Studies on the effect of fructose diphosphatase on phosphofructokinase

Chicken liver fructose diphosphatase was prepared in a homogeneous form. The molecular weights of the enzyme and the dissociated enzyme were determined to be 142,000 and 38,000, respectively, by a high speed sedimentation equilibrium technique and by acrylamide gel electrophoresis in sodium dodecyl sulfate. The effect of fructose diphosphatase on phosphofructokinase was investigated using purified preparations of these enzymes from chickens liver and rabbit muscle. Fructose diphosphatase enhances ATP inhibition of phosphofructokinase. However, fructose diphosphatase which has been treated with homocystine, CoA, p chloromercuribenzoate, or iodacetamide loses this ability to enhance the ATP inhibition of phosphofructokinase. Among various proteins examined, only fructose disphosphatase showed this effect. Phosphofructokinase shows sigmoidal kinetics with respect to fructose 6 P, but in the presence of fructose diphosphatase the sigmoidicity is increased significantly and s(0.5) for fructose 6 P is increased with increasing fructose diphosphatase concentration. ATP inhibition of phosphofructokinase is less at pH 8 than at pH 7.4; however, fructose diphosphatase enhances this inhibition at pH 8. Fructose diphosphatase also increases the inhibition of phosphofructokinase by 3 P glycerate and citrate. The molar ratios of fructose diphosphatase to phosphofructokinase which give 50% inhibition vary from 100 to 400 depending upon the sources of the enzymes. These kinetic studies suggest that fructose diphosphatase may cause conformational changes in phosphofructokinase. In order to examine a possible interaction between these enzymes, the effect of fructose disphosphatase on the fluorescence of phosphofructokinase anilinonaphthol sulfonate complex was investigated. Adenine nucleotides quench the fluorescence of the complex, and fructose diphosphatase enhances the quenching of the fluorescence further. Homocysteine or CoA derivatives of fructose disphosphatases, however, showed no effect on the fluorescence. These fluorescence data are consistent with the kinetic results and suggest that fructose diphosphatase enhances allosteric effects of phosphofructokinase. A possible significance of these observations in the regulation of the futile cycle and also coordinated control of glycolysis and gluconeogenesis is discussed.