Urokinase binds to platelets through a specific saturable, low affinity mechanism

Specific receptors for urokinase have previously been identified on a number of cell types, including peripheral blood monocytes and endothelial cells. In the present study, the existence of specific receptors for single chain urokinase-type plasminogen activator (scu-PA) on gel-filtered human platelets was investigated. Platelets were found to bind ¹²⁵I-labelled recombinant scu-PA (¹²⁵I-rscu-PA) rapidly, specifically, saturably and reversibly, with a binding capacity of 92000±10000 molecules (mean±SEM) per platelet, and an apparent K_d of 130±20nM. Unlabelled rscu-PA and high molecular weight urokinase efficiently completed with binding, whereas low molecular weight urokinase (lacking the epidermal growth factor and kringle domains) did not. Other molecules known to bind to platelets, including fibrinogen, tissue-type plasminogen activator and plasminogen, had little effect on the binding of ¹²⁵I-rscu-PA. Binding was not significantly altered by the addition of 1.5 mM Ca⁺⁺ or by 5mM EDTA. Thus, platelets have a high density, specific mechanism that binds scu-PA and potentially localises fibrinolytic activity to their surface.