Opposite changes with age in liver and muscle in the mitochondrial and soluble glucose-1,6-bisphosphate and 6-phosphogluconate dehydrogenase

Glucose-1,6-bisphosphate (Glc-1,6-P2), the powerful regulator of carbohydrate metabolism, was markedly decreased in liver of adult rats (2 months of age) as compared to young rats (1-2 weeks of age). This regulator was found to be present in both the mitochondrial and soluble fractions of liver. Its concentration in both these fractions was decreased with age. Concomitant to the decrease in Glc-1,6-P2, which is a potent inhibitor of 6-phosphogluconate dehydrogenase, the activity of this enzyme was markedly increased with age in both the mitochondrial and soluble fractions. However, the increase in this enzyme's activity was more pronounced in the mitochondrial fraction. The mitochondrial enzyme was more susceptible to inhibition by Glc-1,6-P2 as compared to the soluble enzyme, and this may explain the greater enhancement in its activity with age in this fraction. The tibialis anterior muscle exhibited changes with age opposite to those found in liver; Glc-1,6-P2 concentration, in both the mitochondrial and soluble fractions of muscle increased with age, and this increase was accompanied by a concomitant reduction in the activity of the mitochondrial and soluble 6-phosphogluconate dehydrogenase. Similar to liver, the mitochondrial enzyme was more affected by age, as it also exhibited a greater susceptibility to inhibition by Glc-1,6-P2.

Inhibition of 6-phosphogluconate dehydrogenase by glucose 1,6-diphosphate in human normal and malignant colon extracts

Increased activity of 6-phosphogluconate dehydrogenase was found in human colon tumors as compared to the adjacent unaffected mucosa. Glucose 1,6-diphosphate (Glc-1,6-P2), an endogenous potent regulator of glucose metabolism, markedly inhibited the activity of 6-phosphogluconate dehydrogenase (6-PGD) in extracts of the normal and malignant human colon. Glc-1,6-P2 also inhibited the activity of hexokinase in these extracts. The endogenous levels of Glc-1,6-P2 in the colon and tumors were measured. Since the pentose cycle can be inhibited by Glc-1,6-P2, means to increase endogenous levels of Glc-1,6-P2 or to introduce it into cells, might result in antitumor effects.

General stability of thermophilic enzymes: studies on 6-phosphogluconate dehydrogenase from Bacillus stearothermophilus and yeast

The thermophilic enzyme 6-phosphogluconate dehydrogenase (6-phospho-D-gluconate:NADP oxidoreductase, decarboxylating, EC 1.1.1.44) from Bacillus stearothermophilus was much more resistant to inactivation under different conditions of temperature, pH, guanidine-hydrochloride, and organic solvents (dioxane, dimethylformamide, acetone) than its mesophilic counterpart from yeast. In addition, the thermophilic enzyme largely withstands proteolysis with trypsin, chymotrypsin, and elastase when compared with the yeast enzyme. It is proposed that thermophilic enzymes are not only thermostable, but also generally more stable to most common protein denaturants than their mesophilic counterparts. Because of their remarkable stability, enzymes isolated from thermophilic microorganisms may be ideally suited for technological applications.

The rate of utilization of glucose via hexosemonophosphate shunt in brain

The concentration of 6-phosphogluconate in the brain increased from 0-24 nmol/g in the controls to 1430 and 1506 nmol/g in rats treated with 50 mg of 6-aminonicotinamide/kg of body weight. A dose-dependent increase in the concentrations of glucose and glucose 6-phosphate as well as of 6-phosphogluconate was found in the brains of 6-aminonicotinamide-treated rats. The biochemical changes and symptoms of neurological disorder in 6-aminonicotinamide-treated rats were not due to hypothermia. The rate of utilization of glucose via the hexosemonophosphate shunt was determined by isolation of gluconate from 6-phosphogluconate and measurement of its [14C]content at short time intervals after injection of [U-14C]glucose into 6-aminonicotinamide-treated rats; it was 16.5 nmol of glucose utilized/min per g of brain, and represented approximately 2.3% of the overall utilization of glucose in the brain. A highly significant correlation was observed between the concentration of 6-phosphogluconate and the concentration of glucose 6-phosphate and free glucose. The validity of this correlation was supported by the results of previous investigations involving several other treatments.

Inhibition of the pentose phosphate shunt by 2,3-diphosphoglycerate in erythrocyte pyruvate kinase deficiency

Pentose phosphate shunt activity was studied by the release of 14CO2 from 14C-1-glucose and 14C-2-glucose in the red cells of five patients with pyruvate kinase deficiency and found to be significantly decreased after new methylene blue stimulation when compared to high reticulocyte controls. Incubated Heinz body formation was increased and the ascorbate cyanide test was positive in blood from these patients. The activity of glucose-6-phosphate dehydrogenase (G6PD) as well as that of 6-phosphogluconate dehydrogenase (6PGD) was inhibited to 20% of baseline in normal red cell haemolysate by 4 mM 2,3-diphosphoglycerate at pH 7.1. 2,3-Diphosphoglycerate was a competitive inhibitor with 6-phosphogluconate (Ki=1.05 mM) and a noncompetitive inhibitor with NADP (Ki=3.3 mM) for 6PGD. Since the intracellular concentrations of glucose-6-phosphate, 6-phosphogluconate and NADP are below their Kms for G6PD and 6PGD, the kinetic data suggest that increased concentrations of 2,3-diphosphoglycerate in pyruvate kinase deficient red cells are sufficiently high to suppress pentose phosphate shunt activity. This suppression may be an additional factor contributing to the haemolytic anaemia of pyruvate kinase deficiency, particularly during periods of infection or metabolic stress.

The effect of inorganic phosphate on the stability of some enzymes

In the presence of 5nM-P1, 6-phosphogluconate dehydrogenase from the yeast Candida utilis is more resistant to proteolysis and to the inactivating action of some chemical and physical agents. P1 also protects other enzymes against proteolysis. A hypothesis for the mechanism of the stabilizing action of P1 is advanced.

Inhibition of 6-phosphogluconate dehydrogenase (decarboxylating) by glucose 1,6-bisphosphate

Glucose 1,6-bisphosphate, the powerful common regulator of several key enzymes in carbohydrate metabolism, was found to exert a potent inhibitory effect on the activity of 6-phosphogluconate dehydrogenase (decarboxylating) from yeast and several rat tissues. These findings suggest that glucose 1,6-bisphosphate may have a regulatory influence on the pentose phosphate pathway.

Purification and properties of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase from a methanol-utilizing yeast, Candida boidinii

Glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate: NADP oxidoreductase, EC 1.1.1.49) and 6-phosphogluconate dehydrogenase (6-phospho-D-gluconate: NADP oxidoreductase, EC 1.1.1.44) were purified approx. 1700 fold and 330 fold, respectively, from Candida boidinii grown on methanol. The final enzyme preparations were homogeneous as judged by polyacrylamide gel electrophoresis. The molecular weights of the enzymes were estimated to be 118 000 and 110 000, respectively. Both enzymes are composed of two probably identical subunits and the molecular weights of the polypeptide chains were calculated to be 61 000 and 58 000, respectively. From a consideration of enzyme activities and types of inhibition by different metabolites the role of these two enzymes in glucose- and methanol-metabolism is discussed.

Selective denaturation of several yeast enzymes by free fatty acids

The denaturation of eight purified yeast enzymes, glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, 3-phosphoglycerate kinase, alcohol dehydrogenase, beta-fructosidase, hexokinase and glucose-6-phosphate isomerase, promoted under controlled conditions by the free fatty acids myristic and oleic, is selective. Glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate:NADP+ 1 oxidoreductase, EC 1.1.1.49) is extremely sensitive to destabilization and was studied in greater detail. Results show that chain length and degree of unsaturation of fatty acids are important to their destabilizing effect, and that ligands of the enzyme can afford protection. The denaturation process results in more than one altered form. These results can be viewed in the perspective of the possibility that amphipathic substances, and in particular free fatty acids, may play a role for enzyme degradation in vivo, by initiating steps of selective denaturation.

Regulation of the oxidative phase of the pentose phosphate cycle in mussels

1. The mechanisms that control the oxidative phase of the pentose phosphate cycle in mussel hepatopancreas were investigated. 2. The effects of GSSG (oxidized glutathione) on the inhibition of glucose 6-phosphate dehydrogenase by NADPH [Eggleston & Krebs (1974) Biochem. J. 138, 425-435] extend to 6-phosphogluconate dehydrogenase. 3. The effect of GSSG on both enzymes increases as the [NADP+1]/[NADPH] ratio decreases; greater percentage deinhibition always was obtained for 6-phosphogluconate dehydrogenase. 4. Increasing concentration of GSSG increased the percentage deinhibition. This effect is more pronounced with 6-phosphogluconate dehydrogenase. 5. We confirmed the apparent imbalance between the activities of the two enzymes [sapag-Hagar, Lagunas & Sols (1973) Biochem. Biophys. Res. Commun, 50, 179-185] in the presence of 10mM-Mg2+. 6. The imbalance practically disappears when the substrate concentrations are less than saturating and Mg2+ approaches physiological concentrations. 7. The addition of GSSG at physiological concentrations allows the activities of both enzymes to be measured at high [NADPH]/[NADP+] ratios ratios and the co-operative action of GSSG and Mg2+ on the imbalance between the two enzymes to be verified. 8. The control of the activity of the two enzymes of the pentose cycle could be carried out by deinhibition of the two dehydrogenases and by the intracellular concentrations of substrates and inorganic ions.

Effectors of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase of rat liver

The lower Vmax of 6PGDH with respect to G6PDH and its higher sensitivity to inhibition by NADPH, suggest the existence of an imbalance between the two dehydrogenases of the pentose phosphate pathway in rat liver. Possible modulators of these activities, particularly in relation with the inhibition by NADPH in physiological conditions, have been investigated. The results suggest that in both cases the inhibition by NADPH is strictly isosteric and that the relative affinities for the reduced and oxidized forms of the pyridine nucleotide are unaffected by glutathion, the intermediates of the pentose phosphate shunt or some divalent ions.

Spastic paresis after 6-aminonicotinamide: metabolic disorders in the spinal cord and electromyographically recorded changes in the hind limbs of rats

In rats the application of 10 mg/kg 6-amino-nicotinamide (6-AN) leads to an accumulation of 6-phosphogluconate, by inhibition of 6-phosphogluconate dehydrogenase in the pentose phosphate pathway, in the cells of the spinal cord. The accumulation reaches its maximum after 18-24 h. It seems that there exists a relationship between the accumulation of 6-phosphogluconate and the lesion of the neuroglia, which is found in electron microscopic studies. Symptoms of a spastic paresis only develop later when the spinal interneurones are destroyed as a consequence of the lesion of the neuroglia. The accumulation of 6-phosphogluconate almost exceeds the 400 fold of the norm. No considerable differences are found between the effects of a dose of 35 mg 6-AN/kg and one of 10 mg 6-AN/kg. Free gluconate is identified enzymically in the cells of the spinal cords of the rats treated with 6-AN. The compound is very probably formed by dephosphorylation and diffuses into the blood. 6-Phosphogluconate is an inhibitor of the phosphoglucose isomerase. Its accumulation shifts the equilibrium towards glucose 6-phosphate. The lactate concentration decreases as compared with the untreated controls. Muscular action potentials are recorded extracellularly with a concentric needle electrode from the musculus gastrocnemius of rats treated with 6-AN. First activations of the electromyograms are found 48 h after the application of 10 mg 6-AN/kg. The electrical activities increase during the time in which a progressive destruction of the interneurones occurs. The electromyogram displays a permanent state of excitation with high amplitudes and an increased frequency. The continuity and intensity of the increased activity recorded by the electromyograph is the most important pathological finding. p-Chlorophenyl-GABA and, still more so, chlorpromazine cause temporary reduction of the excitation processes and an electromyogram nearly at rest. Under the same conditions, haloperidol is only slightly effective. The symptoms developed by the chemical destruction of the interneurones of the spinal cord, with rigidity and spasticity of the hind limbs, are suitable for testing antispastic drugs.

Identification of the chemical groups involved in the binding of periodate-oxidized NADP+ to 6-phosphogluconate dehydrogenase

Periodate-oxidized NADP+ binds specifically and reversibly to the NADP+ binding site of 6-phosphogluconate dehydrogenase (EC 1.1.1.44) from Candida utilis. The inhibition can be stabilized by reduction with sodium borohydride. It has been shown that an aldehydic group of the inhibitor forms a Schiff base with a lysine residue of the enzyme.

An analysis of intermediary metabolism and its control in a fat-synthesizing yeast (Candida 107) growing on glucose or alkanes

Enzymes of glycolysis, pentose phosphate pathway, gluconeogenesis, tricarboxylate acid cycle, glyoxylate by-pass and fatty-acid biosynthesis were assayed in extracts from Candida 107 grown continuously on glucose under carbon limitation, nitrogen limitation and on n-alkanes. The yeast was therefore either in a lipogenic or lipolytic state. Phosphofructokinase was absent under all conditions whereas enzymes of gluconeogenesis, including fructose 1,6-bisphosphatase and the pentose phosphate cycle, were all present. Glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase were specific for NADP+ and were inhibited in a non-competitive manner by NADPH and NADH. Phosphoenolpyruvate, citrate, ATP and acetyl CoA had no inhibitory effects. Thus glucose metabolism appears to be by the pentose phosphate pathway which will rapidly produce NADPH. This can readily be consumed during fatty-acid biosynthesis and, as there appears to be no inhibition of the flow of carbon from glucose to acetyl CoA, fatty-acid synthesis can continue for as long as there is a supply of glucose. These results help to explain the probable causes of fat build-up to high concentrations (about 40% of the cell dry weight) in this and other organisms. In alkane-grown cells, lipogenesis is repressed and carbon is able to flow from the alkanes via acetyl CoA, oxaloacetate and pyruvate into pentoses and hexoses in a unidirectional manner, because of the strong repression of pyruvate kinase and the increased activities of phosphoenolpyruvate kinase and fructose 1,6-biosphosphatase under these conditions. Although there was little change in the total activity of the TCA cycle enzymes under the various growth conditions, isocitrate lyase was induced under lipolytic conditions.