| Abstract: | High-performance liquid chromatography was used to analyze the kinetics of the thrombin-catalyzed release of the activation peptide from the factor XIII zymogen (fibrin-stabilizing factor). The specificity constant (kcat/Km) for this reaction, measured at factor XIII concentrations much below Km, was (0.13-0.16) X 10(6) M-1 s-1 at pH 7.4, mu = 0.15, and 37 degrees C. Separate estimates, obtained from the dependence of the initial rates of release of the activation peptide on the concentration of factor XIII, gave values of 10 (+/- 3) s-1 for kcat and 84 (+/- 30) microM for Km, in terms of ab protomers of the zymogen. The thrombin-mediated release of the activation peptide was dramatically enhanced in the presence of fibrinogen. Furthermore, the time course of release, in relation to that of fibrinopeptide A, suggested that some des-A-fibrinogen species (e.g., alpha 2B beta 2 gamma 2) may be the true activator for promoting the cleavage of the Arg-36 peptide bonds in the a subunits of factor XIII. This observation suggests that generation of factor XIIIa and its substrate (fibrin) is coordinated so that thrombin-mediated zymogen activation proceeds efficiently only after the process of clotting has been initiated by the removal of fibrinopeptide A from fibrinogen. |
