Detailed kinetic characterization of the activation of lys-plasminogen by uPA in the presence of Nalm6 cells. Reaction mixtures contained 15 pmol/L uPA, and both cell and lys-plasminogen concentration were varied. Curve fitting to the standard Michaelis Menten equation was performed to determine the apparent k_cat (A), K_m (B), and k_cat/K_m (C) values for each cell concentration. Improvements in both apparent K_m and k_cat are responsible for the observed stimulation of kinetics by cells under these conditions. Values are shown as fitted estimates from direct fitting to Michaelis Menten equation ± SE of the mean.