Metabolic Pathways Leading to Mercury Methylation in Desulfovibrio desulfuricans LS

The synthesis of methylmercury by Desulfovibrio desulfuricans LS was investigated on the basis of ¹⁴C incorporation from precursors and the measurement of relevant enzyme activities in cell extracts. The previously observed incorporation of C-3 from serine into methylmercury was confirmed by measurement of relatively high activities of serine hydroxymethyltransferase and other enzymes of this pathway. High rates of label incorporation into methylmercury from H¹⁴COO^- and H¹⁴CO₃^- prompted the assay of enzymes of the acetyl coenzyme A (CoA) synthase pathway. These enzymes were found to be present but at activity levels much lower than those reported for acetogens. Propyl iodide inhibited methylmercury and acetyl-CoA syntheses to similar extents, and methylmercury synthesis was found to compete with acetyl-CoA synthesis for methyl groups. On the basis of these findings, we propose that in methylmercury synthesis by D. desulfuricans LS the methyl group is transferred from CH₃-tetrahydrofolate via methylcobalamin. The methyl group may originate from C-3 of serine or from formate via the acetyl-CoA synthase pathway. These pathways are not unique to D. desulfuricans LS, and thus the ability of this bacterium to methylate mercury is most likely associated with the substrate specificity of its enzymes.