Regulation of the Level of Key Enzymes of Glycolysis and Gluconeogenesis in Liver

Abstract

Diets rich in fructose, glycerol, or both were fed to normal and diabetic rats, and the activities of enzymes which catalyze irreversible steps of glycolysis and gluconeogenesis were studied in liver. With these diets, rich in precursors of triosephosphates, the activities of glucose-6-phosphatase and pyruvate kinase in liver were high, while those of glucokinase and phosphoenolpyruvate carboxykinase were low. A metabolic bifurcation to glycolysis and gluconeogenesis, below and above the trioscphosphates level respectively, has then been shown to be easily maintained in liver.It is suggested that the induction of glucokinase and pyruvate kinase in diabetic animals after insulin administration is a sequential process that involves hormonal induction of glucokinase by insulin and secondary metabolite induction of the L isoenzyme of pyruvate kinase by some glycolytic intermediate. The occurrence of independent mechanisms for the regulation in liver of the activity of enzymes which catalyze irreversible steps of glycolysis and gluconeogenesis affords a valuable metabolic plasticity.Changes in the apparent concentration of a number of enzymes in liver have been observed in relation to the nutritional and hormonal conditions of the animal. Some of these changes could be of considerable significance in the regulation of major metabolic pathways. Glucose-6-phosphatase [l] glycerol, both triosephosphate prccursors that by-pass glucokinase and phosphofructokinase [13]. With this approach, non-coordinated variations in the activity of enzymes which catalyze physiologically irreversible steps of glycolysis and gluconeogenesis in liver have been observed in normal and diabetic rats. A preliminary report of this finding has already appeared [14]. MATERIALS AND METHODSWhite rats weighing 140-2OOg were used. Animals were made diabetic with alloxan (intravenously, 40 mg/kg body weight) ; treated animals were considered diabetic when the glucosuria reached values of at least 0.5 g/100 ml.Control rats were fed a stock diet ad libitum. The composition of the fructose, glycerol, and fructoseglycerol diets utilized was: 30°/, casein, 5O/, fat, and 5 0 / , cellulose, supplemented with minerals and vitamins, and 600/, fructose, or 60°/, glycerol, or 30°/, fructose and 30°/, glycerol, respectively. Groups of animals were maintained on these diets for 3 to 5 days. They ate the fructose and the fructoseglycerol diets well, but the glycerol diet not so well. The rats were killed by decapitation. The livers were excised and homogenized with two volumes-of 0.1 M Tris pH 7.2-1 mM EDTA-1 mM ethanethiol in a Kontes-Dual1 Grinder. The homogenates were centrifuged at 3000Oxg for 20 min. All operations were

The Problem of Cell Heterogeneity of Liver Tissue in the Study of Fructose Metabolism

“…A detailed analysis of enzyme activities and substrate affinities in rat liver tissue favours this pathway [3]. Furthermore recent results from experiments in vivo with [4-3H]fructose infusions into mice, demonstrate that glyceraldehyde formed in the liver by aldolic splitting of fructose 1-phosphate is metabolized by phosphorylation into glyceraldehyde 3-phosphate by triokinase [31].…”

“…I n experiments with [2-14C]fructose there is no correlation between the specific 14C activities of phosphoglycerate, phosphoenolpyruvate and intracellular lactate indicating compartmentation of these metabolites. 3. By comparing the specific 14C activities of the C4-C5-C6 triose unit of newly formed glucose (taken as representative of the specific 14C activity of glyceraldehyde-3-phosphate in the "gluconeogenic" compartment) with the specific 14C activity of cellular lactate, the following can be postulated.…”

Regulation of Mammalian Fatty-Acid Synthetase. The Roles of Carbohydrate and Insulin

“…First, liver is not dependent on insulin for maintenance of normal synthetase activity, and, therefore, regulation of fatty-acid synthetase is unlike the regulation of an enzyme such as glucokinase (21). Thus, feeding of fructose led to a marked increase in synthesis of the synthetase in liver of diabetic rats, whereas such feeding does not correct the defect of hepatic glucokinase activity in diabetic animals (21). In contrast to the liver enzyme, fatty-acid synthetase of adipose tissue may be dependent on insulin for regulation of its activity, although this effect of the hormone may be secondary to effects on glycolysis and/or sugar transport (see below).…”

Enzymes Involved in Fructose Metabolism in Liver and the Glyceraldehyde Metabolic Crossroads

“…This low K , value, which contrasts with those of the other enzymes in Table 2, clearly points to a preferential transformation of glyceraldehyde to glyceraldehyde-3-phosphate in fructose metabolism in liver. Furthermore, fructokinase and triokinase have a parallel distribution and development in the rat [39]. I n the absence of cofactor limitation (unlikely except in the case of the NADH dependent reaction) or allosteric effects (none is known for these enzymes), the relative activity of each enzyme at a metabolic crossroads is in principle a function of the respective V, , , , K,, and the common substrate concentration, according to the classical Michaelis equation.…”