Vitamin B 12 and the synthesis of the methyl groups of choline in ochromonas malhamensis

Abstract

Vitamin B 12-deficient cells of Ochromonas malhamensis were used to investigate the effect of vitamin B 12 on choline-methyl group formation. Vitamin B 12 stimulated the incorporation of formate-C 14 and formaldehyde-C 14, but not of methionine-C 14H 3, into the methyl groups of phospholipid choline. The optimum utilization of formate-C 14 occurred when both homocysteine and vitamin B 12 were provided. Methyl-C 14-cobalamin donated its methyl group for choline synthesis; however, a greater C 14 incorporation was realized when homocysteine was provided. The data support the concept that methionine synthesis is an intermediate step in choline-methyl group biosynthesis, and that in Ochromonas as in other organisms, de novo methyl group synthesis is vitamin B 12 dependent.