Inhibition of hexokinase activity by a fructose 2,6-bisphosphate-dependent cytosolic protein from liver

A protein from rat liver confers to glucokinase the property of being antagonistically regulated by fructose 6-phosphate and fructose 1-phosphate

At a concentration of 1 mM, fructose 1-phosphate stimulated about twofold, and glucose 6-phosphate inhibited by about 30%, the phosphorylation of 5 mM glucose in high-speed supernatants prepared from rat liver or from isolated hepatocytes, but did not affect, or barely so, the activity of a partially purified preparation of glucokinase. Anion-exchange chromatography of liver extracts separated glucokinase from a fructose-6-phosphate-sensitive and fructose-1-phosphate-sensitive inhibitor of that enzyme. This inhibitor could be further purified by chromatography on phospho-Ultrogel. It was destroyed by trypsin and was heat-labile. It inhibited glucokinase competitively with respect to glucose and its inhibitory effect was greatly reinforced by fructose 6-phosphate although not by glucose 6-phosphate. Fructose 1-phosphate relieved the enzyme of the inhibitory effect of the regulator and antagonised the effect of fructose 6-phosphate in a competitive manner. It is concluded that the regulator plays a role in the physiological control of the activity of glucokinase, particularly with respect to the stimulatory effect of fructose in isolated hepatocytes (see preceding paper in this journal).

Phosphofructokinase is responsible for the fructose 2,6-bisphosphate inhibition of hexokinase in tissue extracts

Mammalian and yeast hexokinases were reported to be reversibly inhibited by fructose 2,6-bisphosphate in the presence of cytosolic proteins (H. Niemeyer, C. Cerpa, and E. Rabajille (1987) Arch. Biochem. Biophys. 257, 17-26). Reinvestigation of this finding using a radioassay with [14C]glucose as substrate showed no effect of fructose 2,6-bisphosphate on hexokinase activity of rat liver cytosols. Detailed reexamination of the spectrophotometric assay resulted in the observation that the fructose 2,6-bisphosphate-dependent inhibition was a function of the cytosolic phosphoglucose isomerase and phosphofructokinase activities compared to the amount of glucose-6-phosphate dehydrogenase used as auxiliary enzyme. The diminution or loss of the fructose 2,6-bisphosphate-dependent inhibition produced in aged cytosols was restored by addition of crystalline muscle phosphofructokinase, as well as by decreasing the amount of glucose-6-phosphate dehydrogenase in the assay. When phosphoglucose isomerase, phosphofructokinase, and hexokinase activities were separated by DEAE-chromatography of liver cytosol, no fructose 2,6-bisphosphate-dependent inhibition of hexokinase was found in any single fraction of the chromatogram. However, combination of fractions containing both phosphoglucose isomerase and phosphofructokinase displayed the fructose 2,6-bisphosphate-dependent inhibition on either endogenous hexokinase or added yeast hexokinase. From these results we conclude that the activation of phosphofructokinase elicited by fructose 2,6-bisphosphate is responsible for the hexokinase inhibition observed in the coupled spectrophotometric assay.