Steady state kinetics of activation of human and bovine plasminogens by streptokinase and its equimolar complexes with various activated forms of human plasminogen.

The steady state kinetic parameters of activation of human Gluand Lys-plasminogens and bovine plasminogen by streptokinase and its equimolar complexes with Gluand Lys-plasminogen, Lys-plasmin, and the plasmin-derived light(B) chain, as well as urokinase, were determined. Activation rates were measured at pH 6.0 and 30” by determining the initial velocity of Na-Cbz-L-lysine-p-nitrophenyl ester hydrolysis by the plasmin generated. The dissociation constant, K,,,, for activation of human plasminogen, by all the activators studied was comparable (about 1 to 3 PM) while the catalytic rate constant, k,,,, ranged from about 3 to 52 so’ and the k,,,/K,,, values varied from about 1 to 41 PM-’ S-l, with the highest value observed for the preformed light(B) chain.streptokinase complex. Glu-plasminogen activation by streptokinase (Glu-plasminogen streptokinase) and the light(B) chain.streptokinase complex proceeded at one-third the rate of that of Lys-plasminogen activation, with a k,,, for Glu-plasminogen of about 7 and 19 s-l compared to a k,,, for Lys-plasminogen of about 27 and 52 s’, respectively. Lys-plasminogen activation by Gluand Lys-plasminogen . streptokinase and Lys-plasmin streptokinase complexes gave k,,,/K,,, values of about 15 to 22 PM-’ s-*. Lys-plasminogen activation by urokinase was much slower, with a Ic,,,/K,,,, value of about 1 PM-’ ss’. Activation of bovine plasminogen by the streptokinase complexes showed decreased catalytic rates, with k,,,lK,,, values between about 0.5 and 3 PM-’ s’, the lowest value was for the Lys-plasminogen . streptokinase complex. The stoichiometry of the interaction of the light(B) chain with streptokinase was confirmed using both esterase and activator parameters; the equimolar complex formed at pH 6.0 and 30” at concentrations of 3 x 10m9 M, indicating a very low dissociation constant and probably an even lower one for streptokinase binding to both plasminogen and plasmin. Incubation of solutions of the light(B) chain. streptokinase complex at 30”, up to 60 min, showed no change in esterase activity but some loss in activator