Studies on the Terminal Reaction in the Biosynthesis of Methionine.

It has been established from studies with Escherichia coli mutants (1,2) and animal liver (3,4) that 1\Is-methyltetrahydrofolic acid (5-CH3-folate-H4) transfers its methyl group to homocysteine to form methionine. The functions of the complex requirements for this reaction (reduced flavin (1,5), S-adenosylmethionine (AMe)l(4), and a cobalamin compound (6-8)) have not yet been fully elucidated. Recent work by Smith et al. (9) and by the group of Woods (10) have contributed some idea as to the function of the cobalamin in this reaction. The former group, in a notable achievement, reported the chemical synthesis of the B12 coenzyme (5,6 dimethylbenzimidazolylcobamide 5’-deoxyadenosine) and related derivatives in which the 5’-deoxyadenosyl moiety of the coenzyme is replaced by various alkyl groups. Synthetic methyl-Blz was tested by Guest, Friedman, and Woods (10) in a methionine-synthesizing system from E. coli. It was demonstrated that this derivative could replace the vitamin in the enzymatic reaction but, more noteworthy, could also transfer its methyl group to homocysteine to form methionine. In the present report we have investigated and compared the biosynthesis of methionine, in cell-free systems from animal liver and E. co& employing Cl’-5-CH3-folate-H4, Cl’-methyl-B12, and methyl-H3-AMe. Methyl-Blr can transfer its methyl group to homocysteine in all the systems tested, and evidence is presented from studies with the bacterial system that either methyl-B12 or a reduced cobalamin derivative is capable of binding to the apoenzyme.