The capacity of mouse intestinal cecal microflora to methylate inorgan
ic arsenicals (iAs) was examined in vitro under conditions of restrict
ed bacterial growth. Cecal contents incubated under anaerobic conditio
ns at 37 degrees C for 21 hr methylated up to 40% of either 0.1 mu M a
rsenite (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 f
rom iAs(V), suggesting that reduction of iAs(V) to iAs(III) before met
hylation could be rate limiting. Over the concentration range of 0.1 t
o 10 mu M of iAS(IlI) or iAS(V), there was an approximately linear inc
rease in the production of MAs and DMAs. There was evidence of saturat
ion or inhibition of methylation at 100 mu M of either substrate. Subs
trate concentration had little effect on MAs/DMAs ratio. Incubation of
cecal contents at 0 degrees C abolished methylation of either arsenic
al. Under aerobic or anaerobic conditions, cecal tissue homogenates pr
oduced little MAs or DMAs from either arsenical. Addition of potential
methyl group donors, L-methionine and methylcobalamin, into cecal con
tents significantly increased the rate of methylation, especially for
iAs(V). Addition of glutathione, but not L-cysteine, had a similar eff
ect. 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 methylati
on system that might contribute significantly to methylation of iAs in
intact animals.