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Blood coagulation factor V is a single-chain glycoprotein with Mr = 330,000 which plays an important role in the procoagulant and anticoagulant pathways. Thrombin activates factor V into factor Va, a two-chain molecule which is composed of a heavy (Mr = 105,000) and a light chain (Mr = 71,000/74,000). Factor Va accelerates factor Xa-catalysed prothrombin activation more than 1,000-fold and under physiological conditions the cofactor activity of factor Va in prothrombin activation is down-regulated by activated protein C. Factor V can also be activated by a wide variety of snake venoms (e.g. from Vipera species, Naja naja oxiana, Bothrops atrox) and by proteases present in the bristles of a South American caterpillar (Lonomia achelous). Some venoms, notably of Vipera lebetina turanica and Lonomia achelous, contain proteases that are able to inactivate factor V or factor Va. Venom factor V activators are excellent tools in studying the structure-function relationship of factor V(a) and they are also used in diagnostic tests for quantification of plasma factor V levels and for the screening of defects in the protein C pathway. In this review, the structural and functional properties of animal venom factor V activators and inactivators is described.

Protein C (PC) is an anticoagulant protein which, being activated by thrombin, degrades factors V/Va and VIII/VIIIa and releases a tissue-type plasminogen activator. Some Agkistrodon snake venoms contain PC activators which, in experiments, exert an anticoagulant action. An antithrombotic effect of the PC activator from the venom of A. blomhoffi ussuriensis on the model of thrombus formation in the arterio-venous shunt in rats was under investigation. Administration of the PC activator resulted in a dose-dependent prolongation of the thrombus formation time and a decrease in plasma PC activity, which were accompanied by a decrease in factor V activity and APTT prolongation. No reliable changes in the t-PA level, ADP- and epinephrine-induced platelet aggregation were observed. Platelet adhesion to glass beads diminished. We assume that the antithrombotic effect of the PC activator from the A. blomhoffi venom in the platelet-dependent thrombosis model is caused by PC activation and subsequent factor V inactivation as well as by platelet adhesiveness reduction.