Incorporation of radioactivity from [Me- 14 C] methionine and [2- 14 C]glycine into the lipids of Rhodopseudomonas spheroides

Abstract

Radioactivity from L-[Me-$^{14}$C]methionine, in addition to being incorporated into pigments, as found by other workers, was incorporated into phosphatidylcholine, into phosphatidyl N-methylethanolamine and into phosphatidyl N,N-dimethylethanolamine. The latter two compounds were present in trace amounts only. Of the radioactivity in the phospholipid fraction, 90% was in the bases with only minor amounts in the fatty acids. Incorporation was also found into ornithine lipid. The presence of the pathway of phosphatidylcholine biosynthesis which proceeds by the successive methylation of phosphatidylethanolamine was shown by demonstrating a precursor-product relationship between the N-methylated phospholipids and phosphatidylcholine. Evidence for the presence of the enzymes concerned in the biosynthesis of phosphatidylcholine by the pathway mentioned and also for the incorporation of the methyl group into ornithine lipid has been obtained in cell-free systems from Rhodopseudomonas spheroides. The methyl donor was S-adenosylmethionine. Radioactivity from [2-$^{14}$C]glycine was incorporated into ethanolamine, into choline and into fatty acids. Ethionine markedly reduced the incorporation into choline without affecting the labelling of ethanolamine. Thus it seems likely that the methyl carbons of choline were radioactive and that ethionine acted by inhibiting transmethylation from S-adenosylmethionine. It is concluded that the $\alpha $-carbon of glycine gives rise to 'one carbon' fragments and also to acetate and the pathways by which these reactions occur are discussed.