Several Mechanisms Contribute to the Basic Carboxypeptidase Mediated Downregulation of Fibrinolysis.

Activated thrombin-activatable fibrinolysis inhibitor (TAFIa) is an intrinsically unstable basic carboxypeptidase which removes C-terminal lysine residues from plasmin-degraded fibrin. This modification of the fibrin surface attenuates fibrinolysis via several mechanisms. First, plasminogen activation is downregulated due to the loss of high affinity plasminogen binding. Second, the rate of plasmin inhibition is increased due to the loss of the protection from inhibition that degraded fibrin confers on plasmin. Finally, fibrin degradation by plasmin becomes less efficient due to the loss of cleavage cooperativity. Although carboxypeptidase-mediated downregulation of the fibrinolytic process has been studied, the relative contribution of each mechanism to the aggregate antifibrinolytic process remains unclear. We, therefore, undertook the following study to measure the contribution of plasmin protection and fibrin cleavage cooperativity to the overall antifibrinolytic effect. To this end, we studied the lysis of clots formed from purified fibrinogen, thrombin and plasmin in the presence and absence of various concentrations of aprotinin, a tight-binding but reversible plasmin inhibitor, and pancreatic carboxypeptidase B (CPB), a stable carboxypeptidase highly homologous to TAFIa. To determine the overall antifibrinolytic effect, we studied the lysis of clots formed from the same components, but using plasminogen with various concentrations of tissue plasminogen activator (tPA) in place of plasmin. In the absence of the inhibitor, CPB saturably increased the plasmin-mediated lysis time by 1.2-fold of that seen in the absence of CPB, suggesting that the loss of cleavage cooperativity increases lysis time by ~20%. The effect of plasmin protection from aprotinin, mediated by recruitment of plasmin to the degraded fibrin surface, was demonstrated using a fixed concentration of plasmin (20nM) and various concentrations of aprotinin (5–20nM). When plasmin was equimolar with or in excess of aprotinin, CPB saturably prolonged lysis by a factor which increased as the concentration of aprotinin increased: saturating CPB (50nM) increased the lysis times by 1.3, 1.5, 1.9 and 2.3-fold at 5, 10, 15 and 20nM aprotinin. When an excess of aprotinin was included (30nM), CPB saturably increased the plasmin-mediated lysis time identically over a range of plasmin concentrations: saturating CPB (50nM) increased the lysis times 2.4, 2.4, 2.3 and 2.3-fold at plasmin concentrations of 10, 15, 20 and 25nM, respectively. Since the total increase (2.4-fold) results from the loss of plasmin protection in addition to the loss of cleavage cooperativity (1.2-fold), we conclude that the loss of plasmin protection increases lysis time ~ 2.4/1.2 = 2-fold. When plasminogen (200nM) and tPA (25–200pM) were used in place of plasmin, CPB also saturably increased lysis time: saturating CPB (50nM) increased lysis times by ~ 15-fold at 150nM aprotinin and by ~ 17-fold at 300nM aprotinin for 25, 50, 100 and 200pM tPA. Since the total increase in lysis time (~ 16-fold) in these experiments results from the downregulation of plasminogen activation in addition to the loss of cleavage cooperativity (1.2-fold) and the loss of plasmin protection (2-fold), we estimate that the downregulation of plasminogen activation resulting from sustained removal of C-terminal lysine residues by a basic carboxypeptidase during fibrinolysis increases clot lysis time by 16/(1.2 x 2) ~ 6.7-fold.