The regulation of pea-seed phosphofructokinase by phosphoenolpyruvate.

1. Pea-seed phosphofructokinase was purified 27-fold by a combination of fractionation with ethanol and ammonium sulphate. Under the conditions of assay, the enzyme was strongly inhibited by phosphoenolpyruvate. This inhibition was reversed by increasing the concentration of fructose 6-phosphate or magnesium chloride, or by lowering the ATP concentration. 2. Citrate, ADP and AMP inhibited phosphofructokinase and increased the sensitivity to phosphoenolpyruvate inhibition. Sulphate and inorganic phosphate stimulated the enzyme activity and decreased the sensitivity to phosphoenolpyruvate. 3. In the presence of inorganic phosphate and low concentrations of ATP, inhibition by phosphoenolpyruvate ceased and phosphoenolpyruvate became stimulatory. 4. The possible significance of these results in the control of plant carbohydrate metabolism is discussed.