Binding of urokinase to plasma proteinase inhibitors

125I-labelled urokinase was incubated with plasma and plasminogen free plasma, and the incubation mixtures were analyzed by agarose gel electrophoresis. Autoradiography demonstrated that non-reacted urokinase remained around the slit and that complex-formation with inhibitors altered the migration and resulted in two bands, a major one and a minor one. Crossed immunoelectrophoresis combined with autoradiography showed that the major band contained a complex between alpha 2-antiplasmin and urokinase. The minor band contained a complex between alpha 2-macroglobulin and urokinase. Also di-isopropylfluorophosphate-inactivated urokinase was bound to alpha 2-macroglobulin but not to alpha 2-antiplasmin. Thus, an intact active site of urokinase is not necessary for complex formation with alpha 2-macroglobulin.