Robert A. Zakharyan

Oxidation and detoxification of trivalent arsenic species

Arsenic compounds with a +3 oxidation state are more toxic than analogous compounds with a +5 oxidation state, for example, arsenite versus arsenate, monomethylarsonous acid (MMA(III)) versus monomethylarsonic acid (MMA(V)), and dimethylarsinous acid (DMA(III)) versus dimethylarsinic acid (DMA(V)). It is no longer believed that the methylation of arsenite is the beginning of a methylation-mediated detoxication pathway. The oxidation of these +3 compounds to their less toxic +5 analogs by hydrogen peroxide needs investigation and consideration as a potential mechanism for detoxification. Xanthine oxidase uses oxygen to oxidize hypoxanthine to xanthine to uric acid. Hydrogen peroxide and reactive oxygen are also products. The oxidation of +3 arsenicals by the hydrogen peroxide produced in the xanthine oxidase reaction was blocked by catalase or allopurinol but not by scavengers of the hydroxy radical, e.g., mannitol or potassium iodide. Melatonin, the singlet oxygen radical scavenger, did not inhibit the oxidation. The production of H2O2 by xanthine oxidase may be an important route for decreasing the toxicity of trivalent arsenic species by oxidizing them to their less toxic pentavalent analogs. In addition, there are many other reactions that produce hydrogen peroxide in the cell. Although chemists have used hydrogen peroxide for the oxidation of arsenite to arsenate to purify water, we are not aware of any published account of its potential importance in the detoxification of trivalent arsenicals in biological systems. At present, this oxidation of the +3 oxidation state arsenicals is based on evidence from in vitro experiments. In vivo experiments are needed to substantiate the role and importance of H2O2 in arsenic detoxication in mammals.

Enzymatic methylation of arsenic compounds: IV. In vitro and in vivo deficiency of the methylation of arsenite and monomethylarsonic acid in the guinea pig.

Using an in vitro assay which measures the transfer of a radiolabeled methyl moiety of S-[methyl-3H]adenosylmethionine ([3H]SAM) to arsenite or monomethylarsonate (MMA) to yield [methyl-3H]MMA or [methyl-3H]dimethylarsinate (DMA) respectively, guinea pig liver cytosol was found to be deficient in the enzyme activities which methylate these substrates. Moreover, when guinea pigs were given a single intraperitoneal dose of [73As]arsenate (400 micrograms/kg body weight, 25 microCi/kg body weight), very little or no methylated arsenic species were detected in the urine after cation exchange chromatography. The urine collected 0-12 h after arsenate injection contained 98% inorganic arsenic and less than 1% DMA. No MMA was detected in the 0-12 h urine. Urine collected 12-24 h after injection contained approximately 93% inorganic arsenic, 2% MMA and 3% DMA in five of the six animals studied. However, in the 12-24 h urine of one guinea pig, 17% of the radioactivity was DMA, 80% was inorganic arsenic and 3% was MMA. The guinea pig, like the marmoset and tamarin monkeys and unlike most other animals studied thus far, appears to be deficient as far as the enzyme activities that methylate inorganic arsenite. The results of these experiments suggest that there may be a genetic polymorphism associated with the enzymes that methylate inorganic arsenite.

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.

How is Inorganic Arsenic Detoxified

Publisher Summary Arsenate is reduced to arsenite enzymatically by arsenate reductase and nonenzymatically by GSH. An early step in the detoxification appears to be the formation of the Gailer compound, seleno-bis(S-glutathionyl) arsinium ion, which is rapidly formed and excreted in the bile. Arsenite-binding proteins initially may prevent or enhance the accumulation of toxic levels of arsenite. As these binding sites become saturated, the arsenite may be released for methylation, a biotransformation process that results in the increase of urinary arsenic (As). Methylation of As species can occur via SAM and methyltransferases and/or nonenzymatically with methylvitamin B 12 , GSH, and selenite. Methylation by the methylvitamin B 12 system has been shown in vitro only. The substrate for DMA production appears to be MMA III . The lack of methyltransferases in many primates strongly indicates that methylation may not be the primary detoxification pathway for Asi. In fact, the environmental protection agency classifies dimethylarsinic acid, the final urinary metabolite for As in humans, as a probable human carcinogen. The determination of the amino acid sequences of the As methyltransferases needs to be accomplished so that gene probes can be constructed to better study As methyltransferase polymorphism, as it is related to the various responses of people to Asi.

Chapter 17 – Enzymology and toxicity of inorganic arsenic

Publisher Summary The biotransformation of inorganic arsenate to dimethylarsinic acid (DMA) involves a series of enzymatic steps. Recent studies based on amino acid homology and other properties demonstrate that the human liver arsenate reductase and the human purine nucleoside phosphorylase (PNP) are identical proteins. The reaction requires inosine and dihydrolipoic acid. Dihydrolipoic acid is the most potent naturally occurring dithiol in this reaction. PNP is an essential enzyme involved in purine and nucleic acid metabolism. Arsenate reductase/PNP will not reduce monomethylarsonous acid (MMAV).