Identification of the dehydroascorbic acid reductase and thioltransferase (Glutaredoxin) activities of bovine erythrocyte glutathione peroxidase

Selenium-based nanozyme as a fluorescence-enhanced probe and imaging for chlortetracycline in living cells and foods

Developing rapid monitoring methods to detect antibiotic residues in food plays an important role in safeguarding human health. This study presents the development of a novel fluorescence-enhanced detection method for chlortetracycline (CTC) using a GSH-Se nanozyme. A GSH-Se nanozyme prepared using a one-pot hydrothermal method not only possesses excellent fluorescent properties but also exhibits good glutathione peroxidase-like activity. The results show that the addition of CTC leads to a significant enhancement in the fluorescence intensity of GSH-Se, and this increase exhibits a good linear relationship with the concentration of CTC. The linear range of this method is 0.02-1 µM, and the limit of detection (LOD) for CTC was 0.02 µM. Moreover, the cell toxicity of GSH-Se is low and can be used for monitoring and imaging of CTC in cells, and satisfactory results have been obtained in the analysis of actual food samples.

Imaging glutathione depletion in the rat brain using ascorbate-derived hyperpolarized MR and PET probes

Oxidative stress is a critical feature of several common neurologic disorders. The brain is well adapted to neutralize oxidative injury by maintaining a high steady-state concentration of small-molecule intracellular antioxidants including glutathione in astrocytes and ascorbic acid in neurons. Ascorbate-derived imaging probes for hyperpolarized ¹³C magnetic resonance spectroscopy and positron emission tomography have been used to study redox changes (antioxidant depletion and reactive oxygen species accumulation) in vivo. In this study, we applied these imaging probes to the normal rat brain and a rat model of glutathione depletion. We first studied hyperpolarized [1-¹³C]dehydroascorbate in the normal rat brain, demonstrating its robust conversion to [1-¹³C]vitamin C, consistent with rapid transport of the oxidized form across the blood-brain barrier. We next showed that the kinetic rate of this conversion decreased by nearly 50% after glutathione depletion by diethyl maleate treatment. Finally, we showed that dehydroascorbate labeled for positron emission tomography, namely [1-¹¹C]dehydroascorbate, showed no change in brain signal accumulation after diethyl maleate treatment. These results suggest that hyperpolarized [1-¹³C]dehydroascorbate may be used to non-invasively detect oxidative stress in common disorders of the brain.

Old Things New View: Ascorbic Acid Protects the Brain in Neurodegenerative Disorders

Ascorbic acid is a key antioxidant of the Central Nervous System (CNS). Under brain activity, ascorbic acid is released from glial reservoirs to the synaptic cleft, where it is taken up by neurons. In neurons, ascorbic acid scavenges reactive oxygen species (ROS) generated during synaptic activity and neuronal metabolism where it is then oxidized to dehydroascorbic acid and released into the extracellular space, where it can be recycled by astrocytes. Other intrinsic properties of ascorbic acid, beyond acting as an antioxidant, are important in its role as a key molecule of the CNS. Ascorbic acid can switch neuronal metabolism from glucose consumption to uptake and use of lactate as a metabolic substrate to sustain synaptic activity. Multiple evidence links oxidative stress with neurodegeneration, positioning redox imbalance and ROS as a cause of neurodegeneration. In this review, we focus on ascorbic acid homeostasis, its functions, how it is used by neurons and recycled to ensure antioxidant supply during synaptic activity and how this antioxidant is dysregulated in neurodegenerative disorders.

Structural insights into omega-class glutathione transferases: a snapshot of enzyme reduction and identification of a non-catalytic ligandin site

Glutathione transferases (GSTs) are dimeric enzymes containing one active-site per monomer. The omega-class GSTs (hGSTO1-1 and hGSTO2-2 in humans) are homodimeric and carry out a range of reactions including the glutathione-dependant reduction of a range of compounds and the reduction of S-(phenacyl)glutathiones to acetophenones. Both types of reaction result in the formation of a mixed-disulfide of the enzyme with glutathione through the catalytic cysteine (C32). Recycling of the enzyme utilizes a second glutathione molecule and results in oxidized glutathione (GSSG) release. The crystal structure of an active-site mutant (C32A) of the hGSTO1-1 isozyme in complex with GSSG provides a snapshot of the enzyme in the process of regeneration. GSSG occupies both the G (GSH-binding) and H (hydrophobic-binding) sites and causes re-arrangement of some H-site residues. In the same structure we demonstrate the existence of a novel “ligandin” binding site deep within in the dimer interface of this enzyme, containing S-(4-nitrophenacyl)glutathione, an isozyme-specific substrate for hGSTO1-1. The ligandin site, conserved in Omega class GSTs from a range of species, is hydrophobic in nature and may represent the binding location for tocopherol esters that are uncompetitive hGSTO1-1 inhibitors.

Vitamin C transport and its role in the central nervous system

Vitamin C, or ascorbic acid, is important as an antioxidant and participates in numerous cellular functions. Although it circulates in plasma in micromolar concentrations, it reaches millimolar concentrations in most tissues. These high ascorbate cellular concentrations are thought to be generated and maintained by the SVCT2 (Slc23a2), a specific transporter for ascorbate. The vitamin is also readily recycled from its oxidized forms inside cells. Neurons in the central nervous system (CNS) contain some of the highest ascorbic acid concentrations of mammalian tissues. Intracellular ascorbate serves several functions in the CNS, including antioxidant protection, peptide amidation, myelin formation, synaptic potentiation, and protection against glutamate toxicity. The importance of the SVCT2 for CNS function is supported by the finding that its targeted deletion in mice causes widespread cerebral hemorrhage and death on post-natal day 1. Neuronal ascorbate content as maintained by this protein also has relevance for human disease, since ascorbate supplements decrease infarct size in ischemia-reperfusion injury models of stroke, and since ascorbate may protect neurons from the oxidant damage associated with neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's. The aim of this review is to assess the role of the SVCT2 in regulating neuronal ascorbate homeostasis and the extent to which ascorbate affects brain function and antioxidant defenses in the CNS.

Meat Science and Muscle Biology Symposium: manipulating meat tenderness by increasing the turnover of intramuscular connective tissue

Controlled reduction of the connective tissue contribution to cooked meat toughness is an objective that would have considerable financial impact in terms of added product value. The amount of intramuscular connective tissue in a muscle appears connected to its in vivo function, so reduction of the overall connective tissue content is not thought to be a viable target. However, manipulation of the state of maturity of the collagenous component is a biologically viable target; by increasing connective tissue turnover, less mature structures can be produced that are functional in vivo but more easily broken down on cooking at temperatures above 60°C, thus improving cooked meat tenderness. Recent work using cell culture models of fibroblasts derived from muscle and myoblasts has identified a range of factors that alter the activity of the principal enzymes responsible for connective tissue turnover, the matrix metalloproteinases (MMP). Fibroblasts cultured from 3 different skeletal muscles from the same animal show different cell proliferation and MMP activity, which may relate to the different connective tissue content and architecture in functionally different muscles. Expression of MMP by fibroblasts is increased by vitamins that can counter the negative effects of oxidative stress on new collagen synthesis. Preliminary work using in situ zymography of myotubes in culture also indicates increased MMP activity in the presence of epinephrine and reactive oxidative species. Comparison of the relative changes in MMP expression from muscle cells vs. fibroblasts shows that myoblasts are more responsive to a range of stimuli. Muscle cells are likely to produce more of the total MMP in muscle tissue as a whole, and the expression of latent forms of the enzymes (i.e., pro-MMP) may vary between oxidative and glycolytic muscle fibers within the same muscle. The implication is that the different muscle fiber composition of different muscles eaten as meat may influence the potential for manipulation of their connective tissue turnover.

Diphenyl diselenide in its selenol form has dehydroascorbate reductase and glutathione S-transferase-like activity dependent on the glutathione content

Objectives

The antioxidant action of diphenyl diselenide ((PhSe)2) is attributed to the mechanism by which (PhSe)2 has pharmacological activity. Although (PhSe)2 has glutathione peroxidase mimetic activity, the exact mechanism involved in its antioxidant effect has not yet been completely elucidated. In the present study, mechanisms involved in the antioxidant property of (PhSe)2 (1–50 µm) were investigated.

Methods

Dehydroascorbate (DHA) reductase- and glutathione S-transferase (GST)-like activity, 2,2′-diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) radical-scavenging activity and the protection against the oxidation of Fe2+ were evaluated.

Key findings

(PhSe)2 at concentrations equal to, or greater than, 5 µm showed DHA reductase- and GST-like activity. (PhSe)2 was not a scavenger of DPPH or ABTS radicals and did not protect against the oxidation of Fe2+.

Conclusions

These results clearly indicated that DHA reductase- and GST-like activity are the mechanisms involved in the antioxidant effect of (PhSe)2.

Reduction of 1-Cys peroxiredoxins by ascorbate changes the thiol-specific antioxidant paradigm, revealing another function of vitamin C

Peroxiredoxins (Prx) are widely distributed peroxidases that can be divided into 1-Cys and 2-Cys Prx groups based on the number of conserved cysteine residues that participate in their catalytical cycle. Prx have been described to be strictly dependent on thiols, but here, we show that ascorbate (vitamin C) also reduces 1-Cys Prx, but not 2-Cys Prx, from several taxonomic groups. Reduction by ascorbate is partly related to the fact that the oxidized form of 1-Cys Prx is a stable sulfenic acid (Cys-SOH) instead of a disulfide. In addition, a histidine residue in the active site is required. In fact, we engineered a 2-Cys Prx with these two features, and it displayed ascorbate peroxidase activity. These data represent a breakthrough in the thiol-specific antioxidant paradigm. Ascorbate may be the long-sought-after biological reductant of 1-Cys Prx. Because ascorbate is present in high amounts in cells, the ascorbate/protein sulfenic acid pair represents an aspect of redox biochemistry that has yet to be explored in vivo.

Translational control of the ascorbic acid transporter SVCT2 in human platelets

Reactive oxygen species (ROS) and redox state have emerged as physiological mediators, controlling blood coagulation and thrombosis. The redox balance is obviously linked to the presence of antioxidants; in particular, vitamin C appears to be a key modulator of platelet oxidative state, since these cells physiologically accumulate ascorbic acid and, moreover, platelet ascorbate plays a role during aggregation. Here, we showed that platelets could compensate for fluctuations in ascorbate levels by modulating the expression of the Na+-dependent transporter SVCT2. Furthermore, the use of anucleated cells demonstrated, for the first time, that SVCT2 expression could be regulated at the translational level. The control of ascorbic acid uptake, through regulation of its carrier, was not only related to substrate availability, but it also occurred during platelet activation, which was accompanied by vitamin C deprivation and alteration in the redox state. Finally, we showed that changes in intracellular ascorbic acid content had physiological relevance, since they modulate the surface sulfhydryl content and the thrombus viscoelastic properties. Beside its role during aggregation, vitamin C may also have important effects during postaggregatory events.

Vitamin C homeostasis in skeletal muscle cells

In skeletal muscle, vitamin C not only enhances carnitine biosynthesis but also protects cells against ROS generation induced by physical exercise. The ability to take up both ascorbic and dehydroascorbic acid from the extracellular environment, together with the ability to recycle the intracellular vitamin, maintains high cellular stores of ascorbate. In this study, we examined vitamin C transport and recycling, by using the mouse C2C12 and rat L6C5 muscle cell lines, which exhibit different sensitivity to oxidative stress and GSH metabolism. We found that: (1) both cell lines express SVCT2, whereas SVCT1 is expressed at very low levels only in proliferating L6C5 cells; furthermore L6C5 myoblasts are more efficient in ascorbic acid transport than C2C12 myoblasts; (2) C2C12 cells are more efficient in dehydroascorbic acid transport and ascorbyl free radical/dehydroascorbic acid reduction; (3) differentiation is paralleled by decreased ascorbic acid and dehydroascorbic acid transport and reduction and increased ascorbyl free radical reduction; (4) differentiated cells are more responsive to oxidative stress induced by glutathione depletion; indeed, myotubes showed increased SVCT2 expression and thioredoxin reductase-mediated dehydroascorbic acid reduction. From our data, SVCT2 and NADPH-thioredoxin-dependent DHA reduction appears to belong to an inducible system activated in response to oxidative stress.

Biological Role of Vitamin C in Keratinocytes

Epidemiological studies have suggested an association between vitamin C (and other antioxidant vitamins) and cancer risk. However, the mechanisms accounting for prevention have not been extensively investigated. In skin, vitamin C (ascorbic acid) exerts different biological roles, including photoprotective effects and participation in collagen synthesis. This paper reports new findings about additional functions of the vitamin. Vitamin C counteracts oxidative stress via transcriptional and post-translational mechanisms; this modulation may interfere with the activity of redox-sensitive transcription factors, commitment to differentiation or cell cycle arrest, and apoptosis in response to DNA damage. All of these vitamin C-mediated responses might be important in different cell types, allowing for the maintenance of body homeostasis.

Vitamin C Recycling Is Enhanced in the Adaptive Response to Leptin-Induced Oxidative Stress in Keratinocytes

Leptin acts on energy metabolism and plays a role in skin repair and in the modulation of cellular redox balance as well. Here, we investigated the effects of leptin on the redox homeostasis in keratinocytes, by evaluating reactive oxygen species (ROS) generation, glutathione content, antioxidant enzymes, activating protein 1 (AP-1) activity, and expression of AP-1-dependent, differentiation-specific genes. We also evaluated the systems involved in the maintenance of a positive ascorbate/dehydroascorbate ratio, i.e., transport and recycling. Leptin altered the keratinocyte redox state, as evident by enhanced ROS generation, oxidized/reduced glutathione ratio, and AP-1 activity. Still, this phenomenon was temporary. Indeed, we found an adaptive response, as demonstrated by an early induction of catalase and a late induction of specific dehydroascorbate reductase activities. In particular, leptin-treated cells showed an increased ability to reduce dehydroascorbate, both in a NADH, lipoic acid- and in a NADPH, thioredoxin-dependent manner. Our results show that leptin may induce adaptation to oxidative stress in skin, leading to an improved vitamin C homeostasis.

Mechanism of the reaction catalyzed by dehydroascorbate reductase from spinach chloroplasts

Dehydroascorbate reductase (DHAR) reduces dehydroascorbate (DHA) to ascorbate with glutathione (GSH) as the electron donor. We analyzed the reaction mechanism of spinach chloroplast DHAR, which had a much higher reaction specificity for DHA than animal enzymes, using a recombinant enzyme expressed in Escherichia coli. Kinetic analysis suggested that the reaction proceeded by a bi-uni-uni-uni-ping-pong mechanism, in which binding of DHA to the free, reduced form of the enzyme was followed by binding of GSH. The Km value for DHA and the summed Km value for GSH were determined to be 53 +/- 12 micro m and 2.2 +/- 1.0 mm, respectively, with a turnover rate of 490 +/- 40 s-1. Incubation of 10 microm DHAR with 1 mm DHA and 10 microm GSH resulted in stable binding of GSH to the enzyme. Bound GSH was released upon reduction of the GSH-enzyme adduct by 2-mercaptoethanol, suggesting that the adduct is a reaction intermediate. Site-directed mutagenesis indicated that C23 in DHAR is indispensable for the reduction of DHA. The mechanism of catalysis of spinach chloroplast DHAR is proposed.

Cancer drugs, genetic variation and the glutathione-S-transferase gene family

The glutathione-S-transferase (GST) super family comprises multiple isozymes (Alpha, Mu, Pi, Omega, Theta, and Zeta) with compelling evidence of functional polymorphic variation. Over the last two decades, a significant body of data has accumulated linking aberrant expression of GST isozymes with the development and expression of resistance to cancer drugs. Clinical correlation studies show that genetic differences within the human GST isozymes may play a role in cancer susceptibility and treatment. The initial confusion was presented by the fact that not all drugs used to select for resistance were substrates for thioether bond catalysis by GSTs. However, recent evidence that certain GST isozymes possess the capacity to regulate mitogen activated protein kinases presents an alternative explanation. This dual functionality has contributed to the recent efforts to target GSTs with novel small molecule therapeutics. While the ultimate success of these attempts remains to be shown, at least one drug is in late-stage clinical testing. In addition, the concept of designing new drugs that might interfere with protein:protein interactions between GSTs and regulatory kinases provides a novel approach to identify new targets in the search for cancer therapeutics.

Ebselen has dehydroascorbate reductase and thioltransferase-like activities

Ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)-one), a seleno-organic compound, has been reported to mimic glutathione peroxidase (GPX). Since bovine erythrocyte GPX showed dehydroascorbic acid (DHA) reductase and thioltransferase (TTase) activities, ebselen was also examined for DHA reductase and TTase-like activities. Evidence is reported that, in the presence of GSH, ebselen catalyzed the in vitro reduction of DHA to L-ascorbic acid in a dose-dependent manner. Using S-sulfocysteine and GSH as co-substrates, ebselen catalyzed the in vitro formation of glutathione disulfide in a dose-dependent manner, thereby acting as a TTase mimic. 1-Chloro-2,4-dinitrobezene (CDNB), a co-substrate with GSH for glutathione S-transferase, was used to measure rates of adduct formation with ebselen pretreated with GSH and compared with GSH alone. The reaction rate was proportional to ebselen, and ebselen was about 250 times more reactive than GSH on an equimolar basis. The DHA reductase and TTase-like activities, in addition to the powerful nucleophilic reactivity of ebselen selenol, may contribute to ebselen's significant anti-inflammatory and anti-oxidative properties in vivo.

Ascorbic acid: much more than just an antioxidant

Vitamin C (ascorbic acid (AA)) is very popular for its antioxidant properties. Consequently, many other important aspects of this multifaceted molecule are often underestimated or even ignored. In the present paper, we have tried to bring to the foreground some of these aspects, including the peculiarities of the AA biosynthetic pathway in different organisms, the remarkable function of AA as a co-substrate of many important dioxygenases, the role of AA-regenerating enzymes and the known pathways of AA catabolism, as well as the intriguing function of AA in gene expression.

Identification, characterization, and crystal structure of the Omega class glutathione transferases

A new class of glutathione transferases has been discovered by analysis of the expressed sequence tag data base and sequence alignment. Glutathione S-transferases (GSTs) of the new class, named Omega, exist in several mammalian species and Caenorhabditis elegans. In humans, GSTO 1-1 is expressed in most tissues and exhibits glutathione-dependent thiol transferase and dehydroascorbate reductase activities characteristic of the glutaredoxins. The structure of GSTO 1-1 has been determined at 2.0-A resolution and has a characteristic GST fold (Protein Data Bank entry code ). The Omega class GSTs exhibit an unusual N-terminal extension that abuts the C terminus to form a novel structural unit. Unlike other mammalian GSTs, GSTO 1-1 appears to have an active site cysteine that can form a disulfide bond with glutathione.