The high molecular mass, glycoprotein Ib-binding protein flavocetin-A induces only small platelet aggregates in vitro

The direct effects of snake venom glycoprotein (GP) Ib-binding proteins on platelet receptors during the formation of platelet aggregates were determined by a particle counting method using light scattering. Flavocetin-A induces small platelet aggregates, but not medium or large ones. However, neither jararaca GPIb-BP nor tokaracetin induce platelet aggregation. The flavocetin-A dose-response curve for formation of small aggregates is bell-shaped, with maximal effect at 1 to 2 mu g/mL. The formation of small aggregates was not: observed when fixed human platelets were used. Jararaca GPIb-BP, the anti-CPIb monoclonal antibody GUR83-35, prostaglandin I-2, and ethylene diamine-1, N-dimethylformamide all inhibited flavocetin-A-induced small aggregate formation, but acetylsalicylic acid did not. Furthermore, anti-GPIIb/IIIa monoclonal antibodies, Abciximab, and YM337 significantly but partially inhibited aggregate formation, but the anti-von Willebrand factor monoclonal antibody NMC-4 had no effect. The formation of small aggregates required extra-cellular calcium? but flavocetin-A did not elevate cytosolic calcium. These results suggest that flavocetin-A binds to intact platelets, initiating platelet responses and inducing platelet aggregate formation by cross-linking platelets. Consequently, flavocetin-A may be a useful tool to study the mechanism of GPIb-mediated platelet activation and the structure-function relationships of GPIb. (C) 2000 Elsevier Science Ltd. All rights reserved.