The role of dehydroascorbate in disulfide bond formation

Highly efficient export of a disulfide-bonded protein to the periplasm and medium by the Tat pathway using CyDisCo in Escherichia coli

High-value heterologous proteins produced in Escherichia coli that contain disulfide bonds are almost invariably targeted to the periplasm via the Sec pathway as it, among other advantages, enables disulfide bond formation and simplifies downstream processing. However, the Sec system cannot transport complex or rapidly folding proteins, as it only transports proteins in an unfolded state. The Tat system also transports proteins to the periplasm, and it has significant potential as an alternative means of recombinant protein production because it transports fully folded proteins. Most of the studies related to Tat secretion have used the well-studied TorA signal peptide that is Tat-specific, but this signal peptide also tends to induce degradation of the protein of interest, resulting in lower yields. This makes it difficult to use Tat in the industry. In this study, we show that a model disulfide bond-containing protein, YebF, can be exported to the periplasm and media at a very high level by the Tat pathway in a manner almost completely dependent on cytoplasmic disulfide formation, by other two putative Tat SPs: those of MdoD and AmiC. In contrast, the TorA SP exports YebF at a low level.

Oxidative protein folding fidelity and redoxtasis in the endoplasmic reticulum

In eukaryotic cells, oxidative protein folding occurs in the lumen of the endoplasmic reticulum (ER), catalyzed by ER sulfhydryl oxidase 1 (Ero1) and protein disulfide isomerase (PDI). The efficiency and fidelity of oxidative protein folding are vital for the function of secretory cells. Here, we summarize oxidative protein folding in yeast, plants, and mammals, and discuss how the conformation and activity of human Ero1-PDI machinery is regulated through various post-translational modifications (PTMs). We propose that oxidative protein folding fidelity and ER redox homeostasis are maintained by both the precise control of Ero1 oxidase activity and the division of labor between PDI family members. We also discuss how deregulated Ero1-PDI functions contribute to human diseases and can be leveraged for therapeutic interventions.

Introduction of a More Glutaredoxin-like Active Site to PDI Results in Competition between Protein Substrate and Glutathione Binding

Proteins in the thioredoxin superfamily share a similar fold, contain a -CXXC- active site, and catalyze oxidoreductase reactions by dithiol-disulfide exchange mechanisms. Protein disulfide isomerase (PDI) has two -CGHC- active sites. For in vitro studies, oxidation/reduction of PDI during the catalytic cycle is accomplished with glutathione. Glutathione may act as electron donor/acceptor for PDI also in vivo, but at least for oxidation reactions, GSSG probably is not the major electron acceptor and PDI may not have evolved to react with glutathione with high affinity, but merely having adequate affinity for both glutathione and folding proteins/peptides. Glutaredoxins, on the other hand, have a high affinity for glutathione. They commonly have -CXFC- or -CXYC- active site, where the tyrosine residue forms part of the GSH binding groove. Mutating the active site of PDI to a more glutaredoxin-like motif increased its reactivity with glutathione. All such variants showed an increased rate in GSH-dependent reduction or GSSG-dependent oxidation of the active site, as well as a decreased rate of the native disulfide bond formation, with the magnitude of the effect increasing with glutathione concentration. This suggests that these variants lead to competition in binding between glutathione and folding protein substrates.

Ascorbate as a Bioactive Compound in Cancer Therapy: The Old Classic Strikes Back

Cancer is a disease of high mortality, and its prevalence has increased steadily in the last few years. However, during the last decade, the development of modern chemotherapy schemes, new radiotherapy techniques, targeted therapies and immunotherapy has brought new hope in the treatment of these diseases. Unfortunately, cancer therapies are also associated with frequent and, sometimes, severe adverse events. Ascorbate (ascorbic acid or vitamin C) is a potent water-soluble antioxidant that is produced in most mammals but is not synthesised endogenously in humans, which lack enzymes for its synthesis. Ascorbate has antioxidant effects that correspond closely to the dose administered. Interestingly, this natural antioxidant induces oxidative stress when given intravenously at a high dose, a paradoxical effect due to its interactions with iron. Importantly, this deleterious property of ascorbate can result in increased cell death. Although, historically, ascorbate has been reported to exhibit anti-tumour properties, this effect has been questioned due to the lack of available mechanistic detail. Recently, new evidence has emerged implicating ferroptosis in several types of oxidative stress-mediated cell death, such as those associated with ischemia–reperfusion. This effect could be positively modulated by the interaction of iron and high ascorbate dosing, particularly in cell systems having a high mitotic index. In addition, it has been reported that ascorbate may behave as an adjuvant of favourable anti-tumour effects in cancer therapies such as radiotherapy, radio-chemotherapy, chemotherapy, immunotherapy, or even in monotherapy, as it facilitates tumour cell death through the generation of reactive oxygen species and ferroptosis. In this review, we provide evidence supporting the view that ascorbate should be revisited to develop novel, safe strategies in the treatment of cancer to achieve their application in human medicine.

SVCT2 Overexpression and Ascorbic Acid Uptake Increase Cortical Neuron Differentiation, Which Is Dependent on Vitamin C Recycling between Neurons and Astrocytes

During brain development, sodium–vitamin C transporter (SVCT2) has been detected primarily in radial glial cells in situ, with low-to-absent expression in cerebral cortex neuroblasts. However, strong SVCT2 expression is observed during the first postnatal days, resulting in increased intracellular concentration of vitamin C. Hippocampal neurons isolated from SVCT2 knockout mice showed shorter neurites and low clustering of glutamate receptors. Other studies have shown that vitamin C-deprived guinea pigs have reduced spatial memory, suggesting that ascorbic acid (AA) and SVCT2 have important roles in postnatal neuronal differentiation and neurite formation. In this study, SVCT2 lentiviral overexpression induced branching and increased synaptic proteins expression in primary cultures of cortical neurons. Analysis in neuroblastoma 2a (Neuro2a) and human subventricular tumor C3 (HSVT-C3) cells showed similar branching results. SVCT2 was mainly observed in the cell membrane and endoplasmic reticulum; however, it was not detected in the mitochondria. Cellular branching in neuronal cells and in a previously standardized neurosphere assay is dependent on the recycling of vitamin C or reduction in dehydroascorbic acid (DHA, produced by neurons) by glial cells. The effect of WZB117, a selective glucose/DHA transporter 1 (GLUT1) inhibitor expressed in glial cells, was also studied. By inhibiting GLUT1 glial cells, a loss of branching is observed in vitro, which is reproduced in the cerebral cortex in situ. We concluded that vitamin C recycling between neurons and astrocyte-like cells is fundamental to maintain neuronal differentiation in vitro and in vivo. The recycling activity begins at the cerebral postnatal cortex when neurons increase SVCT2 expression and concomitantly, GLUT1 is expressed in glial cells.

Ascorbate is a multifunctional micronutrient whose synthesis is lacking in primates

Ascorbate (vitamin C) is an essential micronutrient in primates, and exhibits multiple physiological functions. In addition to antioxidative action, ascorbate provides reducing power to α-ketoglutarate-dependent non-heme iron dioxygenases, such as prolyl hydroxylases. Demethylation of histones and DNA with the aid of ascorbate results in the reactivation of epigenetically silenced genes. Ascorbate and its oxidized form, dehydroascorbate, have attracted interest in terms of their roles in cancer therapy. The last step in the biosynthesis of ascorbate is catalyzed by l-gulono-γ-lactone oxidase whose gene Gulo is commonly mutated in all animals that do not synthesize ascorbate. One common explanation for this deficiency is based on the increased availability of ascorbate from foods. In fact, pathways for ascorbate synthesis and the detoxification of xenobiotics by glucuronate conjugation share the metabolic processes up to UDP-glucuronate, which prompts another hypothesis, namely, that ascorbate-incompetent animals might have developed stronger detoxification systems in return for their lack of ability to produce ascorbate, which would allow them to cope with their situation. Here, we overview recent advances in ascorbate research and propose that an enhanced glucuronate conjugation reaction may have applied positive selection pressure on ascorbate-incompetent animals, thus allowing them to dominate the animal kingdom.

Ascorbic Acid Route to the Endoplasmic Reticulum: Function and Role in Disease

Significance: Humans cannot synthesize ascorbic acid (AscH₂) (vitamin C), so deficiencies in dietary AscH₂ cause the life-threatening disease of scurvy and many other diseases. After oral ingestion, plasma AscH₂ concentrations are strictly controlled by transporters, which are required for entry into the cell and into intracellular organelles. Recent Advances: Besides its general antioxidant function, AscH₂ is a cofactor for endoplasmic reticulum (ER)-localized collagen hydroxylases. Its important role in ER homeostasis is also highlighted by the fact that AscH₂ deficiency in auxotrophic species triggers ER stress. Critical Issues: Characterizations of the molecular basis of diseases suggest that intracellular AscH₂ deficiency is due not only to limited dietary access but also to its limited intracellular transport and net loss under conditions of intracellular hyperoxidation in the ER. This essay will offer an overview of the different transporters of vitamin C regulating its intracellular concentration, its function inside the ER, and the phenotypes of the diseases that can be triggered by increased depletion of this vitamin in the ER. Future Directions: When considering the benefits of increasing dietary AscH₂, it is important to consider pharmacokinetic differences in the bioavailability between orally and intravenously administered AscH₂: the latter bypasses intestinal absorption and is, therefore, the only route that can lead to the high plasma concentrations that may provide some health effects, and it is this route that needs to be chosen in clinical trials for those diseases associated with a deficiency of AscH₂. Antioxid. Redox Signal. 34, 845-855.

Cysteine 343 in the substrate binding domain is the primary S-Nitrosylated site in protein disulfide isomerase

Abnormal protein accumulations are typical pathological features for neurodegenerative diseases. Protein disulfide isomerase (PDI) is a critical enzyme in oxidative protein folding. S-nitrosylated PDI has been detected in the postmortem brain in neurodegenerative disease patients, but the effect of S-nitrosylation on PDI function and developing neurodegeneration was not clarified in detail. In this study, we identified that in vitro and in vivo S-nitrosylation of C343 in the b' domain of PDI occurs. Reduced recombinant human PDI (hPDI) reacted quickly with S-nitrosocompounds, with an observed increase in the expected S-nitrosylated species and the appearance of the disulfide state of the active sites. Both Mononitrosylated and dinitrosylated were observed from the S-nitrosylation of hPDI. Dinitrosylated species were S-nitrosylated both cysteines at active site. But, at least in part, mononitrosylated species were S-nitrosylated on cysteine 343 in the substrate binding b' domain. Although active site S-nitrosylation is reversible by reduced glutathione, S-nitrosylation of C343 is comparative stable. S-nitrosylation of PDI in SH-SY5Y cells was observed after the S-nitrosocysteine (SNOC) treatment and S-nitrosylated PDI was still detected 24 h after removing SNOC. While wild-type PDI was S-nitrosylated, the level of S-nitrosylation of the C343S mutant in over-expressed cells was substantially lower and only wild-type PDI of S-nitrosylation remained 24 h after removing SNOC in over-expressed cells. Both of in vitro and in vivo results suggested that S-nitrosylation of C343 in PDI may be the causative effect on physiological changes in neurodegerenative disease patients, and may be useful for the drug development for neurodegenerative diseases.

Investigating protein thiol chemistry associated with dehydroascorbate, homocysteine and glutathione using mass spectrometry

RATIONALE

Oxidative stress is an imbalance between reactive free radical oxygen species and antioxidant defenses. Its consequences can lead to numerous pathologies. Regulating oxidative stress is the complex interplay between antioxidant recycling and thiol-containing regulatory proteins. Understanding these regulatory mechanisms is important for preventing onset of oxidative stress. The aim of this study was to investigae S-thiol protein chemistry associated with oxidized vitamin C (dehydroascorbate, DHA), homocysteine (HcySH) and glutathione (GSH) using mass spectrometry.

METHODS

Glutaredoxin-1 (Grx-1) was incubated with DHA, with and without GSH and HcySH. Disulfide formation was followed by electrospray ionization mass spectrometry (ESI-MS) of intact proteins and by LC/ESI-MS/MS of peptides from protein tryptic digestions. The mechanism of DHA-mediated S-thiolation was investigated using two synthetic peptides: AcFHACAAK and AcFHACE. Three proteins, i.e. human hemoglobin (HHb), recombinant peroxiredoxin 2 (Prdx2) and Grx-1, were S-homocysteinylated followed by S-transthiolyation with GSH and investigated by ESI-MS and ESI-MS/MS.

RESULTS

ESI-MS analysis reveals that DHA mediates disulfide formation and S-thiolation by HcySH as well as GSH of Grx-1. LC/ESI-MS/MS analysis allows identification of Grx-1 S-thiolated cysteine adducts. The mechanism by which DHA mediates S-thiolation of heptapeptide AcFHACAAK is shown to be via initial formation of a thiohemiketal adduct. In addition, ESI-MS of intact proteins shows that GSH can S-transthiolate S-homocysteinylated Grx-1_ HHb and Prdx2. The GS-S-protein adducts over time dominate the ESI-MS spectrum profile.

CONCLUSIONS

Mass spectrometry is a unique analytical technique for probing complex reaction mechanisms associated with oxidative stress. Using model proteins, ESI-MS reveals the mechanism of DHA-facilitated S-thiolation, which consists of thiohemiketal formation, disulfide formation or S-thiolation. Furthermore, protein S-thiolation by HcySH can be reversed by reversible GSH thiol exchange. The use of mass spectrometry with in vitro models of protein S-thiolation in oxidative stress may provide significant insight into possible mechanisms of action occurring in vivo.

Pyridine nucleotide regulation of hepatic endoplasmic reticulum calcium uptake

Pyridine nucleotides serve an array of intracellular metabolic functions such as, to name a few, shuttling electrons in enzymatic reactions, safeguarding the redox state against reactive oxygen species, cytochrome P450 (CYP) enzyme detoxification pathways and, relevant to this study, the regulation of ion fluxes. In particular, the maintenance of a steep calcium gradient between the cytosol and endoplasmic reticulum (ER), without which apoptosis ensues, is achieved by an elaborate combination of energy–requiring ER membrane pumps and efflux channels. In liver microsomes, net calcium uptake was inhibited by physiological concentrations of NADP. In the presence of 1 mmol/L NADP, calcium uptake was attenuated by nearly 80%, additionally, this inhibitory effect was blunted by concomitant addition of NADPH. No other nicotinamide containing compounds ‐save a slight inhibition by NAADP‐hindered calcium uptake; thus, only oxidized pyridine nucleotides, or related compounds with a phosphate moiety, had an imposing effect. Moreover, the NADP inhibition was evident even after selectively blocking ER calcium efflux channels. Given the fundamental role of endoplasmic calcium homeostasis, it is plausible that changes in cytosolic NADP concentration, for example, during anabolic processes, could regulate net ER calcium uptake.

Production of Extracellular Matrix Proteins in the Cytoplasm of E. coli: Making Giants in Tiny Factories

Escherichia coli is the most widely used protein production host in academia and a major host for industrial protein production. However, recombinant production of eukaryotic proteins in prokaryotes has challenges. One of these is post-translational modifications, including native disulfide bond formation. Proteins containing disulfide bonds have traditionally been made by targeting to the periplasm or by in vitro refolding of proteins made as inclusion bodies. More recently, systems for the production of disulfide-containing proteins in the cytoplasm have been introduced. However, it is unclear if these systems have the capacity for the production of disulfide-rich eukaryotic proteins. To address this question, we tested the capacity of one such system to produce domain constructs, containing up to 44 disulfide bonds, of the mammalian extracellular matrix proteins mucin 2, alpha tectorin, and perlecan. All were successfully produced with purified yields up to 6.5 mg/L. The proteins were further analyzed using a variety of biophysical techniques including circular dichroism spectrometry, thermal stability assay, and mass spectrometry. These analyses indicated that the purified proteins are most likely correctly folded to their native state. This greatly extends the use of E. coli for the production of eukaryotic proteins for structural and functional studies.

Dehydroascorbic acid S-Thiolation of peptides and proteins: Role of homocysteine and glutathione

Ascorbic acid (vitamin C) plays a significant role in the prevention of oxidative stress. In this process, ascorbate is oxidized to dehydroascorbate (DHA). We have investigated the impact of DHA on peptide/protein intramolecular disulfide formation as well as S-glutathionylation and S-homocysteinylation. S-glutathionylation of peptides/proteins is a reversible, potential regulatory mechanism in oxidative stress. Although the exact role of protein S-homocysteinylation is unknown, it has been proposed to be of importance in pathobiological processes such as onset of cardiovascular disease. Using an in vitro model system, we demonstrate that DHA causes disulfide bond formation within the active site of recombinant human glutaredoxin (Grx-1). DHA also facilities the formation of S-glutathionylation and S-homocysteinylation of a model peptide (AcFHACAAK) as well as Grx-1. We discuss the possible mechanisms of peptide/protein S-thiolation, which can occur either via thiol exchange or a thiohemiketal intermediate. A thiohemiketal DHA-peptide adduct was detected by mass spectrometry and its location on the peptide/protein cysteinyl thiol group was unambiguously confirmed by tandem mass spectrometry. This demonstrates that peptide/protein S-thiolation mediated by DHA is not limited to thiol exchange reactions but also takes place directly via the formation of a thiohemiketal peptide intermediate. Finally, we investigated a potential reducing role of glutathione (GSH) in the presence of S-homocysteinylated peptide/protein adducts. S-homocysteinylated AcFHACAAK, human hemoglobin α-chain and Grx-1 were incubated with GSH. Both peptide and proteins were reduced, and homocysteine replaced with GS-adducts by thiol exchange, as a function of time.

A critical role of the thioredoxin domain containing protein 5 (TXNDC5) in redox homeostasis and cancer development

Correct folding of nascent peptides occurs in the endoplasmic reticulum (ER). It is a complicate process primarily accomplished by the coordination of multiple redox proteins including members of the protein disulfide isomerase (PDI) family. As a critical member of the PDI family, thioredoxin domain containing protein 5 (TXNDC5) assists the folding of newly synthesized peptides to their mature form through series of disulfide bond exchange reactions. Interestingly, TXNDC5 is frequently found overexpressed in specimens of many human diseases including various types of cancer. In this review, we summarized the biochemical function of TXNDC5 in mammalian cells and the recent progress on the understanding of its role and molecular mechanisms in cancer development. Findings of TXNDC5 in the activation of intracellular signaling pathways, stimulation of cell growth & proliferation, facilitation of cell survival and modulation of extracellular matrix to affect cancer cell invasion and metastasis are reviewed. These published studies suggest that strategies of targeting TXNDC5 can be developed as potentially valuable methods for the treatment of certain types of cancer in patients.

Gaussia princeps luciferase: a bioluminescent substrate for oxidative protein folding

Gaussia princeps luciferase (GLuc) generates an intense burst of blue light when exposed to coelenterazine in the absence of ATP. Here we show that this 5-disulfide containing enzyme can be used as a facile and convenient substrate for studies of oxidative protein folding. Reduced GLuc (rGLuc), with 10 free cysteine residues, is completely inactive as a luciferase but >60% bioluminescence activity, compared to controls, can be recovered using a range of oxidizing regimens in the absence of the exogenous shuffling activity of protein disulfide isomerase (PDI). The sulfhydryl oxidase QSOX1 can be assayed using rGLuc in a simple bioluminescence plate reader format. Similarly, low concentrations of rGLuc can be oxidized by millimolar levels of dehydroascorbate, hydrogen peroxide or much lower concentrations of sodium tetrathionate. The oxidative refolding of rGLuc in the presence of a range of glutathione redox buffers is only marginally accelerated by micromolar levels of PDI. This modest rate enhancement probably results from a relatively simple disulfide connectivity in native GLuc; reflecting two homologous domains each carrying two disulfide bonds with a single interdomain disulfide. When GLuc is reoxidized under denaturing conditions the resulting scrambled protein (sGLuc) can be used in a sensitive bioluminescence assay for reduced PDI in the absence of added exogenous thiols. Finally, the general facility by which rGLuc can recover bioluminescent activity in vitro provides a sensitive method for the assessment of inhibitors of oxidative protein folding.

Peroxynitrite preferentially oxidizes the dithiol redox motifs of protein-disulfide isomerase

Protein-disulfide isomerase (PDI) is a ubiquitous dithiol-disulfide oxidoreductase that performs an array of cellular functions, such as cellular signaling and responses to cell-damaging events. PDI can become dysfunctional by post-translational modifications, including those promoted by biological oxidants, and its dysfunction has been associated with several diseases in which oxidative stress plays a role. Because the kinetics and products of the reaction of these oxidants with PDI remain incompletely characterized, we investigated the reaction of PDI with the biological oxidant peroxynitrite. First, by determining the rate constant of the oxidation of PDI's redox-active Cys residues (Cys53 and Cys397) by hydrogen peroxide (k = 17.3 ± 1.3 m-1 s-1 at pH 7.4 and 25 °C), we established that the measured decay of the intrinsic PDI fluorescence is appropriate for kinetic studies. The reaction of these PDI residues with peroxynitrite was considerably faster (k = (6.9 ± 0.2) × 104 m-1 s-1), and both Cys residues were kinetically indistinguishable. Limited proteolysis, kinetic simulations, and MS analyses confirmed that peroxynitrite preferentially oxidizes the redox-active Cys residues of PDI to the corresponding sulfenic acids, which reacted with the resolving thiols at the active sites to produce disulfides (i.e. Cys53-Cys56 and Cys397-Cys400). A fraction of peroxynitrite, however, decayed to radicals that hydroxylated and nitrated other active-site residues (Trp52, Trp396, and Tyr393). Excess peroxynitrite promoted further PDI oxidation, nitration, inactivation, and covalent oligomerization. We conclude that these PDI modifications may contribute to the pathogenic mechanism of several diseases associated with dysfunctional PDI.

Sulfenylation of Human Liver and Kidney Microsomal Cytochromes P450 and Other Drug-Metabolizing Enzymes as a Response to Redox Alteration

The lumen of the endoplasmic reticulum (ER) provides an oxidizing environment to aid in the formation of disulfide bonds, which is tightly regulated by both antioxidant proteins and small molecules. On the cytoplasmic side of the ER, cytochrome P450 (P450) proteins have been identified as a superfamily of enzymes that are important in the formation of endogenous chemicals as well as in the detoxication of xenobiotics. Our previous report described oxidative inhibition of P450 Family 4 enzymes via oxidation of the heme-thiolate cysteine to a sulfenic acid (-SOH) (Albertolle, M. E. et al. (2017) J. Biol. Chem. 292, 11230-11242). Further proteomic analyses of murine kidney and liver microsomes led to the finding that a number of other drug-metabolizing enzymes located in the ER are also redox-regulated in this manner. We expanded our analysis of sulfenylated enzymes to human liver and kidney microsomes. Evaluation of the sulfenylation, catalytic activity, and spectral properties of P450s 1A2, 2C8, 2D6, and 3A4 led to the identification of two classes of redox sensitivity in P450 enzymes: heme-thiolate-sensitive and thiol-insensitive. These findings provide evidence for a mammalian P450 regulatory mechanism, which may also be relevant to other drug-metabolizing enzymes. (Data are available via ProteomeXchange with identifier PXD007913.).

Chemistry and Enzymology of Disulfide Cross-Linking in Proteins

Cysteine thiols are among the most reactive functional groups in proteins, and their pairing in disulfide linkages is a common post-translational modification in proteins entering the secretory pathway. This modest amino acid alteration, the mere removal of a pair of hydrogen atoms from juxtaposed cysteine residues, contrasts with the substantial changes that characterize most other post-translational reactions. However, the wide variety of proteins that contain disulfides, the profound impact of cross-linking on the behavior of the protein polymer, the numerous and diverse players in intracellular pathways for disulfide formation, and the distinct biological settings in which disulfide bond formation can take place belie the simplicity of the process. Here we lay the groundwork for appreciating the mechanisms and consequences of disulfide bond formation in vivo by reviewing chemical principles underlying cysteine pairing and oxidation. We then show how enzymes tune redox-active cofactors and recruit oxidants to improve the specificity and efficiency of disulfide formation. Finally, we discuss disulfide bond formation in a cellular context and identify important principles that contribute to productive thiol oxidation in complex, crowded, dynamic environments.

Glucose transporter type 10-lacking in arterial tortuosity syndrome-facilitates dehydroascorbic acid transport

Loss-of-function mutations in the gene encoding GLUT10 are responsible for arterial tortuosity syndrome (ATS), a rare connective tissue disorder. In this study GLUT10-mediated dehydroascorbic acid (DAA) transport was investigated, supposing its involvement in the pathomechanism. GLUT10 protein produced by in vitro translation and incorporated into liposomes efficiently transported DAA. Silencing of GLUT10 decreased DAA transport in immortalized human fibroblasts whose plasma membrane was selectively permeabilized. Similarly, the transport of DAA through endomembranes was markedly reduced in fibroblasts from ATS patients. Re-expression of GLUT10 in patients' fibroblasts restored DAA transport activity. The present results demonstrate that GLUT10 is a DAA transporter and DAA transport is diminished in the endomembranes of fibroblasts from ATS patients.

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.

Reconstitution of Formylglycine-generating Enzyme with Copper(II) for Aldehyde Tag Conversion

To further our aim of synthesizing aldehyde-tagged proteins for research and biotechnology applications, we developed methods for recombinant production of aerobic formylglycine-generating enzyme (FGE) in good yield. We then optimized the FGE biocatalytic reaction conditions for conversion of cysteine to formylglycine in aldehyde tags on intact monoclonal antibodies. During the development of these conditions, we discovered that pretreating FGE with copper(II) is required for high turnover rates and yields. After further investigation, we confirmed that both aerobic prokaryotic (Streptomyces coelicolor) and eukaryotic (Homo sapiens) FGEs contain a copper cofactor. The complete kinetic parameters for both forms of FGE are described, along with a proposed mechanism for FGE catalysis that accounts for the copper-dependent activity.

Peroxiredoxin 4 improves insulin biosynthesis and glucose-induced insulin secretion in insulin-secreting INS-1E cells

Oxidative folding of (pro)insulin is crucial for its assembly and biological function. This process takes place in the endoplasmic reticulum (ER) and is accomplished by protein disulfide isomerase and ER oxidoreductin 1β, generating stoichiometric amounts of hydrogen peroxide (H2O2) as byproduct. During insulin resistance in the prediabetic state, increased insulin biosynthesis can overwhelm the ER antioxidative and folding capacity, causing an imbalance in the ER redox homeostasis and oxidative stress. Peroxiredoxin 4 (Prdx4), an ER-specific antioxidative peroxidase can utilize luminal H2O2 as driving force for reoxidizing protein disulfide isomerase family members, thus efficiently contributing to disulfide bond formation. Here, we examined the functional significance of Prdx4 on β-cell function with emphasis on insulin content and secretion during stimulation with nutrient secretagogues. Overexpression of Prdx4 in glucose-responsive insulin-secreting INS-1E cells significantly metabolized luminal H2O2 and improved the glucose-induced insulin secretion, which was accompanied by the enhanced proinsulin mRNA transcription and insulin content. This β-cell beneficial effect was also observed upon stimulation with the nutrient insulin secretagogue combination of leucine plus glutamine, indicating that the effect is not restricted to glucose. However, knockdown of Prdx4 had no impact on H2O2 metabolism or β-cell function due to the fact that Prdx4 expression is negligibly low in pancreatic β-cells. Moreover, we provide evidence that the constitutively low expression of Prdx4 is highly susceptible to hyperoxidation in the presence of high glucose. Overall, these data suggest an important role of Prdx4 in maintaining insulin levels and improving the ER folding capacity also under conditions of a high insulin requirement.

The role of ascorbate in protein folding

Ascorbate was linked to protein folding a long time ago. At the first level of this connection, it had been shown that ascorbate functions as an essential cofactor in the hydroxylation enzymes involved in collagen synthesis. Although the hydroxylation reactions catalyzed by the members of the prolyl 4-hydroxylase family are considered to be ascorbate dependent, the hydroxylation of proline alone does not need ascorbate. Prolyl 4-hydroxylases participate in two catalytic reactions: one in which proline residues are hydroxylated, while 2-oxoglutarate is decarboxylated and molecular oxygen is consumed. This reaction is ascorbate independent. However, in another reaction, prolyl 4-hydroxylases catalyze the decarboxylation of 2-oxoglutarate uncoupled from proline hydroxylation but still needing molecular oxygen. At this time, ferrous iron is oxidized and the protein is rendered catalytically inactive until reduced by ascorbate. At the second level of the connection, the oxidation and the oxidized form of ascorbate, dehydroascorbate, is involved in the formation of disulfide bonds of secretory proteins. The significance of the dehydroascorbate reductase activity of protein disulfide isomerase was debated because protein disulfide isomerase as a dehydroascorbate reductase was found to be too slow to be the major route for the reduction of dehydroascorbate (and formation of disulfides) in the endoplasmic reticulum lumen. However, very recently, low tissue ascorbate levels and a noncanonical scurvy were observed in endoplasmic reticulum thiol oxidase- and peroxiredoxin 4-compromised mice. This novel observation implies that ascorbate may be involved in oxidative protein folding and creates a link between the disulfide bond formation (oxidative protein folding) and hydroxylation.

Disulfide bond formation in the cytoplasm

SIGNIFICANCE

Disulfide bond formation is critical for biogenesis of many proteins. While most studies in this field are aimed at elucidating the mechanisms in the endoplasmic reticulum, intermembrane space of mitochondria, and prokaryotic periplasm, structural disulfide bond formation also occurs in other compartments including the cytoplasm. Such disulfide bond formation is essential for biogenesis of some viruses, correct epidermis biosynthesis, thermal adaptation of some extremophiles, and efficient recombinant protein production.

RECENT ADVANCES

The majority of work in this new field has been reported in the past decade. Within the past few years very significant new data have emerged on the catalytic and noncatalytic mechanisms for disulfide bond formation in the cytoplasm. This includes the crystal structure of a key component of viral oxidative protein folding, identification of a missing component in cytoplasmic disulfide bond formation in hyperthermophiles, and introduction of de novo dithiol oxidants in engineered oxidative folding pathways.

CRITICAL ISSUES AND FUTURE DIRECTIONS

While a broad picture of cytoplasmic disulfide bond formation has emerged many critical questions remain unanswered. The individual components in the natural systems are largely known, but the molecular mechanisms by which these processes occur are largely deduced from the mechanisms of analogous components in other compartments. This prevents full understanding and manipulation of these systems, including the potential for novel anti-viral drugs based on the unique features of their sulfhydryl oxidases and the generation of more efficient cell factories for the large-scale production of therapeutic and industrial proteins.

Homologous expression of cytosolic dehydroascorbate reductase increases grain yield and biomass under paddy field conditions in transgenic rice (Oryza sativa L. japonica)

Dehydroascorbate reductase (DHAR, EC 1.8.5.1) maintains redox pools of ascorbate (AsA) by recycling oxidized AsA to reduced AsA. To investigate whether DHAR affects rice yield under normal environmental conditions, cDNA-encoding DHAR (OsDHAR1) was isolated from rice and used to develop OsDHAR1-overexpressing transgenic rice plants, under the regulation of a maize ubiquitin promoter. Incorporation and expression of the transgene in transgenic rice plants was confirmed by genomic polymerase chain reaction (PCR), semi-quantitative reverse transcription PCR (RT-PCR), western blot, and enzyme activity. The expression levels were at least twofold higher in transgenic (TG) rice plants than in control wild-type (WT) rice plants. In addition, OsDHAR1-overexpression in seven-independent homologous transgenic plants, as compared to WT plants, increased photosynthetic capacity and antioxidant enzyme activities under paddy field conditions, which led to an improved AsA pool and redox homeostasis. Furthermore, OsDHAR1 overexpression significantly improved grain yield and biomass due to the increase of culm and root weights and to enhance panicle and spikelet numbers in the same seven independent TG rice plants during the farming season (2010 and 2011) in South Korea. The OsDHAR protein contained the redox-active site (Cys20), as well as the conserved GSH-binding region, GSH-binding motif, glutathione-S-transferase (GST) N-terminal domain, C-terminal domain interface, and GST C-terminal domain. Therefore, our results indicate that OsDHAR1 overexpression, capable of functioning in AsA recycling, and protein folding increases environmental adaptation to paddy field conditions by the improving AsA pool and redox homeostasis, which enhances rice grain yield and biomass.

PRDX4, an endoplasmic reticulum-localized peroxiredoxin at the crossroads between enzymatic oxidative protein folding and nonenzymatic protein oxidation

SIGNIFICANCE

Peroxiredoxin 4 (PRDX4) is an endoplasmic reticulum (ER)-resident peroxiredoxin that has the characteristic of coupling hydrogen peroxide (H(2)O(2)) catabolism with oxidative protein folding. This enzymatic arrangement involves the ingenious use of H(2)O(2) as a substrate to streamline protein metabolism.

RECENT ADVANCES

Mice with compound mutations in Prdx4 and Ero1 have revealed the physiological implication that PRDX4 is a fine-tuned enzymatic mediator of oxidative folding. Remarkably, by simultaneously triggering slow disulfide bond formation and the buildup of H(2)O(2), the lack of PRDX4 and endoplasmic oxidoreductin 1 (ERO1) exposes the thiols of new client proteins to competing H(2)O(2)-mediated oxidation, which leads to an increase in sulfenylated proteins. Such oxygenated thiol derivatives exploit ascorbate as their reductant, thus accelerating its clearance. This relay of events culminates in an altered extracellular matrix (ECM) and a senescent phenotype.

CRITICAL ISSUES AND FUTURE DIRECTIONS

By combining H(2)O(2) metabolism with oxidative folding, PRDX4 protects nascent proteins from an alternative oxidative fate, and cells from the consequences of having misfolded proteins. This highlights the importance of kinetic-regulated disulfide formation at physiological level, and the presence of an exquisite backup system to protect ER redox homeostasis. By altering ECM architecture, ascorbate depletion in the cells triggers an integrated signaling cascade. This sequence of events is part of a multifaceted response linking the ER and the nucleus, which helps cells to overcome ER redox impairment. Furthermore, the relationship between the protein sulfenylation and ascorbate depletion suggests that it would be interesting to explore the metabolism of ascorbate in pathological conditions accompanied by oxidative stress and a defective ECM.

Light-dependent regulation of ascorbate in tomato by a monodehydroascorbate reductase localized in peroxisomes and the cytosol

Ascorbate is a powerful antioxidant in plants, and its levels are an important quality criteria in commercial species. Factors influencing these levels include environmental variations, particularly light, and the genetic control of its biosynthesis, recycling and degradation. One of the genes involved in the recycling pathway encodes a monodehydroascorbate reductase (MDHAR), an enzyme catalysing reduction of the oxidized radical of ascorbate, monodehydroascorbate, to ascorbate. In plants, MDHAR belongs to a multigene family. Here, we report the presence of an MDHAR isoform in both the cytosol and peroxisomes and show that this enzyme negatively regulates ascorbate levels in Solanum lycopersicum (tomato). Transgenic lines overexpressing MDHAR show a decrease in ascorbate levels in leaves, whereas lines where MDHAR is silenced show an increase in these levels in both fruits and leaves. Furthermore, the intensity of these differences is light dependent. The unexpected effect of this MDHAR on ascorbate levels cannot be explained by changes in the expression of Smirnoff-Wheeler pathway genes, or the activity of enzymes involved in degradation (ascorbate peroxidase) or recycling of ascorbate (dehydroascorbate reductase and glutathione reductase), suggesting a previously unidentified mechanism regulating ascorbate levels.

Ascorbate as seen through plant evolution: the rise of a successful molecule?

Ascorbate is a widespread and efficient antioxidant that has multiple functions in plants, traditionally associated with the reactions of photosynthesis. This review aims to look at ascorbate from an evolutionary perspective. Cyanobacteria, algae, and bryophytes contain lower concentrations of ascorbate than higher plants, where the molecule accumulates in high concentrations in both photosynthetic and non-photosynthetic organs and tissues. This increase in ascorbate concentration is paralleled by an increase in the number of isoforms of ascorbate peroxidase and the ascorbate regenerating enzymes mono- and dehydroascorbate reductase. One way of understanding the rise in ascorbate concentrations is to consider ascorbate as a molecule among others that has been subject to selection pressures during evolution, due to its cost or benefit for the cell and the organism. Ascorbate has a low cost in terms of synthesis and toxicity, and its benefits include protection of the glutathione pool and proper functioning of a range of enzymes. The hypothesis presented here is that these features would have favoured increasing roles for the molecule in the development and growth of multicellular organisms. This review then focuses on this diversity of roles for ascorbate in both photosynthetic and non-photosynthetic tissues of higher plants, including fruits and seeds, as well as further functions the molecule may possess by looking at other species. The review also highlights one of the trade-offs of domestication, which has often reduced or diluted ascorbate content in the quest for increased fruit growth and yield, with unknown consequences for the corresponding functional diversity, particularly in terms of stress resistance and adaptive responses to the environment.

Glutathione peroxidases

BACKGROUND

With increasing evidence that hydroperoxides are not only toxic but rather exert essential physiological functions, also hydroperoxide removing enzymes have to be re-viewed. In mammals, the peroxidases inter alia comprise the 8 glutathione peroxidases (GPx1-GPx8) so far identified.

SCOPE OF THE REVIEW

Since GPxs have recently been reviewed under various aspects, we here focus on novel findings considering their diverse physiological roles exceeding an antioxidant activity.

MAJOR CONCLUSIONS

GPxs are involved in balancing the H2O2 homeostasis in signalling cascades, e.g. in the insulin signalling pathway by GPx1; GPx2 plays a dual role in carcinogenesis depending on the mode of initiation and cancer stage; GPx3 is membrane associated possibly explaining a peroxidatic function despite low plasma concentrations of GSH; GPx4 has novel roles in the regulation of apoptosis and, together with GPx5, in male fertility. Functions of GPx6 are still unknown, and the proposed involvement of GPx7 and GPx8 in protein folding awaits elucidation.

GENERAL SIGNIFICANCE

Collectively, selenium-containing GPxs (GPx1-4 and 6) as well as their non-selenium congeners (GPx5, 7 and 8) became key players in important biological contexts far beyond the detoxification of hydroperoxides. This article is part of a Special Issue entitled Cellular functions of glutathione.

Disulfide bond formation in the mammalian endoplasmic reticulum

The formation of disulfide bonds between cysteine residues occurs during the folding of many proteins that enter the secretory pathway. As the polypeptide chain collapses, cysteines brought into proximity can form covalent linkages during a process catalyzed by members of the protein disulfide isomerase family. There are multiple pathways in mammalian cells to ensure disulfides are introduced into proteins. Common requirements for this process include a disulfide exchange protein and a protein oxidase capable of forming disulfides de novo. In addition, any incorrect disulfides formed during the normal folding pathway are removed in a process involving disulfide exchange. The pathway for the reduction of disulfides remains poorly characterized. This work will cover the current knowledge in the field and discuss areas for future investigation.

Endoplasmic reticulum thiol oxidase deficiency leads to ascorbic acid depletion and noncanonical scurvy in mice

Endoplasmic reticulum (ER) thiol oxidases initiate a disulfide relay to oxidatively fold secreted proteins. We found that combined loss-of-function mutations in genes encoding the ER thiol oxidases ERO1α, ERO1β, and PRDX4 compromised the extracellular matrix in mice and interfered with the intracellular maturation of procollagen. These severe abnormalities were associated with an unexpectedly modest delay in disulfide bond formation in secreted proteins but a profound, 5-fold lower procollagen 4-hydroxyproline content and enhanced cysteinyl sulfenic acid modification of ER proteins. Tissue ascorbic acid content was lower in mutant mice, and ascorbic acid supplementation improved procollagen maturation and lowered sulfenic acid content in vivo. In vitro, the presence of a sulfenic acid donor accelerated the oxidative inactivation of ascorbate by an H(2)O(2)-generating system. Compromised ER disulfide relay thus exposes protein thiols to competing oxidation to sulfenic acid, resulting in depletion of ascorbic acid, impaired procollagen proline 4-hydroxylation, and a noncanonical form of scurvy.

Low-molecular-weight oxidants involved in disulfide bond formation

SIGNIFICANCE

The biogenesis of most secreted and outer membrane proteins involves the formation of structure stabilizing disulfide bonds. Hence knowledge of the mechanisms for their formation is critical for understanding a myriad of cellular processes and associated disease states.

RECENT ADVANCES

Until recently it was thought that members of the Ero1 sulfhydryl oxidase family were responsible for catalyzing the majority of disulfide bond formation in the endoplasmic reticulum. However, multiple eukaryotic organisms are now known to show no or minor phenotypes when these enzymatic pathways are disrupted, suggesting that other pathways can catalyze disulfide bond formation to an extent sufficient to maintain normal physiology.

CRITICAL ISSUES AND FUTURE DIRECTIONS

This lack of a strong phenotype raises multiple questions regarding what pathways are acting and whether they themselves constitute the major route for disulfide bond formation. This review critically examines the potential low molecular oxidants that maybe involved in the catalyzed or noncatalyzed formation of disulfide bonds, with an emphasis on the mammalian endoplasmic reticulum, via an examination of their thermodynamics, kinetics, and availability and gives pointers to help guide future experimental work.

The endoplasmic reticulum as the extracellular space inside the cell: role in protein folding and glycosylation

SIGNIFICANCE

Proteins destined to secretion and exposure on the cell surface are synthesized and processed in the extracellular-like environment of the endoplasmic reticulum (ER) of higher eukaryotic cells. Compartmentation plays a crucial role in the post-translational modifications, such as oxidative folding and N-glycosylation in the ER lumen. Transport of the required intermediates across the ER membrane and maintenance of the luminal redox conditions and Ca(2+) ion concentration are indispensable for appropriate protein maturation.

RECENT ADVANCES

Cooperation of enzymes and transporters to maintain a thiol-oxidizing milieu in the ER lumen has been recently elucidated. Ca(2+)-dependence of certain ER chaperones is a subject of intensive research.

CRITICAL ISSUES

Mounting evidence supports the existence of a real barrier between the ER lumen and the cytosol. The unique set of enzymes, selection of metabolites, and characteristic ion and redox milieu of the luminal compartment strongly argue against the general permeability of the ER membrane.

FUTURE DIRECTIONS

Alterations in the luminal environment can trigger the unfolded protein response, a common event in a variety of pathological conditions. Therefore, redox and calcium homeostasis and protein glycosylation in the ER provide novel drug-targets for medical treatment in a wide array of diseases.

Protein disulfide isomerase in redox cell signaling and homeostasis

Thiol proteins may potentially act as redox signaling adaptor proteins, adjusting reactive oxygen species intermediates to specific signals and redox signals to cell homeostasis. In this review, we discuss redox effects of protein disulfide isomerase (PDI), a thioredoxin superfamily oxidoreductase from the endoplasmic reticulum (ER). Abundantly expressed PDI displays ubiquity, interactions with redox and nonredox proteins, versatile effects, and several posttranslational modifications. The PDI family contains >20 members with at least some apparent complementary actions. PDI has oxidoreductase, isomerase, and chaperone effects, the last not directly dependent on its thiols. PDI is a converging hub for pathways of disulfide bond introduction into ER-processed proteins, via hydrogen peroxide-generating mechanisms involving the oxidase Ero1α, as well as hydrogen peroxide-consuming reactions involving peroxiredoxin IV and the novel peroxidases Gpx7/8. PDI is a candidate pathway for coupling ER stress to oxidant generation. Emerging information suggests a convergence between PDI and Nox family NADPH oxidases. PDI silencing prevents Nox responses to angiotensin II and inhibits Akt phosphorylation in vascular cells and parasite phagocytosis in macrophages. PDI overexpression spontaneously enhances Nox activation and expression. In neutrophils, PDI redox-dependently associates with p47phox and supports the respiratory burst. At the cell surface, PDI exerts transnitrosation, thiol reductase, and apparent isomerase activities toward targets including adhesion and matrix proteins and proteases. Such effects mediate redox-dependent adhesion, coagulation/thrombosis, immune functions, and virus internalization. The route of PDI externalization remains elusive. Such multiple redox effects of PDI may contribute to its conspicuous expression and functional role in disease, rendering PDI family members putative redox cell signaling adaptors.

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.

Induction of phase 2 antioxidant enzymes by broccoli sulforaphane: perspectives in maintaining the antioxidant activity of vitamins a, C, and e

Consumption of fruits and vegetables is recognized as an important part of a healthy diet. Increased consumption of cruciferous vegetables in particular has been associated with a decreased risk of several degenerative and chronic diseases, including cardiovascular disease and certain cancers. Members of the cruciferous vegetable family, which includes broccoli, Brussels sprouts, cauliflower, and cabbage, accumulate significant concentrations of glucosinolates, which are metabolized in vivo to biologically active isothiocyanates (ITCs). The ITC sulforaphane, which is derived from glucoraphanin, has garnered particular interest as an indirect antioxidant due to its extraordinary ability to induce expression of several enzymes via the KEAP1/Nrf2/ARE pathway. Nrf2/ARE gene products are typically characterized as Phase II detoxification enzymes and/or antioxidant (AO) enzymes. Over the last decade, human clinical studies have begun to provide in vivo evidence of both Phase II and AO enzyme induction by SF. Many AO enzymes are redox cycling enzymes that maintain redox homeostasis and activity of free radical scavengers such as vitamins A, C, and E. In this review, we present the existing evidence for induction of PII and AO enzymes by SF, the interactions of SF-induced AO enzymes and proposed maintenance of the essential vitamins A, C, and E, and, finally, the current view of genotypic effects on ITC metabolism and AO enzyme induction and function.

Multiple ways to make disulfides

Our concept of how disulfides form in proteins entering the secretory pathway has changed dramatically in recent years. The discovery of endoplasmic reticulum (ER) oxidoreductin 1 (ERO1) was followed by the demonstration that this enzyme couples oxygen reduction to de novo formation of disulfides. However, mammals deficient in ERO1 survive and form disulfides, which suggests the presence of alternative pathways. It has recently been shown that peroxiredoxin 4 is involved in peroxide removal and disulfide formation. Other less well-characterized pathways involving quiescin sulfhydryl oxidase, ER-localized protein disulfide isomerase peroxidases and vitamin K epoxide reductase might all contribute to disulfide formation. Here we discuss these various pathways for disulfide formation in the mammalian ER and highlight the central role played by glutathione in regulating this process.

Real-time monitoring of redox changes in the mammalian endoplasmic reticulum

Redox-sensitive GFPs with engineered disulphide bonds have been used previously to monitor redox status in the cytosol and mitochondria of living cells. The usefulness of these redox probes depends on the reduction potential of the disulphide, with low values suiting the cytosol and mitochondrion, and higher values suiting the more oxidising environment of the endoplasmic reticulum (ER). Here, we targeted a modified redox-sensitive GFP (roGFP1-iL), with a relatively high reduction potential, to the ER of mammalian cells. We showed that the disulphide is partially oxidised, allowing roGFP1-iL to monitor changes in ER redox status. When cells were treated with puromycin, the redox balance became more reducing, suggesting that the release of nascent chains from ribosomes alters the ER redox balance. In addition, downregulating Ero1α prevented normal rapid recovery from dithiothreitol (DTT), whereas downregulating peroxiredoxin IV had no such effect. This result illustrates the contribution of the Ero1α oxidative pathway to ER redox balance. This first report of the use of roGFP to study the ER of mammalian cells demonstrates that roGFP1-iL can be used to monitor real-time changes to the redox status in individual living cells.

Glutathione- and non-glutathione-based oxidant control in the endoplasmic reticulum

The redox-active tripeptide glutathione is an endogenous reducing agent that is found in abundance and throughout the cell. In the endoplasmic reticulum (ER), the ratio of glutathione to glutathione disulfide is lower compared with non-secretory organelles. This relatively oxidizing thiol-disulfide milieu is essential for the oxidative folding of nascent proteins in the ER and, at least in part, maintained by the activity of ER-resident endoplasmic oxidoreductin 1 (Ero1) enzymes that oxidize cysteine side chains at the expense of molecular oxygen. Glutathione disulfide and hydrogen peroxide formed as a consequence of Ero1 activity are widely considered as being inoperative and potentially dangerous by-products of oxidative protein folding in the ER. In contrast to this common view, this Commentary highlights the importance of glutathione- and non glutathione-based homeostatic redox control mechanisms in the ER. Stability in the thiol-disulfide system that prominently includes the protein disulfide isomerases is ensured by the contribution of tightly regulated Ero1 activity, ER-resident peroxidases and the glutathione-glutathione-disulfide redox pair that acts as a potent housekeeper of redox balance. Accordingly, the widely held concept that Ero1-mediated over-oxidation in the ER constitutes a common cause of cellular demise is critically re-evaluated.

Reexamination of the role of interplay between glutathione and protein disulfide isomerase

Protein disulfide isomerase (PDI) has an essential role in the process of disulfide bond formation, where it catalyzes disulfide bond formation, reduction, and isomerization. It is thought that the major route for oxidizing dithiols in folding proteins to disulfides is via Ero1-mediated oxidation of PDI. Since the discovery of Ero1, the role of glutathione in disulfide bond formation has been downplayed. In this study, the role of glutathione in disulfide bond formation was reexamined. Here we have studied in vitro the kinetics of the glutathione-mediated oxidation and reduction of the catalytic a domains of human PDI and yeast Pdi1p. The results obtained from stopped-flow and quenched-flow experiments show that the reactions of PDI and Pdi1p are faster and more complex than previously thought. Our results suggest that the kinetics of oxidation of PDI and Pdi1p by oxidized glutathione are remarkably similar, whereas the kinetics of reduction by reduced glutathione shows clear differences. The data generated here on the rapid reactivity of PDI towards glutathione suggest that reevaluation is required for several aspects of the field of catalyzed disulfide bond formation, including the potential physiological role of glutathione.

Molecular bases of cyclic and specific disulfide interchange between human ERO1alpha protein and protein-disulfide isomerase (PDI)

In the endoplasmic reticulum (ER) of human cells, ERO1α and protein-disulfide isomerase (PDI) constitute one of the major electron flow pathways that catalyze oxidative folding of secretory proteins. Specific and limited PDI oxidation by ERO1α is essential to avoid ER hyperoxidation. To investigate how ERO1α oxidizes PDI selectively among more than 20 ER-resident PDI family member proteins, we performed docking simulations and systematic biochemical analyses. Our findings reveal that a protruding β-hairpin of ERO1α specifically interacts with the hydrophobic pocket present in the redox-inactive PDI b'-domain through the stacks between their aromatic residues, leading to preferred oxidation of the C-terminal PDI a'-domain. ERO1α associated preferentially with reduced PDI, explaining the stepwise disulfide shuttle mechanism, first from ERO1α to PDI and then from oxidized PDI to an unfolded polypeptide bound to its hydrophobic pocket. The interaction of ERO1α with ERp44, another PDI family member protein, was also analyzed. Notably, ERO1α-dependent PDI oxidation was inhibited by a hyperactive ERp44 mutant that lacks the C-terminal tail concealing the substrate-binding hydrophobic regions. The potential ability of ERp44 to inhibit ERO1α activity may suggest its physiological role in ER redox and protein homeostasis.

Oxidative protein folding by an endoplasmic reticulum-localized peroxiredoxin

Endoplasmic reticulum (ER) oxidation 1 (ERO1) transfers disulfides to protein disulfide isomerase (PDI) and is essential for oxidative protein folding in simple eukaryotes such as yeast and worms. Surprisingly, ERO1-deficient mammalian cells exhibit only a modest delay in disulfide bond formation. To identify ERO1-independent pathways to disulfide bond formation, we purified PDI oxidants with a trapping mutant of PDI. Peroxiredoxin IV (PRDX4) stood out in this list, as the related cytosolic peroxiredoxins are known to form disulfides in the presence of hydroperoxides. Mouse embryo fibroblasts lacking ERO1 were intolerant of PRDX4 knockdown. Introduction of wild-type mammalian PRDX4 into the ER rescued the temperature-sensitive phenotype of an ero1 yeast mutation. In the presence of an H(2)O(2)-generating system, purified PRDX4 oxidized PDI and reconstituted oxidative folding of RNase A. These observations implicate ER-localized PRDX4 in a previously unanticipated, parallel, ERO1-independent pathway that couples hydroperoxide production to oxidative protein folding in mammalian cells.

Recycling of peroxiredoxin IV provides a novel pathway for disulphide formation in the endoplasmic reticulum

Disulphide formation in the endoplasmic reticulum (ER) is catalysed by members of the protein disulphide isomerase (PDI) family. These enzymes can be oxidized by the flavoprotein ER oxidoreductin 1 (Ero1), which couples disulphide formation with reduction of oxygen to form hydrogen peroxide (H(2)O(2)). The H(2)O(2) produced can be metabolized by ER-localized peroxiredoxin IV (PrxIV). Continuous catalytic activity of PrxIV depends on reduction of a disulphide within the active site to form a free thiol, which can then react with H(2)O(2). Here, we demonstrate that several members of the PDI family are able to directly reduce this PrxIV disulphide and in the process become oxidized. Furthermore, we show that altering cellular expression of these proteins within the ER influences the efficiency with which PrxIV can be recycled. The oxidation of PDI family members by PrxIV is a highly efficient process and demonstrates how oxidation by H(2)O(2) can be coupled to disulphide formation. Oxidation of PDI by PrxIV may therefore increase efficiency of disulphide formation by Ero1 and also allows disulphide formation via alternative sources of H(2)O(2).

Disulphide production by Ero1α-PDI relay is rapid and effectively regulated

The molecular networks that control endoplasmic reticulum (ER) redox conditions in mammalian cells are incompletely understood. Here, we show that after reductive challenge the ER steady-state disulphide content is restored on a time scale of seconds. Both the oxidase Ero1α and the oxidoreductase protein disulphide isomerase (PDI) strongly contribute to the rapid recovery kinetics, but experiments in ERO1-deficient cells indicate the existence of parallel pathways for disulphide generation. We find PDI to be the main substrate of Ero1α, and mixed-disulphide complexes of Ero1 primarily form with PDI, to a lesser extent with the PDI-family members ERp57 and ERp72, but are not detectable with another homologue TMX3. We also show for the first time that the oxidation level of PDIs and glutathione is precisely regulated. Apparently, this is achieved neither through ER import of thiols nor by transport of disulphides to the Golgi apparatus. Instead, our data suggest that a dynamic equilibrium between Ero1- and glutathione disulphide-mediated oxidation of PDIs constitutes an important element of ER redox homeostasis.

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.