Endogenous plasminogen is activated by tPA to degrade endogenous FXIIIa. Plasma was analyzed by western blot against FXIII-A, after addition of tPA and TXA. (A-B) Time-dependent degradation of endogenous (A) FXIII-A₂* and (B) fibrin(ogen) in normal plasma. (C) The relative amount of intact FXIII-A* from panel A using densitometry, calculated as a percentage of total signal in the lane. *P < .05. (D) Time course for plasminogen-deficient plasma. (E) Time-dependent degradation of FXIIIa in fibrinogen-deficient plasma after adding tPA. (F) Degradation of endogenous FXIII-A₂*, but not FXIII-A₂, in whole-blood clots with tPA. Samples contain combinations of hirudin (H), Innovin (I), tPA (2 µM) (T), TXA (7.5 mM) (X), and α2-antiplasmin (4 µM) (A). n = 3 for all experiments. FDP, fibrin degradation products; MW, molecular weight.