TRANSFORMATION OF CELLULAR FACTOR-XIII INTO AN ACTIVE ZYMOGEN TRANSGLUTAMINASE IN THROMBIN-STIMULATED PLATELETS

The cellular form of blood coagulation factor XIII (FXIII) is present in platelets, monocytes and macrophages. During long-term stimulation of platelets by thrombin cellular FXIII becomes activated and crosslin ks proteins, however, the mechanism of its activation has not been elu cidated. It was shown that, contrary to plasma FXIII, the intracellula r activation of platelet FXIII does not involve proteolysis. FXIII rem ained intact in thrombin-activated platelets, i.e., the activation pep tide was not removed from the molecule. Part of the zymogen FXIII mole cules, however, assumed an active configuration as was demonstrated bo th by the measurement of transglutaminase activity and by active-site- SH titration. These findings clearly indicate that during platelet act ivation, when intracellular Ca2+ concentration is raised, a slow non-p roteolytic transformation of FXIII zymogen into an active transglutami nase occurs