Multiple molecular forms of rat mammary glucose-6-phosphate dehydrogenase: proposed role in turnover of the enzyme

Elevation of Glucose 6-Phosphate Dehydrogenase Activity Induced by Amplified Insulin Response in Low Glutathione Levels in Rat Liver

Weanling male Wistar rats were fed on a 10% soybean protein isolate (SPI) diet for 3 weeks with or without supplementing 0.3% sulfur-containing amino acids (SAA; methionine or cystine) to examine relationship between glutathione (GSH) levels and activities of NADPH-producing enzymes, glucose 6-phosphate dehydrogenase (G6PD) and malic enzyme (ME), in the liver. Of rats on the 10% SPI diet, GSH levels were lower and the enzyme activities were higher than of those fed on an SAA-supplemented diet. Despite the lower GSH level, γ-glutamylcysteine synthetase (γ-GCS) activity was higher in the 10% SPI group than other groups. Examination of mRNAs of G6PD and ME suggested that the GSH-suppressing effect on enzyme induction occurred prior to and/or at transcriptional levels. Gel electrophoresis of G6PD indicated that low GSH status caused a decrease in reduced form and an increase in oxidized form of the enzyme, suggesting an accelerated turnover rate of the enzyme. In primary cultured hepatocytes, insulin response to induce G6PD activity was augmented in low GSH levels manipulated in the presence of buthionine sulfoximine. These findings indicated that elevation of the G6PD activity in low GSH levels was caused by amplified insulin response for expression of the enzyme and accelerated turnover rate of the enzyme molecule.

Quantification of G6PD in small and large intestine of rat during aging

Numerous studies have demonstrated a decrease in glucose-6-phosphate dehydrogenase (G6PD) activity during aging in many cell types, including red blood cells, fibroblasts and lens cells. Moreover, the intracellular activity of G6PD has been shown to be regulated by binding to cell organelles. To investigate whether binding of G6PD to cell organelles is related with the decrease in its activity during aging, distribution patterns of G6PD activity and protein were assessed in small (SI) and large (LI) intestine of 3-month-old and 28-month-old rats. Enzyme activity, as measured spectrophotometrically, did not show any significant change with aging in SI or LI. Enzyme histochemistry, performed by subtracting activity staining of 6-phosphogluconate dehydrogenase (6PGD) from that of G6PD, showed a lower net G6PD activity in SI and LI epithelium of old rats in comparison with young rats. G6PD activity did not change significantly with aging in the muscularis externa of SI and LI. Immunoelectron microscopic analysis of G6PD protein allowed us to measure the density of G6PD molecules in cellular compartments, and the fraction of enzyme bound to cell organelles. In SI and LI epithelia, density of G6PD molecules was higher in old rats than in young rats; however, the fraction of enzyme bound to cell organelles also increased with aging. These data suggest that G6PD activity in epithelium of SI and LI decreases with aging due to the accumulation of significant amounts of enzyme bound to cell organelles, a condition which makes it less active than the soluble enzyme.

Activation of spinach pullulanase by reduction results in a decrease in the number of isomeric forms

Spinach starch debranching enzyme, a limit dextrinase or pullulanase (EC 3.2.1.41), is a monomeric protein of 100 kDa that produces up to seven coexisting and mutually interconvertible isomers of different specific activity, a phenomenon that has been termed microheterogeneity and for which a structural explanation has not yet been presented. The enzyme can be activated by reduction, in particular by thiol reagents, and inactivated by oxidation and the concomitant change of the patterns of its isomeric forms could be quantified by chromatofocusing. The hypothesis was examined that reduction of the enzyme's thiol groups shifts the isomer pattern towards the forms with a higher specific activity while oxidation favours the less active forms. Using TCEP as reductant only the form with the highest specific activity was obtained. This form was almost inaccessible for proteolysis by trypsin while the oxidized and GSH-activated enzyme yielded four peptides when treated with trypsin. Their sequence indicated cleavage predominantly of loops connecting the beta-strands and alpha-helices of the (beta/alpha)(8)-barrel which forms the catalytic site of the pullulanase. Formation of various disulphide bridges between the loops connecting the barrel structures -- predominantly on one side -- may be the reason for the microheterogeneity of the spinach pullulanase. In vivo, the enzyme maintains its activated state due to the high concentration of GSH in the chloroplast. However, the chloroplast's pH shifts from day (pH 8) to night (pH 7) and thus could also alter the activity of the protein in accordance with the required function in starch metabolism.

Glucose-6-phosphate dehydrogenase in small intestine of rabbit: biochemical properties and subcellular localization

Biochemical properties and cellular and subcellular distribution patterns of glucose-6-phosphate dehydrogenase (G6PD) were investigated in small intestine of rabbits. The specific activity of G6PD in fresh homogenates of small intestine was 19 +/- 9 IU/g protein. This value did not change significantly after dialysis. The kinetic and electrophoretic properties of the partially purified enzyme were similar to those found in other rabbit tissues. Enzyme histochemical analysis of G6PD activity using the tetrazolium salt method showed high activity in epithelial cells of villi and crypts of Lieberkuhn. The activity in acinar cells of Brunner's glands was lower than that in epithelium, whereas cells of the muscularis externa showed a very low activity. Immunohistochemical analysis showed that the amounts of G6PD protein were lower in the epithelium than in Brunner's glands and muscularis externa. The differences between distribution patterns of activity and protein of G6PD may reflect the presence of inactive enzyme molecules in Brunner's glands and muscularis externa or posttranslational activation of G6PD in epithelium. Electron microscopic immunocytochemical analysis performed with gold-labelled antibodies showed the presence of G6PD protein throughout the cytoplasm and at smooth endoplasmic reticulum in enterocytes. In Paneth cells and cells of Brunner's glands, G6PD was found in the cytoplasm, at rough endoplasmic reticulum and Golgi complex. Immunolabelling was not found in mitochondria or nuclei. Our findings show that G6PD is heterogeneously distributed in cells of the small intestine and that the enzyme is associated with rough and smooth endoplasmic reticulum to support synthetic functions in these compartments by NADPH production.

Heterogeneous distribution of glucose-6-phosphate dehydrogenase in lingual epithelium

Lingual epithelium undergoes oxidative stress and apoptosis with consequent renewal of superficial keratinized cells by proliferation and differentation of the stem cells of the basal germinative layer. In 3 distinct areas of lingual epithelium of rat and rabbit, the anterior third, central third and posterior third, we determined the activity of hexose monophosphate shunt enzymes and antioxidant enzymes, which are essential for support of cell proliferation and differentation. Enzymatic assays of the epithelium showed that glucose-6-phosphate dehydrogenase (G6PD) activity was highest in the anterior third, whereas activity of glutathione peroxidase, 6-phosphogluconate dehydrogenase, glutathione reductase, superoxide dismutase and catalase was similar over all areas. Histochemical localization of activity and immunohistochemical localization of protein of G6PD showed that all types of papillae had a similar G6PD content; moreover, the presence of different G6PD isoforms in the 3 areas was excluded by electrophoretic analysis. We conclude that the higher G6PD activity in the anterior part of the epithelium is due only to the anatomical organization of the epithelial surface of this area, in which many filiform and fungiform papillae are arranged in a compact manner, which corresponds with a higher number of proliferating and differentiating cells. These processes need products of G6PD activity. This study indicates that G6PD is a good marker for the number of differentiating cells in tongue epithelium.

Postnatal expression of glucose-6-phosphate dehydrogenase in different brain areas

The activity of glucose-6-phosphate dehydrogenase (G6PD) was studied in five brain areas of rats aged 5 to 90 days. The areas studied were: the olfactory bulb (OB), cortex, hippocampus, striatum and septum. The G6PD activity increased more than 2-fold from 5 to 90 days in the OB, while it was almost constant in the other areas. At every stage of development, the G6PD activity was significantly higher in the OB than in the other areas. The G6PD pattern was compared with 6-phosphogluconate dehydrogenase (6PGD), glutathione reductase (GR); glutathione peroxidase (GPX), catalase (CAT) and superoxide dismutase (SOD) in order to find synergistic interactions among activities of these enzymes during development. Over the considered period, the activity of 6PGD increased significantly in the OB, while no significant difference in activity was detected in the other areas. GR increased significantly and progressively at each developmental stage in all areas. GPX showed a progressive increase in the OB, while in other areas a significant increase was detected at 90 days only. CAT and SOD showed a different and independent pattern which differed from the G6PD pattern. CAT showed the highest level of activity at 5 days then progressively decreased or was constant until 90 days; SOD had the highest value at 5 days, than it decreased at 10 days and increased from 10 to 90 days. In all areas, G6PD activity showed three electrophoretic bands, whose relative activity changed with development. At histochemical level, we found a marked G6PD activity in the periglomerular zone of the OB, which increased with age, while other areas showed a homogeneous staining. The present results demonstrate that G6PD activity increases in the OB during the developmental stages and there is a coordinated simultaneous activation of 6PGD, GPX and GR. It is likely that this enzyme induction increases the antioxidant defense of periglomerular cells that are subject to a rapid renewal and thus much more exposed to oxidant stress.

Impact of starvation-refeeding on kinetics and protein expression of trout liver NADPH-production systems

Herein we report on the kinetic and protein expression of glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase, and malic enzyme (ME) in the liver of the trout (Oncorhynchus mykiss) during a long-term starvation-refeeding cycle. Starvation significantly depressed the activity of these enzymes by almost 60%, without changing the Michaelis constant. The time response to this nutritional stimulus increased with fish weight. The sharp decline in G6PDH and ME activities was due to a specific protein-repression phenomenon, as demonstrated by molecular and immunohistochemical analyses. Also, the dimeric banding pattern of liver G6PDH shifted from the fully reduced and partially oxidized forms, predominant in control, to a fully oxidized form, more sensitive to proteolytic inactivation. Refeeding caused opposite effects in both protein concentration and enzyme activities of about twice the control values in the first stages, later reaching the normal enzyme activity levels. Additionally, the partially oxidized form of G6PDH increased. The kinetics of these enzymes were examined in relation to the various metabolic roles of NADPH. These results clearly indicate that trout liver undergoes protein repression-induction processes under these two contrasting nutritional conditions.

A dehydrogenase-mediated recycling system of NADPH in plant peroxisomes

The presence of the two NADP-dependent dehydrogenases of the pentose phosphate pathway has been investigated in plant peroxisomes from pea (Pisum sativum L.) leaves. Both enzymes, glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) and 6-phosphogluconate dehydrogenase (6PGDH; EC 1.1.1.44), were present in the matrix of leaf peroxisomes, and their kinetic properties were studied. G6PDH and 6PGDH showed a typical Michaelis-Menten kinetic saturation curve, and had specific activities of 12.4 and 29.6 mU/mg protein, respectively. The Km values of G6PDH and 6PGDH for glucose 6-phosphate and for 6-phosphogluconate were 107.3 and 10.2 microM, respectively. Dithiothreitol did not inhibit G6PDH activity. By isoelectric focusing of peroxisomal matrices, the G6PDH activity was resolved into three isoforms with isoelectric points of 5.55, 5.30 and 4.85. The isoelectric point of peroxisomal 6PGDH was 5.10. Immunoblot analyses of peroxisomal matrix with an antibody against yeast G6PDH revealed a single cross-reactive band of 56 kDa. Post-embedment, EM immunogold labelling of G6PDH confirmed that this enzyme was localized in the peroxisomal matrices, the thylakoid membrane and matrix of chloroplasts, and the cytosol. The presence of the two oxidative enzymes of the pentose phosphate pathway in plant peroxisomes implies that these organelles have the capacity to reduce NADP+ to NADPH for its re-utilization in the peroxisomal metabolism. NADPH is particularly required for the ascorbate-glutathione cycle, which has been recently demonstrated in plant peroxisomes [Jiménez, Hernández, del Río and Sevilla (1997) Plant Physiol. 114, 275-284] and represents an important antioxidant protection system against H2O2 generated in peroxisomes.

Methods for studying the glucose-6-phosphate dehydrogenase activity in brain areas

This paper reports on the protocols for the spectrophotometric determination of the enzymatic activity, the electrophoretic pattern and the cytochemical assay of glucose-6-phosphate dehydrogenase (G6PD) in different areas of rat brain. For the spectrophotometric assay we used the method of Glock and McLean. Non-denaturing polyacrylamide gel electrophoresis was performed using 1.5 mm thick slab-gel, with a 7.5% acrylamide separating gel and a 4% acrylamide stacking gel. NADP (0.01 mM) was included at the cathode buffer. The cytochemical assay was performed on unfixed cryostat sections, by incubating the slides for 5 min at 37 degrees C in the staining medium. A combination of these techniques offers a means for the study of the effect of age, drugs, oxidizing or reducing agents on the G6PD activity in different brain areas.

Testis-specific expression of a functional retroposon encoding glucose-6-phosphate dehydrogenase in the mouse

The X-chromosomal gene glucose-6-phosphate dehydrogenase (G6pd) is known to be expressed in most cell types of mammalian species. In the mouse, we have detected a novel gene, designated G6pd-2, encoding a G6PD isoenzyme. G6pd-2 does not contain introns and appears to represent a retroposed gene. This gene is uniquely transcribed in postmeiotic spermatogenic cells in which the X-encoded G6pd gene is not transcribed. Expression of the G6pd-2 sequence in a bacterial system showed that the encoded product is an active enzyme. Zymogramic analysis demonstrated that recombinant G6PD-2, but not recombinant G6PD-1 (the X-chromosome-encoded G6PD), formed tetramers under reducing conditions. Under the same conditions, G6PD tetramers were also found in extracts of spermatids and spermatozoa, indicating the presence of G6pd-2-encoded isoenzyme in these cell types. G6pd-2 is one of the very few known expressed retroposons encoding a functional protein, and the presence of this gene is probably related to X chromosome inactivation during spermatogenesis.

Glucose-6-phosphate dehydrogenase activity is higher in the olfactory bulb than in other brain areas

The activity of antioxidant enzymes was measured in the olfactory bulb (OB) of rat and compared with cortex, hippocampus, striatum and septum. Glutathione reductase, glutathione peroxidase, catalase and superoxide dismutase were not significantly different in the five brain areas, while glucose-6-phosphate dehydrogenase (G6PD) and 6-phosphogluconate dehydrogenase activities were four times higher in the OB than in the other areas. This picture prompted us to explore the reasons of the marked increase of G6PD, since it is the enzyme that regulates the operation of the hexose monophosphate shunt. A first approach was to analyze the G6PD electrophoretic pattern. The analysis revealed that the high G6PD activity of the bulb was neither due to new isoenzymes nor to a modification of the equilibrium between the G6PD dimers. We secondly hypothesized an induction of G6PD activity in the OB by oxidant stress. The assay of markers of the oxidant stress, such as thiobarbituric acid reactive substances, oxidized and reduced glutathione, did not confirm this hypothesis. A third approach was the cytochemical analysis of cryostat sections of OB. By this method we identified a particular cell type which was very rich in G6PD and located at the border of the glomerular layer. Thus, we attributed the high G6PD activity of the OB to the consistent presence of periglomerular cells, that probably need a high G6PD activity for their regulatory function in the neurochemical transmission.

Comparative study on glucose-6-phosphate dehydrogenase from rabbit tissues

The activity of glucose-6-phosphate dehydrogenase (G6PD) was measured in bone marrow, spleen, lung, liver, kidney, adipose tissue, brain, heart, muscle, and in the erythroid cell line of rabbit. In tissues, the activity ranged from 6.87 to 0.09 U/g wet tissue, found in bone marrow and muscles, respectively, whereas in the erythroid cell line it ranged from 14.3 to 2.4 U/g cells for erythroblasts and erythrocytes, respectively. The electrophoretic patterns of the tissue crude extracts showed an identical set of three activity bands, and the immunotitration curves obtained with rat antirabbit erythrocyte G6PD antibodies shared the same equivalence point. The enzyme, purified to homogeneity from different tissues, showed no significant differences among the Km values for NADP and G6P. The results give a picture of the variability of the G6PD activity in rabbit tissues and suggest the presence of the same enzyme molecule in each tissue.

Influence of experimental diabetes on the kinetic behaviour of renal cortex hexose monophosphate dehydrogenases

1. Short term (1-2 hr) and long-term (2 days) effects of experimental alloxan induced diabetes on the kinetics of the renal hexose monophosphate shunt dehydrogenases are reported. 2. Alloxan diabetes for 2 days significantly increased kidney weight (16%) adding about 80 mg/day per g of kidney. No significant changes were found in renal growth 1-2 hr after alloxan injection. 3. Under these experimental conditions, the activities of glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase significantly increased (103 and 33% respectively) at all substrate concentrations, without affecting the KmS of either enzyme. 4. There was no effect of alloxan on the activity of these enzymes at 1-2 hr. Saturation curves show that all enzymes exhibited a M-M kinetic without evidence of sigmoidicity. 5. The results suggest that increased renal hexose monophosphate dehydrogenases activities are due to increased concentrations of the rate limiting proteins. 6. The relationship between these changes and renal hypertrophy is also discussed.

Rabbit bone marrow glucose-6-phosphate dehydrogenase during erythroid cell development

Studies were carried out on glucose-6-phosphate dehydrogenase (G6P-DH) during the differentiation of rabbit bone marrow erythroid cells. It was found that G6P-DH, although displaying a 7-fold activity decrease, did not change the relative amounts of its three dimeric forms. Using homogeneous enzyme preparations, we observed that from dividing to non-dividing erythroblasts the following properties remained constant: V max dependence on pH and temperature, Km for G6P dependence on pH, heat stability, 2-deoxy glucose-6-phosphate utilization, molecular weight, while the Km for NADP significantly increased in non-dividing erythroblasts. These results indicate that no shift towards the oxidized form of the enzyme and no substantial modifications of the protein take place during cell differentiation.

NADP-linked dehydrogenases in secreted milk

The activities of glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, isocitrate dehydrogenase, malic enzyme, lactate dehydrogenase and malate dehydrogenase have been determined in secreted milk from sows, rats and rabbits. Within each species, although there was considerable variation in the absolute activities of these enzymes, the relative activities were similar to those observed for, or previously published for mammary homogenates. The only exception was milk glucose 6-phosphate dehydrogenase which tended to lose activity upon prolonged storage in the mammary gland. These results suggest that the pattern of milk enzymes can be an accurate reflection of that occurring in the mammary gland.

Regulation of the multiple molecular forms of rat liver glucose 6-phosphate dehydrogenase by insulin and dietary restriction

Insulin treatment of virgin female rats increased the hepatic activity of glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase to levels 3.4 and 1.5 fold higher than controls. The increase in glucose 6-phosphate dehydrogenase activity was attributed to increased activity of all three dimer species. Thus dimer bands, 1, 2 and 3 of insulin-treated animals were 5, 3 and 2-fold higher respectively than controls. The activity of glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase decreased with fasting to 55% and 72% respectively of controls. The decrease in glucose 6-phosphate dehydrogenase activity reflected a lower activity of dimer bands 2 and 3 only, which were 62% and 39% of control activity respectively after three days fasting. A shift towards band 1 was observed under both conditions of starvation as well as under conditions of insulin treatment.