ROLE OF ENZYMATIC-ACTIVITY IN THE ANTIPLATELET EFFECTS OF A PHOSPHOLIPASE A(2) FROM OPHIOPHAGUS HANNAH SNAKE-VENOM

A phospholipase A(2) (OHV A-PLA(2)) from the venom of Ophiophagus hann ah (King cobra) is an acidic protein exhibiting antiplatelet activity. In in vitro tests, OHV A-PLA(2) 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 ma nner. The antiplatelet effects of OHV A-PLA(2) did not increase when p reincubation times of platelets and OHV A-PLA(2) were prolonged indica ting phospholipid hydrolysis did not significantly contribute to the a ntiplatelet effects. Alkylation of active site His residue using p-bro mophenacyl bromide resulted in the complete loss of enzymatic activity , but the modified enzyme retained more than 30% of its antiplatelet e ffects. These results indicate that the antiplatelet effects of OHV A- PLA(2) appear to be independent of its enzymatic activity, and there a re separate sites responsible for the catalytic and antiplatelet activ ities.