Properties and distribution of phosphatidylinositol-specific phospholipase C in human and horse platelets

Phospholipase C has been studied in homogenates, total particulate and soluble fractions of horse and human platelets. This enzyme, assayed with exogenous L-3-phosphatidyl[14C]inositol, is predominantly localized in the soluble fraction and its distribution parallels that of lactate dehydrogenase. A small percentage of activity present in the particulate fraction seems to be due to contamination with soluble enzyme. Enzyme from horse and human platelets appears identical, having a Km of 0.10-0.15 mM, acid pH optimum (pH 5.5) and showing Ca2+-dependency and weak inhibition by deoxycholate. Analysis of the reaction products shows the formation of myo-inositol 1,2-cyclic phosphate and myo-inositol 1-phosphate in almost equal amounts. Platelet stimulation with thrombin does not seem to induce association of the cytosolic activity to the membranes. The cytosolic activity is not affected by pretreatment of the intact platelets with prostacyclin or thrombin. Degradation of phosphatidylinositol present in a membrane fraction isolated from platelets by cytosolic phospholipase C requires addition of deoxycholate. Our information suggests that the degradation of phosphatidylinositol in stimulated platelets is mainly achieved by exposure of the substrate to the cytosolic enzyme and by an increase of the free Ca2+ concentration needed for optimal phospholipase C activity.