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Factor‐Xa Inactivation by Antithrombin III

Summary. The inactivation of the procoagulant activity of bovine factor Xa by antiproteinase present in antithrombin III preparation follows the pseudo‐first‐order kinetics with respect to factor Xa. The rate of inactivation is devoid of the dimension of factor Xa concentration when antiproteinase in excess is held constant. In the presence of physiological concentrations of factor V, phospholipids and ionic calcium, the affinity of factor Xa for antithrombin III drastically decreases, and the half‐life of the enzyme is very significantly prolonged. This biological stabilization of factor Xa requires the presence of all three substances: factor V, phospholipid and calcium. However, it is quantitatively dictated only by factor V concentrations. Calcium ions and phospholipid required for factor Xa protection from inactivation by antithrombin III have a rather low optimum concentration range, beyond which the original fast rate of inactivation tends to be restored. A direct interaction between antithrombin III and factor V or phospholipid was not found. Thus binding of factor V mediated by phospholipid and calcium ions, which brings about the increase in factor Xa proteolytic activity, also decreases the susceptibility of this enzyme to a biological inactivator in blood.