Summary
Platelet aggregation induced by three thrombin-like enzymes of snake venoms was compared
with that by thrombin. Acutin was isolated from Agkistrodon acutus venom and thrombocytin and batroxobin were from Bothrops atrox venom. The fibrinogenclotting activities were 700,170 and 7 U/mg for batroxobin,
acutin and thrombocytin, respectively. They induced aggregation and ATP release of
washed rabbit platelets. The aggregating activity of thrombin was 102, 104 and 105 times more potent than those of thrombocytin, acutin and batroxobin, respectively.
Plateletactivating potency of the thrombin-like enzymes was correlated with their
effectiveness on the retractility and elasticity of the clots. Platelet aggregation
induced by thrombin or thrombocytin could be inhibited by heparin with antithrombin
III while that by acutin or batroxobin could not. Indomethacin showed weak inhibition
on the aggregation while the ADP-scavenging system, creatine phosphate/creatine phosphokinase,
inhibited the aggregation induced by the three thrombin-like enzymes but not that
by thrombin. Platelet aggregation induced by the thrombin-like enzymes could not be
inhibited by PAF antagonists-BN 52021, kadsurenone or L-652,731. In the presence of
EGTA, only thrombin could induce ATP release from platelets. Thrombin-like enzymes
and low concentration of thrombin did not form thromboxane B2. Nitroprusside and prostaglandin E1 completely inhibited the aggregation, mepacrine and imipramine showed marked inhibition
while verapamil had only weak inhibition. It is concluded that the aggregation induced
by the thrombin-like enzymes is different from that of thrombin and mainly due to
ADP released from platelets.
Keywords
Platelet aggregation - Thrombin-like enzyme - Snake venom - ADP pathway
