Methionine synthetase. Existence and interconversion of two enzyme species.

Abstract

A purification procedure is described which permits the isolation of the vitamin B12‐de‐pendent methionine synthetase (5‐methyl‐tetrahydrofolate‐homocysteine‐transmethylase) from Escherichia coli largely in its native, i. e. methylated form. The enzymatic activity of this species does not depend on the presence of S‐adenosylmethionine. The native enzyme can be demethylated and thus made dependent on S‐adenosylmethionine; this reaction is reversible. The inter‐conversions of the enzyme can be recognized by the spectra. The demethylation is effected by a macromolecular factor present in E. coli cells. By gel filtration, both methylated and demethylated enzyme have been found to possess a molecular weight of about 255000.