Esterase Activities in the Zymogen Moiety of the Streptokinase-Plasminogen Complex

Abstract

Abstract It was shown earlier that streptokinase and human plasminogen form a stoichiometric complex in which the presence of a functional active center can be detected in the zymogen moiety before its proteolytic conversion to plasmin. This center was shown to react with the active site titrant pnitrophenyl-p'-guanidinobenzoate, and to possess activator activity for human plasminogen. We now show that the zymogen moiety of the complex is capable of hydrolyzing lysine and acetyllysine methyl ester, as well as tosylarginine methyl ester, at rates approximately one-half that of the fully developed streptokinase-plasmin complex. At sufficiently high ester concentrations, activation to streptokinaseplasmin can be completely arrested, as the esters compete with plasminogen for the activator center in the streptokinaseplasminogen complex. This inhibition prolongs the life of this transient active complex, and has permitted in this study the determination of kinetic constants for acetyllysine methyl ester hydrolysis, as well as the quantitative determination of the relative activator activities of streptokinaseplasminogen and streptokinase-plasmin. The effects of bovine pancreatic trypsin inhibitor and of soybean trypsin inhibitor were also determined on the esterase activity of plasmin and its streptokinase complexes. Finally, a recent suggestion, that it is the streptokinase moiety of the activator complex which bears the active center, was examined by active site titration: no evidence was found to support this hypothesis.