Marked increase of human platelet phospholipase A2 activity in vitro and demonstration of an endogenous inhibitor.

When soluble and particulate fractions of human platelet homogenate were chromatographed on DEAE-cellulose and hydroxylapatite columns, total phospholipase A2 (PLA2; phosphatide 2-acylhydrolase, EC 3.1.1.4) activity increased sharply. PLA2 activity detected in these partially purified preparations was approximately 12 times greater than that associated with the original homogenate. Chromatography of the particulate enzyme on a DEAE-cellulose column yielded one activity peak. Fractions eluted near the activity peak showed a trace of PLA2 activity but inhibited purified PLA2 stoichiometrically, suggesting the presence of an endogenous "inhibitor" in the homogenate. The inhibitor activity was heat-stable, trypsin insensitive, and extractable by chloroform/methanol, and thus appears to be associated with a lipid(s). PLA2 activity was Ca2+ dependent. The crude enzyme was variably stimulated by calmodulin, whereas the purified enzyme was not, suggesting that the effect of calmodulin on the crude enzyme is indirect. Our results suggest that human platelet PLA2 activity reported in the literature may have been underestimated, apparently due to the presence of an endogenous inhibitor.