Heparin-catalyzed inhibitor/protease reactions: kinetic evidence for a common mechanism of action of heparin.

Three different heparin-catalyzed inhibitor/protease reactions were studied: antithrombin III/thrombin, heparin cofactor II/thrombin, antithrombin III/factor Xa. The three reactions were saturable with respect to both inhibitor and protease. The initial reaction velocity, for each reaction, could be described by the general rate equation for a random-order bireactant enzyme-catalyzed reaction. The kinetic parameters for the heparin-catalyzed antithrombin III/thrombin and antithrombin III/factor Xa reactions differed in terms of apparent maximum velocity (Vmax) and apparent heparin-protease dissociation constant values. The apparent heparin-antithrombin III dissociation constant values were the same for both reactions. The kinetic parameters for the heparin-catalyzed antithrombin III/thrombin and heparin cofactor II/thrombin reactions differed in terms of apparent Vmax and apparent heparin-inhibitor dissociation constant values. The apparent heparin-thrombin dissociation constant values were the same for both reactions. The results are consistent with a general mechanism of action of heparin for the three reactions that, in its simplest form, requires only that both protease and inhibitor bind to heparin for catalysis to occur.