Targeting of Venom Phospholipases: The Strongly Anticoagulant Phospholipase A2 from Naja nigricollis Venom Binds to Coagulation Factor Xa to Inhibit the Prothrombinase Complex

The strongly anticoagulant basic phospholipase A2 (CM-IV) from Naja nigricollis venom has previously been shown to inhibit the prothrombinase complex of the coagulation cascade by a novel nonenzymatic mechanism (S. Stefansson, R. M. Kini, and H. J. Evans Biochemistry 29, 7742–7746, 1990). That work indicated that CM-IV is a noncompetitive inhibitor and thus it interacts with either factor Va or factor Xa, or both. We further examined the interaction of CM-IV and the protein components of the prothrombinase complex. Isothermal calorimetry studies indicate that CM-IV does not bind to prothrombin or factor Va, but only to factor Xa. CM-IV has no effect on the cleavage of prothrombin by factor Xa in the absence of factor Va. However, in the presence of factor Va, CM-IV inhibits thrombin formation by factor Xa. With a constant amount of CM-IV, raising the concentration of factor Va relieved the inhibition. The phospholipase A2 enzyme inhibits by competing with factor Va for binding to factor Xa and thus prevents formation of the normal Xa–Va complex or replaces bound factor Va from the complex. Thus factor Xa is the target protein of this anticoagulant phospholipase A2, which exerts its anticoagulant effect by protein–protein rather than protein–phospholipid interactions.