INOSITOL POLYPHOSPHATE 4-PHOSPHATASE IS INACTIVATED BY CALPAIN-MEDIATED PROTEOLYSIS IN STIMULATED HUMAN PLATELETS

Inositol polyphosphate 4-phosphatase (4-phosphatase), an enzyme that c atalyzes the hydrolysis of the 4 position phosphate of phosphatidylino sitol 3,4-bisphosphate, was shown to be a substrate for the calcium-de pendent protease calpain in vitro and in stimulated human platelets. S timulation of platelets with the calcium ionophore, A23187, resulted i n complete proteolysis of 4-phosphatase and a 75% reduction in enzyme activity. Thrombin stimulation of platelets resulted in partial proteo lysis of 4-phosphatase and a 41% reduction in enzyme activity (n = 8, range of 36-51%). In addition, preincubation with the calpain inhibito r, calpeptin, suppressed the accumulation of phosphatidylinositol 3,4 bisphosphate in thrombin stimulated platelets by 36% (n = 2, range = 3 5-37%). These data suggest that the calpain-mediated inhibition of 4-p hosphatase is involved in the phosphatidylinositol 3,4-bisphosphate ac cumulation in thrombin stimulated platelets.