Monoclonal Antibodies to Human Thrombomodulin Whose Binding Is Calcium Dependent

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Abstract
Four monoclonal antibodies to human thrombomodulin were characterized. Binding of two of these antibodies was dependent on the presence of calcium ions, and approximately 5 mM calcium was required for their maximum binding. These two antibodies inhibited the binding of thrombin to thrombomodulin, thereby inhibiting activation of protein C catalyzed by thrombin-thrombomodulin complex. These two antibodies bind to a major active fragment formed by limited proteolytic digestions of thrombomodulin with elastase and trypsin, suggesting that the antibodies bind to the thrombin-binding site (or its vicinity) located in the epidermal growth factor (EGF)-homology domain. One of the other calcium-independent antibodies also inhibited the binding of thrombin and the activation of protein C, but the inhibition was very weak and was observed only when the antibody was present in a molar excess over thrombomodulin. This antibody did not bind to the protease digests of thrombomodulin. Another calcium-independent antibody did not inhibit either thrombin binding or protein C activation, but bound to the active fragment of protease digests, suggesting that the antibody binds to a region other than the thrombin-binding site in the EGF-homology domain. These observations suggest that thrombomodulin undergoes a calcium-dependent conformational change which may occur in proximity to a thrombin-binding site located in the EGF-homology domain.