The inhibition of activated bovine coagulation factors X and VII by antithrombin III

The inhibition of activated bovine Factors VII and X by antithrombin III has been studied by kinetic methods. The reaction between Factor X_a and antithrombin III is characterized by second-order kinetics, with a rate constant of 3.9 × 10³m^?1s^?1 at pH 7.5 at 37 °C. Inhibition in the presence of excess antithrombin III does not proceed to completion: The decay of Factor X_a deviates from pseudo-first-order kinetics and a final equilibrium is reached, suggesting reversibility. The apparent association constant, at pH 7.5, 37 °C, is 2.3 × 10⁹m^?1. The interaction of three forms of bovine Factor VII with antithrombin III has been studied by the same methods. Factor VII and the two-chain activated form, α-Factor VII_a, and the tissue factor-Factor VII_a complex are not significantly inhibited by plasma levels of antithrombin III, in the either the presence or absence of heparin.