COMPARISON OF THE IN-VITRO FIBRINOLYTIC-ACTIVITIES OF LOW AND HIGH-MOLECULAR-WEIGHT SINGLE-CHAIN UROKINASE-TYPE PLASMINOGEN-ACTIVATOR

The fibrinolytic activity of low molecular weight (LMW) single-chain u rokinase-type plasminogen activator (scu-PA) lacking the epidermal gro wth factor domain and the kringle domain was compared with the activit y of high molecular weight (HMW) scu-PA. LMW scu-PA was 1-5 times less active than HMW scu-PA in a fibrin plate method, in a purified fibrin clot lysis assay and in a plasma clot lysis assay. Time course experi ments in a chromogenic plasminogen activator assay suggested that LMW scu-PA was less sensitive to activation by plasmin than HMW scu-PA. Th is was confirmed in a scu-PA activation test, which showed that at a c oncentration of 40 IU/ml LMW scu-PA required a three-fold higher plasm in concentration for 50% activation in 20 min than did HMW scu-PA. Kin etic experiments in the presence of 0.1 M NaCl showed non-standard Mic haelis-Menten kinetics for the activation by plasmin of both HMW and L MW scu-PA. In contrast, standard kinetics was observed at 0.15 M NaCl, showing a 2.6-fold lower catalytic efficiency for LMW scu-PA than for HMW scu-PA. It is concluded that the plasmin activation of LMW scu-PA is about three times slower than the activation of HMW scu-PA. This e xplains, at least partially, the lower fibrinolytic activity of LMW sc u-PA in comparison with HMW scu-PA.