Plasmin-independent activation of pro-uPA bound to THP-1 cells. (A) Pro-uPA (○, •) or active tc-uPA (□, ▪) was incubated with either U937 (open symbols) or THP-1 (closed symbols) cells for 10 minutes, washed to remove unbound enzyme, and assayed directly for uPA activity using the substrate H-Glu-Gly-Arg-AMC (EGR-AMC). (B) The data for bound pro-uPA are converted to pro-uPA activation from the rate of substrate hydrolysis with time, and expressed as a percentage of the activity of bound tc-uPA. For THP-1 cells (•), uPA activity is extrapolated back to zero activation, which coincides with the start of the incubation with pro-uPA. The inset shows the activation of pro-uPA in the cell supernatant over the same time period, which is below 0.2% for both cell types.