Early changes in the phospholipid metabolism of lymphocytes following stimulation with phytohemagglutinin and with lysolecithin

Following stimulation of rabbit lymphocytes with phytohemagglutinin (PHA) ¹⁴C]oleic acid incorporation was augmented into phospholipids but not into neutral fats. The main phospholipid into which oleate was incoporated was lecithin. Stimulation of lymphocytes with lysolecithin in short-time experiments also increased the incorporation of oleate into lecithin. The distribution of the labeled fatty acids in the newly formed lecithin molecules was determined using snake venom phospholipase A. Of the incorporated oleate 80 % was found in position 1 of the glycerol moiety both in unstimulated and stimulated lymphocytes. In addition to natural 1-acyl]-lysolecithin a sythetic non-metabolized lysolecithin analogue also effected lymphocyte stimulation, suggesting that exogenous lysolecithin acts not as a substrate for lysolecithin-acyltransferase. Microsomal membranes of normal and stimulated lymphocytes were examined for enzyme ectivities involved in the incorporation of long-chain fatty acids into phospholipids especially (a) fatty acid: CoA-ligase, (b) lysolecithin-acyltransferase, (c) phospholipase A and (d) lysophospholipase. After 3 h of cultivation with PHA, fatty acid: CoA-ligase (which is rate-limiting for the incorporation of oleate into phospholipids) and phospholipase A were activated. Lysolecithin produced only an activation of phospholipase A supporting the idea that oleate is incorporated by reacylation of endogenously formed 2-acyl]-lysolecithin and consistent with our findings that de novo synthesis of phospholipids, as measured by ¹⁴C]choline uptake, is low during the early phase of stimulation.