The Anticoagulant Effect of the Human Secretory Phospholipase A2 on Blood Plasma and on a Cell‐Free System is Due to a Phospholipid‐Independent Mechanism of Action Involving the Inhibition of Factor Va

Abstract

Blood platelets play a central role in haemostasis by leading to plug formation and by increasing the efficiency of blood coagulation. We have previously shown that blood platelets contain a group II secretory phospholipase A₂ (sPLA₂ grII) which is released into the extracellular medium upon activation but is unable to stimulate blood platelets. We presently reported an investigation of the putative involvement of the human sPLA₂ grII (hsPLA₂ grII) in the coagulation process, both in the absence and in the presence of activated platelets. We show that this enzyme prolongs the recalcification time of blood plasma even in the presence of activated platelets. The positive action of blood platelets on coagulation is correlated, at least in part, with the appearence at the cellular surface of procoagulant phospholipids which constitute a potential target for hsPLA₂ grII. We therefore investigated the involvement of its enzymatic activity in the anticoagulant effect of this enzyme. We observed that the replacement of CaCl₂ by SrCl₂ to initiate the coagulation cascade did not suppress, but rather increased, the inhibitory action of hsPLA₂ grII. Moreover, hsPLA₂ grII hydrolyzed only a minor proportion of platelet phospholipids, and it did not affect plasma phospholipids. Taken together, these observations strongly suggest that the major action of hsPLA₂ grII on blood coagulation does not involve the hydrolysis of phospholipids, in contrast with the strong anticoagulant effect of the group II venom phospholipase A₂ from Crotalus durrissus terrificus. We next studied which step of the coagulation cascade was affected by hsPLA₂ grII. Using purified coagulation factors, we demonstrated that hsPLA₂ grII strongly inhibited the prothrombinase activity. This inhibitory effect was independent of the presence of phospholipids but required factor Va, leading to the hypothesis that hsPLA₂ grII inhibited this factor. Further, the anticoagulant effect of hsPLA₂ grII was observed on normal and factor‐X‐deficient plasma, but not on factor‐V‐deficient plasma. In conclusion, the anticoagulant action of hsPLA₂ grII is based on a nonenzymatic mechanism of action involving the inhibition of factor Va.