Tissue Factor Pathway Inhibitor and Heparin

The anticoagulant property of heparinized blood results from interactions between heparin, protease inhibitors and clotting enzymes. The inactivation of thrombin, factor Xa and factor IXa by antithrombin (AT) is dramatically accelerated by even low concentrations of heparin. At high concentrations of heparin, inactivation of thrombin by heparin cofactor II (HC II) also contributes to the anticoagulant effect. The relative contribution of these reactions to the total anticoagulant effect is currently under debate. Moreover, their relative significance to the antithrombotic effect of heparin is not obvious. The possibility of additional anticoagulant effects of heparin tends to be ignored in recent literature.