INHIBITION OF ACTIVATED BOVINE COAGULATION FACTOR-X AND FACTOR-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 Xa 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 Xa 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 VIIa, and the tissue factor-Factor VIIa complex are not significantly inhibited by plasma levels of antithrombin III, in the either the presence or absence of heparin. © 1978.