Biosynthesis of Choline and Ethanolamine Phospholipids in Crithidia fasciculata*

The incorporation of radioactivity from 1,2-³⁴C]choline, 1,2-³⁴C]ethanolamine, 3-¹⁴C]serine and methyl-¹⁴C]methionine into lipids was studied in growing cultures of Crithidia fasciculata. Lecithin was formed both from choline and by the methylation of phosphatidylethanolamine. Mono- and dimethylphosphatidylethanolamines were present in no more than trace amounts. Growth of the protozoa in media containing choline (1 mM) did not decrease synthesis by the methylation pathway. Phosphatidylethanolamine was formed from ethanolamine. Radioactivity from serine also was present in both phosphatidylethanolamine and lecithin; however, the presumed intermediate, phosphatidylserine, could not be detected.