Mechanism of action of glucagon on hepatocyte phosphofructokinase activity

Addition of glucagon to isolated hepatocytes reduced the activity of 6-phosphofructokinase (ATP:D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11) and pyruvate kinase (ATP:pyruvate 2-O-phosphotransferase, EC 2.7.1.40). Phosphorylation contributed to the inhibition of pyruvate kinase, but several lines of evidence indicated that this reaction was not responsible for the inhibition of phosphofructokinase. First, the increase in phosphorylation in intact cells induced by increasing the concentration of glucagon did not correlate well with the decrease in enzyme activity. Second, phosphorylation of phosphofructokinase induced by addition of cyclic AMP and Mg²⁺-ATP or by addition of Mg²⁺-ATP and the catalytic subunit of the cyclic AMP-dependent protein kinase to hepatocyte extracts had no effect on enzyme activity. Third, ammonium sulfate precipitation of the enzyme from extracts of cells incubated with glucagon abolished the hormone effect. The effect could be restored, however, by the addition of a phosphofructokinase-free extract from glucagon-treated cells to the ammonium sulfate-treated enzyme from either untreated or glucagon-treated cells. These results suggest that the inhibition of phosphofructokinase by glucagon is due to changes in the level of an allosteric effector(s).