Effect of snake venom procoagulants on snake plasma: implications for the coagulation cascade of snakes.

Several snake venoms contain proteinases that activate zymogens in the coagulation cascade and thus exhibit their procoagulant effects. While most procoagulant proteinases from snake venoms are dissimilar to coagulation factors, Group D (trocarin, notecarin) and C (pseutarin) prothrombin activators are structural and functional homologues of factor Xa and the prothrombinase complex, respectively. We examined the effect of these and other procoagulants from snake venoms as well as mammalian and snake thromboplastins on the coagulation of plasmas of Notechis scutatus, Pseudonaja textilis (both procoagulant venoms), Python reticulatus (non-venomous) and Crotalus atrox (non-procoagulant venom) snakes. The results indicate that the intrinsic pathway seems to be weak or absent only in venomous snakes, while the extrinsic pathway is fully functional in all snakes. Python and Crotalus plasmas have extrinsic pathways similar to that in mammals. In contrast, although Notechis and Pseudonaja plasmas were clotted by a Group C activator, they failed to clot upon the addition of factor Xa and Group D activators. The mechanism of this resistance is still elusive.