Phosphorylation of the 47 KDa protein in gamma-thrombin-stimulated human platelets does not activate phospholipase A2: evidence against lipocortin.

Intact human platelets were stimulated with alpha or gamma thrombin in the presence and absence of epinephrine and the ability of these agonists to stimulate aggregation, arachidonic acid release and protein phosphorylation was measured. Epinephrine alone had no effect on any of these events. Both alpha and gamma thrombin induced platelet aggregation which was potentiated in each case by epinephrine. Similarly, both thrombin species were able to induce the phosphorylation of platelet 20 KDa and 47KDa proteins. The gamma thrombin-induced phosphorylation was slightly enhanced by epinephrine. In contrast, only alpha thrombin was capable of inducing significant arachidonic acid release and the small release induced by gamma thrombin was reduced by epinephrine. These results show that the agonist-induced phosphorylation of the 47KDa protein by protein kinase C does not impart the ability to activate phospholipase A2 in human platelets, and questions the suggestion that the 47KDa protein is lipocortin.