Interaction of viper venom serine peptidases with thrombin receptors on human platelets

The serine peptidases, thrombocytin and PA-BJ, isolated from the venom of B othrops atrox and Bothrops jararaca, respectively, induce platelet aggregat ion and granule secretion without clotting fibrinogen, The specific platele t aggregation activity of each enzyme was about 15 times lower than that of thrombin, This activity was blocked by monoclonal antibodies recognizing p rotease activated receptor 1 (PAR1) and by heparin, but not by hirudin nor thrombomodulin, Both enzymes induced calcium mobilization in platelets and desensitized platelets to the action of thrombin and the SFLLRN peptide. We compared the effect of thrombin, PA-BJ, and thrombocytin on the degradatio n of the soluble N-terminal domain of the PAR1 receptor. The major cleavage site by thrombin and both viper enzymes was Arg41-Ser42. In addition, a ra pid cleavage of the peptide bond at Arg46-Asn47 by the viper enzymes was ob served, resulting in the inactivation of the tethered ligand, PA-BJ and thr ombocytin both cleaved at 41-42 and 46-47 peptide bonds, and fragment 42-10 3 disappeared rapidly. Both viper enzymes caused calcium mobilization in fi broblasts transfected with PAR4 and desensitized these cells to the thrombi n action. In conclusion, both PAR1 and PAR4 mediate the effect of viper ven om serine peptidases on platelets. (C) 2000 Federation of European Biochemi cal Societies. Published by Elsevier Science B.V. All rights reserved.