Reversible inhibitors of TAFIa can both promote and inhibit fibrinolysis

Summary.  The plasma carboxypeptidase activated thrombin‐activable fibrinolysis inhibitor (TAFIa), is thermally unstable at 37 °C, with a half‐life of 8 or 15 min depending on the isoform. The arginine analog, 2‐guanidinoethylmercaptosuccinate (GEMSA), not only inhibits TAFIa but also slows the spontaneous inactivation of the enzyme, thereby reducing the activity of TAFIa, while extending its apparent half‐life. Because, as shown in previous work, the ability of TAFIa to prolong clot lysis can be more dependent on its half‐life than its concentration, in this study we determined whether reversible inhibitors of TAFIa could paradoxically prolong clot lysis. Potato tuber carboxypeptidase inhibitor (PTCI) or GEMSA were titrated into normal pooled human plasma, in the presence of soluble thrombomodulin. Both inhibitors mediate a biphasic antifibrinolytic effect, prolonging clot lysis at lower concentrations and enhancing clot lysis at higher concentrations. The antifibrinolytic effect of GEMSA is maximized at 1 mmol L −1 , increasing clot lysis time from 100 min to 350 min. The antifibrinolytic effect of PTCI is maximized at 100 nmol L −1 , increasing clot lysis time from 100 min to 240 min. To further characterize the nature of this biphasic effect, TAFI at various concentrations was added to TAFI‐immunodepleted human plasma in the presence of PTCI or GEMSA. The magnitude of the effect depends on the concentration of TAFIa, the concentration of inhibitor, and the potency of the inhibitor. We propose that the biphasic antifibrinolytic effect is mediated by the dynamic equilibrium of free TAFIa that inactivates quickly, and TAFIa bound to inhibitor that inactivates slowly. TAFIa inhibitors used as therapeutic agents might not only enhance lysis at higher concentrations, but also stabilize fibrin clots at intermediate concentrations.