Antithrombin III binds to human platelets

The receptor site for antithrombin III (AT III) was investigated in normal human platelets. ¹²⁵I] iodinated AT III was utilized as tracer for the binding assay. Equilibrium of AT III binding was reached within 2 min. The binding capacity was pH-dependent with the optimum around pH 7.0. Binding specificity was demonstrated by inhibition of ¹²⁵I] AT III ligation using an excess amount of non-labeled AT III. The AT III·heparin complex did not supress ¹²⁵I] AT III binding. Analysis of binding data by Scatchard plot revealed a single class of binding sites with K_d of 3.2 × 10^?7 M and binding capacity of 3840 per platelet.