In perfused rat liver, the effects of various hormones on the stimulation of phosphorylation and allosteric properties of purified phosphorfructokinase were investigated. Rat livers were perfused with [32P]phosphate followed with various hormones or cyclicAMP, and 32P-labeled phosphofructokinase was isolated. 32P incorporation into the enzyme and enzyme inhibition by ATP or citrate were determined. Only glucagon increased the 32P incorporation into phosphofructokinase and this increase was approximately threefold. The cyclicAMP level was increased simultaneously approximately four- to fivefold compared to the control perfused liver. Similar results were obtained by perfusing the liver with cyclicAMP (0.1 mm). The phosphorylated phosphofructokinase showed a decrease in the Ki values for ATP (from 0.4 to 0.2 mm) and citrate (from 2 to 0.6 mm). Neither epinephrine nor insulin affected the extent of phosphorylation or the allosteric properties of the enzyme. The half-maximal concentration of glucagon required for phosphorylation of phosphofructokinase and modification of its allosteric properties was approximately 6 × 10−11m. It is concluded that glucagon increases the inhibition of liver phosphofructokinase by ATP and citrate through phosphorylation of the enzyme involving a β-receptor-mediated cyclicAMP-dependent mechanism.
ASJC Scopus subject areas
- Molecular Biology