Further evidence for an active center in streptokinase-plasminogen complex; interaction with pancreatic trypsin inhibitor

Earlier work has shown that streptokinase and human plasminogen form a stoichiometric complex in which the presence of a functional active center can be detected by reaction with the active center-specific reagent, $\text{p}$-nitrophenyl-$\text{p′}$-guanidinobenzoate. The complex possesses activator activity, i.e. it catalyzes the conversion of plasminogen to plasmin. Evidence is presented to show that pancreatic trypsin inhibitor abolishes both the activator activity and the ability to react with the active center-specific reagent. This is accomplished, not by displacement of streptokinase, but by the formation of a ternary complex with streptokinase-plasminogen.