Properties of phosphofructokinase from rat liver and their relation to the control of glycolysis and gluconeogenesis

1. Phosphofructokinase from rat liver has been partially purified by ammonium sulphate precipitation so as to remove enzymes that interfere in one assay for phosphofructokinase. The properties of this enzyme were found to be similar to those of the same enzyme from other tissues (e.g. cardiac muscle, skeletal muscle and brain) that were previously investigated by other workers. 2. Low concentrations of ATP inhibited phosphofructokinase activity by decreasing the affinity of the enzyme for the other substrate, fructose 6-phosphate. Citrate, and other intermediates of the tricarboxylic acid cycle, also inhibited the activity of phosphofructokinase. 3. This inhibition was relieved by either AMP or fructose 1,6-diphosphate; however, higher concentrations of ATP decreased and finally removed the effect of these activators. 4. Ammonium sulphate protected the enzyme from inactivation, and increased the activity by relieving the inhibition due to ATP. The latter effect was similar to that of AMP. 5. Phosphofructokinase was found in the same cellular compartment as fructose 1,6-diphosphatase, namely the soluble cytoplasm. 6. The properties of phosphofructokinase and fructose 1,6-diphosphatase are compared and a theory is proposed that affords dual control of both enzymes in the liver. The relation of this to the control of glycolysis and gluconeogenesis is discussed.     

Effect of calcitonin on fructose 1,6-diphosphatase activity in rat liver: role of cytosolic calcium concentration

The effect of calcitonin (CT) on fructose 1,6-diphosphatase activity in the hepatic cytosol was investigated after a single subcutaneous administration of the hormone to rats. Administration of CT (synthetic [Asu1,7] eel CT; 80 MRC mU/100 g body weight) produced significant increase in fructose 1,6-diphosphatase activity and calcium content in the hepatic cytosol of intact and thyroparathyroidectomized rats. Those alterations were also observed with the dose of CT at physiological level. The binding of calcium by 10 microM EGTA in the hepatic cytosol caused a clear reduction of the increase in fructose 1,6-diphosphatase activity produced by CT administration. The enzyme activity of CT-treated rats was markedly reduced by W-7 (100 microM), calmodulin inhibitor, while that of control rats was not significantly altered by the drug. Meanwhile, fructose 1,6-diphosphatase activity in the hepatic cytosol obtained from normal rats was significantly enhanced by addition of calcium ion (0.1-5.0 microM). This increase was also clearly inhibited by W-7. These results suggest that CT increases fructose 1,6-diphosphatase activity in the hepatic cytosol of rats, and that this hormonal regulation may depend on calmodulin, mediated through calcium increased in the cytosol.