Human Platelets Contain Phospholipase C That Hydrolyzes Polyphosphoinositides

Stimulated human platelets are known to undergo marked and rapid changes in phosphoinositide metabolism consistent with the activation of phospholipase C. Such changes may promote a Ca2+flux after platelets are exposed to agonists. I have examined this enzymatic activity by using disrupted platelets. When human platelets are sonicated and then incubated with phosphatidylinositol 4,5-bisphosphate (PtdIns4, 5P2) or phosphatidylinositol 4-monophosphate (PtdIns4P) in the presence of Ca2+and deoxycholate, marked hydrolysis of these substrates occurs. Characterization of the hydrolysis products by anion exchange and thin-layer chromatography indicates that the bulk of this activity is enzymatic and attributable to phospholipase C. In the absence of Ca2+or deoxycholate, only phosphomonoesterase activity is observed. I partially purified the soluble phospholipase C on DEAE-cellulose in order to minimize phosphomonoesterase activity. Fractions eluting at low salt concentrations contain the highest phospholipase C activity with respect to PtdIns4, 5P2and PtdIns4P and the lowest phosphomonoesterase activity. The enzyme(s) in these fractions is (are) maximally active in the presence of 0.1 mM Ca2+and deoxycholate (1 mg/ml) and display(s) substrate affinities in the order PtdIns > PtdIns4P > PtdIns4, 5P2and maximum rates in the order PtdIns4P > PtdIns4, 5P2> PtdIns. This order of substrate preference appears to differ from that observed for physiologically stimulated cells. Possible reasons for such a discrepancy are discussed.