Time course of wild-type and mutant hAS3MTs. Methylation activity was assayed by production of SAH, which was determined with an EPIgeneous Methyltransferase Assay kit. Curve 1, wild-type hAS3MT; curve 2, hAS3MT C32S; curve 3, hAS3MT C61S; curve 4, hAS3MT C32S/C61S double mutant. Each purified enzyme was added at 1 lM, final concentration, and As(III) and SAM were 10 μM. The data are the mean ± SE (n = 3).