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 c ofactor II (HC II) were analysed as a function of the heparin concentr ation, from 10(-9) to 10(-4)M. The initial concentrations of inhibitor (I) and thrombin (E) were set at equimolar levels (C-I = C-E = 10(-8) M). The experimental data indicate that the reaction of thrombin inhi bition was second-order both in the absence and in the presence of hep arin, and that the apparent rate constant increased at heparin concent rations ranging from 10(-9) to 10(-6) M and decreased at higher concen trations. The data fit with the kinetic model established by Jordan et al. [J. Biol. Chem. 1979, 254, 2902-2913] for the catalysis of the th rombin-AT reaction by a low-molecular-weight heparin fraction. In this model, heparin (H) binds quickly to the inhibitor (I) and forms a hep arin-inhibitor complex (HI), which is more reactive than the free inhi bitor towards thrombin, leading to the formation of an inactive inhibi tor-thrombin complex (IE) and the release of free heparin, in a secon d step which is rate limiting. K-H,K-I,K- the dissociation constant of HI, and k, the second-order rate constant of free thrombin inhibition by HI, were found to be 3.7 x 10(-7) M and 1.3 x 10(9) M(-1) min(-1), respectively, for AT, compared to a K-H,K-I of 2.0 x 10(-6) M and k o f 6.4 x 10(-6) M(-1) min(-1) for HC II. These data indicate that hepar in-HC II complex reactivity is greater than that of the heparin-AT com plex towards thrombin, whereas heparin affinity is stronger for AT. At heparin concentrations higher than 10(-6) M, the decrease in the reac tion rate was in keeping with the formation of a heparin-thrombin comp lex (HE), whose inactivation by the heparin-inhibitor complex (Hl) is slower than that of the free protease. (C) 1997 Elsevier Science Limit ed.