Mechanism of the inhibitory effect of angiostatin on plasminogen activation by its physiologic activators

摘要

The influence of angiostatin K1-4.5, a fragment of the heavy chain of plasmin and a powerful inhibitor of angiogenesis, on kinetic parameters (k_ (Pg) and K_ (Pg)) of human Glu-plasminogen activation under the action of urokinase (uPA) not having affinity for fibrin and fibrin-specific tissue plasminogen activator (tPA) was investigated. Angiostatin does not affect on the k_ (Pg) value, but increases the value of K_ (Pg) plasminogen activation by urokinase. A decrease in the k Pg value and an increase in the K_(Pg) value were found for fibrin-stimulated plasminogen activation by tPA with increasing concentrations of angiostatin. The obtained results show that angiostatin is a competitive inhibitor of the uPA activator activity, while it inhibits the activator activity of tPA with a mixed type. Such an influence of angiostatin on the kinetic constants of the plasminogen activation by urokinase suggests that angiostatin dose-dependent manner replaces plasminogen in the binary enzyme-substrate complex uPA-Pg. In the case of fibrin-stimulated plasminogen activation by tPA, both zymogen and tPA are bound to fibrin with the formation of the effective triple tPA-Pg-fibrin complex. Angiostatin replaces plasminogen both from the fibrin surface and from the enzyme-substrate tPA-Pg complex, which leads to a decrease in k_(Pg) and an increase in K_(Pg) of the plasminogen activation. Inhibition constants by angiostatin (K_ i) of plasminogen-activator activities of uPA and tPA determined by the Dixon method were found to be 0.59 ± 0.04 and 0.12 ± 0.05 μM, respectively.