Cytosolic ATP-Dependent Phosphofructokinase from Spinach

ATP-dependent 6-phosphofructokinase (PFK) activity is present in both chloroplastic and in nonchloroplastic fractions isolated from spinach protoplasts. The activity in the extra-chloroplastic fraction was stimulated 2- to 3.5-fold by 25 mm inorganic phosphate (Pi), the chloroplast-associated activity was inhibited 2- to 5-fold. The Pi stimulated activity was ATP-dependent and was not an artifact due to the presence of fructose 6-P, Pi, pyrophosphatase, and pyrophosphate fructose 6-P 1-phosphotransferase (PFP). PFK activities, which expressed characteristics similar to those separated from protoplasts, could be separated following ammonium sulfate fractionation of crude extracts; the ammonium sulfate treatment also separated both PFK activities from PFP. It is concluded that spinach leaves contain a cytosolic PFK. This activity is relatively stable, is stimulated by Pi over a wide pH range, is not a result of the transformation of another enzyme activity, and has an activity that is similar to, or slightly less than, that of the cytosolic PFP.