ISOLATION AND CHARACTERIZATION OF JARARACA GPIB-BP, A SNAKE-VENOM ANTAGONIST SPECIFIC TO PLATELET GLYCOPROTEIN IB

A platelet glycoprotein Ib-binding protein (GPIb-BP) was isolated from the snake venom of Bothrops jararaca. Jararaca GPIb-BP showed a singl e band with M(r) of 30,000, and two distinct bands with M(r) of 17,000 /13,000 under non-reducing and reducing conditions, respectively, on S DS-polyacrylamide gel electrophoresis. Jararaca GPIb-BP itself induced neither platelet aggregation nor serotonin release from platelets, bu t specifically bound to GPIb (40,629 +/- 2,521 molecules per normal pl atelet, with Kd 39.1 +/- 2.4 nM at saturation). The purified venom pro tein completely inhibited ristocetin- or botrocetin-induced von Willeb rand factor (VWF) binding, and blocked the bovine VWF binding to GPIb, with IC50 values ranging from 28 to 42 nM, without affecting the plat elet aggregation induced by ADP or alpha-thrombin. I-125-jararaca GPIb -BP binding to GPIb was not altered by the presence of human alpha-thr ombin. Jararaca GPIb-BP at a final concentration of 104 nM totally abo lished vWF-dependent shear-induced platelet aggregation (SIPA) at a hi gh shear stress, but had no effect on SIPA at a low shear stress. Redu ced and S-carboxyamidomethylated jararaca GPIb-BP lost its inhibitory activity on SIPA. The NH2-terminal amino acid sequences of the subunit s revealed a high degree of homology with those of several Ca2+-depend ent lectins, especially to those of two functionally opposite venom pr oteins, botrocetin (a vWF-modulator) and alboaggregin-B (a GPIb-modula tor).