Rat brain synaptosomes contain an enzyme, phosphatidylethanolamine N-methyltransferase (EC 2.1.1.17), that catalyzes the methylation of phosphatidylethanolamine to form its mono-, di-, and trimethyl (phosphatidylcholine) derivatives. Synaptosomal phosphatidylethanolamine is much richer in polyunsaturated fatty acids (43.4%) than is synaptosomal phosphatidylcholine (4.6%). It thus seemed possible that the phosphatidylcholine derived via the Nmethylation of phosphatidylethanolamine might also be especially enriched in polyunsaturated fatty acids. To test this hypothesis, we examined the incorporation of PH]methyl groups into various molecular species of phosphatidylcholine, by incubating rat synaptosomes for 10, 30, or 90 min in a medium containing S-adenosyl[methyl-3H]methionine.Phosphatidylcholine was extracted and separated from other lipids by TLC, after which its molecular species were isolated by argentation TLC (which distinguishes among the phospholipid molecules by the degree of unsaturation of their fatty acid moieties.) We found that approximately 65% of the [3H]methyl incorporated into phosphatidylcholine during the incubation period was present in the fraction associated with pentaene or hexaene fatty acids; an additional 30% was present in the tetraene fraction, while the remaining phosphatidylcholine radioactivity was distributed between the dienes and monoenes. Similar distributions were observed among synaptosomes incubated for 10 or 30 min; however, after 90 min the phosphatidyl[3Hjcholine contained proportionately less of the tetraenes. These observations indicate that neuronal phosphatidylcholine molecules formed via N-methylation are especially richer in polyunsaturated fatty acids, and they raise the possibility that these molecules constitute a distinct pool with particular physiologic functions.Three synthetic pathways can produce phosphatidylcholine (PtdCho) molecules in cells: the Kennedy pathway, which combines CDP-choline with 1,2-diacylglycerol; the baseexchange pathway, which exchanges free choline with a serine or ethanolamine in phosphatidylserine or phosphatidylethanolamine (PtdEtn); and the sequential N-methylation of PtdEtn, forming its mono-, di-, and trimethyl (PtdCho) derivatives. The latter is the only pathway capable of producing new choline moieties in the brain, which otherwise depends on the circulation for its choline supply (1, 2). This pathway, catalyzed by phosphatidylethanolamine N-methyltransferase (PtdEtnMeTase; EC 2.1.1.17), has been demnonstrated in brain (3, 4), its activity being highest in synaptosomes (5).The fatty acid composition of synaptosomal PtdEtn, which is the precursor for the PtdCho that is formed via methylation (and a precursor for that formed via the base exchange pathway), differs markedly from that of PtdCho, containing 43.4% polyunsaturated fatty acids as opposed to 4.6% in PtdCho. Two hypotheses have been proposed (6)
