Occurrence of monomethylarsonous acid in urine of humans exposed to inorganic arsenic

Monomethylarsonous acid (MMA(III)) has been detected for the first time in the urine of some humans exposed to inorganic arsenic in their drinking wat er. Our experiments have dealt with subjects in Romania who have been expos ed to 2.8, 29, 84, or 161 mu g of As/L in their drinking water. In the latt er two groups, MMA(III) was 11 and 7% of the urinary arsenic while the mono methylarsonic acid (MMA(V)) was 14 and 13%, respectively. Of our 58 subject s, 17% had MMA(III) in their urine. MMA(III) was not found in urine of any members of the group with the lowest level of As exposure. If the lowest-le vel As exposure group is excluded, 23% of our subjects had MMA(III) in thei r urine. Our results indicate that (a) future studies concerning urinary ar senic profiles of arsenic-exposed humans must determine MMA(III) concentrat ions, (b) previous studies of urinary profiles dealing with humans exposed to arsenic need to be re-examined and re-evaluated, and (c) since MMA(III) is more toxic than inorganic arsenite, a re-examination is needed of the tw o hypotheses which hold that methylation is a detoxication process for inor ganic arsenite and that inorganic arsenite is the major cause of the toxici ty and carcinogenicity of inorganic arsenic.