METABOLIC PATHWAYS LEADING TO MERCURY METHYLATION IN DESULFOVIBRIO-DESULFURICANS LS

The synthesis of methylmercury by Desulfovibrio desulfuricans LS was i nvestigated on the basis of C-14 incorporation from precursors and the measurement of relevant enzyme activities in cell extracts. The previ ously observed incorporation of C-3 from serine into methylmercury was confirmed by measurement of relatively high activities of serine hydr oxymethyltransferase and other enzymes of this pathway. High rates of label incorporation into methylmercury from (HCOO-)-C-14 and (HCO3-)-C -14 prompted the assay of enzymes of the acetyl coenzyme A (CoA) synth ase pathway. These enzymes were found to be present but at activity le vels much lower than those reported for acetogens. Propyl iodide inhib ited methylmercury and acetyl-CoA syntheses to similar extents, and me thylmercury synthesis was found to compete with acetyl-CoA synthesis f or methyl groups. On the basis of these findings, we propose that in m ethylmercury synthesis by D. desulfuricans LS the methyl group is tran sferred from CH3-tetrahydrofolate via methylcobalamin. The methyl grou p 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.