Clearance of thrombin in vivo: significance of alternative pathways.

The clearance of thrombin seems to occur at more than one site and by different mechanisms. This contributes to maintaining thrombin at the right concentrations to act optimally on its various substrates, and thus, to produce the proper amount of proteolytic conversions so that coagulation is precisely controlled. The vascular endothelium plays a major role in thrombin regulation and clearance. It contains heparin-like binding sites and thrombomodulin which serve as cofactors for the thrombin-antithrombin III reaction and the activation of protein C, respectively. In addition, thrombomodulin also serves as a receptor for endothelial cell mediated thrombin endocytosis. Thrombin clearance, which occurs following reaction with antithrombin III or thrombomodulin, probably takes place at different stages in hemostasis.