Isolation and the complete amino acid sequence of lumenal endoplasmic reticulum glucose-6-phosphate dehydrogenase

Absence of hexose-6-phosphate dehydrogenase results in reduced overall glucose consumption but does not prevent 11β-hydroxysteroid dehydrogenase-1-dependent glucocorticoid activation

Hexose-6-phosphate dehydrogenase (H6PD) is thought to be the major source of NADPH within the endoplasmic reticulum (ER), determining 11β-hydroxysteroid dehydrogenase 1 (11β-HSD1) reaction direction to convert inert 11-oxo- to potent 11β-hydroxyglucocorticoids. Here, we tested the hypothesis whether H6pd knock-out (KO) in primary murine bone marrow-derived macrophages results in a switch from 11β-HSD1 oxoreduction to dehydrogenation, thereby inactivating glucocorticoids (GC) and affecting macrophage phenotypic activation as well as causing a more aggressive M1 macrophage phenotype. H6pdKO did not lead to major disturbances of macrophage activation state, although a slightly more pronounced M1 phenotype was observed with enhanced proinflammatory cytokine release, an effect explained by the decreased 11β-HSD1-dependent GC activation. Unexpectedly, ablation of H6pd did not switch 11β-HSD1 reaction direction. A moderately decreased 11β-HSD1 oxoreduction activity by 40-50% was observed in H6pdKO M1 macrophages but dehydrogenation activity was undetectable, providing strong evidence for the existence of an alternative source of NADPH in the ER. H6pdKO M1 activated macrophages showed decreased phagocytic activity, most likely a result of the reduced 11β-HSD1-dependent GC activation. Other general macrophage functions reported to be influenced by GC, such as nitrite production and cholesterol efflux, were altered negligibly or not at all. Importantly, assessment of energy metabolism using an extracellular flux analyzer and lactate measurements revealed reduced overall glucose consumption in H6pdKO M1 activated macrophages, an effect that was GC independent. The GC-independent influence of H6PD on energy metabolism and the characterization of the alternative source of NADPH in the ER warrant further investigations. ENZYMES: 11β-HSD1, EC 1.1.1.146; H6PD, EC 1.1.1.47.

Lipase maturation factor 1 affects redox homeostasis in the endoplasmic reticulum

Lipoprotein lipase (LPL) is a secreted lipase that clears triglycerides from the blood. Proper LPL folding and exit from the endoplasmic reticulum (ER) require lipase maturation factor 1 (LMF1), an ER-resident transmembrane protein, but the mechanism involved is unknown. We used proteomics to identify LMF1-binding partners necessary for LPL secretion in HEK293 cells and found these to include oxidoreductases and lectin chaperones, suggesting that LMF1 facilitates the formation of LPL's five disulfide bonds. In accordance with this role, we found that LPL aggregates in LMF1-deficient cells due to the formation of incorrect intermolecular disulfide bonds. Cells lacking LMF1 were hypersensitive to depletion of glutathione, but not DTT treatment, suggesting that LMF1 helps reduce the ER Accordingly, we found that loss of LMF1 results in a more oxidized ER Our data show that LMF1 has a broader role than simply folding lipases, and we identified fibronectin and the low-density lipoprotein receptor (LDLR) as novel LMF1 clients that contain multiple, non-sequential disulfide bonds. We conclude that LMF1 is needed for secretion of some ER client proteins that require reduction of non-native disulfides during their folding.

Increased glycogen synthase kinase-3β and hexose-6-phosphate dehydrogenase expression in adipose tissue may contribute to glucocorticoid-induced mouse visceral adiposity

BACKGROUND

Increased adiposity in visceral depots is a crucial feature associated with glucocorticoid (GC) excess. The action of GCs in target tissue is regulated by GC receptor (GR) and 11ß-hydroxysteroid dehydrogenase type 1 (11ß-HSD1) coupled with hexose-6-phosphate dehydrogenase (H6pdh). Glycogen synthase kinase-3β (GSK3β) is known to be a crucial mediator of ligand-dependent gene transcription. We hypothesized that the major effects of corticosteroids on adipose fat accumulation are in part medicated by changes in GSK3β and H6pdh.

METHODS

We characterized the alterations of GSK3β and GC metabolic enzymes, and determined the impact of GR antagonist mifepristone on obesity-related genes and the expression of H6pdh and 11ß-HSD1 in adipose tissue of mice exposed to excess GC as well as in in vitro studies using 3T3-L1 adipocytes treated with GCs.

RESULTS

Corticosterone (CORT) exposure increased abdominal fat mass and induced expression of lipid synthase ACC and ACL with activation of GSK3β phosphorylation in abdominal adipose tissue of C57BL/6J mice. Increased pSer⁹ GSK3β was correlated with induction of H6pdh and 11ß-HSD1. Additionally, mifepristone treatment reversed the production of H6pdh and attenuated CORT-mediated production of 11ß-HSD1 and lipogenic gene expression with reduction of pSer⁹ GSK3β, thereby leading to improvement of phenotype of adiposity within adipose tissue in mice treated with excess GCs. Suppression of pSer⁹ GSK3β by mifepristone was accompanied by activation of pThr³⁰⁸ Akt and blockade of CORT-induced adipogenic transcriptor C/EBPα and PPARγ. In addition, mifepristone also attenuated CORT-mediated activation of IRE1α/XBP1. Additionally, reduction of H6pdh by shRNA showed comparable effects to mifepristone on attenuating CORT-induced expression of GC metabolic enzymes and improved lipid accumulation in vitro in 3T3-L1 adipocytes.

CONCLUSION

These findings suggest that elevated adipose GSK3β and H6pdh expression contribute to 11ß-HSD1 mediating hypercortisolism associated with visceral adiposity.

Biochemical characterization of buffalo liver glucose-6-phosphate dehydrogenase isoforms

Glucose-6-phosphate dehydrogenase (G6PD) is a key regulatory enzyme involved in the pentose phosphate pathway. This works represents purification of two buffalo liver glucose-6-phosphate dehydrogenases (BLG6PD1 and BLG6PD2) using combination of ammonium sulfate precipitation and several chromatographic columns. Both enzymes (BLG6PD1 and BLG6PD2) were homogenous on both native PAGE as well as 12% SDS PAGE with molecular weights of 28 and 66 kDa. The molecular weight of BLG6PD1 and BLG6PD2 native forms were determined to be 28 and 66 kDa by gel filtration; indicating monomeric proteins. The K(m) values for BLG6PD1 and BLG6PD2 estimated to be 0.059 and 0.06 mM of β-nicotinamide adenine dinucleotide phosphate. The optimum activity of BLG6PD1 and BLG6PD2 were displayed at pH 8.0 and 8.2 with an isoelectric point (pI) of pH 7.7-7.9 and 5.7-5.9. The divalent cations MgCl2, and CoCl2 act as activators, on the other hand, FeCl2, CuCl2 and ZnCl2 are potent inhibitors of BLG6PD1 and BLG6PD2 activity. NADPH inhibited both isoenzymes competitively with Ki values of 0.012 and 0.030 mM. This study describes a reproducible purification scheme of G6PD from the liver of buffalo as a rich source.

Purification and characterization of glucose-6-phosphate dehydrogenase from camel liver

Glucose-6-phosphate dehydrogenase from camel liver was purified to homogeneity by ammonium sulfate precipitation and a combination of DEAE-cellulose, Sephacryl S-300 gel filtration, and 2', 5' ADP Sepharose 4B affinity chromatography columns. The specific activity of camel liver G6PD is increased to 1.80438 units/mg proteins with 63-fold purification. It turned out to be homogenous on both native PAGE and 12% SDS PAGE, with a molecular weight of 64 kDa. The molecular weight of the native form of camel liver G6PD was determined to be 194 kDa by gel filtration indicating a trimeric protein. The K m value was found to be 0.081 mM of NADP(+). Camel liver G6PD displayed its optimum activity at pH 7.8 with an isoelectric point (pI) of pH 6.6-6.8. The divalent cations MgCl2, MnCl2, and CoCl2 act as activators; on the other hand, CaCl2 and NiCl2 act as moderate inhibitors, while FeCl2, CuCl2, and ZnCl2 are potent inhibitors of camel liver G6PD activity. NADPH inhibited camel liver G6PD competitively with K i value of 0.035 mM. One binding site was deduced for NADPH on the enzyme molecule. This study presents a simple and reproducible purification procedure of G6PD from the camel liver.

Aripiprazole increases NADPH level in PC12 cells: the role of NADPH oxidase

In aripiprazole-treated PC12 cells, we previously showed that the mitochondrial membrane potential (Δψm) was rather increased in spite of lowered cytochrome c oxidase activity. To address these inconsistent results, we focused the NADPH generation by glucose-6-phosphate dehydrogenase (G6PD), a rate-limiting enzyme of the pentose phosphate pathway (PPP), to titrate reactive oxygen species (ROS) that results in the Δψm maintenance. G6PD may be also involved in another inconsistent result of lowered intracellular lactate level in aripiprazole-treated PC12 cells, because PPP competes glucose-6-phosphate with the glycolytic pathway, resulting in the downregulation of glycolysis. Therefore, we assayed intracellular amounts of NADPH, ROS, and the activities of the enzymes generating or consuming NADPH (G6PD, NADP(+)-dependent isocitrate dehydrogenase, NADP(+)-dependent malic enzyme, glutathione reductase, and NADPH oxidase [NOX]) and estimated glycolysis in 50 μM aripiprazole-, clozapine-, and haloperidol-treated PC12 cells. NADPH levels were enhanced only in aripiprazole-treated ones. Only haloperidol increased ROS. However, the enzyme activities did not show significant changes toward enhancing NADPH level except for the aripiprazole-induced decrease in NOX activity. Thus, the lowered NOX activity could have contributed to the aripiprazole-induced increase in the NADPH level by lowering ROS generation, resulting in maintained Δψm. Although the aforementioned assumption was invalid, the ratio of fructose-1,6-bisphosphate to fructose-6-phosphate was decreased by all antipsychotics examined. Pyruvate kinase activity was enhanced only by aripiprazole. In summary, these observations indicate that aripiprazole possibly possesses the pharmacological superiority to clozapine and haloperidol in the ROS generation and the adjustment of glycolytic pathway.

Biochemistry and physiology of hexose-6-phosphate knockout mice

Hexose-6-phosphate dehydrogenase (H6PDH) has emerged as an important factor in setting the redox status of the endoplasmic reticulum (ER) lumen. An important role of H6PDH is to generate a high NADPH/NADP(+) ratio which permits 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) to act as an oxo-reductase, catalyzing the activation of glucocorticoids (GCs). In H6PDH knockout mice 11β-HSD1 assumes dehydrogenase activity and inactivates GCs, rendering the target cell relatively GC insensitive. Consequently, H6PDHKO mice have a phenotype consistent with defects in the permissive and adaptive actions of GCs upon physiology. H6PDHKO mice have also offered an insight into muscle physiology as they also present with a severe vacuolating myopathy, abnormalities of glucose homeostasis and activation of the unfolded protein response due to ER stress, and a number of mechanisms driving this phenotype are thought to be involved. This article will review what we understand of the redox control of GC hormone metabolism regulated by H6PDH, and how H6PDHKO mice have allowed an in-depth understanding of its potentially novel, GC-independent roles in muscle physiology.

Tissue-specific dysregulation of hexose-6-phosphate dehydrogenase and glucose-6-phosphate transporter production in db/db mice as a model of type 2 diabetes

AIMS/HYPOTHESIS

Tissue-specific amplification of glucocorticoid action through 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) affects the development of the metabolic syndrome. Hexose-6-phosphate dehydrogenase (H6PDH) mediates intracellular NADPH availability for 11β-HSD1 and depends on the glucose-6-phosphate transporter (G6PT). Little is known about the tissue-specific alterations of H6PDH and G6PT and their contributions to local glucocorticoid action in db/db mice.

METHODS

We characterised the role of H6PDH and G6PT in pre-receptor metabolism of glucocorticoids by examining the production of the hepatic 11β-HSD1-H6PDH-G6PT system in db/db mice.

RESULTS

We observed that increased production of hepatic H6PDH in db/db mice was paralleled by upregulation of hepatic G6PT production and responded to elevated circulating levels of corticosterone. Treatment of db/db mice with the glucocorticoid antagonist RU486 markedly reduced production of both H6PDH and 11β-HSD1 and improved hyperglycaemia and insulin resistance. The reduction of H6PDH and 11β-HSD1 production by RU486 was accompanied by RU486-induced suppression of hepatic G6pt (also known as Slc37a4) mRNA. Incubation of mouse primary hepatocytes with corticosterone enhanced G6PT and H6PDH production with corresponding activation of 11β-HSD1 and PEPCK: effects that were blocked by RU486. Knockdown of H6pd by small interfering RNA showed effects comparable with those of RU486 for attenuating the corticosterone-induced H6PDH production and 11ß-HSD1 reductase activity in these intact cells. Addition of the G6PT inhibitor chlorogenic acid to primary hepatocytes suppressed H6PDH production.

CONCLUSIONS/INTERPRETATION

These findings suggest that increased hepatic H6PDH and G6PT production contribute to 11β-HSD1 upregulation of local glucocorticoid action that may be related to the development of type 2 diabetes.

Contribution of hexose-6-phosphate dehydrogenase to NADPH content and redox environment in the endoplasmic reticulum

BACKGROUND

Hexose-6-phosphate dehydrogenase (H6PD) has been considered to be a main source of NADPH in the endoplasmic reticulum. It provides reducing equivalents to 11-hydroxysteroid dehydrogenase type 1 for in situ re-activation of glucocorticoids. H6PD null mice indeed show signs of glucocorticoid deficiency, but also suffer from a skeletal myopathy mainly affecting fast twitch muscles, in which the unfolded protein response (UPR) is activated. Thus, H6PD may have additional functions in muscle.

MATERIALS AND METHODS

To determine the contribution of H6PD to total microsomal NADPH content, we measured NADPH in microsomes from liver and quadriceps, gastrocnemius and soleus muscles. To evaluate the effect of H6PD deficiency on microsomal thiol-disulfide redox environment, we measured reduced and oxidized glutathione and free protein thiols.

RESULTS AND CONCLUSIONS

H6PD deficiency decreased but did not eliminate NADPH content in liver and soleus microsomes. Thus there must be other sources of NADPH within the endoplasmic/sarcoplasmic reticulum. Levels of reduced glutathione and free protein thiols were decreased in gastrocnemius muscle from null mice, indicating a more oxidative environment. Such alterations in redox environment may underlie the myopathy and UPR activation in H6PD null mice.

GENERAL SIGNIFICANCE

H6PD plays a role in maintaining normal NADPH levels and redox environment inside the endoplasmic reticulum. Intrinsic differences in ER metabolism may explain the differing effects of H6PD deficiency in different tissues.

Redox control of endoplasmic reticulum function

The lumen of the endoplasmic reticulum constitutes a separate intracellular compartment with a special proteome and metabolome. The redox conditions of the organelle are also characteristically different from those of the other subcellular compartments. The luminal environment has been considered more oxidizing than the cytosol due to the presence of oxidative protein folding. However, recent observations suggest that redox systems in reduced and oxidized states are present simultaneously. The concerted action of membrane transporters and oxidoreductase enzymes maintains the oxidized state of the thiol-disulfide and the reduced state of the pyridine nucleotide redox systems, which are prerequisites for the normal redox reactions localized in the organelle. The powerful thiol-oxidizing machinery of oxidative protein folding continuously challenges the local antioxidant defense. Alterations of the luminal redox conditions, either in oxidizing or reducing direction, affect protein processing, are sensed by the accumulation of misfolded/unfolded proteins, and may induce endoplasmic reticulum stress and unfolded protein response. The activated signaling pathways attempt to restore the balance between protein loading and processing and induce programmed cell death if these attempts fail. Recent findings strongly support the involvement of redox-based endoplasmic reticulum stress in a plethora of human diseases, either as causative agents or as complications.

Hexose-6-phosphate dehydrogenase in the endoplasmic reticulum

Hexose-6-phosphate dehydrogenase (H6PD) is a luminal enzyme of the endoplasmic reticulum that is distinguished from cytosolic glucose-6-phosphate dehydrogenase by several features. H6PD converts glucose-6-phosphate and NADP(+) to 6-phosphogluconate and NADPH, thereby catalyzing the first two reactions of the pentose-phosphate pathway. Because the endoplasmic reticulum has a separate pyridine nucleotide pool, H6PD provides NADPH for luminal reductases. One of these enzymes, 11beta-hydroxysteroid dehydrogenase type 1 responsible for prereceptorial activation of glucocorticoids, has been the focus of much attention as a probable factor in the pathomechanism of several human diseases including insulin resistance and the metabolic syndrome. This review summarizes recent advances related to the functions of H6PD.

The glucocorticoid-activating enzyme 11beta-hydroxysteroid dehydrogenase type 1 has broad substrate specificity: Physiological and toxicological considerations

The primary function of 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) is to catalyze the conversion of inactive to active glucocorticoid hormones and to modulate local glucocorticoid-dependent gene expression. Thereby 11beta-HSD1 plays a key role in the regulation of metabolic functions and in the adaptation of the organism to energy requiring situations. Importantly, elevated 11beta-HSD1 activity has been associated with metabolic disorders, and recent investigations with rodent models of obesity and type 2 diabetes provided evidence for beneficial effects of 11beta-HSD1 inhibitors, making this enzyme a promising therapeutic target. Several earlier and recent studies, mainly performed in vitro, revealed a relatively broad substrate spectrum of 11beta-HSD1 and suggested that this enzyme has additional functions in the metabolism of some neurosteroids (7-oxy- and 11-oxyandrogens and -progestins) and 7-oxysterols, as well as in the detoxification of various xenobiotics that contain reactive carbonyl groups. While there are many studies on the effect of inhibitors on cortisone reduction and circulating glucocorticoid levels and on the transcriptional regulation of 11beta-HSD1 in obesity and diabetes, only few address the so-called alternative functions of this enzyme. We review recent progress on the biochemical characterization of 11beta-HSD1, with a focus on cofactor and substrate specificity and on possible alternative functions of this enzyme.

H6PDH interacts directly with 11beta-HSD1: implications for determining the directionality of glucocorticoid catalysis

Tissue specific amplification of glucocorticoid action through NADPH-dependent reduction of inactive glucocorticoid precursors by 11beta-hydroxysteroid dehydrogenase (11beta-HSD1) contributes to the development of visceral obesity, insulin resistance and Type 2 Diabetes. Hexose-6-phosphate dehydrogenase (H6PDH) is believed to supply NADPH for the reductase activity of 11beta-HSD1 in the lumen of the endoplasmic reticulum (ER), where the two enzymes are co-localized. We report here expression and purification of full-length and truncated N-terminal domain (NTD) of H6PDH in a mammalian expression system. Interestingly, both full-length H6PDH and the truncated NTD are secreted into the culture medium in the absence of 11beta-HSD1. Purified full-length H6PDH is a bi-functional enzyme with glucose-6-phosphate dehydrogenase (G6PDH) activity as well as 6-phosphogluconolactonase (6PGL) activity. Using co-immunoprecipitation experiments with purified H6PDH and 11beta-HSD1, and with cell lysates expressing H6PDH and 11beta-HSD1, we observe direct physical interaction between the two enzymes. We also show the modulation of 11beta-HSD1 directionality by H6PDH using overexpression and siRNA knockdown systems. The NTD retains the ability to interact with 11beta-HSD1 physically as well as modulate 11beta-HSD1 directionality indicating that the NTD of H6PDH is sufficient for the regulation of the 11beta-HSD1 activity.

Insulin and dexamethasone dynamically regulate adipocyte 11beta-hydroxysteroid dehydrogenase type 1

The adipocyte enzyme 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) amplifies local glucocorticoid action by generating active glucocorticoids from inactive metabolites and has emerged as a key player in the pathogenesis of central obesity and metabolic syndrome. However, the regulation of adipocyte 11beta-HSD1 is incompletely understood. Therefore, the present study was designed to investigate the effects of insulin and glucocorticoid as well as their underlying molecular mechanisms on 11beta-HSD1 activity and expression in 3T3-L1 adipocytes and determine whether the in vitro findings could be confirmed in vivo. Our main in vitro findings are 1) insulin stimulated whereas dexamethasone inhibited 11beta-HSD1 activity and expression in a time- and concentration-dependent manner; 2) the effect of dexamethasone was mimicked by both cortisol and corticosterone but blocked by the glucocorticoid receptor antagonist RU486; 3) the p38 MAPK inhibitor SB220025, but not the ERK inhibitor U0126 or the phosphatidylinositol 3-kinase inhibitor LY294002, prevented insulin stimulation of 11beta-HSD1 activity; and 4) although dexamethasone did not alter the half-life of 11beta-HSD1 mRNA, insulin doubled it. Taken together, these in vitro results demonstrate that insulin stimulates adipocyte 11beta-HSD1 through a posttranscriptional mechanism that involves activation of the p38 MAPK signaling pathway, whereas dexamethasone exerts an opposite effect by a glucocorticoid receptor-mediated transcriptional mechanism. In contrast, both insulin and dexamethasone augmented 11beta-HSD1 activity and expression in rat white adipose tissue in vivo, thus confirming the role of insulin but revealing a fundamental difference regarding the role of dexamethasone in regulating adipocyte 11beta-HSD1 between the two model systems.

Maintenance of luminal NADPH in the endoplasmic reticulum promotes the survival of human neutrophil granulocytes

The present study demonstrates the expression of hexose-6-phosphate dehydrogenase and 11 beta-hydroxysteroid dehydrogenase type 1 in human neutrophils, and the presence and activity of these enzymes in the microsomal fraction of the cells. Their concerted action together with the previously described glucose-6-phosphate transporter is responsible for cortisone-cortisol interconversion detected in human neutrophils. Furthermore, the results suggest that luminal NADPH generation by the cortisol dehydrogenase activity of 11 beta-hydroxysteroid dehydrogenase type 1 prevents neutrophil apoptosis provoked by the inhibition of the glucose-6-phosphate transporter. In conclusion, the maintenance of the luminal NADPH pool is an important antiapoptotic factor in neutrophil granulocytes.

A novel selective 11beta-hydroxysteroid dehydrogenase type 1 inhibitor prevents human adipogenesis

Glucocorticoid excess increases fat mass, preferentially within omental depots; yet circulating cortisol concentrations are normal in most patients with metabolic syndrome (MS). At a pre-receptor level, 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) activates cortisol from cortisone locally within adipose tissue, and inhibition of 11beta-HSD1 in liver and adipose tissue has been proposed as a novel therapy to treat MS by reducing hepatic glucose output and adiposity. Using a transformed human subcutaneous preadipocyte cell line (Chub-S7) and human primary preadipocytes, we have defined the role of glucocorticoids and 11beta-HSD1 in regulating adipose tissue differentiation. Human cells were differentiated with 1.0 microM cortisol (F), or cortisone (E) with or without 100 nM of a highly selective 11beta-HSD1 inhibitor PF-877423. 11beta-HSD1 mRNA expression increased across adipocyte differentiation (P<0.001, n=4), which was paralleled by an increase in 11beta-HSD1 oxo-reductase activity (from nil on day 0 to 5.9+/-1.9 pmol/mg per h on day 16, P<0.01, n=7). Cortisone enhanced adipocyte differentiation; fatty acid-binding protein 4 expression increased 312-fold (P<0.001) and glycerol-3-phosphate dehydrogenase 47-fold (P<0.001) versus controls. This was abolished by co-incubation with PF-877423. In addition, cellular lipid content decreased significantly. These findings were confirmed in the primary cultures of human subcutaneous preadipocytes. The increase in 11beta-HSD1 mRNA expression and activity is essential for the induction of human adipogenesis. Blocking adipogenesis with a novel and specific 11beta-HSD1 inhibitor may represent a novel approach to treat obesity in patients with MS.

Direct protein-protein interaction of 11beta-hydroxysteroid dehydrogenase type 1 and hexose-6-phosphate dehydrogenase in the endoplasmic reticulum lumen

Hexose-6-phosphate dehydrogenase (H6PDH) has been shown to stimulate 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1)-dependent local regeneration of active glucocorticoids. Here, we show that coexpression with H6PDH results in a dramatic shift from 11beta-HSD1 oxidase to reductase activity without affecting the activity of the endoplasmic reticular enzyme 17beta-HSD2. Immunoprecipitation experiments revealed coprecipitation of H6PDH with 11beta-HSD1 but not with the related enzymes 11beta-HSD2 and 17beta-HSD2, suggesting a specific interaction between H6PDH and 11beta-HSD1. The use of the 11beta-HSD1/11beta-HSD2 chimera indicates that the N-terminal 39 residues of 11beta-HSD1 are sufficient for interaction with H6PDH. An important role of the N-terminus was indicated further by the significantly stronger interaction of 11beta-HSD1 mutant Y18-21A with H6PDH compared to wild-type 11beta-HSD1. The protein-protein interaction and the involvement of the N-terminus of 11beta-HSD1 were confirmed by Far-Western blotting. Finally, fluorescence resonance energy transfer (FRET) measurements of HEK-293 cells expressing fluorescently labeled proteins provided evidence for an interaction between 11beta-HSD1 and H6PDH in intact cells. Thus, using three different methods, we provide strong evidence that the functional coupling between 11beta-HSD1 and H6PDH involves a direct physical interaction of the two proteins.

Hexose-6-phosphate dehydrogenase and 11beta-hydroxysteroid dehydrogenase-1 tissue distribution in the rat

Intracellular concentrations of the glucocorticoids cortisol and corticosterone are modulated by the enzymes 11beta-hydroxysteroid dehydrogenase (11beta-HSD) 1 and 2. 11beta-HSD1 is a reduced nicotinamide adenine dinucleotide phosphate (NADPH)-dependent microsomal reductase that converts the inactive glucocorticoids cortisone and 11-dehydrocorticosterone to their active forms, cortisol and corticosterone. Hexose-6-phosphate dehydrogenase (H6PDH) is an enzyme that generates NADPH from oxidized NADP (NADP(+)) within the endoplasmic reticulum. In the absence of NADPH or H6PDH to regenerate NADPH, 11beta-HSD1 acts as a dehydrogenase and inactivates glucocorticoids, as does 11beta-HSD2. A monoclonal antibody against H6PDH was produced to study the possibility that 11beta-HSD1 in the absence of H6PDH may be responsible for hydroxysteroid dehydrogenase activity in tissues that do not express significant amounts of 11beta-HSD2. H6PDH and 11beta-HSD1 expression was surveyed in a variety of rat tissues by real-time RT-PCR, Western blot analysis, and immunohistochemistry. H6PDH was found in a wide variety of tissues, with the greatest concentrations in the liver, kidney, and Leydig cells. Although the brain as a whole did not express significant amounts of H6PDH, some neurons were clearly immunoreactive by immunohistochemistry. H6PDH was amply expressed in most tissues examined in which 11beta-HSD1 was also expressed, with the notable exception of the renal interstitial cells, in which dehydrogenase activity by 11beta-HSD1 probably moderates activation of the glucocorticoid receptor because rat renal interstitial cells do not have significant amounts of mineralocorticoid receptors. This antibody against the H6PDH should prove useful for further studies of enzyme activity requiring NADPH generation within the endoplasmic reticulum.

Diabetic pregnancy in rats leads to impaired glucose metabolism in offspring involving tissue-specific dysregulation of 11beta-hydroxysteroid dehydrogenase type 1 expression

Population-based studies have shown that the offspring of diabetic mothers have an increased risk of developing obesity, insulin resistance, type 2 diabetes and hypertension in later life. To investigate mechanism for the high incidence of metabolic diseases in the offspring of diabetic mothers, we focused on the tissue-specific glucocorticoid regulation by 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) and studied offspring born to streptozotocin-induced diabetic rats. The body weights of newborn rats from diabetic mothers were heavier than those from control mothers. Offspring born to diabetic mothers demonstrated insulin resistance and mild glucose intolerance after glucose loading at 10 weeks and showed significantly increased 11beta-HSD1 mRNA and enzyme activity in adipose tissue at 12 weeks of age without obvious obesity. Hepatic 11beta-HSD1 mRNA was also elevated. We propose that the 11beta-HSD1 in adipose tissue and liver may play a key role in the development of metabolic syndrome in the offspring of diabetic mothers. Tissue-specific glucocorticoid dysregulation provides a candidate mechanism for the high incidence of metabolic diseases in the offspring of diabetic mothers. Therefore early analyses before apparent obesity are needed to elucidate the molecular mechanisms that may be programmed during the fetal period.

The glucose-6-phosphate transporter-hexose-6-phosphate dehydrogenase-11beta-hydroxysteroid dehydrogenase type 1 system of the adipose tissue

11beta-hydroxysteroid dehydrogenase type 1, expressed mainly in the endoplasmic reticulum of adipocytes and hepatocytes, plays an important role in the prereceptorial activation of glucocorticoids. In liver endoplasmic reticulum-derived microsomal vesicles, nicotinamide adenine dinucleotide phosphate reduced supply to the enzyme is guaranteed by a tight functional connection with hexose-6-phosphate dehydrogenase and the glucose-6-phosphate transporter (G6PT). In adipose tissue, the proteins and their activities supporting the action of 11beta-hydroxysteroid dehydrogenase type 1 have not been explored yet. Here we report the occurrence of the hexose-6-phosphate dehydrogenase in rat epididymal fat, as detected at the level of mRNA, protein, and activity. In the isolated microsomes, the activity was evident only on the permeabilization of the membrane because of the poor permeability to the cofactor nicotinamide adenine dineucleotide phosphate (NADP(+)), which is consistent with the intralumenal compartmentation of both the enzyme and a pool of pyridine nucleotides. In fat cells, the access of the substrate, glucose-6-phosphate to the intralumenal hexose-6-phosphate dehydrogenase appeared to be mediated by the liver-type G6PT. In fact, the G6PT expression was revealed at the level of mRNA and protein. Accordingly, the transport of glucose-6-phosphate was demonstrated in microsomal vesicles, and it was inhibited by S3483, a prototypic inhibitor of G6PT. Furthermore, isolated adipocytes produced cortisol on addition of cortisone, and the production was markedly inhibited by S3483. The results show that adipocytes are equipped with a functional G6PT-hexose-6-phosphate dehydrogenase-11beta-hydroxysteroid dehydrogenase type 1 system and indicate that all three components are potential pharmacological targets for modulating local glucocorticoid activation.

Transport and transporters in the endoplasmic reticulum

Enzyme activities localized in the luminal compartment of the endoplasmic reticulum are integrated into the cellular metabolism by transmembrane fluxes of their substrates, products and/or cofactors. Most compounds involved are bulky, polar or even charged; hence, they cannot be expected to diffuse through lipid bilayers. Accordingly, transport processes investigated so far have been found protein-mediated. The selective and often rate-limiting transport processes greatly influence the activity, kinetic features and substrate specificity of the corresponding luminal enzymes. Therefore, the phenomenological characterization of endoplasmic reticulum transport contributes largely to the understanding of the metabolic functions of this organelle. Attempts to identify the transporter proteins have only been successful in a few cases, but recent development in molecular biology promises a better progress in this field.

Loss of Peroxisomes Causes Oxygen Insensitivity of the Histochemical Assay of Glucose-6-Phosphate Dehydrogenase Activity to Detect Cancer Cells

Oxygen insensitivity of carcinoma cells and oxygen sensitivity of non-cancer cells in the histochemical assay of glucose-6-phosphate dehydrogenase (G6PD) enables detection of carcinoma cells in unfixed cell smears or cryostat sections of biopsies. The metabolic background of oxygen insensitivity is still not understood completely. In the present study, rat hepatocytes, rat hepatoma cells (FTO-2B), and human colon carcinoma cells (HT29) were used to elucidate these backgrounds. The residual activity in oxygen was 0%, 55%, and 80% in hepatocytes, hepatoma cells, and colon carcinoma cells, respectively. N-ethylmaleimide (NEM), a blocker of SH-groups, did not affect G6PD activity in both carcinoma cell types but reduced G6PD activity in hepatocytes by 40%. Ultrastructural localization of G6PD activity was exclusively in the cytoplasm of carcinoma cells, but in hepatocytes both in cytoplasm and peroxisomes. NEM abolished peroxisomal G6PD activity only. Histochemical assay of catalase activity demonstrated absence of peroxisomes in both carcinoma cell lines. It is concluded that absence of SH-sensitive G6PD activity in peroxisomes in cancer cells is responsible for the oxygen-insensitivity phenomenon.

Why is 11beta-hydroxysteroid dehydrogenase type 1 facing the endoplasmic reticulum lumen? Physiological relevance of the membrane topology of 11beta-HSD1

11Beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) is essential for the local activation of glucocorticoid receptors (GR). Unlike unliganded cytoplasmic GR, 11beta-HSD1 is an endoplasmic reticulum (ER)-membrane protein with lumenal orientation. Cortisone might gain direct access to 11beta-HSD1 by free diffusion across membranes, indirectly via intracellular binding proteins or, alternatively, by insertion into membranes. Membranous cortisol, formed by 11beta-HSD1 at the ER-lumenal side, might then activate cytoplasmic GR or bind to ER-lumenal secretory proteins. Compartmentalization of 11beta-HSD1 is important for its regulation by hexose-6-phosphate dehydrogenase (H6PDH), which regenerates cofactor NADPH in the ER lumen and stimulates oxoreductase activity. ER-lumenal orientation of 11beta-HSD1 is also essential for the metabolism of the alternative substrate 7-ketocholesterol (7KC), a major cholesterol oxidation product found in atherosclerotic plaques and taken up from processed cholesterol-rich food. An 11beta-HSD1 mutant adopting cytoplasmic orientation efficiently catalyzed the oxoreduction of cortisone but not 7KC, indicating access to cortisone from both sides of the ER-membrane but to 7KC only from the lumenal side. These aspects may be relevant for understanding the physiological role of 11beta-HSD1 and for developing therapeutic interventions to control glucocorticoid reactivation.

Hexose-6-phosphate dehydrogenase knock-out mice lack 11 beta-hydroxysteroid dehydrogenase type 1-mediated glucocorticoid generation

The local generation of active glucocorticoid by NADPH-dependent, 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) oxoreductase activity, has emerged as an important factor in regulating hepatic glucose output and visceral adiposity. We have proposed that this NADPH is generated within the endoplasmic reticulum by the enzyme hexose-6-phosphate dehydrogenase. To address this hypothesis, we generated mice with a targeted inactivation of the H6PD gene. These mice were unable to convert 11-dehydrocorticosterone (11-DHC) to corticosterone but demonstrated increased corticosterone to 11-DHC conversion consistent with lack of 11beta-HSD1 oxoreductase and a concomitant increase in dehydrogenase activity. This increased corticosterone clearance in the knock-out mice resulted in a reduction in circulating corticosterone levels. Our studies define the critical requirement of hexose-6-phosphate dehydrogenase for 11beta-HSD1 oxoreductase activity and add a new dimension to the investigation of 11beta-HSD1 as a therapeutic target in patients with the metabolic syndrome.

Purification and kinetics of sheep kidney cortex glucose-6-phosphate dehydrogenase

Glucose-6-phosphate dehydrogenase (G-6-PD) is one of the important enzymes, which is responsible for the production of NADPH and ribose-5-phosphate. NADPH is used for the biosynthetic reactions and protection of the cells from free radicals. We have investigated some properties and kinetic mechanism of the sheep kidney cortex G-6-PD. This enzyme has been purified 1,384-fold with a yield of 16.96% and had a specific activity of 27.69 U/mg protein. The purification procedure consists of 2', 5'-ADP-Sepharose 4B affinity chromatography after ultracentrifugation. The sheep kidney cortex G-6-PD was found to operate according to a Ping Pong Bi Bi mechanism. The kinetic parameters from sheep K(m) values for G-6-P and NADP(+) and V(m) were determined to be 0.041+/-0.0043 mM, 0.0147+/-0.001 mM and 28.23+/-0.86 microMol min(-1) mg protein(-1), respectively. The pH optimum was 7.4 and the optimum temperature was 45 degrees C. In our previous study we have found that lamb kidney cortex G-6-PD enzyme obeys 'Ordered Bi Bi' mechanism. We suggest that kinetic mechanism altered due to the aging since sheep G-6-PD uses a 'ping pong' mechanism.

Minireview: hexose-6-phosphate dehydrogenase and redox control of 11{beta}-hydroxysteroid dehydrogenase type 1 activity

Hexose-6-phosphate dehydrogenase (H6PDH) is a microsomal enzyme that is able to catalyze the first two reactions of an endoluminal pentose phosphate pathway, thereby generating reduced nicotinamide adenine dinucleotide phosphate (NADPH) within the endoplasmic reticulum. It is distinct from the cytosolic enzyme, glucose-6-phosphate dehydrogenase (G6PDH), using a separate pool of NAD(P)+ and capable of oxidizing several phosphorylated hexoses. It has been proposed to be a NADPH regenerating system for steroid hormone and drug metabolism, specifically in determining the set point of 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) activity, the enzyme responsible for the activation and inactivation of glucocorticoids. 11beta-HSD1 is a bidirectional enzyme, but in intact cells displays predominately oxo-reductase activity, a reaction requiring NADPH and leading to activation of glucocorticoids. However, in cellular homogenates or in purified preparations, 11beta-HSD1 is exclusively a dehydrogenase. Because H6PDH and 11beta-HSD1 are coexpressed in the inner microsomal compartment of cells, we hypothesized that H6PDH may provide 11beta-HSD1 with NADPH, thus promoting oxo-reductase activity in vivo. Recently, several studies have confirmed this functional cooperation, indicating the importance of intracellular redox mechanisms for the prereceptor control of glucocorticoid availability. With the increased interest in 11beta-HSD1 oxo-reductase activity in the pathogenesis and treatment of several human diseases including insulin resistance and the metabolic syndrome, H6PDH represents an additional novel candidate for intervention.

Hexose-6-phosphate dehydrogenase determines the reaction direction of 11beta-hydroxysteroid dehydrogenase type 1 as an oxoreductase

The impact of hexose-6-phosphate dehydrogenase (H6PDH) on 11beta-hydroxysteroid dehydrogenase (11beta-HSD) type 1 activity was investigated upon coexpression in HEK-293 cells. Confocal microscopy analysis indicated colocalisation of both enzymes at the lumenal side of the endoplasmic reticulum (ER) membrane. Functional analysis in intact cells revealed fivefold stimulation of 11beta-HSD1 oxoreductase activity and sixfold decrease of dehydrogenase activity upon coexpression with H6PDH, without changing kinetic parameters in cell lysates. Thus, H6PDH directly determines the reaction direction of 11beta-HSD1 in intact cells as an oxoreductase without changing intrinsic catalytic properties of 11beta-HSD1 by regenerating NADPH in the ER-lumen.

Cooperativity between 11β-Hydroxysteroid Dehydrogenase Type 1 and Hexose-6-phosphate Dehydrogenase in the Lumen of the Endoplasmic Reticulum

The functional coupling of 11beta-hydroxysteroid dehydrogenase type 1 and hexose-6-phosphate dehydrogenase was investigated in rat liver microsomal vesicles. The activity of both enzymes was latent in intact vesicles, indicating the intraluminal localization of their active sites. Glucose-6-phosphate, a substrate for hexose-6-phosphate dehydrogenase, stimulated the cortisone reductase activity of 11beta-hydroxysteroid dehydrogenase type 1. Inhibition of glucose-6-phosphate uptake by S3483, a specific inhibitor of the microsomal glucose-6-phosphate transporter, decreased this effect. Similarly, cortisone increased the intravesicular accumulation of radioactivity upon the addition of radiolabeled glucose-6-phosphate, indicating the stimulation of hexose-6-phosphate dehydrogenase activity. A correlation was shown between glucose-6-phosphate-dependent cortisone reduction and cortisone-dependent glucose-6-phosphate oxidation. The results demonstrate a close cooperation of the enzymes based on co-localization and the mutual generation of cofactors for each other.

Increased myocardial dysfunction after ischemia-reperfusion in mice lacking glucose-6-phosphate dehydrogenase

BACKGROUND

Free radical injury contributes to cardiac dysfunction during ischemia-reperfusion. Detoxification of free radicals requires maintenance of reduced glutathione (GSH) by NADPH. The principal mechanism responsible for generating NADPH and maintaining GSH during periods of myocardial ischemia-reperfusion remains unknown. Glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme in the pentose phosphate pathway, generates NADPH in a reaction linked to the de novo production of ribose. We therefore hypothesized that G6PD is essential for maintaining GSH levels and protecting the heart during ischemia-reperfusion injury.

METHODS AND RESULTS

Susceptibility to myocardial ischemia-reperfusion injury was determined in Langendorff-perfused hearts isolated from wild-type mice (WT) and mice lacking G6PD (G6PD(def)) (20% of WT myocardial G6PD activity). During global zero-flow ischemia, cardiac function was similar between WT and G6PD(def) hearts. On reperfusion, however, cardiac relaxation and contractile performance were greatly impaired in G6PD(def) myocardium, as demonstrated by elevated end-diastolic pressures and decreased percent recovery of developed pressure relative to WT hearts. Contractile dysfunction in G6PD(def) hearts was associated with depletion of total glutathione stores and impaired generation of GSH from its oxidized form. Increased ischemia-reperfusion injury in G6PD(def) hearts was reversed by treatment with the antioxidant MnTMPyP but unaffected by supplementation of ribose stores.

CONCLUSIONS

These results demonstrate that G6PD is an essential myocardial antioxidant enzyme, required for maintaining cellular glutathione levels and protecting against oxidative stress-induced cardiac dysfunction during ischemia-reperfusion.

Murine hexose-6-phosphate dehydrogenase: a bifunctional enzyme with broad substrate specificity and 6-phosphogluconolactonase activity

Murine hexose-6-phosphate dehydrogenase has been purified from liver microsomes by affinity chromatography on 2('),5(')-ADP-Sepharose. The purified enzyme has 6-phosphogluconolactonase activity and glucose-6-phosphate dehydrogenase activity and has a native molecular mass of 178 kDa and a subunit molecular mass of 89 kDa. Glucose 6-phosphate, galactose 6-phosphate, 2-deoxyglucose 6-phosphate, glucosamine 6-phosphate, and glucose 6-sulfate are substrates for murine hexose-6-phosphate dehydrogenase, with either NADP or deamino-NADP as coenzyme. This study confirms that hexose-6-phosphate dehydrogenase is a bifunctional enzyme which can catalyze the first two reactions of the pentose phosphate pathway.

Purification of glucose 6-phosphate dehydrogenase from Buffalo (Bubalus bubalis) erythrocytes and investigation of some kinetic properties

Glucose 6-phosphate dehydrogenase (G6PD) was purified from buffalo (Bubalus bubalis) erythrocytes and some characteristics of the enzyme were investigated. The purification procedure was composed of two steps: hemolysate preparation and 2('),5(')-ADP-Sepharose 4B affinity gel chromatography. Thanks to the two consecutive procedures, the enzyme, having a specific activity of 69.7EU/mg proteins, was purified 650-fold with a yield of 31%. Optimal pH, stable pH, optimal temperature, molecular weight, and K(M) and V(max) values for NADP(+) and glucose 6-phosphate (G6-P) substrates were also determined for the enzyme. In addition, K(i) values and the type of inhibition were determined by means of Lineweaver-Burk graphs obtained for such inhibitors as ATP, ADP, NADPH, and NADH.

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.

Hofbauer cell activation and its increased glucose-6-phosphate dehydrogenase activity in second trimester-spontaneous abortion: an ultrastructural dual staining enzyme-cytochemical study

PROBLEM

While activated/phagocytosing phagocytes infiltrating to the chorioamnion are considered to be one of the causal agents of preterm labor onset, whether placental villous macrophages (Hofbauer cells) are activated/phagocytosing in this condition is not known.

METHOD OF STUDY

We concomitantly localized two important phagocytosis-related enzymes, acid phosphatase (ACP) and glucose-6-phosphate dehydrogenase (G6PD), in Hofbauer cells in second trimester placental villi, and compared them with those from infection-related second trimester-spontaneous abortion (miscarriage) placentas.

RESULTS

There were two types of Hofbauer cells. The first cells exhibited ACP stainings confined to the lysosomes, suggesting that they are dormant/non-activated cells. Approximately two-thirds of these cells showed weak G6PD labeling on the cytosolic side of endoplasmic reticula, and G6PD labeling was hardly recognizable in the remaining one-third. The second cells, possessing large phagosomes, showed marked ACP labeling in the phagosomes, suggesting that they are activated/phagocytosing cells. All these cells exhibited G6PD labeling, and in 'bursting cells' (possibly hyperactivated cells) G6PD deposits were marked. The percentage of activated cells in miscarriage placentas was significantly higher (44.8 +/- 6.0%) than that in gestational age-matched controls (17.4 +/- 5.3%).

CONCLUSIONS

These observations indicated that (1) G6PD activity increased in activated/phagocytosing Hofbauer cells, and (2) the percentage of phagocytosing cells increased in infection-related miscarriage placentas. Hofbauer activation and G6PD may play an role in the pathogenesis/pathophysiology of preterm labor onset.

Purification of 6-phosphogluconate dehydrogenase from parsley (Petroselinum hortense) leaves and investigation of some kinetic properties

In this study, 6-phosphogluconate dehydrogenase (E.C.1.1.44; 6PGD) was purified from parsley (Petroselinum hortense) leaves, and analysis of the kinetic behavior and some properties of the enzyme were investigated. The purification consisted of three steps that are preparation of homogenate ammonium sulfate fractionation and on DEAE-Sephadex A50 ion exchange. The enzyme was obtained with a yield of 49% and had a specific activity of 18.3 U (mg proteins)(-1) (Lehninger, A.L.; Nelson, D.L.; Cox, M.M. Principles of Biochemistry, 2nd Ed.; Worth Publishers Inc.: N.Y., 2000, 558-560). The overall purification was about 339-fold. A temperature of +4 degrees C was maintained during the purification process. Enzyme activity was spectrophotometrically measured according to the Beutler method at 340 mn. In order to control the purification of the enzyme, SDS-polyacrylamide gel electrophoresis was carried out in 4% and 10% acrylamide for stacking and running gel, respectively. SDS-polyacrylamide gel electrophoresis showed a single band for enzyme. The molecular weight was found to be 97.5 kDa by Sephadex G-150 gel filtration chromatography. A protein band corresponding to a subunit molecular weight of 24.1 kDa was obtained on SDS-polyacrylamide gel electrophoresis. For the enzymes, the stable pH, optimum pH, and optimum temperature were found as 8.0, 8.0, and 50 degrees C, respectively. In addition, KM and Vmax values for NADP+ and G6-P at optimum pH and 25 degrees C were determined by means of Lineweaver-Burk plots.

Post-translational regulation of glucose-6-phosphate dehydrogenase activity in (pre)neoplastic lesions in rat liver

Glucose-6-phosphate dehydrogenase (G6PD; EC 1.1.1.49) is the key regulatory enzyme of the pentose phosphate pathway and produces NADPH and riboses. In this study, the kinetic properties of G6PD activity were determined in situ in chemically induced hepatocellular carcinomas, and extralesional and control parenchyma in rat livers and were directly compared with those of the second NADPH-producing enzyme of the pentose phosphate pathway, phosphogluconate dehydrogenase (PGD). Distribution patterns of G6PD activity, protein, and mRNA levels were also compared to establish the regulation mechanisms of G6PD activity. In (pre)neoplastic lesions, the V(max) of G6PD was 150-fold higher and the K(m) for G6P was 10-fold higher than in control liver parenchyma, whereas in extralesional parenchyma, the V(max) was similar to that in normal parenchyma but the K(m) was fivefold lower. This means that virtual fluxes at physiological substrate concentrations are 20-fold higher in lesions and twofold higher in extralesional parenchyma than in normal parenchyma. The V(max) of PGD was fivefold higher in lesions than in normal and extralesional liver parenchyma, whereas the K(m) was not affected. Amounts of G6PD protein and mRNA were similar in lesions and in extralesional liver parenchyma. These results demonstrate that G6PD is strongly activated post-translationally in (pre)neoplastic lesions to produce NADPH.

Purification and investigation of some kinetic properties of glucose-6-phosphate dehydrogenase from parsley (Petroselinum hortense) leaves

In this study, glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate: NADP+ oxidoreductase, EC 1.1.1.49; G6PD) was purified from parsley (Petroselinum hortense) leaves, and analysis of the kinetic behavior and some properties of the enzyme were investigated. The purification consisted of three steps: preparation of homogenate, ammonium sulfate fractionation, and DEAE-Sephadex A50 ion exchange chromatography. The enzyme was obtained with a yield of 8.79% and had a specific activity of 2.146 U (mg protein)(-1). The overall purification was about 58-fold. Temperature of +4 degrees C was maintained during the purification process. Enzyme activity was spectrophotometrically measured according to the Beutler method, at 340 nm. In order to control the purification of enzyme, SDS-polyacrylamide gel electrophoresis was carried out in 4% and 10% acrylamide for stacking and running gel, respectively. SDS-polyacrylamide gel electrophoresis showed a single band for enzyme. The molecular weight was found to be 77.6 kDa by Sephadex G-150 gel filtration chromatography. A protein band corresponding to a molecular weight of 79.3 kDa was obtained on SDS-polyacrylamide gel electrophoresis. For the enzymes, the stable pH, optimum pH, and optimum temperature were found to be 6.0, 8.0, and 60 degrees C, respectively. Moreover, KM and Vmax values for NADP+ and G6-P at optimum pH and 25 degrees C were determined by means of Lineweaver-Burk graphs. Additionally, effects of streptomycin sulfate and tetracycline antibiotics were investigated for the enzyme activity of glucose-6-phosphate dehydrogenase in vitro.

The glucose-6-phosphatase system

Glucose-6-phosphatase (G6Pase), an enzyme found mainly in the liver and the kidneys, plays the important role of providing glucose during starvation. Unlike most phosphatases acting on water-soluble compounds, it is a membrane-bound enzyme, being associated with the endoplasmic reticulum. In 1975, W. Arion and co-workers proposed a model according to which G6Pase was thought to be a rather unspecific phosphatase, with its catalytic site oriented towards the lumen of the endoplasmic reticulum [Arion, Wallin, Lange and Ballas (1975) Mol. Cell. Biochem. 6, 75--83]. Substrate would be provided to this enzyme by a translocase that is specific for glucose 6-phosphate, thereby accounting for the specificity of the phosphatase for glucose 6-phosphate in intact microsomes. Distinct transporters would allow inorganic phosphate and glucose to leave the vesicles. At variance with this substrate-transport model, other models propose that conformational changes play an important role in the properties of G6Pase. The last 10 years have witnessed important progress in our knowledge of the glucose 6-phosphate hydrolysis system. The genes encoding G6Pase and the glucose 6-phosphate translocase have been cloned and shown to be mutated in glycogen storage disease type Ia and type Ib respectively. The gene encoding a G6Pase-related protein, expressed specifically in pancreatic islets, has also been cloned. Specific potent inhibitors of G6Pase and of the glucose 6-phosphate translocase have been synthesized or isolated from micro-organisms. These as well as other findings support the model initially proposed by Arion. Much progress has also been made with regard to the regulation of the expression of G6Pase by insulin, glucocorticoids, cAMP and glucose.

A switch in dehydrogenase to reductase activity of 11 beta-hydroxysteroid dehydrogenase type 1 upon differentiation of human omental adipose stromal cells

As exemplified in patients with Cushing's syndrome, glucocorticoids play an important role in regulating adipose tissue distribution and function, but circulating cortisol concentrations are normal in most patients with obesity. However, human omental adipose stromal cells (ASCs) can generate glucocorticoid locally through the expression of the enzyme 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD) type 1 (11 beta-HSD1), which, in intact cells, has been considered to be an oxoreductase, converting inactive cortisone (E) to cortisol (F). Locally produced F can induce ASC differentiation, but the relationship between 11 beta-HSD1 expression and adipocyte differentiation is unknown. Primary cultures of paired omental (om) and sc ASC and adipocytes were prepared from 17 patients undergoing elective abdominal surgery and cultured for up to 14 d. Expression and activity of 11 beta-HSD isozymes were analyzed together with early (lipoprotein lipase) and terminal (glycerol 3 phosphate dehydrogenase) markers of adipocyte differentiation. On d 1 of culture, 11 beta-HSD1 activity in intact om ASCs exceeded oxoreductase activity in every patient (78.9 +/- 24.9 vs. 15.8 +/- 3.7 [mean +/- SE] pmol/mg per hour, P < 0.001), and in sc ASCs, relative activities were similar (40.6 +/- 12.2 vs. 36.9 +/- 8.8). Conversely, in freshly isolated om adipocytes, reductase activity exceeded dehydrogenase activity (23.6 +/- 1.5 vs. 6.2 +/- 0.8 pmol/mg per hour, P < 0.01). Following 14 d of culture in serum-free conditions with addition of 10 nM insulin (Ctr) or insulin with 100 nM F (+F), lipoprotein lipase/18S RNA levels increased in both the Ctr- and +F-treated ASCs, but glycerol 3 phosphate dehydrogenase increased only in the +F cultures. In both cases, however, 11 beta-HSD1 oxoreductase activity exceeded dehydrogenase activity (Ctr: 53.3 +/- 9.0 vs. 32.4 +/- 10.5, P < 0.05; +F: 65.6 +/- 15.6 vs. 37.1 +/- 11.5 pmol/mg per hour, P < 0.05), despite no significant changes in 11 beta-HSD1 mRNA levels. In sc ASCs, dehydrogenase activity was similar to reductase activity in both Ctr- and +F-treated cells. Type 2 11 beta-HSD expression was undetectable in each case. These data show that in intact, undifferentiated om ASCs, 11 beta-HSD1 acts primarily as a dehydrogenase, but in mature adipocytes oxoreductase activity predominates. Because glucocorticoids inhibit cell proliferation, we postulate that 11 beta-HSD1 activity in uncommitted ASCs may facilitate proliferation rather than differentiation. Once early differentiation is initiated, a "switch" to 11 beta-HSD1 oxoreductase activity generates F, thus promoting adipogenesis. Site-specific regulation of the set-point of 11 beta-HSD1 activity may be an important mechanism underpinning visceral obesity.

Enzyme-cytochemically detectable glucose-6-phosphate dehydrogenase in human villous macrophages (Hofbauer cells)

Though various tissue macrophages possess high glucose-6-phosphate dehydrogenase (G6PD) activity, which plays an important role in their phagocytosis/bactericidal function, the presence of this enzyme in human placental villous macrophages (Hofbauer cells) has not been determined. We examined the ultrastructural localization of glucose-6-phosphate dehydrogenase (G6PD) in Hofbauer cells in first and second trimester placental villi, using a newly developed enzyme-cytochemistry (copper-ferrocyanide) method. Electron-dense deposits indicative of G6PD activity were clearly visible in the cytoplasm and on the cytosolic side of the endoplasmic reticulum of Hofbauer cells. Positive and negative cytochemical controls ensured specific detection of enzyme activity. These observations indicated that Hofbauer cells abundantly possessed enzyme-cytochemically detectable G6PD activity. Hofbauer cell G6PD may play a role in placental defense, by supplying NADPH-dependent enzymes (i.e. nitric oxide synthase or NADPH oxidase) with NADPH. This enzyme may also fuel Hofbauer cells with ribose 5-phosphate during their cell proliferation and cell division.

Cytochemically detectable glucose-6-phosphate dehydrogenase is present in human fetal membrane chorion laeve trophoblasts

We examined the ultrastructural localization of glucose-6-phosphate dehydrogenase (G6PD), a NADPH-generating enzyme, in human fetal membranes at various gestational ages, using newly developed enzyme histochemistry (copper-ferrocyanide method). Electron-dense deposits indicative of G6PD activity were clearly visible in the cytoplasm and on the cytosolic side of the endoplasmic reticulum of chorion laeve cytotrophoblasts at various gestational ages. Positive and negative cytochemical controls ensured specific detection of enzyme activity. These observations indicated that chorion laeve trophoblasts were the site of NADPH production. Chorion laeve trophoblast G6PD may play a significant role in fetal membrane physiology, by delivering NADPH to NADPH-dependent oxidoreductases which these cells possess.

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.

Glucose-6-phosphate dehydrogenase-6-phosphogluconolactonase. A novel bifunctional enzyme in malaria parasites

Plasmodium falciparum glucose 6-phosphate dehydrogenase (Pf Glc6PD), compared to other Glc6PDs has an additional 300 amino acids at the N-terminus. They are not related to Glc6PD but are similar to a family of proteins (devb) of unknown function, some of which are encoded next to Glc6PD in certain bacteria. The human devb homologue has recently been shown to have 6-phosphogluconolactonase (6PGL) activity. This suggests Pf Glc6PD may be a bifunctional enzyme, the evolution of which has involved the fusion of adjacent genes. Further functional analysis of Pf Glc6PD has been hampered because parts of the gene could not be cloned. We have isolated and sequenced the corresponding Plasmodium berghei gene and shown it encodes an enzyme (Pb Glc6PD) with the same structure as the P. falciparum enzyme. Pb Glc6PD is 950 amino acids long with significant sequence similarity in both the devb and Glc6PD domains with the P. falciparum enzyme. The P. berghei enzyme does not have an asparagine-rich segment between the N and C halves and it contains an insertion at the same point in the Glc6PD region as the P. falciparum enzyme but the insertion in the P. berghei is longer (110 versus 62 amino acids) and unrelated in sequence to the P. falciparum insertion. Though expression of this enzyme in bacteria produced largely insoluble protein, conditions were found where the full-length enzyme was produced in a soluble form which was purified via a histidine tag. We show that this enzyme has both Glc6PD and 6PGL activities. Thus the first two steps of the pentose phosphate pathway are catalysed by a single novel bifunctional enzyme in these parasites.

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.

Posttranslational regulation of glucose-6-phosphate dehydrogenase activity in tongue epithelium

Expression of glucose-6-phosphate dehydrogenase (G6PD) activity is high in tongue epithelium, but its exact function is still unknown. It may be related either to the high proliferation rate of this tissue or to protection against oxidative stress. To elucidate its exact role, we localized quantitatively G6PD activity, protein and mRNA using image analysis in tongue epithelium of rat and rabbit, two species with different diets. Distribution patterns of G6PD activity were largely similar in rat and rabbit but the activities were twofold lower in rabbit. Activity was two to three times higher in upper cell layers of epithelium than in basal cell layers, whereas basal layers, where proliferation takes place, contained twice as much G6PD protein and 40% more mRNA than upper layers. Our findings show that G6PD is synthetized mainly in basal cell layers of tongue epithelium and that it is posttranslationally activated when cells move to upper layers. Therefore, we conclude that the major function of G6PD activity in tongue epithelium is the formation of NADPH for protection against oxidative stress and that diet affects enzyme expression in this tissue.

The Pseudomonas aeruginosa devB/SOL homolog, pgl, is a member of the hex regulon and encodes 6-phosphogluconolactonase

A cyclic version of the Entner-Doudoroff pathway is used by Pseudomonas aeruginosa to metabolize carbohydrates. Genes encoding the enzymes that catabolize intracellular glucose to pyruvate and glyceraldehyde 3-phosphate are coordinately regulated, clustered at 39 min on the chromosome, and collectively form the hex regulon. Within the hex cluster is an open reading frame (ORF) with homology to the devB/SOL family of unidentified proteins. This ORF encodes a protein of either 243 or 238 amino acids; it overlaps the 5' end of zwf (encodes glucose-6-phosphate dehydrogenase) and is followed immediately by eda (encodes the Entner-Doudoroff aldolase). The devB/SOL homolog was inactivated in P. aeruginosa PAO1 by recombination with a suicide plasmid containing an interrupted copy of the gene, creating mutant strain PAO8029. PAO8029 grows at 9% of the wild-type rate using mannitol as the carbon source and at 50% of the wild-type rate using gluconate as the carbon source. Cell extracts of PAO8029 were specifically deficient in 6-phosphogluconolactonase (Pgl) activity. The cloned devB/SOL homolog complemented PAO8029 to restore normal growth on mannitol and gluconate and restored Pgl activity. Hence, we have identified this gene as pgl and propose that the devB/SOL family members encode 6-phosphogluconolactonases. Interestingly, three eukaryotic glucose-6-phosphate dehydrogenase (G6PDH) isozymes, from human, rabbit, and Plasmodium falciparum, contain Pgl domains, suggesting that the sequential reactions of G6PDH and Pgl are incorporated in a single protein. 6-Phosphogluconolactonase activity is induced in P. aeruginosa PAO1 by growth on mannitol and repressed by growth on succinate, and it is expressed constitutively in P. aeruginosa PAO8026 (hexR). Taken together, these results establish that Pgl is an essential enzyme of the cyclic Entner-Doudoroff pathway encoded by pgl, a structural gene of the hex regulon.

Human mutations in glucose 6-phosphate dehydrogenase reflect evolutionary history

Glucose 6-phosphate dehydrogenase (G6PD) is a cytosolic enzyme encoded by a housekeeping X-linked gene whose main function is to produce NADPH, a key electron donor in the defense against oxidizing agents and in reductive biosynthetic reactions. Inherited G6PD deficiency is associated with either episodic hemolytic anemia (triggered by fava beans or other agents) or life-long hemolytic anemia. We show here that an evolutionary analysis is a key to understanding the biology of a housekeeping gene. From the alignment of the amino acid (aa) sequence of 52 glucose 6-phosphate dehydrogenase (G6PD) species from 42 different organisms, we found a striking correlation between the aa replacements that cause G6PD deficiency in humans and the sequence conservation of G6PD: two-thirds of such replacements are in highly and moderately conserved (50-99%) aa; relatively few are in fully conserved aa (where they might be lethal) or in poorly conserved aa, where presumably they simply would not cause G6PD deficiency. This is consistent with the notion that all human mutants have residual enzyme activity and that null mutations are lethal at some stage of development. Comparing the distribution of mutations in a human housekeeping gene with evolutionary conservation is a useful tool for pinpointing amino acid residues important for the stability or the function of the corresponding protein. In view of the current explosive increase in full genome sequencing projects, this tool will become rapidly available for numerous other genes.

Identification of the cDNA encoding human 6-phosphogluconolactonase, the enzyme catalyzing the second step of the pentose phosphate pathway(1)

We report the sequence of a human cDNA encoding a protein homologous to devB (a bacterial gene often found in proximity to the gene encoding glucose-6-phosphate dehydrogenase in bacterial genomes) and to the C-terminal part of human hexose-6-phosphate dehydrogenase. The protein was expressed in Escherichia coli, purified and shown to be 6-phosphogluconolactonase, the enzyme catalyzing the second step of the pentose phosphate pathway. Sequence analysis indicates that bacterial devB proteins, the C-terminal part of hexose-6-phosphate dehydrogenase and yeast Sol1-4 proteins are most likely also 6-phosphogluconolactonases and that these proteins are related to glucosamine-6-phosphate isomerases.

A rapid method for the purification of glucose-6-phosphate dehydrogenase from bovine lens

This paper describes a simple and rapid method for the purification of glucose-6-phosphate dehydrogenase from bovine lens, together with analysis of the kinetic behaviour and some properties of the enzyme. The purification consisted of two steps, 2',5'-ADP-Sepharose 4B affinity chromatography and DEAE Sepharose Fast Flow ion exchange chromatography in procedure which took two working days. The enzyme was obtained with a yield of 13.7% and had a specific activity of 2.64 U/mg protein. The overall purification was about 19,700-fold. The molecular weight of the enzyme was found to be 62 +/- 3 kDa by Sephadex G-200 gel filtration chromatography. A protein band corresponding to a molecular weight of 69.2 +/- 3.2 kDa was obtained on SDS polyacrylamide slab gel electrophoresis. On chromatofocusing, lens glucose-6-phosphate dehydrogenase gave a single peak at pI 5.14. The activation energy of the reaction catalyzed by the enzyme was calculated from Arrhenius plot as Ea = 5.88 kcal/mol. The pH versus velocity curve had two peaks at pH 7.7 and 9.6. By the double-reciprocal plots and the product inhibition studies, it was shown that the enzyme follows 'Ordered Bi Bi' sequential kinetics. From the graphical and statistical analyses, KmNADP+, KmG-6-P, KiNADPH, Ki6-PGA were estimated to be 0.008 +/- 0.002, 0.035 +/- 0.013, 0.173 +/- 0.007 and 1.771 +/- 0.160 mM, respectively. The observed kinetic behaviour of glucose-6-phosphate dehydrogenase from bovine lens was in accordance with the enzyme from other sources.