Comparative in vitro methylation of trivalent and pentavalent arsenicals

The time course and extent of methylation of 1 mu M arsenite (iAs(III)), arsenate (iAs(V)), methylarsenite (MeAs(III)), methylarsenate (MeAs(V)), and MeAs(III)-diglutathione complex (MeAs(III)(GS)(2)) were examined in an in vitro assay system that contained rat liver cytosol. Precursor arsenicals and methylated metabolites were analyzed by thin-layer chromatography (TLC) or by hydride generation-atomic absorption spectrophotometry (HG-AAS). More than 90% of iAs(III) was converted to a dimethylated species (Me(2)As) during a 90-min incubation at 37 degrees C; the amount of monomethylated metabolite was maximal at 15 min. In contrast, only 40% of iAs(V) was dimethylated during a 90-min incubation. Comparison of the yields of methylated species in the whole in vitro assay system as determined by HG-AAS and in an ultrafiltrate prepared from the in vitro assay system as determined by TLC indicated that nearly 70% of the dimethylated metabolite (possibly Me(2)As(III)) that was produced during a 90-min incubation was bound to proteins (>10 kDa). The percentage of protein-bound arsenic in the assay system incubated at 0 degrees C with trivalent arsenicals was three-to fivefold greater than the binding of corresponding pentavalent species. This indicated that both iAs(III) and trivalent organoarsenicals interact avidly with proteins. Both MeAs(III) prepared by metabisulfte-thiossulfate reduction of MeAs(V) and a MeAs(III)(GS)2 were quantitatively converted to Me(2)As during 90-min incubation. In contrast, only 3% of MeAs(V) was dimethylated during this interval. These results suggest that trivalent arsenicals are preferred substrates for methylation reactions and that the reduction of As from pentavalent to trivalent states may be a critical step in the control of the rate of metabolism of As. (C) 1995 Academic Press, Inc.