A critical appraisal of the effect of oxidized glutathione on hepatic glucose 6-phosphate dehydrogenase activity

Inhibition of glutathione reductase uncovers the activation of NADPH-inhibited glucose-6-phosphate dehydrogenase

Eggleston and Krebs pointed to a paradox in glucose-6-phosphate dehydrogenase (G6PD) regulating process that has not yet been solved, and which originated the term "fine regulation" of G6PD and, therefore, of oxidative phase of pentose phosphate pathway (OPPP). The paradox is that, in basal-like conditions, the activity of G6PD evaluated "in vitro" is very low or nearly null because of the potent inhibiting effect exerted by NADPH, a coenzyme whose concentration in the cell is much higher than that of the substrate NADP⁺ . However, "in vivo," flow through OPPP occurs in basal conditions. Eggleston and Krebs speculated on the possible existence of a system that would reverse the inhibition by NADPH. Such system would involve oxidized glutathione and exclude the participation of glutathione reductase (GR). The present work confirms the experimental results obtained by Eggleston and Krebs and proves that oxidized glutathione (GSSG) in the absence of NADPH is a direct inhibitor of G6PD. In the presence of GSSG, the G6PD activity recovery system suggested can be observed when GR is previously inhibited by alkylating agents. An unknown element with a molecular weight ranging between 12 and 50 kDa has been found to reverse part of G6PD inhibition by NADPH.

Mixed results with mixed disulfides

A period of research with Helmut Sies in the 1980s is recalled. Our experiments aimed at an in-depth understanding of metabolic changes due to oxidative challenges under near-physiological conditions, i.e. perfused organs. A major focus were alterations of the glutathione and the NADPH/NADP(+) system by different kinds of oxidants, in particular formation of glutathione mixed disulfides with proteins. To analyze mixed disulfides, a test was adapted which is widely used until today. The observations in perfused rat livers let us believe that glutathione-6-phosphate dehydrogenase (G6PDH), i.a. might be activated by glutathionylation. Although we did not succeed to verify this hypothesis for the special case of G6PDH, the regulation of enzyme/protein activities by glutathionylation today is an accepted posttranslational mechanism in redox biology in general. Our early experimental approaches are discussed in the context of present knowledge.

The regulation of the oxidative phase of the pentose phosphate pathway: new answers to old problems

There is a paradox in the oxidizing phase of the phosphate pentose pathway that has not yet been solved. The flow through the pathway is reduced in basal conditions; however, it must rise notably when a NADPH supplement is required. The paradox consists of the strong inhibition that the NADPH exerts on the both dehydrogenases of the pathway, especially on the regulating enzyme glucose-6-phosphate dehydrogenase (G6PD). Theoretically, in anabolic situations, the increase of gene expression of G6PD and 6-phosphogluconate dehydrogenase can induce a rise in the production of NADPH, which would cause the immediate inhibition of the enzyme and a drastic flow reduction. However, increasing the flow through oxidative phase of the pentose phosphate pathway (OPPP) has been experimentally demonstrated in many physiological states. However, this situation will be resolved if the NADPH metabolized or otherwise sufficient NADPH is sequestered to relax the inhibition of the dehydrogenases of OPPP and to maintain high ratio of NADPH/NADP(+) needed to ensure the reducing environment of the cell cytoplasm and the contribution of NADPH for anabolic processes. In 1974, the presence of a protein capable of reversing the inhibition of G6PD by NADPH was detected; however, to date, this paradox remains undisclosed. This review deals with the possibility that such reverting action might be similar to the activity of a protein named HSCARG, which is responsible for the abduction of NADPH, thus keeping a portion of the coenzyme away from the catalytic action and, simultaneously, the immune response through the NF-κB (nuclear factor kappa light-chain enhancer of activated B cells) system. The model has many similarities with the hypothesis proposed some 40 years back on the reversion of G6PD inhibition by NADPH.

Impact of glucose-6-phosphate dehydrogenase deficiency on the pathophysiology of cardiovascular disease

Glucose-6-phosphate dehydrogenase (G6PD) catalyzes the rate-determining step in the pentose phosphate pathway and produces NADPH to fuel glutathione recycling. G6PD deficiency is the most common enzyme deficiency in humans and affects over 400 million people worldwide; however, its impact on cardiovascular disease is poorly understood. The glutathione pathway is paramount to antioxidant defense, and G6PD-deficient cells do not cope well with oxidative damage. Limited clinical evidence indicates that G6PD deficiency may be associated with hypertension. However, there are also data to support a protective role of G6PD deficiency in decreasing the risk of heart disease and cardiovascular-associated deaths, perhaps through a decrease in cholesterol synthesis. Studies in G6PD-deficient (G6PDX) mice are mixed and provide evidence for both protective and deleterious effects. G6PD deficiency may provide a protective effect through decreasing cholesterol synthesis, superoxide production, and reductive stress. However, recent studies indicate that G6PDX mice are moderately more susceptible to ventricular dilation in response to myocardial infarction or pressure overload-induced heart failure. Furthermore, G6PDX hearts do not recover as well as nondeficient mice when faced with ischemia-reperfusion injury, and G6PDX mice are susceptible to the development of age-associated cardiac hypertrophy. Overall, the limited available data indicate a complex interplay in which adverse effects of G6PD deficiency may outweigh potential protective effects in the face of cardiac stress. Definitive clinical studies in large populations are needed to determine the effects of G6PD deficiency on the development of cardiovascular disease and subsequent outcomes.

Influence of selenium on glutathione and some associated enzymes in rats with mammary tumor induced by 7,12-dimethylbenz(a)anthracene

A recent finding in epidemiological and laboratory studies suggests that the ratio of selenium to glutathione is lower in breast cancer subjects than its control counterparts. Selenium, an antioxidant and anticarcinogen, can modify the status of glutathione and some associated enzymes by blocking peroxidation of lipids in membranes of cancer subjects. Studies were conducted using female albino rats of Wistar strain bearing mammary tumor induced by 7,12-dimethylbenz(a) anthracene to assess the biological role of selenium on some antioxidant enzymes associated with the maintenance of glutathione status. For induction of mammary tumor, 25 mg DMBA in a 1 ml emulsion of sunflower oil and physiological saline was injected subcutaneously to each rat. One group in each of control and tumor bearing rats, were fed 5 mg sodium selenite/kg diet from the day of tumor induction for 24 weeks. Increase in the reduced glutathione concentration was preceded by significant increase in the oxidized glutathione as well as in the activities of gamma-glutamylcysteine synthetase, glutathione peroxidase, glutathione reductase, glutathione S-transferase, and glucose-6-phosphate dehydrogenase by selenium administration in rats bearing tumor. However, selenium administration to rats bearing tumor decreased the activity of gamma-glutamyl transpeptidase. These observations clearly demonstrate the influence of dietary selenium supplementation in correcting abnormal changes in glutathione turnover and some associated enzymes in tumor induced rats.

The modulation of the oxidative phase of the pentose phosphate pathway in mouse liver

The glucose-6-phosphate dehydrogenase from mouse liver is fully inhibited in vitro by physiological concentrations of NADPH. This suggests that the oxidative phase of the pentose phosphate pathway requires some deinhibitory system. In order to investigate regulation of the pentose phosphate pathway, various parameters (intermediate concentrations, mass-action ratios of reactions, etc.) were measured in liver from control mice and from meal-fed mice. Assays were also carried out to detect any molecules causing the reverse of glucose-6-phosphate dehydrogenase inhibition by NADPH. The liver of meal-fed mice show greater glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase activities. They also had greater concentrations of several metabolic intermediates and triglycerides than the control animals (P < 0.001). These results prove that the diet increases the flow of the pentose phosphate pathway in a lipogenic sense. The glutathione reductase does not change with the diet, suggesting that this enzyme does not participate in the modulating process. Unlike rat liver, no molecules causing the reverse of glucose-6-phosphate dehydrogenase inhibition by NADPH were detected. These data suggest that the increase of flow of the pentose phosphate pathway during lipogenesis is obtained by an increase in enzyme synthesis.

Diamide-induced alterations of intracellular thiol status and the regulation of glucose metabolism in the developing rat conceptus in vitro

Direct oxidation of embryonic reduced glutathione (GSH) by a thiol oxidant, diamide, has been demonstrated to result in increased glutathione disulfide (GSSG) and protein-glutathione mixed disulfide (protein-S-SG) formation, which is accompanied by embryotoxicity and reductions in amniotic fluid volume. The altered functions of critical proteins or enzymes caused by the formation of protein-S-SG perturb cellular metabolism and may be involved in the embryotoxicity produced by GSH oxidation. The present study investigates changes in the metabolism of glucose through glycolysis and the pentose phosphate shunt pathways (PPP) and their related enzymes under the oxidative conditions produced by diamide exposure in organogenesis-stage rat conceptus (gestational day 10) in vitro. The metabolism of glucose via the PPP, measured as amounts of CO2 production from D-[1-14C]-glucose, was significantly increased in the conceptus exposed to 100-500 microM diamide to levels 2.5-3-fold those of controls. It was found that these substantial increases in the PPP activity did not correlate well with a moderate activation of glucose 6-phosphate dehydrogenase (G6PD) activity, the key enzyme in the PPP pathway. Changes in glycolysis due to diamide treatment were also determined by measurements of lactate production from D-[U-14C]-glucose. Production of lactate by the conceptus exposed to 250-500 microM diamide for 60 min was reduced (to approximately 54% of control values) concomitantly with a significant inhibition of the glycolytic enzymes, glyceraldehyde 3-phosphate dehydrogenase (GPD) and phosphofructokinase (PFK), indicating an overall decrease in glycolysis. Diamide was found to produce a differential effect on the enzymatic activities determined in this study, with greater degrees of inhibition seen in the tissue supernatants from the visceral yolk sac (VYS) compared to those from the embryo. Activities of GPD and PFK were decreased to approximately 22% and 43% control values, respectively, when determined in the supernatants from the VYS of the conceptus exposed to 500 microM diamide for 60 min. In addition, more than 90% of the GPD activity in the VYS, but not the embryo, was rapidly inhibited by the thiol alkylating agent N-ethylmaleimide (NEM, 100 microM) within 15 min of the exposure. In contrast to diamide and NEM, no alterations in lactate production were seen in the conceptus treated with the GSH depletor L-buthionine-S,R-sulfoximine (1 mM) for 5 hr in the culture media. Further experiments demonstrated that the activity of the GPD, inhibited by a 30-min incubation with 500 microM diamide, can be reversed after removal of diamide and that this effect was potentiated by subsequent treatment with dithiothreitol (30 mM), a thiol reducing agent. These results indicated the involvement of thiol/disulfide status in regulation of the metabolism of glucose in the developing conceptus and support the hypothesis that GSH oxidation and protein-S-SG formation could be a critical event associated with mechanisms of embryotoxicity elicited by oxidative stress. It was suggested in this study that, under these experimental conditions, embryotoxicity induced by diamide is primarily mediated via altered VYS functions, including disrupted energy production (glycolysis).

Purification, characterization and kinetic mechanism of glucose-6-phosphate dehydrogenase from mouse liver

1. Glucose-6-phosphate dehydrogenase (G6PDH EC 1.1.1.49) from mouse liver has been purified 1100-fold by extraction, ion-exchange chromatography on DE-52, absorption chromatography on Bio-Gel HTP and gel filtration through sepharose 6 HR 10/30. The purified enzyme showed a single band in silver stained SDS-PAGE. 2. The native and subunit molecular weight were 117 and 31 kDa respectively. 3. The kinetic studies and the patterns obtained from the inhibition by-products suggest that the enzyme follows an ordered sequential kinetic mechanism. 4. The reduced Km values for the substrates favour the operativity of the enzyme. The "fine control" of the enzymatic activity was exerted by the NADPH, whose Ki is several fold lower than the in vivo concentration.

Effect of L-penicillamine hydantoin, an analogue of glutathione, on rat liver glutathione peroxidase, reductase and transferase reactions

In soluble fractions prepared from rat liver homogenates, L-penicillamine hydantoin appeared to be, on the basis of SH consumption measurements, a substrate for glutathione peroxidase but not transferase reactions. When glutathione is incubated with rat liver soluble proteins in the presence of penicillamine hydantoin, formation of oxidized glutathione is inhibited. Calculations from Lineweaver-Burk plots point out that inhibition by L-penicillamine hydantoin of the peroxide-dependent oxidations of glutathione is mixed, since both apparent Km and Vmax values are modified. Preincubation of rat liver soluble proteins with L-penicillamine hydantoin led to a progressive inactivation of glutathione peroxidase. The kinetics of this inactivation process with respect to time and inactivator concentration were studied. Inclusion in the preincubation mixture of SH-containing molecules such as dithiothreitol, L-cysteine or glutathione protected the enzyme against inactivation. However, none of these molecules and neither hydantoin, Triton X-100, phenol, nor dialysis could reverse the enzyme from inactivated to activated form. Mitochondrial glutathione peroxidase was inhibited and inactivated by L-penicillamine hydantoin to the same extent as its cytosolic counterpart. Modifications by penicillamine hydantoin of various subcellular markers enzymes (lactate dehydrogenase, N-acetyl beta-glucosaminidase, arylsulfatase C, butyryl-CoA dehydrogenase, lauryl-CoA and glycolate oxidases) were of weak amplitude consisting of either inhibition, inactivation or stimulation.

Glucose-6-phosphate dehydrogenase. Characteristics revealed by the rat liver enzyme structure

The primary structure of glucose-6-phosphate dehydrogenase from rat liver has been determined, showing the mature polypeptide to consist of 513 amino acid residues, with an acyl-blocked N-terminus. This structure is homologous to those of both other eutherian and marsupial mammals (human and opossum), thus characterizing a mammalian type enzyme to which the human form, notwithstanding its large number of genetic variants, conforms. The mammalian type differs from the fruit fly enzyme by about 50%. Known mutant forms exhibit further differences, widely distributed along the polypeptide chain. Structural patterns show glucose-6-phosphate dehydrogenases to consist of a few variable regions intermixed with relatively constant segments.

Molecular diversity of glucose-6-phosphate dehydrogenase: rat enzyme structure identifies NH2-terminal segment, shows initiation from sites nonequivalent in different organisms, and establishes otherwise extensive sequence conservation

The NH2-terminal region of rat liver glucose-6-phosphate dehydrogenase (EC 1.1.1.49) is shown to differ radically from a reported amino acid sequence for the fruit fly enzyme and from one for the human enzyme. The results indicate considerable differences in the translational start point. However, a close relationship with another reported sequence for the human enzyme is established, now showing agreement between an indirectly deduced and a directly analyzed NH2-terminal structure of this enzyme type. The results provide evidence of one structural motif common to mammalian species but also suggest that genetic inconstancy 5' to, or at the start of, the region coding for the enzyme protein could be a source of intra- and interspecies diversity. This is of interest in relation to the large number of genetic variants of human glucose-6-phosphate dehydrogenase.

Glutathione reductase directly mediates the stimulation of yeast glucose-6-phosphate dehydrogenase by GSSG

Yeast glucose-6-phosphate dehydrogenase was inhibited by low NADPH concentrations in cell-free extracts, and de-inhibited by GSSG; extensive dialysis of the crude extract did not diminish the GSSG effect. Immunoprecipitation of glutathione reductase abolished the de-inhibition of glucose-6-phosphate dehydrogenase by GSSG. Purified glucose-6-phosphate dehydrogenase was inhibited by NADPH but not de-inhibited by GSSG, and upon addition of pure glutathione reductase GSSG completely de-inhibited the glucose-6-phosphate dehydrogenase.

Hormones, glutathione status and protein S-thiolation

The formation of mixed disulfides between proteins and glutathione has been discussed as a potentially interesting metabolic signal. The S-thiolation of proteins with glutathione has been observed in several systems in vitro. We have correlated the increase in glutathione disulfide (GSSG) with the amount of protein mixed disulfides. The methodological aspects are briefly presented; normal values for protSSG are about 20-30 nmol per g wet weight of liver. Several processes have been related to changes in the thiol redox state. The stimulation of flux through the pentose phosphate pathway during the metabolism of t-butyl hydroperoxide is presented, and the increase in cellular activity of glucose-6-phosphate dehydrogenase is correlated with the increase in the level of protSSG. Hormonal stimulation of GSH efflux from the liver by vasopressin or by alpha-adrenergic agonists such as phenylephrine or epinephrine is presented and discussed in relation to physiological states of peripheral (non hepatic) GSH utilization. Preliminary work relates the release of GSH to the perturbations in thiol redox state in inflammation and in exercise.

Regulation of the pentose phosphate cycle. Cofactor that controls the inhibition of glucose-6-phosphate dehydrogenase by NADPH in rat liver

A cofactor of Mr 10(4), characterized as a polypeptide, was found to co-operate with GSSG to prevent the inhibition of glucose-6-phosphate dehydrogenase by NADPH, in order to ensure the operation of the oxidative phase of the pentose phosphate pathway, in rat liver [Eggleston & Krebs (1974) Biochem. J. 138, 425-435; Rodriguez-Segade, Carrion & Freire (1979) Biochem. Biophys. Res. Commun. 89, 148-154]. This cofactor has now been partially purified by ion-exchange chromatography and molecular gel filtration, and characterized as a protein of Mr 10(5). The lighter cofactor reported previously was apparently the result of proteolytic activity generated during the tissue homogenization. The heavier cofactor was unstable, and its amount increased in livers of rats fed on carbohydrate-rich diet. Since the purified cofactor contained no glutathione reductase activity, the involvement of this enzyme in the deinhibitory mechanism of glucose-6-phosphate dehydrogenase by NADPH should be ruled out.

tert.-Butyl hydroperoxide metabolism and stimulation of the pentose phosphate pathway in isolated rat hepatocytes

The metabolism of tert.-butyl hydroperoxide (TBHP) by the glutathione peroxidase/reductase system in isolated hepatocytes results in the rapid depletion of reduced glutathione and NADPH. The regeneration of NADPH can occur through the pentose phosphate pathway, but only when the pathway is stimulated, for example, by NADP+ and possibly oxidized glutathione, both of which can be elevated in hepatocytes exposed to TBHP. TBHP is a cytotoxicant and the role of NADPH and the pentose phosphate pathway in protecting hepatocytes from TBHP-induced injury is unknown. Isolated rat hepatocytes exposed to TBHP (0.5 mM) for 30 min metabolized more [1-14C]glucose to 14CO2 than control (638.2 +/- 96.2 vs 306.9 +/- 69.5 dpm/10(6) cells) whereas 14CO2 evolution from [6-14C]glucose was unchanged, indicating that TBHP increases the activity of the pentose phosphate pathway and not glycolysis. TBHP (0.25 mM) metabolism also resulted in a rapid oxidation of hepatocyte NADPH from 2.85 +/- 0.32 to 0.55 +/- 0.24 nmol/10(6) cells which rapidly returned to 3.58 +/- 0.27 nmol NADPH/10(6) cells. Inhibition of the pentose phosphate pathway with 6-aminonicotinamide (70 mg/kg; 5 hr prior to hepatocyte isolation) inhibited TBHP-stimulated 14CO2 evolution from [1-14C]glucose and decreased the rate of NADP+ reduction. Hepatocytes isolated from 6-aminonicotinamide-treated animals were more susceptible to TBHP-induced cell injury than were control hepatocytes. These data demonstrate the following: The metabolism of TBHP by isolated hepatocytes stimulated the activity of the pentose phosphate pathway; and inhibition of the pentose phosphate pathway with 6-aminonicotinamide potentiated the toxicity of TBHP to isolated rat hepatocytes. These results suggest that the regeneration of NADPH by the pentose phosphate pathway may play a significant role in protecting hepatocytes from TBHP-induced damage.

Alteration of enzymes in ageing human fibroblasts in culture. IV. Effect of glutathione on the alteration of glucose-6-phosphate dehydrogenase

Alteration and inactivation of glucose-6-phosphate dehydrogenase (G6PD) can be induced in human fibroblasts by incubation of a cell supernatant at 4 degrees C and pH 7.4. When added in such conditions, glutathione (GSH) had a stabilizing effect on the enzyme. On the other hand, substances which are known to deplete the cells of their GSH content, dramatically increase the inactivation rate. When analysed by gel filtration after 24 h of incubation at 4 degrees C, the inactive G6PD appears as a dimeric protein when GSH is present, while as a monomer in the control experiment. Reactivation of the monomers was stimulated with GSH. The heat inactivation of the dimeric fraction first started with a sharp activity increase of 20%. This increase vanished when the enzyme was first reactivated before the thermolability experiment. We propose that what is called altered G6PD is the expression of a quick reactivation of an inactive, labile dimer. Finally, a schematic view of the G6PD alteration is proposed.

Quantitative analysis of intermediary metabolism in hepatocytes incubated in the presence and absence of glucagon with a substrate mixture containing glucose, ribose, fructose, alanine and acetate

Hepatocytes were isolated from the livers of fed rats and incubated, in the presence and absence of 100 nM-glucagon, with a substrate mixture containing glucose (10 mM), fructose (4 mM), alanine (3.5 mM), acetate (1.25 mM), and ribose (1 mM). In any given incubation one substrate was labelled with 14C. Incorporation of 14C into glucose, glycogen, CO2, lactate, alanine, glutamate, lipid glycerol and fatty acids was measured after 20 and 40 min of incubation under quasi-steady-state conditions [Borowitz, Stein & Blum (1977) J. Biol. Chem. 252, 1589-1605]. These data and the measured O2 consumption were analysed with the aid of a structural metabolic model incorporating all reactions of the glycolytic, gluconeogenic, and pentose phosphate pathways, and associated mitochondrial and cytosolic reactions. A considerable excess of experimental measurements over independent flux parameters and a number of independent measurements of changes in metabolite concentrations allowed for a stringent test of the model. A satisfactory fit to the data was obtained for each condition. Significant findings included: control cells were glycogenic and glucagon-treated cells glycogenolytic during the second interval; an ordered (last in, first out) model of glycogen degradation [Devos & Hers (1979) Eur. J. Biochem. 99, 161-167] was required in order to fit the experimental data; the pentose shunt contributed approx. 15% of the carbon for gluconeogenesis in both control and glucagon-treated cells; net flux through the lower Embden-Meyerhof pathway was in the glycolytic direction except during the 20-40 min interval in glucagon-treated cells; the increased gluconeogenesis in response to glucagon was correlated with a decreased pyruvate kinase flux and lactate output; fluxes through pyruvate kinase, pyruvate carboxylase, and phosphoenolpyruvate carboxykinase were not coordinately controlled; Krebs cycle activity did not change with glucagon treatment; flux through the malic enzyme was towards pyruvate formation except for control cells during interval II; and 'futile' cycling at each of the five substrate cycles examined (including a previously undescribed cycle at acetate/acetyl-CoA) consumed about 26% of cellular ATP production in control hepatocytes and 21% in glucagon-treated cells.

The pentose phosphate cycle is regulated by NADPH/NADP ratio in rat liver

The changes in the activity of the pentose phosphate cycle produced by the activation or inhibition of different NADPH-consuming pathways have been studied. The inhibition of fatty acid synthesis by kynurenate produced to the same extent, inhibition of the pentose phosphate cycle activity and an increase (about twofold) in the NADPH/NADP ratio. The addition of ter-butyl-hydroperoxide or paraquat, which is metabolized via NADPH-consuming pathways, produced the activation of the pentose phosphate cycle and a decrease in the NADPH/NADP ratio (about threefold). The plot of the NADPH/NADP ratio versus the pentose phosphate cycle activity gave a straight line with a regression index of 0.999. The regulation of the pentose phosphate cycle mainly by the intracellular NADPH/NADP ratio is discussed.

Glucose-6-phosphate dehydrogenase from rabbit erythrocytes

Glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate: NADP+ 1-oxidoreductase, EC 1.1.1.49) was purified from rabbit erythrocytes. Initial velocity studies and product and and dead-end inhibitor studies with this enzyme are consistent with a rapid equilibrium random mechanism with an enzyme-NADPH-glucose 6-phosphate dead-end complex.