Trivalent arsenicals are bound to proteins during reductive methylation

Inorganic arsenic is converted to methylated metabolites, and most is excreted in urine as dimethylarsinic acid in humans and animals. The present study was conducted to investigate the metabolism of arsenic and identify hepatic and renal metabolites of arsenic after an intravenous injection of arsenite (0.5 mg As/kg body weight) in rats. Similar levels of arsenic were found in the soluble (SUP) and nonsoluble sediment (SED) fractions of both organs after 1 h. More than 80% of the SUP arsenic was bound to high molecular weight (HMW) proteins in both organs. Arsenic bound to the HMW and SED proteins were oxidized with H2O2 and released in the pentavalent forms (arsenate, monomethylarsonic, and dimethylarsinic acids). The relative ratios of the three arsenicals changed depending on organ, fraction (HMW and SED), and time. Since the arsenic metabolites/intermediates were liberated from proteins by oxidation with H2O2 and recovered in the pentavalent forms, and only tri- but not pentavalent arsenicals were bound to proteins in vitro, it was deduced that arsenic metabolites bound to proteins during the successive methylation pathway are in the trivalent forms; that is, successive methylation reaction takes place with simultaneous reductive rather than stepwise oxidative methylation. Thus, on the basis of the present observations, it was proposed that inorganic arsenic was successively methylated reductively in the presence of glutathione, rather than a stepwise oxidative methylation, and pentavalent arsenicals (MMA(V) and DMA(V)) were present as end products of metabolism, rather than intermediates. We also discussed the in vitro formation of dimethylthioarsenicals after incubating dimethylarsinous acid with liver homogenate.