Ca(2+)-dependent regulation of fibrinolytic system activation on fibrin(ogen) D-domains

In the present study, we investigated whether calcium content modulation in D-domains of fibrin(ogen) was involved in fibrinolytic process activation. To investigate the effect of Ca2+-dependent changes in D-domains two types of fibrinogen fragments D and cross-linked fibrin fragments DD were obtained from plasmin hydrolysate of human fibrin(ogen): chelator-treated and without chelating agents. The study of plasminogen activation by tissue-type plasminogen activator on D- and DD-fragments had shown the intensification of plasmin formation in case of EDTA pretreatment of fragments. The proenzyme activation rate on DD also increased in the presence of EGTA in concentration-dependent manner. Potentiating effect of EGTA-pretreated DD-fragment on plasminogen activation by tPA was decreased in the presence of Ca2+. Activation rate reduction was observed according to the increase of CaCl2 concentration in the reaction medium. The intensification of plasminogen activation potentiation by chelator-treated fibrin(ogen) D-domain containing fragments and subsequent potentiation decrease in the presence of Ca2+ indicated the requirement of Ca2+-dependent changes in D-domains for plasminogen activation sites exposure and initiation of fibrinolysis.