Kinetics of active center formation in dog plasminogin by streptokinase and activity of a modified streptokinase.

The rate of activation of dog plasminogen by excess streptokinase showed a significant delay as compared to the rate of activation with catalytic amounts of streptokinase. Studies of the reaction at high streptokinase levels with the active center reagent, p-nitrophenyl-p-guanidinobenzoate showed that only a fraction (13%) of the potential active centers were developed in a equimolar mixture of streptokinase and dog plasminogen in 15s and more than 10 min were required for the formation of 1 mol of active sites. In the first 15s, the yield of active sites could not be increased by increasing streptokinase 10-fold over the molar concentration of plasminogen, suggesting that active center development rather than complex formation was the rate-limiting step. The delayed reactivity seen in this system provides an interesting model for the study of conformationally induced active center formation. With catalytic amounts of streptokinase, the activation proceeded rapidly but reached a plateau, indicating the loss of activator activity in the reaction mixture. With successive additions of fresh streptokinase, complete activation was achieved. Polyacrylamide gel electorphoresis showed that a stable streptokinase-plasmin complex formed. However, in contrast to the human plasmin-steptokinase complex, a potent plasminogen activator in which streptokinase is found as a residue of 37,000 daltons, dog plasmin-streptokinase complex contained a residue of 25,700 daltons and the complex was inactive against canine and human plasminogen. The 25,700 fragment along, however, showed considerable activator activity when tested with human and dog plasminogens.