Norepinephrine stimulation of phospholipid methylation in rat cortical synaptosomes: fact or artifact?

Synaptosomes from rat cerebral cortex incubated with 3H-S-adenosyl-L-methionine (3H-SAM) displayed an increase in chloroform- extractable tritium when norepinephrine was added to the reaction mixture. The products of this mixture were maximally generated from intact synaptosomes, only partially inhibited by propranolol, and not enhanced by exogenous phospholipids. Thin layer chromatographic analysis of these chloroform extracts in three solvent systems yielded large norepinephrine- stimulated peaks of radioactivity that did not consistently co-chromatograph with authentic methylated phospholipid standards: phosphatidylmonomethylethanolamine (PME), phosphatidyldimethylethanolamine (PDE), and phosphatidylcholine (PC). Further, attempts to identify these peaks of radioactivity using as standards several putative methylated products of varied chemical classes, failed to elucidate likely candidates. It appears that while norepinephrine markedly stimulates the amount of tritium extracted into the chloroform phase, careful and positive structural elucidation of formed products is required before it can be concluded that these are indeed methylated phospholipids.