The reciprocal pro-enzyme activation system of plasmin, urokinase-type plasminogen activator (uPA) and their respective zymogens is a potent mechanism in the generation of extracellular proteolytic activity. Plasminogen activator inhibitor type 1 (PAI-1) acts as a negative regulator. This system is complicated by a poorly understood intrinsic reactivity of the uPA pro-enzyme (pro-uPA) before proteolytic activation, directed against both plasminogen and PAI-1. We have studied the integrated activation mechanism under the repression of PAI-1 in a purified system. A covalent reaction between pro-uPA and PAI-1 was positively demonstrated but the reaction of PAI-1 with two-chain uPA was found to be at least 1000-fold faster. However, in spite of this very fast inhibition, two-chain uPA still became the dominant plasminogen activator when plasminogen was incubated with pro-uPA and PAI-1. The activity pattern observed under these conditions revealed an initial lag phase, followed by a continuous generation of minute amounts of active two-chain uPA, this uPA having a short lifetime before inhibition but still succeeding to generate new plasmin activity, thus preventing a complete inactivation of the feedback system. This property of the activation system was retained even in the simultaneous presence of PAI-1 and α2-antiplasmin. Addition of soluble uPA receptor to the system did not change the role of pro-uPA and the same pattern was observed when pro-uPA was bound to the uPA receptor on U937 cells. The present mechanism maintains the system at standby level and may be triggered to increased activity without the need for an external initiating event.

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