Liver Glucose 6-phosphate Dehydrogenase Research Articles

Comparative effects of pravastatin and rosuvastatin on carbohydrate metabolism in an experimental diabetic rat model.

Statin treatment may increase the risk of diabetes; there is insufficient data on how statins affect glucose regulation and glycemic control and the effects of statins on liver enzymes related to carbohydrate metabolism have not been fully studied. Therefore, we aimed to compare the effects of the statin derivatives, pravastatin, and rosuvastatin, on carbohydrate metabolism in an experimental diabetic rat model. Female Wistar albino rats were used and diabetes was induced by intraperitoneal injection of streptozotocin. Thereafter, 10 and 20 mg kg-1 day-1 doses of both pravastatin and rosuvastatin were administered by oral gavage to the diabetic rats for 8 weeks. At the end of the experiment, body masses, the levels of fasting blood glucose, serum insulin, insulin resistance (HOMA-IR), liver glycogen, and liver enzymes related to carbohydrate metabolism were measured. Both doses of pravastatin significantly in creased the body mass in diabetic rats, however, rosuvastatin, especially at the dose of 20 mg kg-1 day-1 reduced the body mass signi ficantly. Pravastatin, especially at a dose of 20 mg kg-1 day-1, caused significant increases in liver glycogen synthase and glucose 6-phosphate dehydrogenase levels but significant decreases in the levels of glycogen phosphorylase, lactate dehydrogenase, and glucose-6-phosphatase. Hence, pravastatin partially ameliorated the adverse changes in liver enzymes caused by diabetes and, especially at the dose of 20 mg kg-1 day-1, reduced the fasting blood glucose level and increased the liver glycogen content. However, rosuvastatin, especially at the dose of 20 mg kg-1 day-1, significantly reduced the liver glycogen synthase and pyruvate kinase levels, but increased the glycogen phosphorylase level in diabetic rats. Rosuvastatin, 20 mg kg-1 day-1 dose, caused significant decreases in the body mass and the liver glycogen content of diabetic rats. It can be concluded that pravastatin, especially at the dose of 20 mg kg-1 day-1 is more effective in ameliorating the negative effects of diabetes by modulating carbohydrate metabolism.

Alteration of Glutathione and Antioxidant Status with Exercise in Unfed and Refed Rats

The influences of food deprivation and refeeding on glutathione (GSH) status, antioxidant enzyme activity and lipid peroxidation in response to an acute bout of exercise were investigated in the liver and skeletal muscles of male Sprague-Dawley rats randomly divided into three groups: starved for 48 h without refeeding; starved for 48 h and refed for 24 or 48 h. Half of each group of rats was exercised on a treadmill until exhaustion and killed immediately, whereas the other half group was killed at rest. Food-deprived rats had significantly lower liver GSH concentration and GSH:glutathione disulfide (GSSG) ratio. Malondialdehyde concentrations in the liver and skeletal muscle were both higher in the starved than in the refed rats (P < 0.05). Refed rats had significantly greater liver GSH level, γ-glutamylcysteine synthetase and glucose 6-phosphate dehydrogenase activities and plasma insulin concentration than unfed rats. Exercised 24- and 48-h refed rats had 27% and 31% lower liver GSH (P < 0.05), respectively, and a 21% lower GSH:GSSG ratio (P < 0.05) than their rested counterparts. Plasma insulin concentrations were significantly lower, whereas glucagon concentrations were greater in the exercised than in the rested rats. Muscle GSH concentration was significantly lower in the food-deprived than in the refed rats (P < 0.05) but was unaffected by exercise. Exercised 24-h refed rats had significantly elevated muscle GSSG concentration compared with rested rats, along with a higher GSH peroxidase and a lower γ-glutamyltranspeptidase activity (P < 0.05). These data indicate that both food deprivation-refeeding and exhaustive exercise influence liver and skeletal muscle glutathione status and that these changes may be controlled by hepatic glutathione synthesis and release due to hormonal stimulation.

Purification of Glucose 6-Phosphate Dehydrogenase from Capoeta umbla Gill and Liver Tissues and Inhibition Effects of Some Metal Ions on Enzyme Activity

In this study, the in vitro effects of some metal ions (Ag+, Cd2+, Cu2+, Fe2+, Ni2+ and Pb2+) on freshwater fish Capoeta umbla liver and gill glucose 6-phosphate dehydrogenase (G6PD) have been investigated. For this purpose, C. umbla liver and gill G6PD enzymes were purified, with a specific activity of 31.52 and 22.83 EU/mg protein, 3353.2 and 736.5 fold in a yield of 19.28% and 23.15%, respectively. In order to control the enzyme purification SDS-PAGE showed a single band for the enzyme. In addition, in vitro effects of metal ions on enzyme activity were researched. As a result, Ag+, Cd2+, Cu2+, Ni2+ and Pb2+ inhibited fish liver G6PD; Ag+, Cd2+, Fe2+, Ni2+ and Pb2+ inhibited fish gill G6PD. Besides, it was found that the most effective inhibitor of G6PD enzyme within metal ions used was Ag+. Our results also demonstrate that these metals might be dangerous at low micromolar concentrations for fish G6PD enzyme.

Temefos’a Maruz Kalan Gökkuşağı Alabalıklarında, Antioksidan Cevaplar

In this study, rainbow trout was exposed to temephos (TE) during 96h acute toxicity conditions. The toxic compound was given in doses of 3 mg L-1 and 6 mg L-1. Furthermore, we assessment antioxidant enzymes activity (glucose-6-phosphate dehydrogenase (G6PD), catalase (CAT), glutathione reductase (GR), superoxide dismutase (SOD), malondialdehyde (MDA) level in liver and acetylcholinesterase (AChE) in brain. CAT, SOD, GR, G6PD, and MDA in liver were demonstrated significant effects by the treatments (P&amp;lt;0.05). TE exposure resulted in the inhibition of AChE activity, which, compared with the control group, decreased in all doses (P&amp;lt;0.05). The results showed that temephos inhibited liver tissue’s enzyme activities and increased MDA levels. In the light of these data, we can say that temephos toxicity should be taken into consideration in aquatic environments.

Synergetic Effects of Lactobacillus plantarum and β-Glucan on Digestive Enzyme Activity, Intestinal Morphology, Growth, Fatty Acid, and Glucose-Related Gene Expression of Genetically Improved Farmed Tilapia

The current study was conducted to evaluate the synergetic effects of heat-killed Lactobacillus plantarum (HK L-137) and β-glucan (BG) on digestive enzyme activity and intestinal morphology of genetically improved farmed tilapia (GIFT) with focus on insulin-like growth factor I (IGF-I), fatty acid synthase (FAS), and glucose-6-phosphate dehydrogenase (G6PD). For 12weeks, fish fed the control, or three diets incorporated with 100 HK L-137, 100 BG, or 50 HK L-137 + 50 BGmg/kg (HK L-137, BG, and HK L-137/BG diets). After final sampling, fish fed HK L-137 or HK L-137/BG diets exhibited significantly (P < 0.05) increased final body weight and weight gain while the specific growth rate and feed efficiency ratio enhanced only in HK L-137/BG group. Mucosal and villi lengths and muscle thickness significantly (P < 0.05) increased by HK L-137 or/and BG for the middle intestine. Lipase and protease improved significantly (P <0.05) in fish fed both HK L-137 and BG when compared to the control group. Interestingly, qRT-PCR revealed a significant (P < 0.05) upregulation in the IGF-1 gene expression in fish fed HK L-137 or/and BG additives compared to the control. Muscle and liver G6PD gene expression were upregulated significantly (P <0.05) in fish fed HK L-137/BG diet as compared to the control group. In addition, feeding HK L-137 or both additives effectively elevated the hematocrit, hemoglobin, and WBCs and decreased triglyceride and glucose levels. Accordingly, the use of both HK L-137 and BG is an efficient scheme to reach economically feasible and sustainable tilapia production.

Nutritional and metabolic responses in common dentex (Dentex dentex) fed on different types and levels of carbohydrates

The present study was aimed to evaluate the capacity of common dentex (Dentex dentex) to efficiently use dietary carbohydrates. So, the effects of different type and levels of carbohydrates on growth performance, feed utilization, fish composition, plasma metabolites and key metabolic pathways in liver and white muscle of common dentex are presented. Nine isonitrogenous (43%) and isoenergetic (22MJkg−1) diets were formulated combining three types, pregelatinized starch (PS), dextrin (Dx) and maltodextrin (Mx), and three levels (12, 18 and 24%) of carbohydrates. Growth performance was not significantly influenced by treatments. The best feed utilization was observed in 18% Mx group. Higher hepatic lipid content was found in fish fed lower dietary carbohydrate levels. PS induced higher liver and white muscle hexokinase and pyruvate kinase activities compared to the lower values observed for Mx. Malic enzyme and glucose 6-phosphate dehydrogenase in liver and white muscle were lower in Mx group. The influence of dietary carbohydrates source was more noticeable than those induced by the carbohydrate level, when glycolysis and lipogenesis pathways were considered. Common dentex is able to use properly dietary carbohydrates, although optimal dietary inclusion levels are below 24%. The greater protein-sparing effect was promoted by the less complex carbohydrate (maltodextrin) and the best feed utilization indices were obtained at intermediate levels of inclusion (18%).

A-7. 肝Glucose 6-Phosphate Dehydrogenaseの誘導に及ぼすCyclic AMPの作用

A-7. 肝Glucose 6-Phosphate Dehydrogenaseの誘導に及ぼすCyclic AMPの作用

Effects of diet supplementation with white tea and methionine on lipid metabolism of gilthead sea bream juveniles (Sparus aurata)

A growth trial was performed with gilthead sea bream juveniles (Sparus aurata) to evaluate the effect of diet supplementation with white tea and methionine on fish performance and lipid metabolism. For that purpose, four diets were formulated: a fish meal-based diet (Control) and diets identical to the control diet but supplemented with 2.9% white tea (Tea), 0.3% methionine (Met) or 2.9% white tea plus 0.3% methionine (Tea+Met). Growth performance and feed efficiency parameters, whole-body and liver composition, plasma metabolites concentration and liver glucose 6-phosphate dehydrogenase (G6PDH), malic enzyme (ME) and fatty acid synthetase (FAS) activities were determined. Feed intake was higher in fish fed methionine-supplemented diets, whereas this parameter and growth was decreased in fish fed white tea supplementation. Feed efficiency and protein efficiency ratio were not affected by diet composition. Plasma HDL cholesterol and total lipids concentration were higher in fish fed white tea-supplemented diets. Whole-body lipid, plasma glucose, liver glycogen concentration and liver G6PDH, ME and FAS activities were lower in fish fed white tea-supplemented diets. Results of the present study indicate that methionine seems to act as a feed attractant in diets for sea bream juveniles. Additionally, white tea is an important modulator of lipid metabolism in sea bream juveniles.

Effects of laparoscopic Roux-en-Y gastric bypass on glucose-6 phosphate dehydrogenase activity in obese type 2 diabetics

Glucose-6-phosphate dehydrogenase (G6PD) is the rate-limiting enzyme of the pentose phosphate pathway that provides the majority of NADPH required for lipid biosynthesis. G6PD overexpression has been implicated in insulin resistance, hyperlipidemia, and increased oxidative stress in animals. This study examines G6PD expression in obese diabetic and nondiabetic subjects pre- and post-laparoscopic Roux-en-Y gastric bypass (LRYGB). Patients undergoing LRYGB were recruited for the IRB-approved study and placed in either the diabetic (n = 11) or nondiabetic group (n = 16) (diabetic, HbA1c > 6.5%; nondiabetic, HbA1c < 6.0%). Blood samples were collected at baseline and throughout the first 3 postoperative months. Liver, adipose, and omental samples were taken during surgery. Results are expressed as mean ± SEM and were compared statistically using the Mann-Whitney test. The two groups were not significantly different at baseline except for fasting glucose and HbA1c. G6PD activity (nm/min/mg protein) was significantly higher in red blood cells (RBCs) (3.12 ± 1.39 vs. 0.67 ± 0.14) and liver (17.23 ± 2.40 vs. 9.74 ± 2.18) in diabetics compared to nondiabetics. There was good correlation between increased liver G6PD activity and the severity of diabetes as measured by HbA1c (r (2) = 0.525) and fasting glucose (r (2) = 0.542). No significant difference was found in the adipose or omental G6PD expression. Both groups experienced a significant increase in G6PD blood activity shortly following surgery (1 week) followed by a reduction 3 months after surgery. These results are the first ever seen in human subjects and demonstrate increased G6PD activity in diabetics compared to nondiabetics. These results suggest a correlation between G6PD activity and the severity of type 2 diabetes. The early increases in G6PD activity after LRYGB were unexpected and longer follow-up is needed to determine the effects of LRYGB on G6PD activity.

Increasing stocking density causes inhibition of metabolic–antioxidant enzymes and elevates mRNA levels of heat shock protein 70 in rainbow trout

The objective of this study was to determine the effects of stocking density on the activity of glucose 6-phosphate dehydrogenase (G6PD), 6-phosphogluconate dehydrogenase (6PGD) and glutathione reductase (GR) enzymes in liver, muscle, gill and kidney tissues of rainbow trout. Fish were reared at different stocking densities (15kg/m3, 20kg/m3, 25kg/m3 and 30kg/m3). After adaptation period of 30days, the experiment was carried out for two months. Stocking density of the control group was 15kg/m3. Increasing stocking density caused inhibition in the activities of the enzymes except for kidney G6PD and 6PGD. Activity of both pentose phosphate pathway enzymes unexpectedly increased only in kidney whereas inhibition was observed in other tissues. Since the most powerful and gradual attenuation was observed in muscle tissue for all enzymes, we performed quantitative Real Time PCR to examine the expression of heat shock protein (Hsp70) gene in muscle in order to understand whether the decrease in enzyme activities is associated with stress. The mRNA expression data showed that Hsp70 expression levels significantly elevated at 25kg/m3 and 30kg/m3 stocking densities. Overall results indicate that increasing stocking density blocks the activity of metabolic and antioxidant enzymes and causes considerable stress in rainbow trout. The most susceptible tissue to increasing stocking density was observed to be the muscle.

Effects of Some Metal Ions on Trout Liver Glucose 6-phosphate Dehydrogenase

Effects of Some Metal Ions on Trout Liver Glucose 6-phosphate Dehydrogenase

An Extract of thePleurotus ostreatusMushroom Bolsters the Glutathione Redox System in Various Organs of Aged Rats

This study is to investigate the putative effect of an extract of the oyster mushroom, Pleurotus ostreatus, on reduced glutathione (GSH) and its metabolic enzymes in major organs of male albino rats. A significant (P < .05) decrease in the level of GSH was observed in liver, kidneys, heart, and brain of aged rats compared to young rats. Activities of glutathione S-transferase (GST), glutathione reductase (GR), and glucose 6-phosphate dehydrogenase (G6PDH) were significantly (P < .05) lower in the liver, kidneys, heart, and brain of aged rats. The isoform pattern of these enzymes in aged rats also revealed variations in relative concentrations, presumably due to oxidative stress during aging. Administration of the extract of P. ostreatus to aged rats resulted in a significant (P < .05) increase in the levels of GSH and elevated activities of GST, GR, and G6PDH in liver, kidney, heart, and brain tissues. An increased staining intensity of isoforms of GST and G6PDH was also noted in aged rats that had been treated with the mushroom extract compared to aged untreated rats. The results of this study may suggest that an extract of P. ostreatus, a potential antioxidant, can prevent the oxidation of GSH and protect its related enzymes during aging.

Effects of some drugs on hepatic glucose 6-phosphate dehydrogenase activity in Lake Van Fish ( Chalcalburnus Tarischii Pallas, 1811)

Inhibitory effects of some drugs on hepatic glucose 6-phosphate dehydrogenase from Lake Van fish ( chalcalburnus tarischii pallas, 1811) were investigated. For this purpose, initially liver glucose 6-phosphate dehydrogenase was purified 899-fold in a yield of 46.24% by using 2′,5′-ADP Sepharose 4B affinity gel. In order to control the purification of enzyme was done SDS polyacrylamide gel electrophoresis. SDS polyacrylamide gel electrophoresis showed a single band for enzyme. A constant temperature (+4 °C) was maintained during the purification process. Enzyme activity was determined with the Beutler method by using a spectrophotometer at 340 nm. Vankomycine, sulfanylamide, sulfanylacetamide, nidazole, ciprofloxacin, amoxicillin and KMnO 4 were used as drugs. These drugs exhibited inhibitory effects on the enzyme. IC 50 values of vankomycine, sulfanylamide, sulfanylacetamide, nidazole, ciprofloxacin, amoxicillin and KMnO 4 were 1.88, 0.037, 0.032, 1.178, 2.26, 643.5 and 0.0002 mM, and the K i constants 1.18 ± 0.148, 0.119 ± 0.021, 0.075 ± 0.015, 1.15 ± 0.21, 7.69 ± 0.67, 1007 ± 69, and 0.001 ± 0.00022 mM, respectively. While vankomycine and nidazole showed competitive inhibition, others displayed noncompetitive inhibition. K i constants and IC 50 values for drugs were determined by Lineweaver–Burk graphs and plotting activity percentage versus [ I], respectively.

Scaling effects on metabolism of a teleost

Scaling effects on citrate synthase (CS), glucose-6-phosphate dehydrogenase (G6-PDH), RNA. RNA/DNA ratio and protein contents of brain, liver and skeletal muscle were studied in a teleost, Clarias batrachus. The activity of white skeletal muscle CS decreased significantly as a function of increasing body mass of the fish. It shows that the fulfilment of energy demand in white skeletal muscle is not dependent on aerobic metabolism. The activity of liver G6-PDH decreased with the increasing body mass showing reduction in NADPH generation for lipogenic activity. However, increase in G6-PDH activity showed enhancement in reductive synthesis in skeletal muscle of the larger-sized individuals. A positive scaling of RNA, RNA/DNA ratio and protein contents reflects changes in macromolecular turnover for ATP-supplying enzymes and proteins.

Fenvalerate-induced changes in a catfish, Clarias batrachus: metabolic enzymes, RNA and protein

We studied the effects of a sublethal concentration of pyrethroid insecticide fenvalerate on metabolic enzymes, RNA and protein of brain, liver and skeletal muscle of the freshwater catfish, Clarias batrachus. Exposure to fenvalerate gradually decreased the activity of citrate synthase (CS), glucose 6-phosphate dehydrogenase (G6-PDH) and lactate dehydrogenase (LDH) in brain, liver and skeletal muscle up to 21 days. The maximum decrease in enzyme activity was 23–47%. Withdrawal of fenvalerate from the medium for 21 days restored enzyme activity to their control level in all three tissues. RNA and protein content in brain, liver and skeletal muscle decreased significantly with exposure of fenvalerate up to 21 days. The maximum decrease in RNA and protein was 22–32%. Withdrawal of fenvalerate from the medium for 21 days restored the RNA and protein contents to control levels. The present study suggests that fenvalerate impairs cellular metabolism and its biochemical effects are reversible after withdrawal of fenvalerate.

Growth, digestibility and fatty acid utilization in large Atlantic salmon ( Salmo salar) fed varying levels of n-3 and saturated fatty acids

Dietary fatty acids strategies of metabolic relevance were studied in large Atlantic salmon. Fish with an average weight of 1.8 kg were fed four experimental diets with the same basal composition but coated with different oils, according to a 2×2 factorial design. The two factors were the level of saturated fatty acids (SAFA) in the diet (30% or 19% of the total fatty acids) and the level of n-3 fatty acids (35% or 20%). The oils used were a pure fish oil (herring oil), or combinations of the fish oil with a n-3 fatty acid concentrate and/or palm stearin rich in SAFA. All diets contained the same concentration of fat, protein and carbohydrates. Productive measurements were recorded and apparent digestibility coefficients (ADC) of fat and selected fatty acids calculated. Heart muscle mitochondrial l-3-hydroxyacyl-CoA dehydrogenase (L3HOAD), and liver glucose 6-phosphate dehydrogenase (G6PD) and malic enzyme (ME) were used as markers to determine the effects of dietary fat type on fat catabolism and synthesis, respectively. Significant differences in growth were observed among fish fed the different diets, with the specific growth rate (SGR) being higher in the groups fed diets with low levels of n-3 fatty acids ( P<0.003). Fat digestibility was highest in fish fed the low saturated–low fatty acid n-3 diet (93.31%) and lowest in fish fed the high saturated–high n-3 diet (79.11%). All enzymatic activities were higher in the groups fed the diets with low levels of n-3 fatty acids. Dietary polyunsaturated fatty acids (PUFA) were negatively correlated with all enzymatic activities. Monounsaturated fatty acids (MUFA) positively correlated with both the ME and L3HOAD activities, especially 16:1n-7, 20:1 and 22:1, which showed the highest Pearson correlation coefficients. Dietary SAFA negatively correlated with enzymatic activities, except 14:0, which positively correlated with ME ( r=0.95; P<0.0001), and L3HOAD ( r=0.81; P<0.0002). The results of this study suggest that the lipid metabolism of large Atlantic salmon is readily influenced by the fatty acids supplied in the diet according to their unsaturation and chain length. Dietary 16:1n-7, 20:1 and 22:1 are preferred substrates for heart mitochondrial β-oxidation.

Effects of some antibiotics on glucose 6-phosphate dehydrogenase in sheep liver

In vitro effects of penicillin, sulbactam, cefazolin, and amikacine on the activity of the enzyme glucose-6-phosphate dehydrogenase in sheep liver were investigated. Glucose 6-phosphate dehydrogenase was purified from sheep liver, using a simple and rapid method. The purification consisted of two steps, preparation of homogenate and 2&amp;rsquo;, 5&amp;rsquo;-ADP Sepharose 4B affinity chromatography. As a result of the two consecutive procedures, the enzyme, having the specific activity of 11.76 EU/mg proteins, was purified with a yield of 35.72% and 1.913 fold. In order to control the enzyme purification SDS polyacrylamide gel electrophoresis (SDS-PAGE) was done. SDS-PAGE showed a single band for the enzyme. In addition, I50 values of the antibiotics were determined by plotting activity % vs. antibiotic concentrations. I50 values were 17.71 mM for penicillin, 27.38 mM for sulbactam, 28.88 mM for cefazolin, and 30.59 mM for amikacine.

Metabolic effects of changes in the dietary protein: carbohydrate ratio in eel (Angilla anguilla) and trout (Oncorhynchus mykiss)

The metabolic consequences of the isoenergetic replacement of dietaryprotein by carbohydrates (CHO) were compared in studies of European eel andrainbow trout. Diets with 45/20, 37/30, 29/40 and 21/50 percentprotein/carbohydrate were assessed during a 12-week experiment. The CHO sourcewas pre-gelatinized corn starch. Fish of initial average weight 45gwere fed to satiation twice daily. Weight-gain and feed-efficiency data weregenerally better in trout than in eel, presumably because trout digestibilitycoefficients for protein and energy were higher than in eel. In both species,regardless of physiological differences in digestion and absorption, feedintakeand nutrient digestibility were lower when dietary CHO was higher, but theinter-species differences decreased with increasing dietary CHO.Pyruvate kinase activity (PK) in trout was not affected by dietary CHO content,while blood-glucose and liver-glycogen levels significantly rose withincreasingdietary CHO. In contrast, blood-glucose levels appeared to be regulated ineels,perhaps by an accelerated glycolysis rate, revealed by changes in PK.Gluconeogenic activity was inhibited in trout fed a diet containing20–30%CHO, while in eels this activity was not inhibited by dietary replacement ofprotein by carbohydrate. Liver glucose 6-phosphate dehydrogenase activity(G6PDH) was inhibited in trout fed increasing dietary CHO, whereas in eelsG6PDHoperated at a high rate regardless of dietary composition. These results mayindicate that eels have a better capacity for metabolising high-CHO/low-proteindiets than do trout.

Dog liver glucose-6-phosphate dehydrogenase: purification and kinetic properties

Glucose-6-phosphate dehydrogenase (G6PD) catalyses the first step of the pentose phosphate pathway which generates NADPH for anabolic pathways and protection systems in liver. G6PD was purified from dog liver with a specific activity of 130 U mg −1 and a yield of 18%. PAGE showed two bands on protein staining; only the slower moving band had G6PD activity. The observation of one band on SDS/PAGE with M r of 52.5 kDa suggested the faster moving band on native protein staining was the monomeric form of the enzyme. Dog liver G6PD had a pH optimum of 7.8. The activation energy, activation enthalpy, and Q 10, for the enzymatic reaction were calculated to be 8.96, 8.34 kcal mol −1, and 1.62, respectively. The enzyme obeyed “Rapid Equilibrium Random Bi Bi” kinetic model with K m values of 122±18 μM for glucose-6-phosphate (G6P) and 10±1 μM for NADP. G6P and 2-deoxyglucose-6-phosphate were used with catalytic efficiencies ( k cat/ K m) of 1.86×10 6 and 5.55×10 6 M −1 s −1, respectively. The intrinsic K m value for 2-deoxyglucose-6-phosphate was 24±4 mM. Deamino-NADP (d-NADP) could replace NADP as coenzyme. With G6P as cosubstrate, K m d-ANADP was 23±3 mM; K m for G6P remained the same as with NADP as coenzyme (122±18 μM). The catalytic efficiencies of NADP and d-ANADP (G6P as substrate) were 2.28×10 7 and 6.76×10 6 M −1 s −1, respectively. Dog liver G6PD was inhibited competitively by NADPH ( K i=12.0±7.0 μM). Low K i indicates tight enzyme:NADPH binding and the importance of NADPH in the regulation of the pentose phosphate pathway.

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.