Some factors influencing phosphatidylcholine formation.

Conversion of ethanolamine to choline (l), with methionine serving as the methyl donor (2), has been established in the intact animal. Stetten (1) observed that the labeled nitrogen of ethanolamine-N15 was retained in the choline molecule, and we determined that the carbon chain of ethanolamine is the precursor in vivo of the choline carbon chain (3). Conversion of ethanolamine to choline by in vitro preparations is in a more doubtful state. The synthesis of choline in vitro from methionine and ethanolamine with rat or guinea pig kidney and liver slices or homogenates (4, 5) has been questioned by Veitch and Zweig (6). These authors have demonstrated that the disappearance of methionine, as judged by the McCarthy-Sullivan method, was not the result of transmethylation but was actually due to the oxidative deamination of the methionine, mainly by n-amino acid oxidase present in the tissue preparations. Similarly, the test for choline by precipitating it as the reineckate salt has been claimed to be unreliable, as ethanolamine and other bases that occur give insoluble reineckates (6). However, choline has been reported to be synthesized from n-methionine and 2-dimethylaminoethanol by rat liver slices in the presence of 0.001 M KCN, but not by liver homogenates (7). Since the earlier in vitro experiments did not employ isotopic tracer methods and apparently did not utilize a reliable procedure for the isolation of choline, it appeared that the problem of the synthesis of choline from ethanolamine in vitro warranted reinvestigation. In view of the lack of knowledge of the formation of the phosphatides, the present study of the metabolism of ethanolamine in vitro and the formation of choline was so organized as to permit the investigation to deal with the factors influencing the formation of lecithin or phosphatidylcholine.