Relative degradation of different molecular species of phosphatidylcholine in thrombin-stimulated human platelets

The relative degradation of the various molecular species of [3H]phosphatidylcholine in response to thrombin was studied in human platelets following prelabeling with [3H]glycerol and compared to results obtained following labeling with [14C]oleic, [14C]linoleic, or [14C]arachidonic acids. This was of interest since previous work using radioactive fatty acids had led to the conclusion that the 1-acyl-2-arachidonoyl species of phosphatidylcholine is exclusively hydrolyzed in thrombin-stimulated platelets. Within 90 s, the thrombin-dependent release of [14C]arachidonic acid from phosphatidylcholine amounted to 25% but only 3 and 6% for oleic and linoleic acids, respectively, in general agreement with previous work. However, for [3H]glycerol-labeled phosphatidylcholine, all molecular species (saturates, monoenes, dienes, trienes, tetraenes, and greater than tetraenes) were subject to significant hydrolysis in the presence of thrombin within 90 s, ranging from 12-24% across the various classes. Furthermore, the degradation of the tetraenoic species (1-acyl-2-arachidonoyl) of [3H]phosphatidylcholine was found to be only 1.5 and 1.4 times that for the monoenoic (predominantly 1-acyl-2-oleoyl) and dienoic (predominantly 1-acyl-2-linoleoyl) species, respectively. A much heavier proportional labeling of plasma membrane relative to whole platelet phosphatidylcholine was observed with [3H]glycerol as compared to [14C] oleate or [14C]arachidonate. These results indicate that the 1-acyl-2-arachidonoyl species of phosphatidylcholine are not exclusively degraded by phospholipase A2 activity in thrombin-stimulated platelets and suggest that the differential compartmentation of molecular species of phosphatidylcholine according to their metabolic origins can influence their apparent susceptibility to hydrolysis.