Kinetic studies of three different molecular forms of urokinase for the activation of native human plasminogen

The kinetic parameters of three different molecular forms of urokinase (UK) for the activation of native Glu-plasminogen were compared. The apparent Michaelis constant (Km. app.) of each UK was almost of the same order of magnitude (31-38 microM), but the catalytic constants (kc) were observed to be different: UKh (high molecular weight form, molecular weight 53,000), 2.4 +/- 0.2 s-1; UK+ (low molecular weight form, molecular weight 33,000), 0.83 +/- o.10 s-1, and UKl (trypsin-digested form, molecular weight 36,000), 0.91 +/- 0.18 s-1. The overall second order rate constant, kc/Km calculated for UKh was 7.7 X 10(4) M-1 s-1, higher than for UKl (2.2 X 10(4) M-1 s-1) or UKt (2.4 X 10(4) M-1 s-1), indicating the possibility of a much higher degree of enzymatic specificity and efficiency.