ENZYMATIC CATALYSIS OF MERCURY METHYLATION BY DESULFOVIBRIO-DESULFURICANS LS

The recently defined role of methylcobalamin in Hg2+ methylation by De sulfovibrio desulfuricans LS enabled us to reexamine the question of w hether the principal source of methylmercury is spontaneous transmethy lation or an enzymatically catalyzed process. In cell extracts of D. d esulfuricans LS, over 95% of the Co-57 label was associated with macro molecules rather than with free cobalamin. Both gel filtration and ele ctrophoresis of cell extracts identified a single corrinoid protein of 40 kDa in size. This finding, in combination with the previously repo rted light-reversible propyl iodide inhibition of the Hg2+ methylation process, led us to propose that this 40-kDa corrinoid protein is the in vivo methyl donor in D. desulfuricans LS. Under reducing conditions , cell extracts containing the corrinoid protein produced (CH3Hg+)-C-1 4 from Hg2+ and 5-(CH3)-C-14-tetrahydrofolate with a maximum specific activity of 0.73 nmol min-1 mg of cell protein-1. The sequence of meth yl transfer was from methyltetrahydrofolate to the corrinoid protein t o Hg2+. The rate of methylation versus the Hg2+ concentration followed Michaelis-Menten kinetics, with an apparent K(m) of 0.87 mM HgCl2. Th e activity was oxygen sensitive, and Hg2+ methylation was optimal at 3 5-degrees-C and pH 6.5. The observation of saturation kinetics and the 600-fold-higher rate of Hg2+ methylation (at pH 7.0) by cell extracts , compared with transmethylation by free methylcobalamin, proved that in vivo Hg2+ methylation is an enzymatically catalyzed process.