Mechanism of thrombin inhibition by antithrombin and heparin cofactor II in the presence of heparin

The kinetics of thrombin inhibition by antithrombin (AT) and heparin cofactor II (HC II) were analysed as a function of the heparin concentration, from 10−9 to 10−4image. The initial concentrations of inhibitor (I) and thrombin (E) were set at equimolar levels (CI = CE = 10−8 m). The experimental data indicate that the reaction of thrombin inhibition was second-order both in the absence and in the presence of heparin, and that the apparent rate constant increased at heparin concentrations ranging from 10−9 to 10−6 image and decreased at higher concentrations. The data fit with the kinetic model established by Jordan et al. [J. Biol. Chem. 1979, 254, 2902–2913] for the catalysis of the thrombin-AT reaction by a low-molecular-weight heparin fraction. In this model, heparin (H) binds quickly to the inhibitor (I) and forms a heparin-inhibitor complex (HI), which is more reactive than the free inhibitor towards thrombin, leading to the formation of an inactive inhibitor-thrombin complex (I*E) and the release of free heparin, in a second step which is rate limiting. KH, I, the dissociation constant of HI, and k, the second-order rate constant of free thrombin inhibition by HI, were found to be 3.7 × 10−7 m and 1.3 × 109 image−1 min−1, respectively, for AT, compared to a KH, I of 2.0 × 10−6 image and k of 6.4 × 109 image−1 min−1 for HC II. These data indicate that heparin-HC II complex reactivity is greater than that of the heparinAT complex towards thrombin, whereas heparin affinity is stronger for AT. At heparin concentrations higher than 10−6 image, the decrease in the reaction rate was in keeping with the formation of a heparinthrombin complex (HE), whose inactivation by the heparin-inhibitor complex (HI) is slower than that of the free protease.