Studies on the Origin of the Methyl Groups of Choline in Ochromonas malhamensis*

Abstract

SYNOPSIS. Five‐ to 6‐day‐old resting cells of Ochromonas malhamensis were incubated at pH 6.5 with glucose and appropriate C14 precursors of the methyl groups of phospholipid‐choline. Under the experimental conditions L‐methionine‐C14H3 was the most efficient source of choline‐methyl groups, followed by formate‐C14, formaldehyde‐C14 and DL‐serine‐3‐C14, respectively. Glycine‐2‐C14 was not incorporated into choline. Both L‐methionine‐C14H3 and formate‐C14 served as precursors for the methyl groups of monomethylethanolamine, dimethylethanolamine and choline. Addition of non‐radio‐active L‐methionine depressed the incorporation of formate‐C14 into choline‐methyl groups by 50%. The results support the hypothesis that methionine can be the source of all 3 methyl groups of choline, and that formate is probably converted to the methyl group of methionine before transmethylation to choline. However, an alternate pathway from single‐carbon sources cannot be excluded.