Complete inhibition of fibrinolysis by sustained carboxypeptidase B activity: the role and requirement of plasmin inhibitors

Summary.  Background:  The antifibrinolytic effect of activated thrombin‐activatable fibrinolysis inhibitor (TAFIa) and carboxypeptidase B (CPB) displays threshold behavior. When CPB was used to simulate conditions mimicking continuous TAFIa activity, it affected the lysis of plasma clots differently to clots formed from a minimal fibrinolytic system comprising fibrinogen, plasminogen and α 2 ‐antiplasmin. Whereas CPB saturably prolonged clot lysis in the purified system, the effect of CPB did not appear saturable in plasma clots. Methods:  To rationalize this difference, we investigated the effects of α 2 ‐antiplasmin, α 2 ‐macroglobulin, antithrombin and aprotinin on CPB‐mediated antifibrinolysis. Results:  CPB alone prolonged fibrinolysis in a saturable manner and the efficacy of CPB increased with decreasing tissue‐type plasminogen activator (t‐PA) concentration. The inhibitors by themselves did not halt fibrinolysis and the potency of each inhibitor in the absence of CPB mirrored their solution‐phase plasmin inhibitory potentials: α 2 ‐antiplasmin ≈ aprotinin >> α 2 ‐macroglobulin >> antithrombin. With both CPB and inhibitor present, a synergistic effect was observed. The antifibrinolytic sensitivity to CPB was related to the plasmin inhibitory potential of the inhibitor. Conclusions:  Fibrinolysis could be completely inhibited by α 2 ‐antiplasmin, α 2 ‐macroglobulin and antithrombin, but not aprotinin, in the presence of CPB, and occurred only when the irreversible inhibitor or pool of inhibitors were in excess of plasminogen. Western blot analysis indicated that the CPB‐mediated shutdown of fibrinolysis was a result of plasminogen consumption prior to clot lysis. The CPB concentration required for fibrinolytic shutdown was dependent on t‐PA concentration and the inhibitory potential of the irreversible inhibitor pool.