Action of Snake Venom Phospholipase A on Isolated Platelet Membranes. ∗

Abstract

Platelets play a significant part in the coagulation process by contributing a phos-pholipid (or more likely a phospholipopro-tein) to the interaction of coagulation factors, which eventually leads to the formation of a prothrombin activator. This clot-promoting property of platelets is ordinarily present in latent form (1-3), but through an unknown mechanism the latency is lost as the coagulation process gains momentum. The physical form in which procoagulant material from platelets interacts with clotting factors is also unknown. Actual release from platelets has been suggested(4), and alternatively it has been proposed that the activity becomes available to the coagulation process as a catalytic lipoprotein surface on the plasma membrane of the platelet(5). These protein-lipo-protein interactions basically involve problems of cellular lipoprotein availability.

A further example of variations in the availability of cellular lipoproteins is revealed by the studies of Condrea and associates(6) and Kirschmann et al(7). These investigators have shown that, whereas the phospholipids of osmotically haemolyzed erythrocytes and intact platelets were hydrolzyed by N. naja phospholipase A, intact red cells were unaffected. Phospholipase A from Russell viper venom was also inactive against intact but active against haemolyzed red cells. Palestinian viper venom phospholipase A did not platelets, intact erythrocytes, or osmotically haemolyzed red cells, and was only active against red cells disrupted by sonication, The results obtained with N. naja and Russell viper venom suggested similarities between lipid availability for the coagulation process and susceptibility to venom action. Specifically, all the cell preparations which resisted the action of venom were also inactive in in vitro coagulation systems. On the other hand, venom-sensitive cells were specifically those which were active in coagulation.

It therefore seemed pertinent to investigate the basis for the difference in venom sensitivity of intact platelets and erythrocytes.