A Kinetic Analysis of the Tissue Plasminogen Activator and DSPAα1 Cofactor Activities of Untreated and TAFIa-treated Soluble Fibrin Degradation Products of Varying Size

Abstract The kinetics of tissue plasminogen activator (t-PA) and DSPAα1-catalyzed plasminogen activation using untreated and TAFIa-treated fibrin degradation products (FDPs), ranging in weight average molecular weight (M w) from 0.48 × 106 to 4.94 × 106 g/mol, were modeled according to the steady-state template model. The FDPs served as effective cofactors for both activators. The intrinsic catalytic efficiencies of both t-PA (17.4 × 105 m − 1 s− 1) and DSPAα1 (6.0 × 105 m − 1 s− 1) were independent of FDP M w. The intrinsic catalytic efficiency of t-PA was 12-fold higher than that measured under identical conditions with intact fibrin as the cofactor. At sub-saturating levels of cofactor and substrate, rates were strongly dependent on FDP M w with DSPAα1 but not t-PA. Loss of activity with decreasing FDPM w correlated with loss of finger-dependent binding of the activators to the FDPs. TAFIa treatment of the FDPs resulted in 90- and 215-fold decreases in the catalytic efficiencies of t-PA (0.20 × 105 m − 1 s− 1) and DSPAα1 (0.028 × 105 m − 1 s− 1), yielding cofactors that were still 30- and 50-fold better than fibrinogen with t-PA and DSPAα1, respectively. Our results show that for both activators the products released during fibrinolysis are very effective cofactors for plasminogen activation, and both t-PA and DSPAα1 cofactor activity are strongly down-regulated by TAFIa.