Binding of thrombin to cultured human endothelial cells. Nonequilibrium aspects.

The long-term incubation of 125I-thrombin with cultured human endothelial cells is studied and compared to the previously described rapid equilibrium binding in this system (Awbrey, B. J., Hoak, J. C., and Owen, W. G. (1979) J. Biol. Chem. 254, 4092-4095). Following rapid initial binding of thrombin, a much slower association of thrombin with endothelium is seen. As opposed to equilibrium binding, this process is 1) irreversible, 2) active-site specific, and 3) associated with covalent linkage to an Mr 30,000 protein. In addition this nonequilibrium binding appears unrelated to high-affinity equilibrium binding since blocking the high-affinity binding sites with diisopropylphosphoryl thrombin has no effect on nonequilibrium binding. The physiological role of this process is unknown.