The inhibition of clotting complexes of the extrinsic coagulation cascade by the phospholipase A2 isoenzymes from Naja nigricollis venom.

Phospholipase A2 (PLA2) isoenzymes from Naja nigricollis venom exhibit anticoagulant activity with varying potencies. To determine which complexes in the extrinsic coagulation cascade are inhibited by these PLA2 enzymes, we examined their effects on the coagulation of bovine plasma initiated by the addition of thromboplastin, Russell's viper venom (RVV) or thrombin. The weakly anticoagulant PLA2 enzymes, CM-I and CM-II, prolonged clotting initiated by thromboplastin, but not that initiated by RVV or thrombin. The strongly anticoagulant enzyme, CM-IV, prolonged clotting initiated by both thromboplastin and RVV, but not clotting initiated by thrombin. To confirm the differences in their inhibitory properties, we examined the effect of these PLA2 enzymes on reconstituted extrinsic tenase and prothrombinase complexes. The weakly anticoagulant enzymes inhibited the tenase complex, but did not inhibit the prothrombinase complex, whereas the strongly anticoagulant enzyme inhibited both complexes. Thus the enzymes showed distinct differences in their inhibition patterns in the extrinsic coagulation cascade. Their dissimilarity in inhibition of the two phospholipid dependent activation steps probably reflects the difference in phospholipid requirements and/or mechanism of inhibition between the two complexes. Inhibition of successive amplification steps in the extrinsic coagulation cascade by CM-IV is consistent with its potency as a strongly anticoagulant PLA2.