The antithrombin activity of heparin

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Abstract
The antithrombin action of heparin on human thrombin was studied, with and without the addition of defibrinated plasma. Heparin in concentrations above 2.0 units/ml forms a reversible complex with thrombin. This complex has less thrombin activity than free thrombin. Heparin in concentrations above 0.1 unit/ml catalyses a rapid combination between thrombin and a cofactor present in defibrinated plasma. Heparin cofactor and the normal antithrombin of plasma are shown to be related but not identical, both occurring in the alpha-globulin fraction. A theory suggesting that normal antithrombin consists of heparin cofactor as well as another independent thrombin inactivator is proposed to account for the observed facts.