The Effects Of An Anticoagulant Phospholipase A2 On The Procoagulant Activity Of Phospholipids And Platelets
Although phospholipids readily substitute for platelets in many in vitro blood coagulation tests, their participation in normal platelet procoagulant activity is uncertain. Phospholipase A2 (PLA2) from Naja nigricollis nigricollis venom, a known anticoagulant, blocked the enhancement of the rate of prothrombin conversion to thrombin by both phospholipids and platelets. The rate of inhibition of the phospholipid procoagulant activity by PLA2 was reduced by indomethacin, an inhibitor of PLA2. At concentrations of PLA2 which inhibited phospholipid procoagulant activity, conversion of purified prothrombin by factors Xa and Va without phospholipid was unaffected.Unactivated, washed platelets combined with coagulation factors II, Va and Xa initially produce thrombin at a slow rate before the platelets become activated but after this lag period the rate of thrombin generation increases. PLA2 added at the same time as the coagulation factors increased the lag period and decreased the rate of thrombin generation during and after the lag. Preincubation of platelets with PLA2 further decreased only the lag rate and this inhibition was partially blocked by 300 uM indomethacin. At a concentration of about one ng/ml, PLA2 reduced the post-lag thrombin generation rate of 3 × 10-9 platelets/ml in half but had no effect on platelet aggregation induced by thrombin, ADP or collagen.These results combined with the known specificity of PLA2 support the theory that phospholipids are involved in platelet procoagulant activity. Furthermore, the ineffectiveness of preincubating PLA2 with platelets on the post-lag procoagulant activity suggests that the phospholipids involved in this post-activation process become accessible during the lag period.