The phospholipase A2 from human platelets

Studies on a purified phospholipase A2 (PLA2) from human platelets show that the enzyme, which is copurified with the plasma membrane fraction, has a MW of ∼50 K Dalton, requires Ca++, and has a pH optimum of 9.4. Under optimal conditons, PLA2 activity corresponds to at least 13 nmol/min/109 platelets. Unsaturated PL are preferred substrates and the enzyme is considerably more active on the aggregated form of the substrate than on the monomers. The specific activity is markedly affected by the quality of the interface, showing variations of more than 10-fold between different substrate forms. In the absence of detergents, a 4-fold increase in rate is observed when both products are present. Maximal rates are obtained at 20 mole percent of products to substrate. 1,2-Diglyceride and phosphatidic acid stimulate the hydrolysis of PC by the purified enzyme, however, in these forms of the substrate, neither of them are hydrolyzed. Activation of this enzyme by some intermediate of the phospholipase C pathway might play a role in the stimulus-linked release of platelet arachidonic acid.