Methylation of Arsenic by Human and Murine Arsenic Methyltransferases

Enzymatic methylation catalyzed by arsenic (As) methyltransferase (AS3MT) is the central pathway for the metabolism and detoxification of inorganic As (iAs). In both mice and humans, AS3MT converts iAs to monomethyl-As (MAs) and dimethyl-As (DMAs) using S-adenosylmethionine as a methyl group donor. However, mice are much more efficient in methylating iAs than humans. What causes this difference remains unclear. To explore this question, I studied the Michaelis-Menten kinetics of iAs methylation and the methylation of its intermediate metabolite, MAs, by human and murine AS3MTs. Michaelis-Menten kinetics were characterized in-vitro for recombinant human and murine AS3MTs and in primary hepatocytes from wild-type (WT) and humanized (Hu) C57BL/6N mice that express murine and human AS3MT.The rates of iAs and MAs methylation and the proportions of methylated metabolites differed between the murine and humanized in vitro and cell culture systems. In the in vitro system, only small differences in Michaelis-Menten constants (Km and Vmax) were found between the murine and human enzymes using iAsIII or MAsIII as a substrate. However, there were major differences in Michaelis-Menten kinetics between WT and humanized hepatocytes. In WT hepatocytes, iAsIII was quickly converted to DMAs while humanized hepatocytes produced only negligible amounts of DMAs. The kinetics data suggests that humanized hepatocytes have higher affinity (i.e., lower Km) for iAsIII than WT hepatocytes, but lower capacity (i.e., lower Vmax) to methylate iAs to DMAs. In comparison, WT hepatocytes had both greater affinity and methylation capacity for MAsIII. Overall, the methylation kinetics in the in-vitro systems did not correlate with what was observed in the hepatocyte cultures. PCR analysis showed that WT hepatocytes expressed significantly more AS3MT mRNA than humanized hepatocytes.These results suggest that the known differences in the primary structure of murine and human AS3MTs have only a minor effect on iAs methylation efficiency, and that AS3MT expression is a key factor responsible for the difference in iAs methylation in primary hepatocytes. Other cellular factors and processes (e.g., endogenous cofactors of AS3MT or membrane transporters) may also play a role in influencing As metabolism in hepatocytes, though these factors were not investigated in the present study.