Interspecies differences in metabolism of arsenic by cultured primary hepatocytes

Department of Nutrition, University of North Carolina, Chapel Hill, NC 27599-7461, USA.
Toxicology and Applied Pharmacology (Impact Factor: 3.71). 02/2010; 245(1):47-56. DOI: 10.1016/j.taap.2010.01.015
Source: PubMed


Biomethylation is the major pathway for the metabolism of inorganic arsenic (iAs) in many mammalian species, including the human. However, significant interspecies differences have been reported in the rate of in vivo metabolism of iAs and in yields of iAs metabolites found in urine. Liver is considered the primary site for the methylation of iAs and arsenic (+3 oxidation state) methyltransferase (As3mt) is the key enzyme in this pathway. Thus, the As3mt-catalyzed methylation of iAs in the liver determines in part the rate and the pattern of iAs metabolism in various species. We examined kinetics and concentration-response patterns for iAs methylation by cultured primary hepatocytes derived from human, rat, mice, dog, rabbit, and rhesus monkey. Hepatocytes were exposed to [(73)As]arsenite (iAs(III); 0.3, 0.9, 3.0, 9.0 or 30 nmol As/mg protein) for 24 h and radiolabeled metabolites were analyzed in cells and culture media. Hepatocytes from all six species methylated iAs(III) to methylarsenic (MAs) and dimethylarsenic (DMAs). Notably, dog, rat and monkey hepatocytes were considerably more efficient methylators of iAs(III) than mouse, rabbit or human hepatocytes. The low efficiency of mouse, rabbit and human hepatocytes to methylate iAs(III) was associated with inhibition of DMAs production by moderate concentrations of iAs(III) and with retention of iAs and MAs in cells. No significant correlations were found between the rate of iAs methylation and the thioredoxin reductase activity or glutathione concentration, two factors that modulate the activity of recombinant As3mt. No associations between the rates of iAs methylation and As3mt protein structures were found for the six species examined. Immunoblot analyses indicate that the superior arsenic methylation capacities of dog, rat and monkey hepatocytes examined in this study may be associated with a higher As3mt expression. However, factors other than As3mt expression may also contribute to the interspecies differences in the hepatocyte capacity to methylate iAs.

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Available from: David J Thomas, Oct 17, 2014
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    • "Thus, the depletion of GSH may be one mechanism through which As could lead to oxidative stress. However, given the well-known species differences in As metabolism and susceptibility to As-induced health effects (Drobná et al. 2010), additional data derived from human population studies are needed. We conducted a cross-sectional study of 378 Bangladeshi adults to test the primary hypothesis that chronic As exposure is associated with reductions in GSH and increases in GSSG. "
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    ABSTRACT: In vitro and rodent studies have shown that arsenic (As) exposure can deplete glutathione (GSH) and induce oxidative stress. GSH is the primary intracellular antioxidant; it donates an electron to reactive oxygen species producing glutathione disulfide (GSSG). Cysteine (Cys) and cystine (CySS) are the predominant thiol/disulfide redox couple found in human plasma. Arsenic, GSH, and Cys are linked in several ways. First, GSH is synthesized via the transulfuration pathway and Cys is the rate limiting substrate. Second, intermediates of the methionine cycle regulate both the transulfuration pathway and As methylation. Third, GSH serves as the electron donor for reduction of arsenate to arsenite. Fourth, As has a high affinity for sulfhydryl groups and therefore binds to GSH and Cys. To test the hypotheses that As exposure is associated with decreases in GSH and Cys and increases in GSSG and CySS, i.e., a more oxidized environment. For this cross-sectional study, the Folate and Oxidative Stress Study, we recruited a total of 378 participants from each of 5 water As categories: <10 (n=76), 10-100 (n=104), 101-200 (n=86), 201-300 (n=67), and >300 µg/L (n=45). Concentrations of GSH, GSSG, Cys, and CySS were measured using HPLC. An interquartile range (IQR) increase in water As was negatively associated with blood GSH (mean change = -25.4 µmol/L; 95% CI: -45.3, -5.31) and plasma CySS (mean change = -3.00 µmol/L; 95% CI: -4.61, -1.40). Similar associations were observed with urine and blood As. There were no significant associations between As exposure and blood GSSG or plasma Cys. The observed associations are consistent with the hypothesis that As may influence concentrations of GSH and other non-protein sulfhydryls through binding and irreversible loss in bile and/or possibly in urine.
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    • "While these studies provide strong experimental evidence that nutritional manipulation of one-carbon metabolism influences As methylation, excretion, and toxicity, the As doses were high, and the dietary deficiencies were severe. Moreover, there are marked species variations in the efficiency of As methylation [92]. "
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    ABSTRACT: Exposure to arsenic (As) through drinking water is a substantial problem worldwide. The methylation of As, a reactive metalloid, generates monomethyl- (MMA) and dimethyl-arsenical (DMA) species. The biochemical pathway that catalyzes these reactions, one-carbon metabolism, is regulated by folate and other micronutrients. Arsenic methylation exerts a critical influence on both its urinary elimination and chemical reactivity. Mice having the As methyltransferase null genotype show reduced urinary As excretion, increased As retention, and severe systemic toxicity. The most toxic As metabolite in vitro is MMA(III), an intermediate in the generation of DMA(V), a much less toxic metabolite. These findings have raised the question of whether As methylation is a detoxification or bioactivation pathway. Results of population-based studies suggest that complete methylation of inorganic As to DMA is associated with reduced risk for As-induced health outcomes, and that nutrients involved in one-carbon metabolism, such as folate, can facilitate As methylation and elimination.
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    • "Arsenic undergoes extensive methylation in humans and most other mammals (Drobná et al., 2010). It was originally thought that As methylation was a detoxification process because it increases the rate of whole-body clearance of arsenic and the methylated As V species monomethylarsonic acid (MMA V ) and dimethylarsinic acid (DMA V ) are less toxic than As V and As III (Drobná et al., 2010). However, in vitro studies have revealed that the trivalent methylated species MMA III and DMA III are substantially more potent toxicants than As III , resulting in methylation being viewed as an activation pathway (Kumagai and Sumi, 2007). "
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