Characterization of the binding of human tissue-type plasminogen activator to platelets

Cell surface binding sites for the constituent proteins of the fibrinolytic system may play a role in the localization and regulation of fibrinolysis. In the present study, specific binding of recombinant human tissue-type plasminogen activator (rt-PA) to human blood platelets was identified and characterized. ¹²⁵I-labeled rt-PA was found to bind specifically, saturably, and reversibly to the surface of gel-filtered platelets, reaching equilibrium within 5 min at 22°C. Scatchard analysis revealed a single class of binding sites. Unstimulated platelets bound 120,000 ± 24,000 (mean ± S.D.) molecules/platelet with an apparent K(d) of 340 ± 25 nM, whereas thrombin-stimulated platelets bound 290,000 ± 32,000, molecules/platelet with an apparent K(d) of 800 ± 60 mM. Binding of 0.1 μM ¹²⁵I-rt-PA was > 90% reversible by a 50-fold excess of unlabeled rt-PA. Binding was not inhibited by fibrinogen or single chain urokinase-type plasminogen activator, but plasminogen partially competed for binding of ¹²⁵I-rt-PA to platelets (up to 40% displacement). These findings indicate that the platelet surface possesses a large number of specific, low affinity binding sites for t-PA and provide further evidence for the role of platelets in localization and regulation of fibrinolysis.