Role of enzymatic activity in the antiplatelet effects of a phospholipase A2 from Ophiophagus hannah snake venom.

A phospholipase A2 (OHV A-PLA2) from the venom of Ophiophagus hannah (King cobra) is an acidic protein exhibiting antiplatelet activity. In in vitro tests, OHV APLA2 showed a marked inhibitory effect on platelet aggregation induced by ADP, collagen and arachidonic acid in both human whole blood and platelet-rich plasma in a dose-dependent manner. The antiplatelet effects of OHV A-PLA2 did not increase when preincubation times of platelets and OHV A-PLA2 were prolonged indicating phospholipid hydrolysis did not significantly contribute to the antiplatelet effects. Alkylation of active site His residue using p-bromophenacyl bromide resulted in the complete loss of enzymatic activity, but the modified enzyme retained more than 30% of its antiplatelet effects. These results indicate that the antiplatelet effects of OHV A-PLA2 appear to be independent of its enzymatic activity, and there are separate sites responsible for the catalytic and antiplatelet activities.