Possible involvement of Ca2+-activated, phospholipid-dependent protein kinase in platelet activation.

Ca2+-activated, phospholipid-dependent protein kinase recently found in rat brain (Takai, Y., Kishimoto, A., Iwasa, Y., Kawahara, Y., Mori, T., & Nishizuka, Y. (1979) J. Biol. Chem. 254, 3692-3695) is present in large quantities in human platelets. The activation of this enzyme appears to be initiated by unsaturated diacylglycerol and intimately related to phosphatidylinositol hydrolysis which is induced by thrombin. The enzyme is selectively and profoundly inhibited by several phospholipid-interacting compounds such as imipramine, verapamil, and tetracaine, which concomitantly inhibit aggregation and release reaction in parallel manners. It is suggestive that this protein kinase may be involved in the transmembrane control of intracellular events eventually leading to platelet activation.