Properties of phosphatidylinositol kinase of human platelets

We have previously reported that phosphatidylinositol (PI) kinase of intact platelet may be activated by either elevating intracellular cAMP content or lowering cytosolic Ca2+ (Thrombos. Res. 44, 155, 1986). Further studies were conducted to elucidate properties of platelet PI-kinase, especially possible regulation by A-kinase or Ca2+. The activity of the enzyme in platelet homogenate was markedly inhibited by a very low Ca2+, while Mg2+ was absolutely required for the activity. The activity was not affected by the presence of A-kinase catalytic subunit or protein kinase inhibitor but it was inhibited by cAMP as well as other compounds containing adenosine. These results suggest that the platelet PI-kinase is not regulated directly by A-kinase but by Ca2+ and that the regulation by Ca2+ may act as a negative feedback system in activated platelets.