Characterization of the binding of FK633 to the platelet fibrinogen receptor

I-125-fibrinogen bound to ADP-activated fixed platelets in a saturable manner. The Scatchard plot was curvilinear but nonlinear model fitting of the data suggested that there was only one binding site with K-A=4.19+/-1.3x10(6) M-l and a maximum number of binding sites of 3.9+/-1.1x10(4) molecules/platelet. The GPIIb/IIIa antagonists RGDS and FK633 both inhibited I-125-fibrinogen binding (50 mu g/ml) in a dose-dependent manner. FK633 had a K-B value of 2.5+/- 0.48x10(7) M-1 (K-b=39.9 nM) and an IC50 value of 64 nM, while RGDS had an K-B Of 2.55+/-0.76x10(4) M-1 (K-b=39.2 mu M) With an IC50 value of 63 mu M. At concentrations below its IC50 value FK633 was a competitive antagonist of fibrinogen binding. However, at concentrations above its IC50 value it was a noncompetitive antagonist. The IC50 values of FK633 remained constant over a wide range of fibrinogen concentrations while its K-B value changed-increasing from 1.8+/-0.6x10(7) M-1 at 10 mu g/ml I-125-fibrinogen to 4.5+/-1.6x10(7) M-1 at 300 mu g/ml and decreasing to 0.3+/-0.2x10(7) M-1 at 2.4 mg/ml. Thus, FK633 is a reversible, noncompetitive antagonist of fibrinogen binding to the platelet GPIIb/IIIa receptor. (C) 1998 Elsevier Science Ltd.