In vitro methylation of inorganic arsenic in mouse intestinal cecum

The capacity of mouse intestinal cecal microflora to methylate inorganic arsenicals (iAs) was examined in vitro under conditions of restricted bacterial growth. Cecal contents incubated under anaerobic conditions at 37 degrees C for 21 hr methylated up to 40% of either 0.1 mu M arsenite (iAs(III)) or 0.1 mu M arsenate (iAs(V)). Methylarsenic (MAs) was the predominant metabolite; however, about 3% of either substrate was converted to dimethylarsenic (DMAs). Over the first 6 hr, the rate of methylation was several times greater for iAs(III) than for iAs(V). There was a 3-hr delay in the production of methylated metabolites from iAs(V), suggesting that reduction of iAs(V) to iAs(III) before methylation could be rate limiting. Over the concentration range of 0.1 to 10 mu M of iAS(IlI) or iAS(V), there was an approximately linear increase in the production of MAs and DMAs. There was evidence of saturation or inhibition of methylation at 100 mu M of either substrate. Substrate concentration had little effect on MAs/DMAs ratio. Incubation of cecal contents at 0 degrees C abolished methylation of either arsenical. Under aerobic or anaerobic conditions, cecal tissue homogenates produced little MAs or DMAs from either arsenical. Addition of potential methyl group donors, L-methionine and methylcobalamin, into cecal contents significantly increased the rate of methylation, especially for iAs(V). Addition of glutathione, but not L-cysteine, had a similar effect. Selenite, a recognized inhibitor of iAs methylation in mammalian tissues, inhibited methylation of either substrate by cecal contents. These data suggest that cecal microflora are a high capacity methylation system that might contribute significantly to methylation of iAs in intact animals.