Human Platelets Possess an Inducible and Saturable Receptor Specific for Fibrinogen

Fibrinogen is required for ADP-dependent aggregation of human platelets; but the molecular basis for the requirement is not: known. Fibrinogen, rendered free of fibronectin, plasminogen, VIII, II and XIII, was radiolabelled (¹²⁵I-fg) and used as the probe for a platelet receptor. Platelets were isolated by differential centrifugation, washed in calcium-free Tyrode’s solution containing 0.2% albumin and isolated by exclusion from Sepharose CL 2B. ¹²⁵I-fg bound to platelets only after ADP stimulation, and binding was characteristic of a saturable receptor. Specificity was established by demonstration that fibrinogen completely inhibited binding of ¹²⁵I-fg, competing with the same affinity, whereas other proteins were without effect. SDS polyacrylamide gel electrophoresis of the platelets after reduction demonstrated ¹²⁵I polypeptide chains with migration identical to Aα, Bβ and γ chains; and fibrinopeptide A was on the bound ¹²⁵I-fg indicating that fibrinogen per se is bound. Scatchard analysis of binding at optimal concentration of ADP (≥ 5 μM) indicated 4,644 ± 186 receptors with a dissociation constant Kd = 1.3 x 10^-7 M, comparable to that estimated directly from the aggregation reactions. Receptors induced at different ADP concentrations appeared to have the same affinity for fibrinogen, indicating the probable existence of only a single class of receptors. We conclude that a specific saturable receptor is rapidly induced on human platelets by ADP, and postulate that this event may play a Significant role in the observed in vitro aggregation phenomenon and perhaps the primary hemostatic function of platelets. Supportedby NIH grant HL-16411.