Inactivation of α- and β-thrombin by antithrombin-III, α2-macroglobulin and α1-proteinase inhibitor

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Abstract
Inactivation of .alpha.- and .beta.-thrombin [bovine] 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 2 forms of thrombin were different. Heparin facilitated complex-formation between .alpha.-thrombin and antithrombin-III, but inactivation of .beta.-thrombin by antithrombin was only slightly influenced even at a heparin concentration 2 orders of magnitude higher. .alpha.2-Macroglobulin inhibited both .alpha.- and .beta.-thrombin activity similarly, the amount of .alpha.- and .beta.-thrombin inactivated and 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. The inactivation of .beta.-thrombin, if it occurs in the plasma, is apparently also controlled by plasma inhibitors.

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