Asymmetry of arachidonic acid metabolism in the phospholipids of the human platelet membrane as studied with purified phospholipases.

Human platelets were incubated with high density lipoproteins (HDL) doubly labelled with either free [14C]arachidonate/[3H]arachidonoylphosphatidylcholine or free [14C]oleate/[3H]oleoylphosphatidylcholine. Whereas [14C]arachidonate was incorporated at a 10-15-times higher rate than [14C]oleic acid, the exchange of both species of phosphatidylcholine occurred to the same extent. In both cases, free 3H-labelled fatty acids were generated during the labelling procedure, indicating phospholipase A2 hydrolysis. A redistribution of radioactivity to other phospholipids was noted after exchange of [3H]arachidonoylphosphatidylcholine only. (2) The exchange of phosphatidylcholine to platelets was confirmed using [14C]choline-labelled dipalmitoyl-and 1-palmitoyl-2-arachidonoylphosphatidylcholines. (3) Non-lytic degradation of platelet phospholipids by phospholipases revealed that free fatty acids were incorporated at the inside of the cells, whereas exchange was taking place on the platelet outer surface. However, 2-arachidonoylphosphatidylcholine displayed a more rapid movement towards the cell inside. The above findings suggest a topological asymmetry for the two pathways (acylation and exchange) of fatty acid renewal in platelets. The possible mechanisms and physiological relevance of the translocation of the external arachidonic acid pool across the membrane are discussed.