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 microg/mL. The formation of small aggregates was not observed when fixed human platelets were used. Jararaca GPIb-BP, the anti-GPIb monoclonal antibody GUR83-35, prostaglandin I2, and ethylene diamine-N,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 extracellular 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.
Taniuchi
Y
Y
Institute for Drug Discovery Research, Tsukuba Research Center, Yamanouchi Pharmaceutical Co., Ltd., Ibaraki, Japan. taniuchi@yamanouchi.co.jp
Kawasaki
T
T
Fujimura
Y
Y
D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism / Q000494:pharmacology* / Q000502:physiology; D003435:Crotalid Venoms / Q000378:metabolism / Q000494:pharmacology*; D004546:Elapid Venoms / Q000494:pharmacology; D006801:Humans; D010974:Platelet Aggregation / Q000187:drug effects*; D010975:Platelet Aggregation Inhibitors / Q000494:pharmacology; D019038:Platelet Glycoprotein GPIb-IX Complex / Q000378:metabolism; D030162:Reptilian Proteins; D012542:Scattering, Radiation
D002352:Carrier Proteins; D003435:Crotalid Venoms; D004546:Elapid Venoms; D010975:Platelet Aggregation Inhibitors; D019038:Platelet Glycoprotein GPIb-IX Complex; D030162:Reptilian Proteins; C402798:flavocetin A; C099427:GPIb-BP, Jararaca; D002118:Calcium