Thrombin binds to a high-affinity approximately 900 000-dalton site on human platelets.

The functional sizes of the binding sites for thrombin on human platelets and isolated membranes have been determined by the technique of radiation inactivation: similar results were obtained. Independent studies using different radiation doses (0, 3, and 48 Mrad) and different thrombin concentrations (10(-10), 10(-8), and 10(-6) M) confirmed the presence of three binding sites with functional sizes of 900 000, 30 000, and 4000 daltons. The binding site of lowest apparent size (4000 daltons) probably corresponds to what has been termed nonspecific binding since its dissociation constant (2900 nM) is well outside the physiological range. The site of intermediate size (30 000 daltons) is also probably not involved in platelet activation since its dissociation constant (11 nM) is also beyond the concentration range required for activation, although it may be involved in other aspects of platelet-thrombin interaction. The sites with the largest functional size are probably important in platelet function since their dissociation constant (0.3 nM) is in the range required for platelet activation. The functional size of these sites (900 000 daltons) suggests that the high-affinity site for thrombin binding to platelets may involve a multimolecular complex of membrane components.