Factor XIIIa Formation Promoted by Complexing of a - Thrombin , Fibrin , and Plasma Factor XIII

Fibrin polymers (des A,B fibrinogen) reduced the concentration of a-thrombin required for 50% activation of plasma factor XIII (a2b2 tetramer) by approximately 1 00-fold. In the presence of fibrin, the amount of ‘y-thrombin required for activation was not affected. Catalytically inactive i-Pr2Pand D-Phe-PrO-Arg-CH2-a-thrOmbin were found to inhibit over 95% of the activation by a-thrombin in the presence of fibrin. Unlike plasma factor XIII, the concentration of athrombin required for 50% activation of platelet factor XIII (a2 dimer) was lower, and the activation was not enhanced by fibrin. However. when the a2 platelet factor XIII was incubated with purified b-chains, the aand ‘y-thrombin concentrations required for activation increased tenfold