A transitional state of pro-urokinase that has a higher catalytic efficiency against glu-plasminogen than urokinase

Plasminogen activation by single-chain urokinase-type plasminogen activator or pro-urokinase (pro-UK) is accompanied by the generation of two-chain urokinase (UK) by plasmin which provides a positive feedback. In the present study, the time course of the activation of Glu-plasminogen and of Lys-plasminogen (10 microM) by pro-UK (1.0 nM) was studied. In the presence of native plasminogen (Glu-plasminogen), three distinct phases with different rates of plasmin generation were observed. The initial phase was slow and corresponded to the intrinsic activity of pro-UK as reflected by the activity of a plasmin-resistant mutant (Lys158----Ala). This was followed by a second phase which had the most rapid rate. The third phase had a plasminogen activation rate which was significantly slower than the second and paralleled the rate of activation by UK (1.0 nM). The second phase coincided with the time at which there was only about 50% conversion of pro-UK to UK, whereas the final phase coincided with essentially complete conversion. In the presence of fibrin fragment E-2 (20 microM), previously shown to strongly promote plasminogen activation by pro-UK, the identical phenomenon was observed, but at one-tenth the concentration of pro-UK. The most rapid rate of plasmin generation again coincided with transitional (25-60%) pro-UK to UK conversion. To further examine this phenomenon, the rate of pro-UK to UK conversion was controlled by using kallikrein in the presence of a plasmin inhibitor. In this experiment, the activation of Glu-plasminogen bound to solid-phase fibrin was measured. A similar three-phase sequence was observed, the highest rate of plasmin generation coinciding with about 45% conversion of pro-UK to UK. A mechanism for this transitional state phenomenon was postulated based on the established significantly higher affinity of pro-UK than of UK for Glu-plasminogen. This exceptional property for a proenzyme may enable a transient activity to be generated during the transition from pro-UK to UK corresponding to the more favorable KM of pro-UK and the kcat of UK. This hypothesis was supported by the results from experiments in which Lys-plasminogen was substituted for the Glu form. No transitional state activity was observed, consistent with the relatively high KM of pro-UK against Lys-plasminogen.