Antithrombin III and heparin inactivation in thrombin involving reactions

The inhibitory capacity of antithrombin III (AT III) was measured by a quantitative method independent of the velocity of inhibition. When AT III was in excess of thrombin in plasma or in purified system the capacity of inhibitor decreased quantitatively in proportion to the amount of thrombin neutralized. Heparin present in reaction together with thrombin invariably induced a more extensive utilization of inhibitor than thrombin alone. The extent of this additional loss of inhibitory capacity was to a limited degree related to the concentration of heparin. Heparin itself was neutralized in thrombin-AT III reaction losing its anticoagulant property in proportion to the amount of thrombin bound by inhibitor. This quantitative neutralization of heparin occurred not only when the anticoagulant participated in thrombin-AT III binding but also when heparin was added to a medium containing a preformed thrombin-AT III complex. These results suggest that acceleration of binding and increased utilization of binding capacity are the two regular effects of heparin on thrombin-involving reactions of AT III. Both of these effects may be abolished by quantitative binding of heparin to thrombin-AT III complex.