ANTITHROMBOTIC EFFECT OF CROTALIN, A PLATELET MEMBRANE GLYCOPROTEIN IB ANTAGONIST FROM VENOM OF CROTALUS-ATROX

A potent platelet glycoprotein Ib (GPIb) antagonist, crotalin, with a molecular weight of 30 kD was purified from the snake venom of Crotalu s atrox. Crotalin specifically and dose dependently inhibited aggregat ion of human washed platelets induced by ristocetin with IC50 Of 2.4 m u g/ml. (83 nmol/L). It was also active in inhibiting ristocetin-induc ed platelet aggregation of platelet-rich plasma (IC50, 6.3 mu g/mL). I -125-crotalin bound to human platelets in a saturable and dose-depende nt manner with a kd value of 3.2+/-0.1 x 10(-7) mol/L, and its binding site was estimated to be 58,632+/-3,152 per platelet. Its binding was specifically inhibited by a monoclonal antibody, AP1 raised against p latelet GPlb. Crotalin significantly prolonged the latent period in tr iggering platelet aggregation caused by low concentration of thrombin (0.03 U/mL), and inhibited thromboxane B-2 formation of platelets stim ulated either by ristocetin plus von Willebrand factor (vWF), or by th rombin (0.03 U/mL). When crotalin was intravenously (IV) administered to mice at inn to 300 mu g/kg, a dose-dependent prolongation on tail b leeding time was observed. The duration of crotalin in prolonging tail bleeding time lasted for 4 hours as crotalin was given at 300 mu g/kg . In addition, its in vivo antithrombotic activity was evidenced by pr olonging the latent period in inducing platelet-rich thrombus formatio n by irradiating the mesenteric venules of the fluorescein sodium-trea ted mice. When administered IV at 100 to 300 mu g/kg, crotalin dose de pendently prolonged the time lapse in inducing platelet-rich thrombus formation. In conclusion, crotalin specifically inhibited vWF-induced platelet agglutination in the presence of ristocetin because crotalin selectively bound to platelet surface receptor-glycoprotein Ib, result ing in the blockade of the interaction of vWF with platelet membrane G Plb. In addition, crotalin is a potent antithrombotic agent because it pronouncedly blocked platelet plug formation in vivo, (C) 1998 by The American Society of Hematology.