The Effects of Human Thrombomodulin on the Inactivation of Thrombin by Its Serum Inhibitors

Summary

Thrombomodulin is an endothelial cell protein which accelerates thrombin-dependent protein C activation by over 1000 fold. In this study, the effect of thrombomodulin on the inactivation of thrombin by its serum inhibitors was evaluated.

¹²⁵I-thrombin was incubated at 37° C with serum and the resulting complexes separated by SDS-PAGE. Antithrombin III was the major complex formed with some ¹²⁵I-thrombin bound to heparin cofactor II and highermolecular weight fractions. Inclusion of thrombomodulin at increasing concentrations inhibited ¹²⁵I-thrombin binding to antithrombin III and the higher molecular weight fractions but had little effect on thrombin-heparin cofactorII complex formation. Similar results were obtained using a purified antithrombin III/heparin cofactor II system.

Kinetic studies, using purified antithrombin III, revealed that thrombomodulin acts as a weak competitive inhibitor towards antithrombin III (K_i = 39 nM).

We postulate that in the microcirculation, where the ratio of thrombomodulin to antithrombin III is relatively high, thrombin bound to thrombomodulin may be protected from inactivation by antithrombin III and can thus promote efficient activation of protein C.

Keywords

Thrombomodulin - Thrombin - Inhibitors

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