Exposure of Platelet Fibrinogen-Binding Sites by Collagen, Arachidonic Acid, and ADP: Inhibition by a Monoclonal Antibody to the Glycoprotein lib-Illa Complex

Following stimulation with adenosine diphosphate (ADP). collagen. or arachidonic acid. unstirred human platelet suspensions bind ‘25l-fibrinogen in a reaction that reaches completion within 30 mm. Scatchard analysis of these binding data reveals two sets of binding sites with all 3 agents: a high affinity site (Kd 0.029-0.045 MM) binding 1 000-i 600 fibrinogen molecules per platelet. and a lower affinity site (Kd 1 .2-2.0 MM) binding 46.000-76.000 fibrinogen molecules per platelet. At a concentration of apyrase that inhibited ADP-induced fibrinogen binding by >85%. fibrinogen binding induced by collagen and arachidonic acid was only partially affected. This suggests that fibrinogen binding induced by collagen or arachidonic acid does not require released ADP. We isolated a monoclonal antibody. B59.2, which precipitated the glycoprotein lIb-Illa complex from solubilized platelet membranes. Binding of labeled antibody to platelets before or after exposure to ADP. collagen. or arachidonic acid showed a single class of