Studies on the binding of heparin to prothrombin and thrombin and the effect of heparin-binding on thrombin activity

Heparin binds with high affinity to thrombin, active as well as inactive, but not to prothrombin. The heparin-binding site therefore must be unmasked or created during prothrombin activation. The binding of heparin to thrombin has no effect on the amidase activity of the enzyme, as measured with a chromogenic substrate. All heparin molecules bind to thrombin with similar affinity, regardless of their affinity for antithrombin. The binding of heparin to antithrombin has been shown previously to be correlated with anticoagulant activity. The lack of a similar correlation between the binding of heparin molecules to thrombin and their anticoagulant activity demonstrates that heparin cannot function by binding only to thrombin, and in this manner facilitate the reaction of the latter with antithrombin. Binding of heparin to antithrombin is thus an essential feature of the anticoagulant activity of the polysaccharide.