Sensitivity of thrombin and plasmin to heparin and antithrombin III.

Thrombin and plasmin are inactivated by antithrombin III at different rates. Heparin, at a catalytic amount, increases primarily the inactivation rate of thrombin. The two proteinases compete for heparin, i.e. heparin is preferably attached by thrombin. Furthermore, fibrinogen as well as fibrin protect plasmin against inactivation by antithrombin III. However, at high concentration of heparin, plasmin inactivation by antithrombin III is accelerated even in the presence of fibrinogen or fibrin.