Summary
Reproducible density-gradient ultracentrifugation reference profiles of the ratezonal type were established individually for radioiodinated fibrinogen, plasminogen, and urokinase. Two- and three-component radioiodinated mixtures were then studied to determine whether stable complexes would form between any two components and thus change the density-gradient distribution profiles.
Urokinase did not form stable complexes with plasminogen, fibrinogen, or fibrin, nor with any plasminogen-fibrinogen or plasminogen-fibrin mixtures.
Plasminogen was found to form stable complexes with fibrinogen and with fibrin. Urokinase added to such mixtures resulted in fibrinogenolysis and fibrinolysis, but not in stable urokinase binding.
The data support the proposition that urokinase does not activate plasminogen directly but requires the formation of plasminogen-fibrin(ogen) complexes as a prelude (conformational change?) to plasmin conversion and subsequent fibrin(ogen)olysis.
