[alpha]-Thrombin within fibrin clots: Inactivation of thrombin by antithrombin-III

Abstract

To demonstrate that human [alpha]-thrombin is effectively inactivated by human antithrombin III (AT) during the production of a fibrin clot we measured the amount of a-thrombin activity which can be recovered from a clot generated from purified human proteins. We discovered that 0.05-0.07% of the original a-thrombin activity is recovered from a fibrin clot produced from a reaction mixture where the initial concentrations of AT and [alpha]-thrombin were chosen at a ratio (17.5) to allow complete conversion of fibrinogen to fibrin. These results indicated that [alpha]-thrombin is successfully inactivated by AT during the production of a fibrin clot. Further, when an amount of [alpha]-thrombin equal to that recovered from a fibrin clot is introduced into a solution of fibrinogen and AT identical to that utilized to produce the clot only 4% of the fibrinogen is converted to fibrin. These results suggest that i) when a fibrin clot is dissolved during fibrinolytic therapy little active [alpha]-thrombin should be released from the clot and ii) this amount of thrombin is insufficient to catalyze rethrombosis without proposing de novo production of thrombin. The action on factors XI, VIII, and V of the small amount of thrombin released upon thrombolysis, however, may provide the stimulus for de novo production of sufficient thrombin to catalyze rethrombosis.