Inactivation of alpha- and beta-thrombin by antithrombin-III, alpha 2-macroglobulin and alpha 1-proteinase inhibitor.

Inactivation of alpha- and beta-thrombin by alpha 2-macroglobulin, by alpha 1-proteinase inhibitor and by antithrombin-III and heparin was studied. The amount of alpha- and beta-thrombin inactivated by antithrombin-III was proportional to the concentration of the inhibitor, but the inactivation rates of the two forms of thrombin were different. Heparin facilitated complex-formation between alpha-thrombin and antithrombin-III, whereas inactivation of beta-thrombin by antithrombin was only slightly influenced, even at a heparin concentration two orders of magnitude higher. alpha 2-Macroglobulin inhibited both alpha- and beta-thrombin activity similarly, i.e. the amount of alpha- and beta-thrombin inactivated as well as the rates of their inhibition were the same. alpha 1-Proteinase inhibitor also formed a complex with alpha- and beta-thrombin, similarly to antithrombin-III, although the inactivation of the enzyme needed high inhibitor concentration and long incubation time. These results suggest that the inactivation of beta-thrombin, if it occurs in the plasma, is also controlled by plasma inhibitors.