N5-Methyltetrahydrofolate-Homocysteine Transmethylase

Methyl iodide and S-adenosyl-l-ethionine were found to satisfy partially the requirement for catalytic amounts of S-adenosyl-l-methionine in transmethylation from N5-methyltetrahydrofolate to homocysteine. Both methyl iodide and S-adenosyl-l-ethionine were about one-third as effective as S-adenosyl-l-methionine when the reducing system consisted of reduced flavin mononucleotide in the presence of dithiothreitol. However, methyl iodide was only 14% as effective as S-adenosyl-l-methionine in a 2-mercaptoethanol-vitamin B12-reducing system; S-adenosyl-l-ethionine exhibited negligible cofactor activity in this reducing system. In the presence of reduced flavin mononucleotide and dithiothreitol, S-methyl-l-methionine and ethyl iodide permitted a slow but significant rate of catalysis, whereas S-adenosyl-l-homocysteine was inactive. 14CH3-Methionine was identified as the radioactive transmethylation product from N5-methyl-14C-H4-folate when unlabeled methyl iodide was substituted for S-adenosyl-l-methionine in the reduced flavin mononucleotide system. From the cofactor activities given by various related compounds in a reduced flavin mononucleotide-dithiothreitol-reducing system, it is tentatively suggested that S-adenosyl-l-methionine activates the vitamin B12 enzyme by methylation. A reactive methyl or ethyl group appeared to be the essential structural feature for S-adenosyl-l-methionine-like cofactor activity.