Induction Of Glucose-6-phosphate Dehydrogenase Research Articles

Effects of cadmium on glutathione synthesis in hepatopancreas of freshwater crab, Sinopotamon yangtsekiense

Cadmium (Cd) is one of the most deleterious heavy metals in aquatic systems that could promote oxidative damage. To explore the effects of Cd exposure of a freshwater crab ( Sinopotamon yangtsekiense) on hepatopancreatic glutathione (GSH) synthesis, crabs were exposed to the reagent with a dose range of 7.25–116.00 mg L −1 for 48 h. The concentrations of GSH, oxidized glutathione (GSSG), NADPH and NADP +, as well as the activities of enzymes involved in GSH synthesis, i.e. glutathione reductase (GR), glucose-6-phosphate dehydrogenase (G6PD), γ-glutamylcysteine synthetase (γ-GCS) were determined. Progressive depletion of cellular GSH content was observed with the increasing of Cd concentrations, while the level of GSSG remained constant. In response to Cd exposure, crabs showed significant induction of G6PD and NADPH, however, only up to moderate exposures. GR activity remained at a steady level at all exposure concentrations. The activity of γ-GCS was significantly positively correlated with the Cd concentration. These results suggested that GSH synthesis could be activated against reactive oxygen species induced by lower Cd exposure; under the higher Cd exposure conditions, an inhibition of NADPH-dependant redox cycling and de novo GSH synthesis led to significant decrease in GSH content.

Genetic definition of the Escherichia coli zwf "soxbox," the DNA binding site for SoxS-mediated induction of glucose 6-phosphate dehydrogenase in response to superoxide

In Escherichia coli K-12, transcription of zwf, the gene for glucose 6-phosphate dehydrogenase, is subject to growth rate-dependent regulation and is activated by SoxS in response to superoxide stress. To define genetically the site of SoxS activation, we undertook a detailed deletion analysis of the zwf promoter region. Using specifically targeted 5' and 3' deletions of zwf sequences, we localized the SoxS activation site to a 21-bp region upstream of the zwf promoter. This minimal "soxbox" was able to confer paraquat inducibility when placed upstream of a normally unresponsive gnd-lacZ protein fusion. In addition, we used these findings as the basis for resecting unnecessary sequences from the region upstream of the promoters of two other SoxS-regulated genes, sodA and nfo. Like the zwf soxbox, the regions required for SoxS activation of sodA and nfo appear to lie just upstream or overlap the -35 hexamers of the corresponding promoters. Importantly, the sequence boundaries established here by deletion analysis agree with the primary SoxS recognition sites of zwf, sodA, and nfo that we previously identified in vitro by gel mobility shift and DNase I protection assays with a purified MalE-SoxS fusion protein.

Free fatty acid inhibition of the insulin induction of glucose-6-phosphate dehydrogenase in rat hepatocyte monolayers.

Rat hepatocytes in monolayer culture were utilized to determine if the decrease in glucose-6-phosphate dehydrogenase (G6PD) activity resulting from the ingestion of fat can be mimicked by the addition of fatty acids to a chemically, hormonally defined medium. G6PD activity in cultured hepatocytes was induced several-fold by insulin. Dexamethasone or T3 did not amplify the insulin induction of G6PD. Glucose alone increased G6PD activity in cultured hepatocytes from fasted donors by nearly 500%. Insulin in combination with glucose induced G6PD an additional two-fold. The increase in G6PD activity caused by glucose was greater in hepatocytes isolated from 72 hr-fasted rats as compared to fed donor rats. Such a response was reminiscent of the "overshoot" phenomenon in which G6PD activity is induced well above the normal level by fasting-refeeding rats a high glucose diet. Addition of linoleate to the medium resulted in a significant suppression of insulin's ability to induce G6PD, but linoleate had no effect on the induction of G6PD activity by glucose alone. A shift to the right in the insulin-response curve for the induction of G6PD also was detected for the induction of malic enzyme and acetyl-CoA carboxylase. Arachidonate (0.25 mM) was a significantly more effective inhibitor of the insulin action than linoleate was. Apparently rat hepatocytes in monolayer culture can be utilized as a model to investigate the molecular mechanism by which fatty acids inhibit the production of lipogenic enzymes. In part, this mechanism of fatty acid inhibition involves desensitization of hepatocytes to the lipogenic action of insulin.

Primary cultures of hepatocytes in serum and hormone-free medium: identification of conditions which stimulate an in vivo-like induction of G6PD.

Recent results from several laboratories suggest that complex interactions between hormones and dietary carbohydrate may be responsible for regulating the induction of several hepatic lipogenic enzymes. Elucidation of these interactions requires the ability to culture hepatocytes for several days in serum-free medium where the hormones or carbohydrate or both present is strictly controlled. The functional response of primary adult rat hepatocytes was examined in a medium without exposure to serum, hormones, or carbohydrates and on three substrata commonly used to culture cells in a defined medium. Hepatocytes cultured on a floating collagen gel in which is embedded a nylon mesh possess cell attachment and morphologic characteristics superior to either cells cultured on a collagen-coated or fibronectin(Fn)-coated substratum. Cells cultured on the gel-mesh system retain insulin responsivity, as measured by protein synthesis rates and glucose-6-phosphate dehydrogenase (G6PD) induction, for at least 6 d in culture. Under these conditions, insulin, dexamethasone, and fructose increase G6PD specific activity to levels comparable to that seen in an induced animal. Hepatocytes cultured on the gel-mesh system tolerate restricted medium conditions better than cells cultured on collagen or Fn-coated substratum, and remain viable for sufficient times to allow, for the first time, full expression and maximal induction (i.e. like in vivo) of G6PD in cultured cells. This system represents a satisfactory model for in vivo liver metabolism and a superior system for studying the effects of hormones and metabolites on G6PD levels, as well as other nutritional-hormonally regulated enzymes.

Independence of Glycogen Accumulation and Glucose-6-Phosphate Dehydrogenase Induction in Rat Liver

The responses of liver glycogen and glucose-6-phosphate dehydrogenase (G6PD) (EC 1.1.1.49) to a high glucose, adequate protein diet were compared between rats previously starved 2 days, then refed a high protein, carbohydrate-free diet for 2 days, and rats previously fed the high protein diet for 4 days. Glycogen levels increased dramatically during the first day the high carbohydrate diet was fed, then decreased gradually on the second day. The response was the same regardless of whether the rats had been starved more before the high protein diet was fed. Liver G6PD activity also increased when the high carbohydrate diet was fed, and continued to increase on the second day. The increase in G6PD, however, was significantly greater in the rats which had been starved before the high protein diet was fed. It is suggested that some process occurs during starvation that predisposes the induction of G6PD upon refeeding a high carbohydrate diet, over and above any effect of glycogen accumulation and breakdown. Glucose or glucose-6-phosphate derived from glycogen does not appear to be the primary inducer of G6PD in rat liver.

The association of lysosomal enzymes with nuclei during enzyme induction in rat liver

Male rats of the Holtzman strain were fasted for 3 days and refed a diet high in carbohydrate (68.9%). The induction of liver glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase was monitored for up to 48 h after refeeding. Induction occurred by 24 h, and by 48 h, 4.2- and 1.5-fold increases were observed for glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, respectively, compared with that of livers of pellet-fed rats. After refeeding, lysosomes increased in fragility as judged by an increased release of acid phosphatase activity during standard homogenization. Fragility was greatest 3 h after refeeding, but normal fragility was observed 24 h after refeeding. Nuclei were isolated from the liver samples before and after refeeding. Those isolated just before refeeding revealed small latent acid phosphatase activity (4–6%). However, after refeeding the carbohydrate-rich diet, a transient and significant ( P < 0.01) increase in the latent activity occurred that was maximal (20%) at 1 h, returning to normal by 24 h. Cross-mixing the 800 g nuclear pellet from livers of animals starved for 3 days with the 800 g supernatant fraction from livers of animals refed the carbohydrate-containing diet did not alter the nuclear lysosomal-free (overt) or latent (detergent-released) enzyme activity. Similarly, mixing the 800 g nuclear pellet from livers of animals refed for 1 h with the 800 g supernatant fraction from livers of animals starved for 3 days, but not refed, did not change the nuclear lysosomalfree or latent enzyme activity. Purified nuclei, further washed in hypotonic buffer, lost acid phosphatase activity, but those isolated from livers of rats refed for 1 h retained 10% of the enzyme latency, whereas all latency was lost from those isolated from uninduced rats. A second lysosomal enzyme, β-galactosidase, became associated with the nuclei with the same temporal pattern as for acid phosphatase. However, no variation in nuclear content of cytosolic lactate dehydrogenase occurred as a result of feeding the high-carbohydrate diet to starved rats. When similarly starved rats were refed a diet high in lipid and carbohydrate-free, no induction of glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase was observed. Lysosomes were not temporarily fragile and purified nuclei did not exhibit increased latency of acid phosphatase activity. Though the evidence presented does not establish a direct correlation between lysosome migration to nuclei as a required function in enzyme induction, the temporal and specific nature of the phenomenon support the hypothesis that liver lysosomal enzymes participate in early signals in the induction of enzymes of lipogenesis.

Regulation of Glucose-6-phosphate Dehydrogenase and 6-Phosphogluconate Dehydrogenase in the Meal-fed Rat

Meal-feeding of a high sucrose diet produces a diurnal cycle (i.e., food response) in glucose-6-phosphate dehydrogenase (G6PD) and 6-phosphogluconate dehydrogenase (6PGD) levels resulting in an elevated level of these enzymes at approximately 12 hours after the start of a 2-hour meal and a return to base level by 24 hours. The effects of actinomycin D and cycloheximide on the 12-hour increases in G6PD and 6GPD were determined. Cycloheximide completely blocked the increase in G6PD if administered 2 or 4 hours after start of the meal, while actinomycin D completely blocked the increase in G6PD if administered at 2 hours and almost completely at 4 hours after start of the meal. These results were obtained previously with starved rats refed a sucrose diet. The diurnal increases in G6PD and 6PGD in meal-fed rats and the induction of G6PD in starved-refed rats thus appear to be regulated by the same mechanism requires RNA synthesis within 4 hours after start of re-feeding. The response of 6PGD to cycloheximide and to actinomycin D at 2 or 4 hours after start of the meal is essentially the same as that of G6PD. These data suggest that the increases in G6PD and 6PGD (and other enzymes) brought about by carbohydrate refeeding AFTER starvation or by carbohydrate meal-feeding on a diurnal cycle are mediated by a rapid change in RNA synthesis. This appears most compatible with a coordinate control of gene expression through messenger RNA synthesis.

Effect of Dietary Fat and Sucrose on the Activities of Several Rat Hepatic Enzymes and Their Diurnal Response to a Meal

Regulation of the cytoplasmic enzymes, pyruvate kinase (PK), glucokinase (GK), phosphoenolpy ruvate carboxykinase (PEPCK), fructose-1,6-diphosphatase (FDP), ATP citrate-lyase (ATP-CL), NAD-malate dehydrogenase (NAD-MD), NADP-malate dehydrogenase (NADP-MD), glutamic-pyruvic transaminase (GPT), glucose-6-phosphate dehydrogenase (G6PD), and 6-phosphogluconate dehydrogenase (6PGD), in rat liver by dietary fat (F diet) and dietary sucrose (S diet) was investigated. Mealfeeding the S diet to adult rats for 5 and 9 months resulted in a diurnal dietary response (i.e., food response) variation of FDP, GK, ATP-CL, 6PGD, and PK, while meal-feeding the S diet to young rats resulted in diurnal dietary response variation of ATP-CL, G6PD, NADP-MD, 6PGD, GPT, and PK. Meal-feeding the fat diet results in essentially no diurnal variation in enzyme activity. The overall effect of meal-feeding, as compared with ad libitum feeding, of the S diet was to increase the levels of G6PD, ATP-CL, and NADP-MD and to decrease the level of PEck in the meal-fed rats. Young rats meal-fed the two diets have higher enzyme activities than meal-fed adult rats for the observed enzymes (except for GPT and NAD-MD). In general, hepatic levels of the enzymes studied are low in the F diet-fed animals and markedly higher for the S diet-fed animals. These results suggest that dietary carbohydrate specifically induces those enzymes involved in carbohydrate metabolism, whereas dietary fat does not affect their levels. On the basis of prior evidence for an early requirement of RNA synthesis for sucrose induction of G6PD, this widespread induction of liver enzymes by carbohydrate must indicate either increased synthesis of ribosomal RNA with later regulation of synthesis specifically of these enzymes or increased synthesis of a rather large group of specific messenger RNAs i.e., coordinate genetic control of a number of these enzyme messenger RNAs.

Induction of Glucose-6-Phosphate Dehydrogenase by a 90% Carbohydrate Diet and 8-Azaguanine Insensitive Induction of Glucose-6-Phosphate Dehydrogenase Following a Transfer from the 90% Carbohydrate Diet to a 90% Protein Diet

A protein-free 90% sucrose diet was refed for several days to rats starved for 7 days. 8-Azaguanine was then injected intraperitoneally concomitant with a change to a 90% protein diet to determine if glucose-6-phosphate dehydrogenase (G6PD) mRNA was synthesized during the refeeding of an inadequate protein diet. The refeeding of the 90% sucrose diet alone resulted in an induction of G6PD to 2-3 times the prestarvation level. Glucose could be substituted for sucrose with equal effect on the induction of G6PD. After the change to the 90% protein diet a further increase in G6PD activity occurred which was insensitive to 8-azaguanine and thus presumably was independent of mRNA synthesis. This 8-azaguanine insensitive increase in G6PD was greater after a 4-day refeeding of the 90% sucrose diet than after only a 2-day refeeding of the 90% sucrose diet. This indicates that synthesis of G6PD mRNA occurred while refeeding the 90% sucrose diet. Reducing the starvation period to only 2 days resulted in no induction of G6PD by the 90% sucrose diet and the absence of an 8-azaguanine insensitive induction upon changing to the 90% protein diet. J. Nutr. 104; 12-17,

Uterine tissue differentiation: oestrogen and progesterone induction of glucose 6-phosphate dehydrogenase.

Uterine tissue differentiation: oestrogen and progesterone induction of glucose 6-phosphate dehydrogenase.

Biochemical adaptation of rat liver in response to marginal oxygen toxicity.

1. Hepatic glucose 6-phosphate dehydrogenase activity was increased in rats exposed to 5lb/in(2) (equivalent to 27000ft), 100% O(2) when compared with control animals in a 14.7lb/in(2) (sea level), air environment. Glyceraldehyde 3-phosphate dehydrogenase, isocitrate dehydrogenase, and succinate dehydrogenase were not affected by the 5lb/in(2), 100% O(2) environment. 2. Animals exposed to the hyperoxic environment consumed food, expired CO(2) and gained weight at the same rate as normoxic control animals. Additionally, blood glucose and liver glycogen concentrations were unchanged in the hyperoxic animals. The only readily apparent physiological difference in the hyperoxic animals was a decreased haematocrit. 3. The increase in glucose 6-phosphate dehydrogenase was eliminated by the injection of actinomycin D or cycloheximide. 4. Expiration of (14)CO(2) from [1-(14)C]glucose was approximately the same in hyperoxic and normoxic rats. However, (14)CO(2) expiration from [6-(14)C]glucose was markedly decreased in the animals exposed to the hyperoxic environment. 5. Calculations of the relative importance of the pentose phosphate pathway versus the tricarboxylic acid cycle plus glycolysis indicated that the livers from animals in the 5lb/in(2), 100% O(2) environment metabolized twice as much carbohydrate by way of the pentose phosphate pathway as did those from the sea-level air control animals. 6. In livers of rats exposed to 5lb/in(2), 100% O(2) the concentrations of pyruvate, citrate and 2-oxoglutarate were increased, that of isocitrate was slightly elevated, whereas the concentrations of succinate, fumarate and malate were decreased. 7. An inactivation of both tricarboxylic acid cycle lipoate-containing dehydrogenases, pyruvate and 2-oxoglutarate, under hyperoxic conditions is proposed. 8. The adaptive significance of the induction of glucose 6-phosphate dehydrogenase and the resultant production of NADPH under hyperoxic conditions is discussed.

Differential responses of glucose 6-phosphate dehydrogenase and lipogenesis to fat-free diets

Fasting and refeeding rats on a glucose-casein diet increased both the activity of glucose 6-phosphate dehydrogenase and the conversion of glucose into fatty acids in liver, whereas refeeding on glucose alone caused a comparable rise in the lipogenic rate without altering the dehydrogenase activity, indicating that the induced levels of the dehydrogenase are not necessarily required for the increased generation of NADPH under the adaptive hyperlipogenesis. This was partially proved by the finding that the oxidation of carbon 1 of glucose was preferentially increased over carbon 6 in the liver of glucose-refed rats. The activities of malic enzyme, citrate cleavage enzyme and glucokinase were elevated substantially by refeeding glucose with or without added protein. These results were interpreted as indicating that the response of enzymes catalyzing the carbon chain-supplying reactions in fatty acid synthesis from glucose is of a primary importance for the adaptive hyperlipogenesis and that the induction of glucose 6-phosphate dehydrogenase by the glucose-casein diet is rather representing a metabolic consequence secondary to the enhanced fatty acid synthesis.