The levels of nicotinamide nucleotides in liver microsomes and their possible significance to the function of hexose phosphate dehydrogenase

Subcellular Localization of Thioredoxin/Thioredoxin Reductase System-A Missing Link in Endoplasmic Reticulum Redox Balance

The lumen of the endoplasmic reticulum (ER) is usually considered an oxidative environment; however, oxidized thiol-disulfides and reduced pyridine nucleotides occur there parallelly, indicating that the ER lumen lacks components which connect the two systems. Here, we investigated the luminal presence of the thioredoxin (Trx)/thioredoxin reductase (TrxR) proteins, capable of linking the protein thiol and pyridine nucleotide pools in different compartments. It was shown that specific activity of TrxR in the ER is undetectable, whereas higher activities were measured in the cytoplasm and mitochondria. None of the Trx/TrxR isoforms were expressed in the ER by Western blot analysis. Co-localization studies of various isoforms of Trx and TrxR with ER marker Grp94 by immunofluorescent analysis further confirmed their absence from the lumen. The probability of luminal localization of each isoform was also predicted to be very low by several in silico analysis tools. ER-targeted transient transfection of HeLa cells with Trx1 and TrxR1 significantly decreased cell viability and induced apoptotic cell death. In conclusion, the absence of this electron transfer chain may explain the uncoupling of the redox systems in the ER lumen, allowing parallel presence of a reduced pyridine nucleotide and a probably oxidized protein pool necessary for cellular viability.

Spatiotemporal compartmentalization of hepatic NADH and NADPH metabolism

Compartmentalization is a fundamental design principle of eukaryotic metabolism. Here, we review the compartmentalization of NAD+/NADH and NADP+/NADPH with a focus on the liver, an organ that experiences the extremes of biochemical physiology each day. Historical studies of the liver, using classical biochemical fractionation and measurements of redox-coupled metabolites, have given rise to the prevailing view that mitochondrial NAD(H) pools tend to be oxidized and important for energy homeostasis, whereas cytosolic NADP(H) pools tend to be highly reduced for reductive biosynthesis. Despite this textbook view, many questions still remain as to the relative size of these subcellular pools and their redox ratios in different physiological states, and to what extent such redox ratios are simply indicators versus drivers of metabolism. By performing a bioinformatic survey, we find that the liver expresses 352 known or predicted enzymes composing the hepatic NAD(P)ome, i.e. the union of all predicted enzymes producing or consuming NADP(H) or NAD(H) or using them as a redox co-factor. Notably, less than half are predicted to be localized within the cytosol or mitochondria, and a very large fraction of these genes exhibit gene expression patterns that vary during the time of day or in response to fasting or feeding. A future challenge lies in applying emerging new genetic tools to measure and manipulate in vivo hepatic NADP(H) and NAD(H) with subcellular and temporal resolution. Insights from such fundamental studies will be crucial in deciphering the pathogenesis of very common diseases known to involve alterations in hepatic NAD(P)H, such as diabetes and fatty liver disease.

Cortisol promotes endoplasmic glucose production via pyridine nucleotide redox

Both increased adrenal and peripheral cortisol production, the latter governed by 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1), contribute to the maintenance of fasting blood glucose. In the endoplasmic reticulum (ER), the pyridine nucleotide redox state (NADP/NADPH) is dictated by the concentration of glucose-6-phosphate (G6P) and the coordinated activities of two enzymes, hexose-6-phosphate dehydrogenase (H6PDH) and 11β-HSD1. However, luminal G6P may similarly serve as a substrate for hepatic glucose-6-phophatase (G6Pase). A tacit belief is that the G6P pool in the ER is equally accessible to both H6PDH and G6Pase. Based on our inhibition studies and kinetic analysis in isolated rat liver microsomes, these two aforesaid luminal enzymes do share the G6P pool in the ER, but not equally. Based on the kinetic modeling of G6P flux, the ER transporter for G6P (T1) preferentially delivers this substrate to G6Pase; hence, the luminal enzymes do not share G6P equally. Moreover, cortisol, acting through 11β-HSD1, begets a more reduced pyridine redox ratio. By altering this luminal redox ratio, G6P flux through H6PDH is restrained, allowing more G6P for the competing enzyme G6Pase. And, at low G6P concentrations in the ER lumen, which occur during fasting, this acute cortisol-induced redox adjustment promotes glucose production. This reproducible cortisol-driven mechanism has been heretofore unrecognized.

Composition of the redox environment of the endoplasmic reticulum and sources of hydrogen peroxide

The endoplasmic reticulum (ER) is a metabolically active organelle, which has a central role in proteostasis by translating, modifying, folding, and occasionally degrading secretory and membrane proteins. The lumen of the ER represents a separate compartment of the eukaryotic cell, with a characteristic proteome and metabolome. Although the redox metabolome and proteome of the compartment have not been holistically explored, it is evident that proper redox conditions are necessary for the functioning of many luminal pathways. These redox conditions are defined by local oxidoreductases and the membrane transport of electron donors and acceptors. The main electron carriers of the compartment are identical with those of the other organelles: glutathione, pyridine and flavin nucleotides, ascorbate, and others. However, their composition, concentration, and redox state in the ER lumen can be different from those observed in other compartments. The terminal oxidases of oxidative protein folding generate and maintain an "oxidative environment" by oxidizing protein thiols and producing hydrogen peroxide. ER-specific mechanisms reutilize hydrogen peroxide as an electron acceptor of oxidative folding. These mechanisms, together with membrane and kinetic barriers, guarantee that redox systems in the reduced or oxidized state can be present simultaneously in the lumen. The present knowledge on the in vivo conditions of ER redox is rather limited; development of new genetically encoded targetable sensors for the measurement of the luminal state of redox systems other than thiol/disulfide will contribute to a better understanding of ER redox homeostasis.

The human NAD metabolome: Functions, metabolism and compartmentalization

The metabolism of NAD has emerged as a key regulator of cellular and organismal homeostasis. Being a major component of both bioenergetic and signaling pathways, the molecule is ideally suited to regulate metabolism and major cellular events. In humans, NAD is synthesized from vitamin B3 precursors, most prominently from nicotinamide, which is the degradation product of all NAD-dependent signaling reactions. The scope of NAD-mediated regulatory processes is wide including enzyme regulation, control of gene expression and health span, DNA repair, cell cycle regulation and calcium signaling. In these processes, nicotinamide is cleaved from NAD⁺ and the remaining ADP-ribosyl moiety used to modify proteins (deacetylation by sirtuins or ADP-ribosylation) or to generate calcium-mobilizing agents such as cyclic ADP-ribose. This review will also emphasize the role of the intermediates in the NAD metabolome, their intra- and extra-cellular conversions and potential contributions to subcellular compartmentalization of NAD pools.

Retinol dehydrogenases: membrane-bound enzymes for the visual function

Retinoid metabolism is important for many physiological functions, such as differenciation, growth, and vision. In the visual context, after the absorption of light in rod photoreceptors by the visual pigment rhodopsin, 11-cis retinal is isomerized to all-trans retinal. This retinoid subsequently undergoes a series of modifications during the visual cycle through a cascade of reactions occurring in photoreceptors and in the retinal pigment epithelium. Retinol dehydrogenases (RDHs) are enzymes responsible for crucial steps of this visual cycle. They belong to a large family of proteins designated as short-chain dehydrogenases/reductases. The structure of these RDHs has been predicted using modern bioinformatics tools, which allowed to propose models with similar structures including a common Rossman fold. These enzymes undergo oxidoreduction reactions, whose direction is dictated by the preference and concentration of their individual cofactor (NAD(H)/NADP(H)). This review presents the current state of knowledge on functional and structural features of RDHs involved in the visual cycle as well as knockout models. RDHs are described as integral or peripheral enzymes. A topology model of the membrane binding of these RDHs via their N- and (or) C-terminal domain has been proposed on the basis of their individual properties. Membrane binding is a crucial issue for these enzymes because of the high hydrophobicity of their retinoid substrates.

Crosstalk and barriers between the electron carriers of the endoplasmic reticulum

SIGNIFICANCE

The lumen of the endoplasmic reticulum (ER) constitutes a separate compartment with a special proteome and metabolome. The characteristic redox environment required for the optimal functioning of local pathways is defined by the redox couples of the main electron carriers. These molecules, glutathione, pyridine nucleotides, and ascorbic acid, are present within the ER, but their composition, concentration, and redox state are characteristically different from those observed in other subcellular compartments. Spatial and kinetic barriers contribute to the generation and maintenance of this special redox environment.

RECENT ADVANCES

The ER redox has usually been considered from the perspective of oxidative protein folding, one of the major functions of the ER. Thus, the lumen has been described as a relatively oxidizing subcellular compartment.

CRITICAL ISSUES

The ER redoxome has been scantily mapped. However, recent observations suggest that the redox systems in reduced and oxidized states are present simultaneously. The concerted actions of transmembrane uptake processes and local oxidoreductases as well as the absence of specific transport and enzyme activities maintain the oxidized state of the thiol-disulfide systems and the reduced state of the pyridine nucleotide redox systems. These states are prerequisites for the normal redox reactions localized in the ER.

FUTURE DIRECTIONS

An outline of the interactions between the major electron carriers of the ER will contribute to a better understanding of human diseases related to ER redox homeostasis.

Decreased prereceptorial glucocorticoid activating capacity in starvation due to an oxidative shift of pyridine nucleotides in the endoplasmic reticulum

Redox state of pyridine nucleotides of the endoplasmic reticulum (ER) lumen was determined in different nutritional conditions. NADPH-dependent cortisone reduction and NADP(+)-dependent cortisol oxidation were measured in rat liver microsomes, by utilizing the luminal 11β-hydroxysteroid dehydrogenase type 1 activity. Cortisone reduction decreased, while cortisol oxidation increased during onward starvation, showing that the luminal NADPH/NADP(+) ratio was substantially decreased. Cortisone or metyrapone addition caused a smaller decrease in NADPH fluorescence in microsomes from starved rats. The results demonstrate that nutrient supply is mirrored by the redox state of ER luminal pyridine nucleotides.

Modulation of UDP-glucuronosyltransferase activity by endogenous compounds

Glucuronidation is one of the major pathways of metabolism of endo- and xenobiotics. UDP-Glucuronosyltransferase (UGT)-catalyzed glucuronidation accounts for up to 35% of phase II reactions. The expression and function of UGT is modulated by gene regulation, post-translational modifications and protein-protein association. Many studies have focused on drug-drug interactions involving UGT, and there are a number of reports describing the inhibition of UGT by xenobiotics. However, studies about the role of endogenous compounds as an inhibitor or activator of UGT are limited, and it is important to understand any change in the function and regulation of UGT by endogenous compounds. Recent studies in our laboratory have shown that fatty acyl-CoAs are endogenous activators of UGT, although fatty acyl-CoAs had been considered as inhibitors of UGT. Further, we have also suggested that adenine and related compounds are endogenous allosteric inhibitors of UGT. In this review, we summarize the endogenous modulators of UGT and discuss their relevance to UGT function.

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.

Visualization of subcellular NAD pools and intra-organellar protein localization by poly-ADP-ribose formation

Poly-ADP-ribose polymerases (PARPs) use NAD(+) as substrate to generate polymers of ADP-ribose. We targeted the catalytic domain of human PARP1 as molecular NAD(+) detector into cellular organelles. Immunochemical detection of polymers demonstrated distinct subcellular NAD(+) pools in mitochondria, peroxisomes and, surprisingly, in the endoplasmic reticulum and the Golgi complex. Polymers did not accumulate within the mitochondrial intermembrane space or the cytosol. We demonstrate the suitability of this compartment-specific NAD(+) and poly-ADP-ribose turnover to establish intra-organellar protein localization. For overexpressed proteins, genetically endowed with PARP activity, detection of polymers indicates segregation from the cytosol and consequently intra-organellar residence. In mitochondria, polymer build-up reveals matrix localization of the PARP fusion protein. Compared to presently used fusion tags for subcellular protein localization, these are substantial improvements in resolution. We thus established a novel molecular tool applicable for studies of subcellular NAD metabolism and protein localization.

Subcellular and tissue localization of NAD kinases from Arabidopsis: compartmentalization of de novo NADP biosynthesis

The de novo biosynthesis of the triphosphopyridine NADP is catalyzed solely by the ubiquitous NAD kinase family. The Arabidopsis (Arabidopsis thaliana) genome contains two genes encoding NAD+ kinases (NADKs), annotated as NADK1, NADK2, and one gene encoding a NADH kinase, NADK3, the latter isoform preferring NADH as a substrate. Here, we examined the tissue-specific and developmental expression patterns of the three NADKs using transgenic plants stably transformed with NADK promoter::glucuronidase (GUS) reporter gene constructs. We observed distinct spatial and temporal patterns of GUS activity among the NADK::GUS plants. All three NADK::GUS transgenes were expressed in reproductive tissue, whereas NADK1::GUS activity was found mainly in the roots, NADK2::GUS in leaves, and NADK3::GUS was restricted primarily to leaf vasculature and lateral root primordia. We also examined the subcellular distribution of the three NADK isoforms using NADK-green fluorescent protein (GFP) fusion proteins expressed transiently in Arabidopsis suspension-cultured cells. NADK1 and NADK2 were found to be localized to the cytosol and plastid stroma, respectively, consistent with previous work, whereas NADK3 localized to the peroxisomal matrix via a novel type 1 peroxisomal targeting signal. The specific subcellular and tissue distribution profiles among the three NADK isoforms and their possible non-overlapping roles in NADP(H) biosynthesis in plant cells are discussed.

Biochemical factors governing the steady-state estrone/estradiol ratios catalyzed by human 17beta-hydroxysteroid dehydrogenases types 1 and 2 in HEK-293 cells

Human 17beta-hydroxysteroid dehydrogenase types 1 and 2 (17betaHSD1 and 17betaHSD2) regulate estrogen potency by catalyzing the interconversion of estrone (E1) and estradiol (E2) using nicotinamide adenine dinucleotide (phosphate) cofactors NAD(P)(H). In intact cells, 17betaHSD1 and 17betaHSD2 establish pseudo-equilibria favoring E1 reduction or E2 oxidation, respectively. The vulnerability of these equilibrium steroid distributions to mutations and to altered intracellular cofactor abundance and redox state, however, is not known. We demonstrate that the equilibrium E2/E1 ratio achieved by 17betaHSD1 in intact HEK-293 cell lines is progressively reduced from 94:6 to 10:90 after mutagenesis of R38, which interacts with the 2'-phosphate of NADP(H), and by glucose deprivation, which lowers the NADPH/NADP(+) ratio. The shift to E2 oxidation parallels changes in apparent K(m) values for purified 17betaHSD1 proteins to favor NAD(H) over NADP(H). In contrast, mutagenesis of E116 (corresponding to R38 in 17betaHSD1) and changes in intracellular cofactor ratios do not alter the greater than 90:10 E1/E2 ratio catalyzed by 17betaHSD2, and these mutations lower the apparent K(m) of recombinant 17betaHSD2 for NADP(H) only less than 3-fold. We conclude that the equilibrium E1/E2 ratio maintained by human 17betaHSD1 in intact cells is governed by NADPH saturation, which is strongly dependent on both R38 and high intracellular NADPH/NADP(+) ratios. In contrast, the preference of 17betaHSD2 for E2 oxidation strongly resists alteration by genetic and metabolic manipulations. These findings suggest that additional structural features, beyond the lack of a specific arginine residue, disfavor NADPH binding and thus support E2 oxidation by 17betaHSD2 in intact cells.

Kinetic analysis of human enzyme RDH10 defines the characteristics of a physiologically relevant retinol dehydrogenase

Human retinol dehydrogenase 10 (RDH10) was implicated in the oxidation of all-trans-retinol for biosynthesis of all-trans-retinoic acid, however, initial assays suggested that RDH10 prefers NADP(+) as a cofactor, undermining its role as an oxidative enzyme. Here, we present evidence that RDH10 is, in fact, a strictly NAD(+)-dependent enzyme with multisubstrate specificity that recognizes cis-retinols as well as all-trans-retinol as substrates. RDH10 has a relatively high apparent K(m) value for NAD(+) (~100 microm) but the lowest apparent K(m) value for all-trans-retinol (~0.035 microm) among all NAD(+)-dependent retinoid oxidoreductases. Due to its high affinity for all-trans-retinol, RDH10 exhibits a greater rate of retinol oxidation in the presence of cellular retinol-binding protein type I (CRBPI) than human microsomal RoDH4, but like RoDH4, RDH10 does not recognize retinol bound to CRBPI as a substrate. Consistent with its preference for NAD(+), RDH10 functions exclusively in the oxidative direction in the cells, increasing the levels of retinaldehyde and retinoic acid. Targeted small interfering RNA-mediated silencing of endogenous RDH10 or RoDH4 expression in human cells results in a significant decrease in retinoic acid production from retinol, identifying both human enzymes as physiologically relevant retinol dehydrogenases. The dual cis/trans substrate specificity suggests a dual physiological role for RDH10: in the biosynthesis of 11-cis-retinaldehyde for vision as well as the biosynthesis of all-trans-retinoic acid for differentiation and development.

Redox-based endoplasmic reticulum dysfunction in neurological diseases

The redox homeostasis of the endoplasmic reticulum lumen is characteristically different from that of the other subcellular compartments. The concerted action of membrane transport processes and oxidoreductase enzymes maintain the oxidized state of the thiol-disulfide and the reducing state of the pyridine nucleotide redox systems, which are prerequisites for the normal functions of the organelle. The powerful thiol-oxidizing machinery allows oxidative protein folding but continuously challenges the local antioxidant defense. Alterations of the cellular redox environment either in oxidizing or reducing direction affect protein processing 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 apoptosis if the attempt fails. Recent findings strongly support the involvement of this mechanism in brain ischemia, neuronal degenerative diseases and traumatic injury. The redox changes in the endoplasmic reticulum are integral parts of the pathomechanism of neurological diseases, either as causative agents, or as complications.

Metyrapone prevents cortisone-induced preadipocyte differentiation by depleting luminal NADPH of the endoplasmic reticulum

Preadipocyte differentiation is greatly affected by prereceptorial glucocorticoid activation catalyzed by 11beta-hydroxysteroid dehydrogenase type 1 in the lumen of the endoplasmic reticulum. The role of the local NADPH pool in this process was investigated using metyrapone as an NADPH-depleting agent. Metyrapone administered at low micromolar concentrations caused the prompt oxidation of the endogenous NADPH, inhibited the reduction of cortisone and enhanced the oxidation of cortisol in native rat liver microsomal vesicles. However, in permeabilized microsomes, it only slightly decreased both NADPH-dependent cortisone reduction and NADP(+)-dependent cortisol oxidation. Accordingly, metyrapone administration caused a switch in 11beta-hydroxysteroid dehydrogenase activity from reductase to dehydrogenase in both 3T3-L1-derived and human stem cell-derived differentiated adipocytes. Metyrapone greatly attenuated the induction of 11beta-hydroxysteroid dehydrogenase type 1 and the accumulation of lipid droplets during preadipocyte differentiation when 3T3-L1 cells were stimulated with cortisone, while it was much less effective in case of cortisol or dexamethasone. In conclusion, the positive feedback of glucocorticoid activation during preadipocyte differentiation is interrupted by metyrapone, which depletes NADPH in the endoplasmic reticulum. The results also indicate that the reduced state of luminal pyridine nucleotides in the endoplasmic reticulum is important in the process of adipogenesis.

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.

Isocitrate dehydrogenase: A NADPH-generating enzyme in the lumen of the endoplasmic reticulum

The aim of the present study was the investigation of the occurrence of NADPH-generating pathways in the endoplasmic reticulum others then hexose-6-phosphate dehydrogenase. A significant isocitrate and a moderate malate-dependent NADP+ reduction were observed in endoplasmic reticulum-derived rat liver microsomes. The isocitrate-dependent activity was very likely attributable to the appearance of the cytosolic isocitrate dehydrogenase isozyme in the lumen. The isocitrate dehydrogenase activity of microsomes was present in the luminal fraction; it showed a strong preference towards NADP+ versus NAD+, and it was almost completely latent. Antibodies against the cytosolic isoform of isocitrate dehydrogenase immunorevealed a microsomal protein of identical molecular weight; the microsomal enzyme showed similar kinetic parameters and oxalomalate inhibition as the cytosolic one. Measurable luminal isocitrate dehydrogenase activity was also present in microsomes from rat epididymal fat. The results suggest that isocitrate dehydrogenase is an important NADPH-generating enzyme in the endoplasmic reticulum.

Inhibitory effects of adenine nucleotides and related substances on UDP-glucuronosyltransferase: structure-effect relationships and evidence for an allosteric mechanism

The inhibitory effects of nucleotides and related substances on rat hepatic UDP-glucuronosyltransferase (UGT) were studied. ATP and NADP+ markedly reduced 4-methylumbelliferone (4-MU) UGT activity only when detergent-treated rat liver microsomes were used as the enzyme source. The IC50 values of adenine, ATP, NAD+ and NADP+ were estimated to be below 20 microM, whereas AMP had no inhibitory effect. From the kinetic behavior observed, these adenine-related compounds were assumed to inhibit UGT activity non-competitively without competing with either 4-MU or UDP-glucuronic acid. Among guanine, cytosine and their related nucleotides, only triphosphate nucleotides (CTP and GTP) exhibited potent UGT inhibition, although the effect of GTP was weak. Estradiol 3- and 17-glucuronidation were also inhibited by the inhibitors of 4-MU UGT. The only exception was that estradiol 17-glucuronidation activity was inhibited by AMP (IC50=31 microM). In addition, AMP antagonized the inhibitory effects of adenine, ATP, and NADP+ on 4-MU and estradiol 3- glucuronidation activities. These results suggest that (1) a number of cellular nucleotides present within the endoplasmic reticulum regulate UGT function; and (2) these substances bind to a common allosteric site on UGT to reduce catalytic function.

Endoplasmic reticulum stress

Stress is the imbalance of homeostasis, which can be sensed even at the subcellular level. The stress-sensing capability of various organelles including the endoplasmic reticulum (ER) has been described. It has become evident that acute or prolonged ER stress plays an important role in many human diseases; especially those involving organs/tissues specialized in protein secretion. This article summarizes the emerging role of ER stress in diverse human pathophysiological conditions such as carcinogenesis and tumor progression, cerebral ischemia, plasma cell maturation and apoptosis, obesity, insulin resistance, and type 2 diabetes. Certain components of the ER stress response machinery are identified as biomarkers of the diseases or as possible targets for therapeutic intervention.

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.

Stress on redox

Redox imbalance in the endoplasmic reticulum lumen is the most frequent cause of endoplasmic reticulum stress and consequent apoptosis. The mechanism involves the impairment of oxidative protein folding, the accumulation of unfolded/misfolded proteins in the lumen and the initiation of the unfolded protein response. The participation of several redox systems (glutathione, ascorbate, FAD, tocopherol, vitamin K) has been demonstrated in the process. Recent findings have attracted attention to the possible mechanistic role of luminal pyridine nucleotides in the endoplasmic reticulum stress. The aim of this minireview is to summarize the luminal redox systems and the redox sensing mechanisms of the endoplasmic reticulum.

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.

Endoplasmic reticulum: a metabolic compartment

Several biochemical reactions and processes of cell biology are compartmentalized in the endoplasmic reticulum (ER). The view that the ER membrane is basically a scaffold for ER proteins, which is permeable to small molecules, is inconsistent with recent findings. The luminal micro-environment is characteristically different from the cytosol; its protein and glutathione thiols are remarkably more oxidized, and it contains a separate pyridine nucleotide pool. The substrate specificity and activity of certain luminal enzymes are dependent on selective transport of possible substrates and co-factors from the cytosol. Abundant biochemical, pharmacological, clinical and genetic data indicate that the barrier function of the lipid bilayer and specific transport activities in the membrane make the ER a separate metabolic compartment.

Uncoupled redox systems in the lumen of the endoplasmic reticulum. Pyridine nucleotides stay reduced in an oxidative environment

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.

Structure and function of retinol dehydrogenases of the short chain dehydrogenase/reductase family

All-trans-retinol is the common precursor of the active retinoids 11-cis-retinal, all-trans-retinoic acid (atRA) and 9-cis-retinoic acid (9cRA). Genetic and biochemical data supports an important role of the microsomal members of the short chain dehydrogenases/reductases (SDRs) in the first oxidative conversion of retinol into retinal. Several retinol dehydrogenases of this family have been reported in recent years. However, the structural and functional data on these enzymes is limited. The prototypic enzyme RDH5 and the related enzyme CRAD1 have been shown to face the lumen of the endoplasmic reticulum (ER), suggesting a compartmentalized synthesis of retinal. This is a matter of debate as a related enzyme has been proposed to have the opposite membrane topology. Recent data indicates that RDH5, and presumably other members of the SDRs, occur as functional homodimers, and need to interact with other proteins for proper intracellular localization and catalytic activity. Further analyses on the compartmentalization, membrane topology, and functional properties of microsomal retinol dehydrogenases, will give important clues about how retinoids are processed.

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.

Properties of Short-Chain Dehydrogenase/Reductase RalR1: Characterization of Purified Enzyme, Its Orientation in the Microsomal Membrane, and Distribution in Human Tissues and Cell Lines

Recently, we reported the first biochemical characterization of a novel member of the short-chain dehydrogenase/reductase superfamily, retinal reductase 1 (RalR1) (Kedishvili et al. (2002) J. Biol. Chem. 277, 28909-28915). In the present study, we purified the recombinant enzyme from the microsomal membranes of insect Sf9 cells, determined its catalytic efficiency for the reduction of retinal and the oxidation of retinol, established its transmembrane topology, and examined the distribution of RalR1 in human tissues and cell lines. Purified RalR1-His(6) exhibited the apparent K(m) values for all-trans-retinal and all-trans-retinol of 0.12 and 0.6 microM, respectively. The catalytic efficiency (k(cat)/K(m)) for the reduction of all-trans-retinal (150,000 min(-1) mM(-1)) was 8-fold higher than that for the oxidation of all-trans-retinol (18,000 min(-1) mM(-1)). Protease protection assays and site-directed mutagenesis suggested that the enzyme is anchored in the membrane by the N-terminal signal-anchor domain, with the majority of the polypeptide chain located on the cytosolic side of the membrane. An important feature that prevented the translocation of RalR1 across the membrane was the positively charged R(25)K motif flanking the N-terminal signal-anchor. The cytosolic orientation of RalR1 suggested that, in intact cells, the enzyme would function predominantly as a reductase. Western blot analysis revealed that RalR1 is expressed in a wide variety of normal human tissues and cancer cell lines. The expression pattern and the high catalytic efficiency of RalR1 are consistent with the hypothesis that RalR1 contributes to the reduction of retinal in various human tissues.

Inhibition of microsomal glucose-6-phosphate transport in human neutrophils results in apoptosis: a potential explanation for neutrophil dysfunction in glycogen storage disease type 1b

Mutations in the gene of the hepatic glucose-6-phosphate transporter cause glycogen storage disease type 1b. In this disease, the altered glucose homeostasis and liver functions are accompanied by an impairment of neutrophils/monocytes. However, neither the existence of a microsomal glucose-6-phosphate transport, nor the connection between its defect and cell dysfunction has been demonstrated in neutrophils/monocytes. In this study we have characterized the microsomal glucose-6-phosphate transport of human neutrophils and differentiated HL-60 cells. The transport of glucose-6-phosphate was sensitive to the chlorogenic acid derivative S3483, N-ethylmaleimide, and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, known inhibitors of the hepatic microsomal glucose-6-phosphate transporter. A glucose-6-phosphate uptake was also present in microsomes from undifferentiated HL-60 and Jurkat cells, but it was insensitive to S3483. The treatment with S3484 of intact human neutrophils and differentiated HL-60 cells mimicked some leukocyte defects of glycogen storage disease type 1b patients (ie, the drug inhibited phorbol myristate acetate-induced superoxide anion production and reduced the size of endoplasmic reticulum Ca(2+) stores). Importantly, the treatment with S3484 also resulted in apoptosis of human neutrophils and differentiated HL-60 cells, while undifferentiated HL-60 and Jurkat cells were unaffected by the drug. The proapoptotic effect of S3483 was prevented by the inhibition of nicotinamide adenine dinucleotide phosphate oxidase or by antioxidant treatment. These results suggest that microsomal glucose-6-phosphate transport has a role in the antioxidant protection of neutrophils, and that the genetic defect of the transporter leads to the impairment of cellular functions and apoptosis.

Biosynthesis of 9-cis-retinoic acid in vivo. The roles of different retinol dehydrogenases and a structure-activity analysis of microsomal retinol dehydrogenases

Retinoic acid is generated by a two-step mechanism. First, retinol is converted into retinal by a retinol dehydrogenase, and, subsequently, retinoic acid is formed by a retinal dehydrogenase. In vitro, several enzymes are suggested to act in this metabolic pathway. However, little is known regarding their capacity to contribute to retinoic acid biosynthesis in vivo. We have developed a versatile cell reporter system to analyze the role of several of these enzymes in 9-cis-retinoic acid biosynthesis in vivo. Using a Gal4-retinoid X receptor fusion protein-based luciferase reporter assay, the formation of 9-cis-retinoic acid from 9-cis-retinol was measured in cells transfected with expression plasmids encoding different combinations of retinol and retinal dehydrogenases. The results suggested that efficient formation of 9-cis-retinoic acid required co-expression of retinol and retinal dehydrogenases. Interestingly, the cytosolic alcohol dehydrogenase 4 failed to efficiently catalyze 9-cis-retinol oxidation. A structure-activity analysis showed that mutants of two retinol dehydrogenases, devoid of the carboxyl-terminal cytoplasmic tails, displayed greatly reduced enzymatic activities in vivo, but were active in vitro. The cytoplasmic tails mediate efficient endoplasmic reticulum localization of the enzymes, suggesting that the unique milieu in the endoplasmic reticulum compartment is necessary for in vivo activity of microsomal retinol dehydrogenases.

Gene structure, expression analysis, and membrane topology of RDH4

The murine retinol dehydrogenase RDH4 oxidizes several cis-isomers of retinol into their corresponding aldehydes. We have determined the structure of the murine gene, investigated the temporal and spatial expression of the enzyme, and analyzed the membrane topology of the enzyme. The gene has four translated exons, and several alternatively spliced exons in the 5'-untranslated region were identified. Immunohistochemical analysis showed expression of RDH4 in developing and adult mouse eye, particularly in the retinal pigment epithelium. In nonocular adult tissues, including liver, kidney, lung, and skin, RDH4 expression was widespread. The results suggest that RDH4 may have a dual and tissue-specific role in oxidation of 9-cis- and 11-cis-isomers of retinol into 9-cis-retinal and 11-cis-retinal, respectively. Furthermore, the lumenal orientation of the enzyme domain in the ER suggests that oxidation of both cis-isomers of retinol occurs in the ER.

Intracellular localization and membrane topology of 11-cis retinol dehydrogenase in the retinal pigment epithelium suggest a compartmentalized synthesis of 11-cis retinaldehyde

11-cis retinol dehydrogenase (EC 1.1.1.105) catalyses the last step in the biosynthetic pathway generating 11-cis retinaldehyde, the common chromophore of all visual pigments in higher animals. The enzyme is abundantly expressed in retinal pigment epithelium of the eye and is a member of the short chain dehydrogenase/reductase superfamily. In this work we demonstrate that a majority of 11-cis retinol dehydrogenase is associated with the smooth ER in retinal pigment epithelial cells and that the enzyme is an integral membrane protein, anchored to membranes by two hydrophobic peptide segments. The catalytic domain of the enzyme is confined to a lumenal compartment and is not present on the cytosolic aspect of membranes. Thus, the subcellular localization and the membrane topology of 11-cis retinol dehydrogenase suggest that generation of 11-cis retinaldehyde is a compartmentalized process.

Glucose dehydrogenase, glucose-6-phosphate dehydrogenase and hexokinase in liver of rainbow trout (Salmo gairdneri). Effects of starvation and temperature variations

1. Activities of trout liver glucose dehydrogenase (GDH, EC 1.1.1.47) and glucose-6-phosphate dehydrogenase (G6PD, EC 1.1.1.49) were increased after a sudden drop in water temperature, but not in long-time cold acclimated as compared with warm acclimated trout. 2. Possibly, the activities of GDH and G6PD were temporarily increased in connection with metabolic adaptation to the lower temperature. 3. The activities of GDH and G6PD were not changed by the stress of handling. 4. Partially purified trout liver GDH has a lower activation energy with glucose than with glucose-6-phosphate as substrate, and the Km (glucose) decreases with decreasing assay temperature. 5. At low temperatures, the activity of trout liver GDH with glucose as substrate may be comparable to that of glucose-6-phosphate. 6. Partially purified beef liver GDH has a high activation energy with glucose as substrate, and the Km (glucose) does not change with the assay temperature. 7. Hexokinase (HK, EC 2.7.1.1) and GDH activities were unchanged when trout were deprived of food for 4 weeks. Apparently, the trout liver glucose utilization did not adapt to the starvation.

Adaptation of an enzymatic cycling assay for NADP(H) measurement to the COBAS-FARA centrifugal analyzer

NADP(H) measurements by enzymatic amplification are described in which the interface step between cycling (glucose-6-phosphate and glutamic dehydrogenases) and indicator (6-phosphogluconic dehydrogenase) enzymes has been reconfigured, permitting the entire operation to run as a continuous assay on a centrifugal fast analyzer. This is accomplished by using the sequential load feature of the analyzer and incorporating either sodium dodecyl sulfate (SDS) or SDS and hydrogen peroxide as kill reagents to replace the thermal step (destruction of cycle enzymes by boiling). The ability of SDS to render a cycle inoperative during the run time of the indicator enzyme depends on the inherent resistivity and absolute amount of its enzyme proteins to this surfactant. Criteria used to judge the efficacy of a potential kill reagent are based on the sample blank time-response curve and the cycle product recovery by the indicator enzyme. Various other enzyme cycling systems which can be fitted to the centrifugal fast analyzer are highlighted.

The activation of glucose dehydrogenase by p-chloromercuribenzoate

p-Chloromercuribenzoate alters various reactions of rat liver glucose (hexose phosphate) dehydrogenase differently. The reagent has little effect on the glucose: NAD or the glucose: NADP oxidoreductases, doubles the rates of oxidations of galactose-6-phosphate and glucose-6-phosphate by NADP and greatly stimulates the oxidations of glucose-6-phosphate and galactose-6-phosphate by NAD. The reagent appears to react with a sulfhydryl group of the enzyme since activation is reversed and prevented by mercaptoethanol. The direct reaction of the reagent with the enzyme is indicated by its lower thermal stability in the presence of the p-chloromercuribenzoate. The size of the enzyme appears to be the same when determined by sucrose gradient centrifugation in the presence or absence of p-chloromercuribenzoate. In microsomes, the oxidation of NADH or NADPH hampers measurements of glucose dehydrogenase. Since p-chloromercuribenzoate inhibits microsomal oxidation of reduced nicontinamide nucleotides, it is possible to assay for glucose dehydrogenase accurately in the presence of the mercurial in microsomes and microsomal extracts and thus measure the effectiveness of a detergent in extracting the enzyme from microsomes.

The topology of phosphogluconate dehydrogenases in rat liver microsomes

Rat liver microsomes are known to contain a 6-phosphogluconate dehydrogenase which differs from the 6-phosphogluconate dehydrogenase in the soluble fraction. Microsomes which were washed once bind the soluble phosphogluconate dehydrogenase more tightly than they do glucose-6-phosphate dehydrogenase. Microsomes washed three times in 0.15 M Tris-HCl, pH 8.0, contain only the microsomal 6-phosphogluconate dehydrogenase. Two observations show that this dehydrogenase is located in the cisternae. First, this dehydrogenase is inactive in intact, three times washed microsomes. Second, proteolytic inactivation of 6-phosphogluconate dehydrogenase like that of the cisternal enzyme glucose-6-phosphatase requires disruption of the membrane. Under the conditions used, detergent did not affect the proteolytic inactivation of NADPH-cytochrome c reductase, an enzyme located on the external surface. The excellent correspondence between the activations of hexose phosphate dehydrogenase and 6-phosphogluconate dehydrogenase in microsomes at various stages of disruption of the microsomal membrane produced by detergent supports the earlier contention that these two dehydrogenases are reducing NADP in the same region of the microsomes. A similar experiment which shows an exact correspondence between the activations of 6-phosphogluconate dehydrogenase and mannose-6-phosphatase with increasing concentrations of detergent indicates that the activation of the dehydrogenase can be explained solely by the penetration of the substrates to the active dehydrogenase within the microsomes and strongly suggests that the dehydrogenase is catalytically active in the cisternae.