Kinetic analysis of the interactions between plasminogen activator inhibitor 1 and both urokinase and tissue plasminogen activator

Plasminogen activator inhibitor 1 (PAI-1) was purified from medium conditioned by cultured bovine aortic endothelial cells by successive chromatography on concanavalin A Sepharose, Sephacryl S-200, Blue B agarose, and Bio-Gel P-60. As shown previously for conditioned media ( C. M. Hekman and D. J. Loskutoff (1985) J. Biol Chem. 260, 11581–11587 ) the purified PAI-1 preparation contained latent inhibitory activity which could be stimulated 9.4-fold by sodium dodecyl sulfate and 45-fold by guanidine-HCl. The specific activity of the preparation following treatment with 0.1% sodium dodecyl sulfate was 2.5 × 10 3 IU/mg. The reaction between purified, guanidine-activated PAI-1 and both urokinase and tissue plasminogen activator (tPA) was studied. The second-order rate constants (pH 7.2, 35 °C) for the interaction between guanidine-activated PAI-1 and urokinase (UK), and one- and two-chain tPA are 1.6 × 10 8, 4.0 × 10 7, and 1.5 × 10 8 M −1 s −1, respectively. The presence of CNBr fibrinogen fragments had no affect on the rate constants of either one- or two-chain tPA. Steady-state kinetic analysis of the effect of PAI-1 on the rate of plasminogen activation revealed that the initial UK PAI-1 interaction can be competed with plasminogen suggesting that the UK PAI-1 interaction may involve a competitive type of inhibition. In contrast, the initial tPA PAI-1 interaction can be competed only partially with plasminogen, suggesting that the tPA PAI-1 interaction may involve a mixed type of inhibition. The results indicate that PAI-1 interacts more rapidly with UK and tPA than any PAI reported to date and suggest that PAI-1 is the primary physiological inhibitor of single-chain tPA. Moreover, the interaction of PAI-1 with tPA differs from its interaction with UK, and may involve two Sites on the tPA molecule.