Thrombin-activable fibrinolysis inhibitor attenuates (DD)E-mediated stimulation of plasminogen activation by reducing the affinity of (DD)E for tissue plasminogen activator - A potential mechanism for enhancing the fibrin specificity of tissue plasminogen activator

A complex of D-dimer noncovalently associated with fragment E ((DD)E), a de gradation product of crosslinked fibrin that binds tissue plasminogen activ ator (t-PA) and plasminogen (Pg) with affinities similar to those of fibrin , compromises the fibrin specificity of t-PA by stimulating systemic Pg act ivation. In this study, we examined the effect of thrombin-activable fibrin olysis inhibitor (TAFI), a latent carboxypeptidase B (CPB)-like enzyme, on the stimulatory activity of (DD)E. Incubation of (DD)E with activated TAFI (TAFIa) or CPB (a) produces a 96% reduction in the capacity of (DD)E to sti mulate t-PA-mediated activation of Glu- or Lys-Pg by reducing k(cat) and in creasing K-m for the reaction; (b) induces the release of 8 mol of lysine/m ol of (DD)E, although most of the stimulatory activity is lost after releas e of only 4 mol of lysine/mol (DD)E; and (c) reduces the affinity of (DD)E for Glu-Pg, Lys-Pg, and t-PA. by 2-, 4-, and 160-fold, respectively. Becaus e TAFIa- or CPB-exposed (DD)E produces little stimulation of Glu-Pg activat ion by t-PA, (DD)E is not degraded into fragment E and D-dimer, the latter of which has been reported to impair fibrin polymerization. These data sugg est a novel role for TAFIa. By attenuating systemic Pg activation by (DD)E, TAFIa renders t-PA more fibrin-specific.