Methyl derivatives of folic acid as intermediates in the methylation of homocysteine by Escherichia coli

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Abstract
The products formed from N5N10-methylenetetrahydropteroyl-monoglutamate and -triglutamate respectively, by an enzyme (A) absent from auxotrophs of E. coli responding to methionine only (but present in other strains), was isolated by chromatographic methods and characterized as the corresponding N5-methyl derivatives. The other reactant was reduced flavin adenine dinucleotide (FADH2). N5 -Methyltetrahydropteroyltriglutamate methylates homocysterine in the presence of an enzyme-B which is absent in cobalamin/methionine auxotrophs. The monoglutamate derivative does not act as substrate for this enzyme, but is effective (as is also the triglutamate) with extracts of organisms grown in the presence of cobalamin and possessing a cobamide-containing enzyme. The metabolic lesion in the methionine auxotrophs that respond to methionine only can be definitely ascribed to the absence of enzyme-A (N5N10-methylenetetrahydrofolate reductase). There are alternative enzymic mechanisms for the formation of methionine from homocysteine. The point of divergence is subsequent to the formation of the methyl group.