In vitro Inactivation and Stabilization of Fibrinolytic Activity of Urokinase

When incubated at 30°C and pH 7.4, urokinase lost fibrinolytic activity (i.e., the plasminogen-activating activity measured by the time of fibrin clot lysis) but completely retained amidase activity. The enzyme inactivation rate depended on the urokinase concentration and, at concentrations of more than 1.5 μM, was described by a second order equation, which indicated that the enzyme underwent autolytic degradation (k_aut = 3.8 × 10^-3 M^-1 min^-1). During incubation, urokinase (54 kDa) was converted into its low-molecular-mass form (33 kDa) and products of the A-chain degradation. The amidase activity did not correlate with the fibrinolytic activity in the cases when the enzyme molecule underwent local unfolding or partial degradation. The optimum mixture of agents for stabilizing the fibrinolytic activity of urokinase was found.