Hexose-6-Phosphate and 6-Phosphogluconate Dehydrogenases of Rat Liver Microsomes. Involvement in NADPH and Carbon Dioxide Generation in the Luminal Space of Microsomal Vesicles1

Hino, Yukinobu; Minakami, Shigeki

doi:10.1093/oxfordjournals.jbchem.a133963

Cited by 22 publications

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“…3). Our observations are in agreement with the findings of Hino and Minakami (37), who have reported a dormant NADPH-generating H6PDH activity in the lumen, which can be activated by oxidants, i.e. by oxidizing intraluminal NADPH.…”

Section: Discussionsupporting

confidence: 93%

Uncoupled Redox Systems in the Lumen of the Endoplasmic Reticulum

Piccirella

Czegle

Lizák

et al. 2006

Journal of Biological Chemistry

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The redox state of the intraluminal pyridine nucleotide pool was investigated in rat liver microsomal vesicles. The vesicles showed cortisone reductase activity in the absence of added reductants, which was dependent on the integrity of the membrane. The intraluminal pyridine nucleotide pool could be oxidized by the addition of cortisone or metyrapone but not of glutathione. On the other hand, intraluminal pyridine nucleotides were slightly reduced by cortisol or glucose 6-phosphate, although glutathione was completely ineffective. Redox state of microsomal protein thiols/disulfides was not altered either by manipulations affecting the redox state of pyridine nucleotides or by the addition of NAD(P) ؉ or NAD(P)H. The uncoupling of the thiol/disulfide and NAD(P) ؉ / NAD(P)H redox couples was not because of their subcompartmentation, because enzymes responsible for the intraluminal oxidoreduction of pyridine nucleotides were distributed equally in smooth and rough microsomal subfractions. Instead, the phenomenon can be explained by the negligible representation of glutathione reductase in the endoplasmic reticulum lumen. The results demonstrated the separate existence of two redox systems in the endoplasmic reticulum lumen, which explains the contemporary functioning of oxidative folding and of powerful reductive reactions.The lumen of the endoplasmic reticulum (ER) 3 is a separate metabolic compartment of the eukaryotic cell (1). Because of the limited and selective permeability of the ER membrane and because of special intraluminal reactions, it differs from the cytosol in numerous parameters. One of the most characteristic differences is that luminal thiols (including protein thiols and glutathione) are present in a more oxidized state than the cytosolic ones. Although the ratio between reduced and oxidized glutathione is 100:1 in the cytosol, this value is around 1-2:1 in the ER lumen (2). The redox potential calculated from these values is Ϫ0.24 and Ϫ0.18 V, respectively. Recent observations suggest an even larger difference between the redox potentials of the two compartments (3).The typical redox potential of a given compartment is a major determinant of rate and direction of redox reactions. The oxidizing environment in the ER lumen is both a prerequisite and a consequence of the oxidative folding of secretory and membrane proteins (4 -6). By generalizing the observations related to the redox state of thiols found in the ER of cells engaged in protein secretion, it is now supposed that the redox conditions in the ER lumen are uniformly oxidizing. However, several intraluminal reactions have been described, which require reducing equivalents. Such reactions involve the isomerization of disulfide bonds (7), the steps of the vitamin K cycle (8), the biotransformation of several endogenous ketones and aldehydes (9), and the reactivation of steroids. One of the main sources of intraluminal reducing equivalents is hexose-6-phosphate dehydrogenase (H6PDH), which generates NADPH at the expense of the oxidation of ...

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Section: Discussionsupporting

confidence: 93%

Uncoupled Redox Systems in the Lumen of the Endoplasmic Reticulum

Piccirella

Czegle

Lizák

et al. 2006

Journal of Biological Chemistry

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“…Moreover, CBX inhibition of hepatic 11b-HSD1 reductase activity itself decreased NADP regeneration from NADPH in ER by preventing the conversion of 11-dehydrocorticosterone to corticosterone, thereby impairing an effective pathway for supplying NADP as cofactor to H6PDH linked to the reduction of H6PDH activity (Ferguson et al 1999). These findings agree with the notion that H6PDH activity is dependent on luminal NADP availability, which can be regenerated by oxidizing luminal NADPH in a reaction catalyzed by 11b-HSD1 (Hino & Minakami 1982, Csala et al 2006). Our data demonstrate that inhibition of 11b-HSD1 may play an important role in control of CBX-induced suppression of hepatic H6PDH in DIO mice, although further studies using specific 11b-HSD1 inhibitors would be expected to provide additional evidence.…”

Section: Discussionsupporting

confidence: 82%

Reduction of hepatic glucocorticoid receptor and hexose-6-phosphate dehydrogenase expression ameliorates diet-induced obesity and insulin resistance in mice

Liu

Nakagawa²,

Wang³

et al. 2008

Journal of Molecular Endocrinology

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Intracellular glucocorticoid (GC) receptor (GR) function determines tissue sensitivity to GCs and strongly affects the development of type 2 diabetes and obesity. 11b-hydroxysteroid dehydrogenase type 1 (11b-HSD1) mediates intracellular steroid exposure to mouse liver GR by prereceptor reactivation of GCs and is crucially dependent on hexose-6-phosphate dehydrogenase (H6PDH)-generating NADPH system. Pharmacological inhibition of 11b-HSD1 improves insulin intolerance and obesity. Here, we evaluated the potential beneficial effects of 11b-HSD1 inhibitor carbenoxolone (CBX) in diet-induced obese (DIO) and insulin-resistant mice by examining the possible influence of CBX on the expression of GR, 11b-HSD1, and H6PDH in vivo and in vitro in hepatocytes. Treatment of DIO mice with CBX markedly reduced hepatic GR mRNA levels and reduced weight gain, hyperglycemia, and insulin resistance. The reduction of hepatic GR gene expression was accompanied by CBX-induced inhibition of both 11b-HSD1 and H6PDH activity and mRNA in the liver. Moreover, CBX treatment also suppressed the expression of both phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase enzyme (G6Pase) mRNA and improved hepatic [1,[2][3] H] deoxy-D-glucose uptake in DIO mice. In addition, the treatment of primary cultures of hepatocytes with increasing concentrations of CBX led to a dose-dependent downregulation of GR mRNA levels, which correlated with the suppression of both 11b-HSD1 and H6PDH activity and their gene expression. Addition of CBX to primary hepatocytes also resulted in suppression of both PEPCK and G6Pase mRNA levels. These findings suggest that CBX exerts some of its beneficial effects, at least in part, by inhibiting hepatic GR and H6PDH expression.

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“…For example some GSD1b patients present with no apparent leucocyte defect [6,7,13] The physiological role of GPT is unknown. One possibility is that it supplies hexose phosphates to hexose-phosphate dehydrogenase, which is present in the lumen of the ER [31][32][33]. Alternatively, it may serve as a G6P sensor in non-gluconeogenic cells, as speculated in [10].…”

Section: Discussionmentioning

confidence: 99%

Glucose 6-phosphate transport in fibroblast microsomes from glycogen storage disease type 1b patients: evidence for multiple glucose 6-phosphate transport systems

Leuzzi

Fulceri

Marcolongo

et al. 2001

Biochemical Journal

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In liver endoplasmic reticulum the intralumenal glucose-6-phosphatase activity requires the operation of a glucose 6-phosphate transporter (G6PT1). Mutations in the gene encoding G6PT1 cause glycogen storage disease type 1b, which is characterized by a loss of glucose-6-phosphatase activity and impaired glucose homoeostasis. We describe a novel glucose 6-phosphate (G6P) transport activity in microsomes from human fibroblasts and HeLa cells. This transport activity is unrelated to G6PT1 since: (i) it was similar in microsomes of skin fibroblasts from glycogen storage disease type 1b patients homozygous for mutations of the G6PT1 gene, and in microsomes from human control subjects; (ii) it was insensitive to the G6PT1 inhibitor chlorogenic acid; and (iii) it was equally active towards G6P and glucose 1-phosphate, whereas G6PT1 is highly selective for G6P. Taken together, our results provide evidence for the presence of multiple transporters for G6P (and other hexose phosphoesters) in the endoplasmic reticulum.

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Hexose-6-Phosphate and 6-Phosphogluconate Dehydrogenases of Rat Liver Microsomes. Involvement in NADPH and Carbon Dioxide Generation in the Luminal Space of Microsomal Vesicles1

Cited by 22 publications

References 0 publications

Uncoupled Redox Systems in the Lumen of the Endoplasmic Reticulum

Uncoupled Redox Systems in the Lumen of the Endoplasmic Reticulum

Reduction of hepatic glucocorticoid receptor and hexose-6-phosphate dehydrogenase expression ameliorates diet-induced obesity and insulin resistance in mice

Glucose 6-phosphate transport in fibroblast microsomes from glycogen storage disease type 1b patients: evidence for multiple glucose 6-phosphate transport systems

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