When ¹⁴C]formate was fed, unk] MME, phosphoryldimethylethanolamine (unk] DME) and unk] choline were the most prominent methylated products at short labeling times, after which ¹⁴C appeared in free choline and in betaine. Phosphatidylcholine labeled more slowly than unk] choline, choline, and betaine, and behaved as a minor end product. Very little ¹⁴C entered the free methylethanolamines. When ¹⁴C]unk] MME was supplied, a small amount was hydrolyzed to the free base but the major fate was conversion to unk] DME, unk] choline, free choline, and betaine; label also accumulated slowly in phosphatidylcholine. Label from supplied ¹⁴C]unk] choline entered choline and betaine rapidly, while phosphatidylcholine labeled only slowly and to a small extent.

These results are consistent with the pathway unk] MME →unk] DME → unk] choline → choline → → betaine, with a minor side branch leading from unk] choline into phosphatidylcholine. This contrasts markedly (a) with the pathway of stress-induced choline and betaine synthesis in barley, in which phosphatidylcholine apparently acts as an intermediate (Hitz, Rhodes, Hanson 1981, Plant Physiol 68: 814-822); (b) with choline biogenesis in mammalian liver and microorganisms. Computer modeling of the experimental data pointed strongly to regulation at the unk] choline → choline step, and also indicated that the rate of unk] choline synthesis is subject to feedback inhibition by unk] choline.