmol/mg protein. In our studies, amino acids other than cysteine (including methionine and histidine) did not bind arsenite at all. Peptides modeled on the estrogen receptor with two or more nearby free sulfhydryls (two or five intervening amino acids) had low K(d) values in the 1-4 microM range. Peptides containing single sulfhydryls or two sulfhydryls spaced 17 amino acids apart had higher K(d) values in the 100-200 microM range, demonstrating lower affinity. With the exception of peptide 24 which had an unusually high B(max) value of 234 nmol/mg, the binding capacity of the studied peptides was proportional to the number of free cysteines. Binding of trivalent arsenicals to peptides and proteins can contribute to arsenic toxicity and carcinogenicity via altered peptide/protein structure and enzyme function.

Evidence of hemoglobin binding to arsenic as a basis for the accumulation of arsenic in rat blood

Four trivalent arsenic species, inorganic arsenite (iAs(III)), monomethylarsonous acid (MMA(III)), dimethylarsinous acid (DMA(III)), and phenylarsine oxide (PhAs(III)O), have shown increasing binding affinity with the hemoglobin (Hb) of rats and humans. The binding stoichiometry was consistent with the number of reactive cysteine residues in the alpha and beta chains of Hb. Comparing the binding affinity of rat Hb and human Hb for the same trivalent arsenic species, rat Hb was 3-16 times stronger than human Hb as demonstrated by their apparent binding constants. Comparative experiments involving incubation of human and rat red blood cells (RBC) with iAs(III), MMA(III), and DMA(III) showed that 15-30-fold more arsenic species were bound to the Hb of rat RBC than that of human RBC. In vivo experiments using rats fed with an arsenic-supplemented diet showed that arsenic in RBC of the rats was predominantly found in the protein-bound form. Further characterization by nanoelectrospray mass spectrometry of the arsenic species in the RBC of these rats confirmed that most arsenic was bound to the alpha chain of Hb. Taken together, these results suggest that the stronger binding affinity of these arsenic species to rat Hb is responsible for the accumulation of arsenic in rat blood. The results provide a chemical basis to explain the previously observed intriguing difference in the retention of arsenic in the human and the rat. The techniques and approaches described can be applied to the studies of arsenic interactions with other functional proteins.

Prospective protective role of melatonin against arsenic-induced metabolic toxicity in Wistar rats

Subchronic exposure to arsenic is associated with alteration of glucose homeostasis. Arsenic treatment (as sodium arsenite) of male Wistar rats (weighing 130-150 g) at a dose of 5.55 mg kg(-1) body weight (equivalent to 35% of LD(50)) (i.p.) per day for a period of 30 days produced hypoglycemia, with associated increased urinary excretion of glucose and depletion of liver glycogen and pyruvic acid contents. Mobilization of free amino acids from kidney to liver was facilitated by arsenic treatment. Arsenic exposure significantly decreased the glutamate-pyruvate transaminase activity in kidney. Glucose 6-phosphatase activity in liver tissue was also significantly decreased after arsenic treatment. In addition to these, liver lactate dehydrogenase activity was elevated due to arsenic treatment. Melatonin supplementation (i.p.) at a dose of 10 mg kg(-1) day(-1) for last five days prior to sacrifice reversed most of the above changes caused by arsenic. Melatonin, being a potent free radical scavenger may reduce arsenic-induced free radical production, and thereby, eliminating its toxic effects. So, arsenic-induced hypoglycemia, with associated glycogenolytic as well as glycolytic activities of liver can be partially counteracted by melatonin supplementation. Accordingly, it may be suggested that melatonin can serve as a prospective protective agent against arsenic-induced metabolic toxicity.

Role of glutathione in reduction of arsenate and of γ-glutamyltranspeptidase in disposition of arsenite in rats

Arsenate (AsV), the environmentally prevalent form of arsenic, is converted sequentially in the body to arsenite (AsIII), monomethylarsonic acid (MMAsV), monomethylarsonous acid (MMAsIII), and dimethylarsinic acid (DMAsV) and some trimethylated metabolites. Although the biliary excretion of arsenic in rats is known to be glutathione (GSH)-dependent, involving transport of arsenic-GSH conjugates, the role of GSH in the reduction of AsV to the more toxic AsIII in vivo has not been defined. Therefore, we studied how the fate of AsV is influenced by buthionine sulfoximine (BSO), which depletes GSH in tissues. Control and BSO-treated rats were given AsV (50 micromol/kg, i.v.) and arsenic metabolites in bile, urine, blood and tissues were analysed by HPLC-HG-AFS. BSO increased retention of AsV in blood and tissues and decreased appearance of AsIII in blood, bile (by 96%) and urine (by 63%). The biliary excretion of MMAsIII was also nearly abolished, the appearance of MMAsIII and MMAsV in the blood was delayed and the renal concentrations of these monomethylated arsenicals were decreased by BSO. Interestingly, appearance of DMAsV in blood and urine remained unchanged and the concentrations of this metabolite in the kidneys and muscle were even increased in response to BSO. To test the role of gamma-glutamyltranspeptidase (GGT) in arsenic disposition, the effect of the of the GGT inhibitor acivicin was investigated in rats injected with AsIII (50 micromol/kg, i.v.). Acivicin lowered the hepatic and renal GGT activities and increased the biliary as well as urinary excretion of GSH, but failed to alter the disposition (i.e. blood and tissue concentrations, biliary and urinary excretion) of AsIII and its metabolites. In conclusion, shortage of GSH decreases not only the hepatobiliary transport of arsenic, but also reduction of AsV and the formation of monomethylated arsenic, while not hindering the production of dimethylated arsenic. While GSH plays an important role in the disposition and toxicity of arsenic, GGT, which hydrolyses GSH and GSH conjugates, apparently does not influence the fate of the GSH-reactive trivalent arsenicals in rats.

The metabolism of inorganic arsenic oxides, gallium arsenide, and arsine: a toxicochemical review

The aim of this review is to compare the metabolism, chemistry, and biological effects to determine if either of the industrial arsenicals (arsine and gallium arsenide) act like the environmental arsenic oxides (arsenite and arsenate). The metabolism of the arsenic oxides has been extensively investigated in the past 4 years and the differences between the arsenic metabolites in the oxidation states +III versus +V and with one or two methyl groups added have shown increased importance. The arsenic oxide metabolism has been compared with arsine (oxidation state -III) and arsenide (oxidation state between 0 to -III). The different metabolites appear to have different strengths of reaction for binding arsenic (III) to thiol groups, their oxidation-reduction reactions and their forming an arsenic-carbon bond. It is unclear if the differences in parameters such as the presence or absence of methyl metabolites, the rates of AsV reduction compared to the rates of AsIII oxidation, or the competition of phosphate and arsenate for cellular uptake are large enough to change biological effects. The arsine rate of decomposition, products of metabolism, target organ of toxic action, and protein binding appeared to support an oxidized arsenic metabolite. This arsine metabolite was very different from anything made by the arsenic oxides. The gallium arsenide had a lower solubility than any other arsenic compound and it had a disproportionate intensity of lung damage to suggest that the GaAs had a site of contact interaction and that oxidation reactions were important in its toxicity. The urinary metabolites after GaAs exposure were the same as excreted by arsenic oxides but the chemical compounds responsible for the toxic effects of GaAs are different from the arsenic oxides. The review concludes that there is insufficient evidence to equate the different arsenic compounds. There are several differences in the toxicity of the arsenic compounds that will require substantial research.

Arsenate reductase II. Purine nucleoside phosphorylase in the presence of dihydrolipoic acid is a route for reduction of arsenate to arsenite in mammalian systems

An arsenate reductase has been partially purified from human liver using ion exchange, molecular exclusion, hydroxyapatite chromatography, preparative isoelectric focusing, and electrophoresis. When SDS-beta-mercaptoethanol-PAGE was performed on the most purified fraction, two bands were obtained. One of these bands was a 34 kDa protein. Each band was excised from the gel and sequenced by LC-MS/MS, and sequest analyses were performed against the OWL database SWISS-PROT with PIR. Mass spectra analysis matched the 34 kDa protein of interest with human purine nucleoside phosphorylase (PNP). The peptide fragments equal to 40.1% of the total protein were 100% identical to the corresponding regions of the human purine nucleoside phosphorylase. Reduction of arsenate in the purine nucleoside arsenolysis reaction required both PNP and dihydrolipoic acid (DHLP). The PNP rate of reduction of arsenate with the reducing agents GSH or ascorbic acid was negligible compared to that with the naturally occurring dithiol DHLP and synthetic dithiols such as BAL (British anti-lewisite), DMPS (2,3-dimercapto-1-propanesulfonate), or DTT (alpha-dithiothreitol). The arsenite production reaction of thymidine phosphorylase had approximately 5% of such PNP activity. Phosphorylase b was inactive. Monomethylarsonate (MMAV) was not reduced by PNP. The experimental results indicate PNP is an important route for the reduction of arsenate to arsenite in mammalian systems.

Induction of vascular endothelial growth factor expression and hypoxia-inducible factor 1alpha protein by the oxidative stressor arsenite

Recent evidence suggests that vascular endothelial growth factor (VEGF) expression is up-regulated by oxidative stressors through activation of hypoxia-inducible Factor 1 (HIF-1). To investigate whether this is a general phenomenon, we studied the effects of the sulfhydryl reagent arsenite on VEGF expression in human ovarian cancer cells. Arsenite potently induces the production of reactive oxygen species (ROS) in several cell systems and directly interacts with sulfhydryl groups of cellular thiols. We report that arsenite induces VEGF mRNA and protein levels in normoxic H134 and OVCAR-3 cells. Arsenite also increases HIF-1alpha protein levels, suggesting a role for HIF-1 in the induction of VEGF expression. Pretreatment with the ROS inhibitors catalase and mannitol attenuated arsenite-induced ROS production, but did not affect induction of VEGF mRNA and HIF-1alpha protein. In contrast, pretreatment with the thiol antioxidants glutathione or N-acetylcysteine completely abrogated both effects, whereas a potentiation was observed by depletion of intracellular glutathione. These results demonstrate that arsenite-induced VEGF mRNA and HIF-1alpha protein expression is independent of increased ROS production but critically regulated by the cellular reduced glutathione content. In addition, these data suggest the involvement of a thiol-sensitive mechanism in the regulation of VEGF mRNA expression and HIF-1alpha protein in human ovarian cancer cells.

Arsenic induces apoptosis in rat cerebellar neurons via activation of JNK3 and p38 MAP kinases

Primary cultures of rat cerebellar neurons were used to study mechanisms of arsenic neurotoxicity. Exposure to 5, 10, or 15 microM sodium arsenite reduced cerebellar neuron viability and induced nuclear fragmentation and condensation as well as DNA degradation to oligonucleosome fragments. Exposure to 1 or 5 mM dimethylarsinic acid caused similar changes. Therefore, both inorganic arsenite and organic dimethylarsinic acid induce apoptosis in cerebellar neurons, with the inorganic form being more toxic. Cotreatment with cycloheximide or actinomycin D, inhibitors of protein or RNA synthesis, respectively, or with the caspase inhibitor zVAD, completely blocked arsenite-induced cerebellar neuron apoptosis. This implies that arsenite-induced cerebellar neuron apoptosis requires new gene expression and caspase activation. Interestingly, sodium arsenite selectively activated p38 and JNK3, but not JNK1 or JNK2 in cerebellar neurons. Blocking the p38 or JNK signaling pathways using the inhibitors SB203580 or CEP-1347 protected cerebellar neurons against arsenite-induced apoptosis. These data suggest that arsenite neurotoxicity may be due to apoptosis caused by activation of p38 and JNK3 MAP kinases.

Pharmacokinetics, metabolism, and carcinogenicity of arsenic

The carcinogenicity of arsenic in humans has been unambiguously demonstrated in a variety of epidemiological studies encompassing geographically diverse study populations and multiple exposure scenarios. Despite the abundance of human data, our knowledge of the mechanism(s) responsible for the carcinogenic effects of arsenic remains incomplete. A deeper understanding of these mechanisms is highly dependent on the development of appropriate experimental models, both in vitro and in vivo, for future mechanistic investigations. Suitable in vitro models would facilitate further investigation of the critical chemical species (arsenate/arsenite/MMA/DMA) involved in the carcinogenic process, as well as the evaluation of the generation and role of ROS. Mechanisms underlying the clastogenic effects of arsenic, its role in modulating DNA methylation, and the phenomenon of inducible tolerance could all be more completely investigated using in vitro models. The mechanisms involved in arsenic's inhibition of ubiquitin-mediated proteolysis demand further attention, particularly with respect to its effects on cell proliferation and DNA repair. Exploration of the mechanisms responsible for the protective or anticarcinogenic effects of arsenic could also enhance our understanding of the cellular and molecular interactions that influence its carcinogenicity. In addition, appropriate in vivo models must be developed that consider the action of arsenic as a promoter and/or progressor. In vivo models that allow further investigation of the comutagenic effects of arsenic are also especially necessary. Such models may employ initiation-promotion-progression bioassays or transgenic animals. Both in vitro and in vivo models have the potential to greatly enhance our current understanding of the cellular and molecular interactions of arsenic and its metabolites in target tissues. However, refinement of our knowledge of the mechanistic aspects of arsenic carcinogenicity is not alone sufficient; an understanding of the pharmacokinetics and target tissue doses of the critical chemical species is essential. Additionally, a more thorough characterization of species differences in the tissue kinetics of arsenic and its methylated metabolites would facilitate the development of more accurate and relevant PBPK models. Improved models could be used to further investigate the existence of a methylation threshold for arsenic and its relevance to arsenic carcinogenicity in humans. The significance of alterations in relative tissue concentrations of SAM and SAH deserves further attention, particularly with respect to their role in modulating methyltransferases involved in arsenic metabolism and DNA methylation. The importance of genetic polymorphisms and nutrition in influencing methyltransferase activities must not be overlooked. In vivo models are necessary to evaluate these factors; transgenic or knockout models would be particularly useful in the investigation of methylation polymorphisms. Further evaluation of methylation polymorphisms in human populations is also warranted. Other in vivo models incorporating dietary manipulation could provide valuable insight into the role of nutrition in the carcinogenicity of arsenic. With more complete knowledge of the pharmacokinetics of arsenic metabolism and the mechanisms associated with its carcinogenic effects, development of more reliable risk assessment strategies are possible. Integration of data, both pharmacokinetic and mechanistic in nature, will lead to more accurate descriptions of the interactions that occur between the active chemical species and cellular constituents which lead to the development of cancer. This knowledge, in turn, will facilitate the development of more accurate and reliable risk assessment strategies for arsenic.

Arsenite induces apoptosis of murine T lymphocytes through membrane raft-linked signaling for activation of c-Jun amino-terminal kinase

Because of its dual roles in acute toxicity and in therapeutic application in cancer treatment, arsenic has recently attracted a renewed attention. In this study, we report NaAsO(2)-induced signal cascades from the cell surface to the nucleus of murine thymic T lymphocytes that involve membrane rafts as an initial signal transducer. NaAsO(2) induced apoptosis through fragmentation of DNA, activation of caspase, and reciprocal regulation of Bcl-2/Bax with the concomitant reduction of membrane potential. We demonstrated that NaAsO(2)-induced caspase activation is dependent on curcumin-sensitive c-Jun amino-terminal kinase and barely dependent on SB203580-sensitive p38 kinase or PD98059-sensitive extracellular signal-regulated kinase. Additionally, staurosporine, which severely inhibited the activation of mitogen-activated protein (MAP) family kinases and c-Jun, partially blocked the NaAsO(2)-mediated signal for poly(ADP-ribose) polymerase (PARP) degradation. Potentially as the initial cell surface event for intracellular signaling, NaAsO(2) induced aggregation of GPI-anchored protein Thy-1 and superoxide production. This Thy-1 aggregation and subsequent activation of MAP family kinase and c-Jun and the degradation of PARP induced by NaAsO(2) were all inhibited by DTT, suggesting the requirement of interaction between arsenic and protein sulfhydryl groups for those effects. beta cyclodextrin, which sequestrates cholesterol from the membrane rafts, inhibited NaAsO(2)-induced activation of protein tyrosine kinases and MAP family kinases, degradation of PARP, and production of superoxide. In addition, beta cyclodextrin dispersed NaAsO(2)-induced Thy-1 clustering. These results suggest that a membrane raft integrity-dependent cell surface event is a prerequisite for NaAsO(2)-induced protein tyrosine kinase/c-Jun amino-terminal kinase activation, superoxide production, and downstream caspase activation.

Involvement of CD95-independent caspase 8 activation in arsenic trioxide-induced apoptosis

Arsenic trioxide (As2O3)-treatment is effective in acute promyelocytic leukemia (APL) patients with t(15;17). Clinically achievable concentrations of As2O3 induce apoptosis in NB4, an APL cell line, in vitro. Here, to study the mechanism of As2O3-induced apoptosis, we established an As2O3-resistant subline, NB4/As. Growth of NB4/As was inhibited by 50% after 2 day-treatment (IC50) at 1.6 microM As2O3, whereas IC50 of NB4 was 0.3 microM. Degradation of PML-RARalpha and change of the PML-subcellular localization were similarly induced by As2O3 in NB4 and NB4/As, suggesting that their contribution to apoptosis is small. Treatment with 1 microM As2O3 induced the activation of caspase 3 as well as a loss of mitochondrial transmembrane potential (deltapsim) in NB4 but not in NB4/As. Caspase 8 and Bid were also activated by As2O3 in NB4 but not in NB4/As. In NB4, an inhibitor of caspase 8 blocked not only the activation of caspase 3 but also the loss of deltapsim. Neither cell line expressed CD95/Fas, and agonistic anti-Fas antibody (CH-11) failed to cause apoptosis. Neither antagonistic anti-CD95/Fas antibody nor anti-Fas ligand antibodies influenced the As2O3-induced apoptosis. NB4/As had a higher concentration of intracellular glutathione (GSH) than NB4 (96 vs 32 nmol/mg). Reduction of the GSH level by buthionine sulfoxide (BSO) completely restored the sensitivity to As2O3 in NB4/As. Furthermore, caspase activation and the loss of deltapsim were recovered by combination treatment with BSO. These findings suggest that the As2O3 treatment activates caspase 8 in a CD95-independent but GSH concentration-dependent manner. In combination with BSO, As2O3 might be applied to therapy of leukemia/cancers which are insensitive to the clinically achievable concentrations of As2O3.

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.

Enzymatic reduction of arsenic compounds in mammalian systems: reduction of arsenate to arsenite by human liver arsenate reductase

An arsenate (As(V)) reductase has been partially purified from human liver. Its apparent molecular mass is approximately 72 kDa. The enzyme required a thiol and a heat stable cofactor for activity. The cofactor is less than 3 kDa in size. The thiol requirement can be satisfied by dithiothreitol (DTT). However, the extent of stimulation of reductase activity by glutathione, thioredoxin, or reduced lipoic acid was negligible compared to that of DTT. The heat stable cofactor does not appear to be Cu(2+), Mn(2+), Zn(2+), Mg(2+), or Ca(2+). The enzyme does not reduce monomethylarsonic acid (MMA(V)). The isolation and characterization of this enzyme demonstrates that in humans, the reduction of arsenate to arsenite is enzymatically catalyzed and is not solely the result of chemical reduction by glutathione as has been proposed in the past.

Different mechanisms of c-Jun NH(2)-terminal kinase-1 (JNK1) activation by ultraviolet-B radiation and by oxidative stressors

Irradiation of mammalian cells with ultraviolet-B radiation (UV-B) triggers the activation of a group of stress-activated protein kinases known as c-Jun NH(2)-terminal kinases (JNKs). UV-B activates JNKs via UV-B-induced ribotoxic stress. Because oxidative stress also activates JNKs, we have addressed the question of whether the ribotoxic and the oxidative stress responses are mechanistically similar. The pro-oxidants sodium arsenite, cadmium chloride, and hydrogen peroxide activated JNK1 with slow kinetics, whereas UV-B potentiated the activity of JNK1 rapidly. N-acetyl cysteine (a scavenger of reactive oxygen intermediates) abolished the ability of all oxidative stressors tested to activate JNK1, but failed to affect the activation of JNK1 by UV-B or by another ribotoxic stressor, the antibiotic anisomycin. In contrast, emetine, an inhibitor of the ribotoxic stress response, was unable to inhibit the activation of JNK1 by oxidative stressors. Although UV-A and long wavelength UV-B are the spectral components of the ultraviolet solar radiation that cause significant oxidative damage to macromolecules, the use of a filter to eliminate the radiation output from wavelengths below 310 nm abolished the activation of JNK1 by UV. Our results are consistent with the notion that UV-B and oxidative stressors trigger the activation of JNK1 through different signal transduction pathways.

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.

Methylation of inorganic arsenic in different mammalian species and population groups

Thousands of people in different parts of the world are exposed to arsenic via drinking water or contaminated soil or food. The high general toxic of arsenic has been known for centuries, and research during the last decades has shown that arsenic is a potent human carcinogen. However, most experimental cancer studies have failed to demonstrate carcinogenicity in experimental animals, indicating marked variation in sensitivity towards arsenic toxicity between species. It has also been suggested that there is a variation in susceptibility among human individuals. One reason for such variability in toxic response may be variation in metabolism. Inorganic arsenic is methylated in humans as well as animals and micro-organisms, but there are considerable differences between species and individuals. In many, but not all, mammalian species, inorganic arsenic is methylated to methylarsonic acid (MMA) and dimethylarsinic acid (DMA), which are more rapidly excreted in urine than is the inorganic arsenic, especially the trivalent form (AsIII, arsenite) which is highly reactive with tissue components. Absorbed arsenate (AsV) is reduced to trivalent arsenic (AsIII) before the methyl groups are attached. It has been estimated that as much as 50-70% of absorbed AsV is rapidly reduced to AsIII, a reaction which seems to be common for most species. In most experimental animal species, DMA is the main metabolite excreted in urine. Compared to human subjects, very little MMA is produced. However, the rate of methylation varies considerably between species, and several species, e.g. the marmoset monkey and the chimpanzee have been shown not to methylate inorganic arsenic at all. In addition, the marmoset monkey accumulates arsenic in the liver. The rat, on the other hand, has an efficient methylation of arsenic but the formed DMA is to a large extent accumulated in the red blood cells. As a result, the rat shows a low rate of excretion of arsenic. In both human subjects and rodents exposed to DMA, about 5% of the dose is excreted in the urine as trimethylarsine oxide. It is obvious from studies on human volunteers exposed to specified doses of inorganic arsenic that the rate of excretion increases with the methylation efficiency, and there are large inter-individual variations in the methylation of arsenic. Recent studies on people exposed to arsenic via drinking water in northern Argentina have shown unusually low urinary excretion of MMA. Furthermore, children had a lower degree of methylation of arsenic than adults. Some studies indicate a lower degree of arsenic methylation in men than in women, especially during pregnancy. Whether the observed differences in methylation of arsenic are associated with variations in the susceptibility of arsenic remains to be investigated.

On-column formation of arsenic-glutathione species detected by size-exclusion chromatography in conjunction with arsenic-specific detectors

The 'retention analysis method', which is based on size-exclusion chromatography (SEC) in conjunction with an arsenic-specific detector (graphite furnace atomic absorption spectrometer, GFAAS), was used to study the effect of pH (range 2.0-10.0), temperature (4, 25 and 37 degrees C), and the concentration of glutathione in the mobile phase (0.5-7.5 mM) on the formation of arsenic-glutathione species after injection of sodium arsenite using phosphate-buffered saline solutions as mobile phases. The formation of arsenic-GSH species was facilitated by low temperatures (4 degrees C), pH 6.0-8.0 and high concentrations of glutathione (7.5 mM) in the mobile phase. Simulating the physicochemical parameters found inside human red blood cells (approximately 3.0 mM glutathione, 37 degrees C, pH 7.4) and hepatocytes (approximately 7.5 mM glutathione, 37 degrees C, pH 7.4), SEC-GFAAS provided evidence for the formation of arsenic-glutathione species under these conditions. In addition, the 'chelating agent', sodium DL-2,3-dimercapto- -propanesulfonate (1.0 and 2.0 mM) was demonstrated to bind arsenous acid stronger in the presence of glutathione (7.5 mM) under these conditions (PBS buffer, pH 7.4, 37 degrees C).