Interaction of streptokinase and human plasminogen. IV. Further gel electrophoretic studies on the combination of streptokinase with human plasminogen or human plasmin.

Streptokinase is shown to react with either human plasmin or human plasminogen to form an equimolar product which migrates in starch gel electrophoresis at pH 6.0 or 8.0 with a migration rate intermediate between the rates of migration of the two proteins. When streptokinase was present in molar excess in the reaction mixtures, the uncombined streptokinase displayed a migration rate at pH 6.0 (but not at pH 8.0) which was different from that of native streptokinase. Inhibition of the enzymatic activity of the reaction product with diisopropyl phosphorofluoridate did not cause dissociation of the complex, although no reaction product was observed in starch gel electrophoresis with mixtures of streptokinase and diisopropylphosphoryl plasmin. When equimolar mixtures of human plasminogen or human plasmin and streptokinase were subjected to electrophoresis at pH 6.0 in starch gels containing 6.4 M urea, the equimolar product was not observed; instead, plasmin and modified streptokinase were in evidence. On the other hand, when electrophoresis was carried out at pH 8.0 to 8.5 in ureastarch gels, reaction mixtures of human plasmin or human plasminogen and streptokinase displayed a number of electrophoretically distinct products which were derived from streptokinase. This marked degradation was not observed with human plasminogen-streptokinase reaction mixtures when the proteins were dissolved in 6.4 M urea prior to interaction and incubation, but it was noted in reaction mixtures made by mixing solutions of human plasmin and streptokinase which were individually dissolved in 6.4 M urea. The equimolar reaction product formed by reaction of streptokinase with human plasminogen was subjected to gel filtration chromatography in lysine-tris(hydroxymethyl)aminomethane buffer, and the isolated main peak was shown to display potent bovine plasminogen activator activity and weak proteolytic activity. The sequence of events occurring in the interaction of streptokinase with human plasminogen or human plasmin is discussed on the basis of the present findings and those re-