Effect of plasminogen kringles on scuPA/suPAR activity. A plasminogen fragment composed of kringles 1-3 (2 μM) was incubated with 5 nM scuPA/suPAR complex (Comp.), scuPA (5 nM), or tcuPA (5nM). Glu-plasminogen (20 nM) and the plasmin chromogenic substrate (0.5 mM) were added and the OD was measured continuously over the next 20 minutes.