This is the first member of the serine protease family that has strong anticoagulant activity and lacks enzymatic activity. It inhibits activities of three blood coagulation complexes: (1) prothrombinase complex (composed of blood coagulation factors Va and Xa (F5 and F10)) (IC(50)=164.1 nM), (2) intrinsic tenase complex (composed of factors VIIIa and IXa (F8 and F9)), and (3) extrinsic tenase complex (composed of tissue factor and factor VIIa (F7)). The toxin has also been observed to bind prothrombin, factor FVa, non-activated and activated forms of factors FVII (F7) (FVII and FVIIa), factor FVIIIa (F8), factors FIX and FIXa (F9) and factors FX and FXa (F10). The toxin inhibits the activity of the intrinsic tenase complex mainly by competing with FIXa (F9) for binding to FVIIIa (F8)