FIG. 4.
Activity of phosphofructokinase in cell lines with different extracellular CF activity. smlA mutant, wild-type (WT), and countin mutant cells were starved by shaking in PBM and collected 6 h later. The activity of phosphofructokinase was measured by varying the concentration of fructose-6-phosphate in the assay buffer and by coupling the reaction to the consumption of NADPH. Values are means ± standard error of the mean from five independent assays. The lines show Michaelis-Menten equations fit to the data with nonlinear regression. The Vmax of wild-type cells was 0.6 nmol/mg of protein/min, and the Km was 0.01 mM. The arrow indicates the physiological level of fructose-6-phosphate (≈43 μM).