Targeting of venom phospholipases: The strongly anticoagulant phospholipase A(2) from Naja nigricollis venom binds to coagulation factor Xa to inhibit the prothrombinase complex

The strongly anticoagulant basic phospholipase A(2) (CM-IV) from Naja nigri collis venom has previously been shown to inhibit the prothrombinase comple x of the coagulation cascade by a novel nonenzymatic mechanism (S. Stefanss on, R.M. Kini, and H.J.Evans Biochemistry 29, 7742-7746, 1990). That work i ndicated that CM-IV is a noncompetitive inhibitor and thus it interacts wit h either factor Va or factor Xa, or both. We further examined the interacti on of CM-IV and the protein components of the prothrombinase complex. Isoth ermal 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 pres ence of factor Va, CM-IV inhibits thrombin formation by factor Xa. With a c onstant amount of CM-IV, raising the concentration of factor Va relieved th e inhibition. The phospholipase A(2) enzyme inhibits by competing with fact or 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 A(2), which exerts its anticoagulant effect by protein-protein rather than protein-phospholipi d interactions. (C) 1999 Academic Press.