Increase in hepatic mixed disulphide and glutathione disulphide levels elicited by paraquat

ADME/T-based strategies for paraquat detoxification: Transporters and enzymes

Paraquat (PQ) is a toxic, organic herbicide for which there is no specific antidote. Although banned in some countries, it is still used as an irreplaceable weed killer in others. The lack of understanding of the precise mechanism of its toxicity has hindered the development of treatments for PQ exposure. While toxicity is thought to be related to PQ-induced oxidative stress, antioxidants are limited in their ability to ameliorate the untoward biological responses to this agent. Summarized in this review are data on the absorption, distribution, metabolism, excretion, and toxicity (ADME/T) of PQ, focusing on the essential roles of individual transporters and enzymes in these processes. Based on these findings, strategies are proposed to design and test specific and effective antidotes for the clinical management of PQ poisoning.

Role of protein S-Glutathionylation in cancer progression and development of resistance to anti-cancer drugs

The survival, functioning and proliferation of mammalian cells are highly dependent on the cellular response and adaptation to changes in their redox environment. Cancer cells often live in an altered redox environment due to aberrant neo-vasculature, metabolic reprogramming and dysregulated proliferation. Thus, redox adaptations are critical for their survival. Glutathione plays an essential role in maintaining redox homeostasis inside the cells by binding to redox-sensitive cysteine residues in proteins by a process called S-glutathionylation. S-Glutathionylation not only protects the labile cysteine residues from oxidation, but also serves as a sensor of redox status, and acts as a signal for stimulation of downstream processes and adaptive responses to ensure redox equilibrium. The present review aims to provide an updated overview of the role of the unique redox adaptations during carcinogenesis and cancer progression, focusing on their dependence on S-glutathionylation of specific redox-sensitive proteins involved in a wide range of processes including signalling, transcription, structural maintenance, mitochondrial functions, apoptosis and protein recycling. We also provide insights into the role of S-glutathionylation in the development of resistance to chemotherapy. Finally, we provide a strong rationale for the development of redox targeting drugs for treatment of refractory/resistant cancers.

In VitroModels to Study Mechanisms of Lung Toxicity

Several functioning in vitro systems of varying complexity are currently in use for the study of mechanisms of lung toxicity. The isolated perfused lung is the model closest to the in vivo situation. It is a suitable model for combining metabolic and functional studies. It is, for instance, possible to relate changes in lung mechanics and lung perfusion flow to the release of various mediators during exposure of the lung to various agents. A simpler model may be constructed from lung slices which are less viable but suitable for uptake as well as metabolism studies.

Specific lung cells such as Clara cells and type II pneumocytes have been isolated and cultured and are valuable tools for studies of the molecular mechanisms of lung toxicity, particularly in cases of cell-specific toxicity. There is, however, a great need to develop techniques for the isolation and culture of other types of lung cells and also to improve the culturing techniques for those already isolated.

Role of Glutathionylation in Infection and Inflammation

Glutathionylation, that is, the formation of mixed disulfides between protein cysteines and glutathione (GSH) cysteines, is a reversible post-translational modification catalyzed by different cellular oxidoreductases, by which the redox state of the cell modulates protein function. So far, most studies on the identification of glutathionylated proteins have focused on cellular proteins, including proteins involved in host response to infection, but there is a growing number of reports showing that microbial proteins also undergo glutathionylation, with modification of their characteristics and functions. In the present review, we highlight the signaling role of GSH through glutathionylation, particularly focusing on microbial (viral and bacterial) glutathionylated proteins (GSSPs) and host GSSPs involved in the immune/inflammatory response to infection; moreover, we discuss the biological role of the process in microbial infections and related host responses.

Redox Proteomics Applied to the Thiol Secretome

SIGNIFICANCE

Secreted proteins are important both as signaling molecules and potential biomarkers. Recent Advances: Protein can undergo different types of oxidation, both in physiological conditions or under oxidative stress. Several redox proteomics techniques have been successfully applied to the identification of glutathionylated proteins, an oxidative post-translational modification consisting in the formation of a mixed disulfide between a protein cysteine and glutathione. Redox proteomics has also been used to study other forms of protein oxidation.

CRITICAL ISSUES

Because of the highest proportion of free cysteines in the cytosol, redox proteomics of protein thiols has focused, so far, on intracellular proteins. However, plasma proteins, such as transthyretin and albumin, have been described as glutathionylated or cysteinylated. The present review discusses the redox state of protein cysteines in relation to their cellular distribution. We describe the various approaches used to detect secreted glutathionylated proteins, the only thiol modification studied so far in secreted proteins, and the specific problems presented in the study of the secretome.

FUTURE DIRECTIONS

This review focusses on glutathionylated proteins secreted under inflammatory conditions and that may act as soluble mediators (cytokines). Future studies on the redox secretome (including other forms of oxidation) might identify new soluble mediators and biomarkers of oxidative stress. Antioxid. Redox Signal. 26, 299-312.

Mixed results with mixed disulfides

A period of research with Helmut Sies in the 1980s is recalled. Our experiments aimed at an in-depth understanding of metabolic changes due to oxidative challenges under near-physiological conditions, i.e. perfused organs. A major focus were alterations of the glutathione and the NADPH/NADP(+) system by different kinds of oxidants, in particular formation of glutathione mixed disulfides with proteins. To analyze mixed disulfides, a test was adapted which is widely used until today. The observations in perfused rat livers let us believe that glutathione-6-phosphate dehydrogenase (G6PDH), i.a. might be activated by glutathionylation. Although we did not succeed to verify this hypothesis for the special case of G6PDH, the regulation of enzyme/protein activities by glutathionylation today is an accepted posttranslational mechanism in redox biology in general. Our early experimental approaches are discussed in the context of present knowledge.

Glutathione - From antioxidant to post-translational modifier

Helmut Sies is one of the leading investigators in the multiple roles of glutathione (GSH) in biology. He has pioneered work on the role of GSH in preventing oxidative stress, in transport of GSSG, in protection of protein thiols from irreversible oxidation through mixed disulfide formation and demonstrated a role of protein glutathionylation in response to hormonal stimulation well before redox signaling became a major subject of investigation. Here I will describe the roles of GSH in several aspects of biology, the work of my laboratory in those findings, and how Helmut Sies work influenced our studies.

Elevation of Glucose 6-Phosphate Dehydrogenase Activity Induced by Amplified Insulin Response in Low Glutathione Levels in Rat Liver

Weanling male Wistar rats were fed on a 10% soybean protein isolate (SPI) diet for 3 weeks with or without supplementing 0.3% sulfur-containing amino acids (SAA; methionine or cystine) to examine relationship between glutathione (GSH) levels and activities of NADPH-producing enzymes, glucose 6-phosphate dehydrogenase (G6PD) and malic enzyme (ME), in the liver. Of rats on the 10% SPI diet, GSH levels were lower and the enzyme activities were higher than of those fed on an SAA-supplemented diet. Despite the lower GSH level, γ-glutamylcysteine synthetase (γ-GCS) activity was higher in the 10% SPI group than other groups. Examination of mRNAs of G6PD and ME suggested that the GSH-suppressing effect on enzyme induction occurred prior to and/or at transcriptional levels. Gel electrophoresis of G6PD indicated that low GSH status caused a decrease in reduced form and an increase in oxidized form of the enzyme, suggesting an accelerated turnover rate of the enzyme. In primary cultured hepatocytes, insulin response to induce G6PD activity was augmented in low GSH levels manipulated in the presence of buthionine sulfoximine. These findings indicated that elevation of the G6PD activity in low GSH levels was caused by amplified insulin response for expression of the enzyme and accelerated turnover rate of the enzyme molecule.

Unveiling the roles of the glutathione redox system in vivo by analyzing genetically modified mice

Redox status affects various cellular activities, such as proliferation, differentiation, and death. Recent studies suggest pivotal roles of reactive oxygen species not only in pathogenesis under oxidative insult but also in intracellular signal transduction. Glutathione is present in several millimolar concentrations in the cytoplasm and has multiple roles in the regulation of cellular homeostasis. Two enzymes, γ-glutamylcysteine synthetase and glutathione synthetase, constitute the de novo synthesis machinery, while glutathione reductase is involved in the recycling of oxidized glutathione. Multidrug resistant proteins and some other transporters are responsible for exporting oxidized glutathione, glutathione conjugates, and S-nitrosoglutathione. In addition to antioxidation, glutathione is more positively involved in cellular activity via its sulfhydryl moiety of a molecule. Animals in which genes responsible for glutathione metabolism are genetically modified can be used as beneficial and reliable models to elucidate roles of glutathione in vivo. This review article overviews recent progress in works related to genetically modified rodents and advances in the elucidation of glutathione-mediated reactions.

Quantification of redox conditions in the nucleus

Many nuclear proteins contain thiols, which undergo reversible oxidation and are critical for normal function. These proteins include enzymes, transport machinery, structural proteins, and transcription factors with conserved cysteine in zinc fingers and DNA-binding domains. Uncontrolled oxidation of these thiols causes dysfunction, and two major thiol-dependent antioxidant systems provided protection. The redox states of these systems, including the small redox active protein thioredoxin-1 (Trx1) and the abundant, low molecular weight thiol antioxidant glutathione (GSH), in nuclei provide means to quantify nuclear redox conditions. Redox measurements are obtained under conditions with excess thiol-reactive reagents. Here we describe a suite of methods to measure nuclear redox state, which include a redox Western blot technique to quantify the redox state of Trxl, a biotinylated iodoacetamide (BIAM) method for thioredoxin reductase-1 (TrxR1), GSH redox measurement using total protein S-glutathionylation, and a redox isotope-coded affinity tag (ICAT) method for measuring oxidation of specific cysteines in high-abundance nuclear proteins.

Paraquat poisonings: mechanisms of lung toxicity, clinical features, and treatment

Paraquat dichloride (methyl viologen; PQ) is an effective and widely used herbicide that has a proven safety record when appropriately applied to eliminate weeds. However, over the last decades, there have been numerous fatalities, mainly caused by accidental or voluntary ingestion. PQ poisoning is an extremely frustrating condition to manage clinically, due to the elevated morbidity and mortality observed so far and due to the lack of effective treatments to be used in humans. PQ mainly accumulates in the lung (pulmonary concentrations can be 6 to 10 times higher than those in the plasma), where it is retained even when blood levels start to decrease. The pulmonary effects can be explained by the participation of the polyamine transport system abundantly expressed in the membrane of alveolar cells type I, II, and Clara cells. Further downstream at the toxicodynamic level, the main molecular mechanism of PQ toxicity is based on redox cycling and intracellular oxidative stress generation. With this review we aimed to collect and describe the most pertinent and significant findings published in established scientific publications since the discovery of PQ, focusing on the most recent developments related to PQ lung toxicity and their relevance to the treatment of human poisonings. Considerable space is also dedicated to techniques for prognosis prediction, since these could allow development of rigorous clinical protocols that may produce comparable data for the evaluation of proposed therapies.

Emerging potential of thioredoxin and thioredoxin interacting proteins in various disease conditions

Reactive oxygen species (ROS) are known to be mediators of intracellular signaling pathways. However the excessive production of ROS may be detrimental to the cell as a result of the increased oxidative stress and loss of cell function. Hence, well tuned, balanced and responsive antioxidant systems are vital for proper regulation of the redox status of the cell. The cells are normally able to defend themselves against the oxidative stress induced damage through the use of several antioxidant systems. Even though the free radical scavenging enzymes such as superoxide dismutase (SOD) and catalase can handle huge amounts of reactive oxygen species, should these systems fail some reactive molecules will evade the detoxification process and damage potential targets. In such a scenario, cells recruit certain small molecules and proteins as 'rescue specialists' in case the 'bodyguards' fail to protect potential targets from oxidative damage. The thioredoxin (Trx) system thus plays a vital role in the maintenance of a reduced intracellular redox state which is essential for the proper functioning of each individual cell. Trx alterations have been implicated in many diseases such as cataract formation, ischemic heart diseases, cancers, AIDS, complications of diabetes, hypertension etc. The interactions of Trx with many different proteins and different metabolic and signaling pathways as well as the significant species differences make it an attractive target for therapeutic intervention in many fields of medical science. In this review, we present, the critical roles that thioredoxins play in limiting oxidant stress through either its direct effect as an antioxidant or through its interactions with other key signaling proteins (thioredoxin interacting proteins) and its implications in various disease models.

Thioltransferases

A family of small molecular weight proteins with thiol-disulfide exchange activity have been discovered, widely distributed from E. coli to mammalian systems, called thioltransferases or glutaredoxins. There are no substantiated reports of thioltransferases-glutaredoxins in plants; however, partially purified dehydroascorbate reductase from peas had thiol-disulfide exchange catalytic activity using glutathione as reductant and S-sulfocysteine as thiosulfate cosubstrate (unpublished data). Thus, this class of proteins is universally distributed. Based on mutagenesis studies, a sequence of Cys-Pro-Tyr(Phe)-Cys- followed by Arg-Lys- or Lys alone is critical for both the thiol-disulfide exchange reaction and the dehydroascorbate reductase activity. The dithiol-disulfide loop represented by this structure is unique since the cystine closer to the N-terminus has a highly acidic thiol pKa (3.8 as determined for the pig liver enzyme) that contributes to the protein's high S- nucleophilicity. Compared with the microbial enzyme, the mammalian thioltransferases (glutaredoxins) are extended at both N and C termini by 10-12 amino acid residues, including a second pair of cysteines toward the C-terminus with no known special function. Yeast thioltransferase is more like mammalian enzymes in length (106 amino acids) but more like E. coli glutaredoxin in being unblocked at the N-terminus and having only one set of cysteines; that is, at the active center. The three mammalian enzymes, for which sequences are available, are blocked at the N-terminus by an acetyl group linked to alanine with no known special function other than possibly to impart greater cellular turnover stability. A report of carbohydrate (8.6%) content in rat liver thioltransferase has not been verified by more sensitive methods of carbohydrate analysis, nor has carbohydrate been identified in samples of purified glutaredoxin from any source. Thiol transferase and glutaredoxin are two names for the same protein based on similarity of amino acid sequence, immunochemical cross-reactivity, and other enzyme properties. The inability of thioltransferase from some mammalian sources to act as an electron carrier in ribonucleotide reductase systems, whether homologous or heterologous in origin, remains to be explained in future studies.

Thioredoxin-1 and posttranslational modifications

Thioredoxin-1 is a 12 kDa protein that consists of a redox regulatory domain containing the active cysteine residues 32 and 35. These cysteines are conserved from bacteria to human. Unlike thioredoxins from lower species, mammalian thioredoxin-1 contains three additional nonactive cysteine residues at positions 62, 69, and 73 (for human thioredoxin-1). Key biological functions of thioredoxin-1 are antioxidative, anti-apoptotic, and pro-proliferative properties. Thioredoxin-1 is regulated by the ability of the thioredoxin reductase to reduce oxidized thioredoxin-1 at cysteines 32 and 35. However, posttranslational modifications of thioredoxin-1, including glutathionylation, thiol-oxidation, and S-nitros(yl)ation, at the nonactive cysteines importantly contribute to the regulation and functions of thioredoxin-1. This review focuses on the posttranslational modifications of the active and nonactive cysteines and their contribution for functional regulation of thioredoxin-1.

Thioredoxins, glutaredoxins, and glutathionylation: new crosstalks to explore

Oxidants are widely considered as toxic molecules that cells have to scavenge and detoxify efficiently and continuously. However, emerging evidence suggests that these oxidants can play an important role in redox signaling, mainly through a set of reversible post-translational modifications of thiol residues on proteins. The most studied redox system in photosynthetic organisms is the thioredoxin (TRX) system, involved in the regulation of a growing number of target proteins via thiol/disulfide exchanges. In addition, recent studies suggest that glutaredoxins (GRX) could also play an important role in redox signaling especially by regulating protein glutathionylation, a post-translational modification whose importance begins to be recognized in mammals while much less is known in photosynthetic organisms. This review focuses on oxidants and redox signaling with particular emphasis on recent developments in the study of functions, regulation mechanisms and targets of TRX, GRX and glutathionylation. This review will also present the complex emerging interplay between these three components of redox-signaling networks.

The Anti-Inflammatory Drug, Nimesulide (4-Nitro-2-phenoxymethane-sulfoanilide), Uncouples Mitochondria and Induces Mitochondrial Permeability Transition in Human Hepatoma Cells: Protection by Albumin

Like other nonsteroidal anti-inflammatory drugs, nimesulide (4-nitro-2-phenoxymethane-sulfoanilide) triggers hepatitis in a few recipients. Although nimesulide has been shown to uncouple mitochondrial respiration and cause hepatocyte necrosis in the absence of albumin, mechanisms for cell death are incompletely understood, and comparisons with human concentrations are difficult because 99% of nimesulide is albumin-bound. We studied the effects of nimesulide, with or without a physiological concentration of albumin, in isolated rat liver mitochondria or microsomes and in human hepatoma cells. Nimesulide did not undergo monoelectronic nitro reduction in microsomes. In mitochondria incubated without albumin, nimesulide (50 microM) decreased the mitochondrial membrane potential (DeltaPsim), increased basal respiration, and potentiated the mitochondrial permeability transition (MPT) triggered by calcium preloading. In HUH-7 cells incubated for 24 h without albumin, nimesulide (1 mM) decreased the DeltaPsim and cell NADPH and increased the glutathione disulfide/reduced glutathione ratio and cell peroxides; nimesulide triggered MPT, ATP depletion, high cell calcium, and caused mostly necrosis, with rare apoptotic cells. Coincubation with either cyclosporin A (an MPT inhibitor) or the combination of fructose-1,6-diphosphate (a glycolysis substrate) and oligomycin (an ATPase inhibitor) prevented the decrease in DeltaPsim, ATP depletion, and cell death. A physiological concentration of albumin abolished the effects of nimesulide on isolated mitochondria or HUH-7 cells. In conclusion, the weak acid, nimesulide, uncouples mitochondria and triggers MPT and ATP depletion in isolated mitochondria or hepatoma cells incubated without albumin. However, in the presence of albumin, only a fraction of the drug enters cells or organelles, and uncoupling and toxicity are not observed.

Oxidoreduction of protein thiols in redox regulation

Protein cysteines can undergo various forms of oxidation, some of them reversible (disulphide formation, glutathionylation and S-nitrosylation). While in the past these were viewed as protein damage in the context of oxidative stress, there is growing interest in oxidoreduction of protein thiols/disulphides as a regulatory mechanism. This review discusses the evolution of the concept of redox regulation from that of oxidative stress and the redox state of protein cysteines in different cellular compartments.

Gene expression profiling reveals a signaling role of glutathione in redox regulation

Proteins can form reversible mixed disulfides with glutathione (GSH). It has been hypothesized that protein glutathionylation may represent a mechanism of redox regulation, in a fashion similar to that mediated by protein phosphorylation. We investigated whether GSH has a signaling role in the response of HL60 cells to hydrogen peroxide (H2O2), in addition to its obvious antioxidant role. We identified early changes in gene expression induced at different times by H2O2 treatment, under conditions that increase protein glutathionylation and minimal toxicity. We then investigated the effect of prior GSH depletion by buthionine sulfoximine and diethylmaleate on this response. The analysis revealed 2,016 genes regulated by H2O2. Of these, 215 genes showed GSH-dependent expression changes, classifiable into four clusters displaying down- or up-regulation by H2O2, either potentiated or inhibited by GSH depletion. The modulation of 20 selected genes was validated by real-time RT-PCR. The biological process categories overrepresented in the largest cluster (genes whose up-regulation was inhibited by GSH depletion) were NF-kappaB activation, transcription, and DNA methylation. This cluster also included several cytokine and chemokine ligands and receptors, the redox regulator thioredoxin interacting protein, and the histone deacetylase sirtuin. The cluster of genes whose up-regulation was potentiated by GSH depletion included two HSPs (HSP40 and HSP70) and the AP-1 transcription factor components Fos and FosB. This work demonstrates that GSH, in addition to its antioxidant and protective function against oxidative stress, has a specific signaling role in redox regulation.

Thiol-disulfide balance: from the concept of oxidative stress to that of redox regulation

Originally, small thiols, including glutathione, were viewed as protective antioxidants, acting as free radical scavengers in the context of oxidative damage. Recently, there is a growing literature showing that protein glutathionylation (formation of protein-glutathione mixed disulfides) and other forms of cysteine oxidation may be a means of redox regulation under physiological conditions. This review discusses the importance of protein oxidation in redox regulation in view of the recent data originating from the application of redox proteomics to identify redox-sensitive targets.

The extracellular microenvironment plays a key role in regulating the redox status of cell surface proteins in HIV-infected subjects

There is an overwhelming interest in the study of the redox status of the cell surface affecting redox signaling in the cells and also predicting the total redox status of the cells. Measuring the total surface thiols (cell surface molecule thiols, csm-SH) we have shown that the overall level of surface thiols is tightly controlled. In vitro, the total concentration of intracellular glutathione (iGSH) seems to play a regulatory role in determination of the amounts of reduced proteins on cells. In addition, short term exposure of the cell surface to glutathione disulfide (GSSG, oxidized GSH) seems to reduce the overall levels of csm-SH suggesting that the function of some cysteine containing proteins on the cell surface may be regulated by the amount of GSSG secreted from the cells or the GSSG available in the extracellular environment. Examination of peripheral blood mononuclear cells (PBMCs) from healthy or HIV-infected subjects failed to reveal a similar correlation between the intra- and extracellular thiol status of cells. Although there is a relatively wide variation between individuals in both csm-SH and iGSH there is no correlation between the iGSH and csm-SH levels measured for healthy and HIV-infected individuals. There are many reports suggesting different redox active proteins on the cell surface to be the key players in the total cell surface redox regulation. However, we suggest that the redox status of the cells is regulated through a complex and tightly regulated mechanism that needs further investigation. In the mean time, overall surface thiol measurements together with case specific protein determinations may offer the most informative approach. In this review, we discuss our own results as well as results from other laboratories to argue that the overall levels of surface thiols on the exofacial membrane are regulated primarily by redox status of the cell surface microenvironment.

Fish tolerance to organophosphate-induced oxidative stress is dependent on the glutathione metabolism and enhanced by N-acetylcysteine

Dichlorvos (2,2-dichlorovinyl dimethyl phosphate, DDVP) is an organophosphorus (OP) insecticide and acaricide extensively used to treat external parasitic infections of farmed fish. In previous studies we have demonstrated the importance of the glutathione (GSH) metabolism in the resistance of the European eel (Anguilla anguilla L.) to thiocarbamate herbicides. The present work studied the effects of the antioxidant and glutathione pro-drug N-acetyl-L-cysteine (NAC) on the survival of a natural population of A. anguilla exposed to a lethal concentration of dichlorvos, focusing on the glutathione metabolism and the enzyme activities of acetylcholinesterase (AChE) and caspase-3 as biomarkers of neurotoxicity and induction of apoptosis, respectively. Fish pre-treated with NAC (1 mmol kg(-1), i.p.) and exposed to 1.5 mg l(-1) (the 96-h LC85) of dichlorvos for 96 h in a static-renewal system achieved an increase of the GSH content, GSH/GSSG ratio, hepatic glutathione reductase (GR), glutathione S-transferase (GST), glutamate:cysteine ligase (GCL), and gamma-glutamyl transferase (gammaGT) activities, which ameliorated the glutathione loss and oxidation, and enzyme inactivation, caused by the OP pesticide. Although NAC-treated fish presented a higher survival and were two-fold less likely to die within the study period of 96 h, Cox proportional hazard models showed that hepatic GSH/GSSG ratio was the best explanatory variable related to survival. Hence, tolerance to a lethal concentration of dichlorvos can be explained by the individual capacity to maintain and improve the hepatic glutathione redox status. Impairment of the GSH/GSSG ratio can lead to excessive oxidative stress and inhibition of caspase-3-like activity, inducing cell death by necrosis, and, ultimately, resulting in the death of the organism. We therefore propose a reconsideration of the individual effective dose or individual tolerance concept postulated by Gaddum 50 years ago for the log-normal dose-response relationship. In addition, as NAC increased the tolerance to dichlorvos, it could be a potential antidote for OP poisoning, complementary to current treatments.

Paraquat and menadione exposure of rainbow trout (Oncorhynchus mykiss)--studies of effects on the pentose-phosphate shunt and thiamine levels in liver and kidney

Possible xenobiotic interactions with thiamine were studied in salmonid fish, by repeatedly injecting two model substances, paraquat and menadione, into juvenile rainbow trout (Oncorhynchus mykiss). These two substances were chosen because of their well-known ability to redox-cycle and cause depletion of NADPH in several biological systems. Depletion of NADPH increases metabolism through the pentose-phosphate shunt and may thereby increase the need for thiamine diphosphate by heightened transketolase activity. A special food was produced with lower thiamine content than commercial food, usually enriched with thiamine, which could mask an effect on the thiamine level. After 9 weeks of exposure, glucose-6-phosphate dehydrogenase, transketolase, glutathione reductase and ethoxyresorufin O-deethylase were analysed in liver and kidney cellular sub-fractions as well as analysis of total thiamine concentrations in liver, kidney and muscle. The results showed that paraquat caused a large increase in hepatic glutathione reductase activity and induced hepatic glucose-6-phosphate dehydrogenase activity, i.e., the rate-limiting enzyme in the oxidative part of the pentose-phosphate shunt. Despite this paraquat exposure did not affect transketolase activity and total thiamine concentration.

Functional glutaredoxin (thioltransferase) activity in rat brain and liver mitochondria

Glutaredoxin (Grx) is a specific and efficient catalyst of glutathione-dependent deglutathionylation of protein-SS-glutathione mixed disulfides. Grx has been identified in brain cytosol, but the presence of activity in subcellular organelles has not been reported. Increases in protein glutathionylation are likely to occur in mitochondria during oxidative stress and it is, therefore, important to know if this organelle contains the enzyme activity needed to reverse such protein thiolation. Grx-like activity in the P1 supernatant from rat brain and liver was doubled in the presence of Triton-X 100 suggesting a releasable pool of Grx. Brain and liver homogenates were subfractionated into cytosolic, mitochondrial and microsomal fraction, their purity determined by biochemical assay and EM and assayed for Grx-like activity. The data presented demonstrate that mitochondria contain functional Grx-like activity.

Inhibition of brain mitochondrial respiration by dopamine: involvement of H(2)O(2) and hydroxyl radicals but not glutathione-protein-mixed disulfides

Examination of the downstream mediators responsible for inhibition of mitochondrial respiration by dopamine (DA) was investigated. Consistent with findings reported by others, exposure of rat brain mitochondria to 0.5 mm DA for 15 min at 30 degrees C inhibited pyruvate/glutamate/malate-supported state-3 respiration by 20%. Inhibition was prevented in the presence of pargyline and clorgyline demonstrating that mitochondrial inhibition arose from products formed following MAO metabolism and could include hydrogen peroxide (H(2) O(2) ), hydroxyl radical, oxidized glutathione (GSSG) or glutathione-protein mixed disulfides (PrSSG). As with DA, direct incubation of intact mitochondria with H(2) O(2) (100 microm) significantly inhibited state-3 respiration. In contrast, incubation with GSSG (1 mm) had no effect on O(2) consumption. Exposure of mitochondria to 1 mm GSSG resulted in a 3.3-fold increase in PrSSG formation compared with 1.4- and 1.5-fold increases in the presence of 100 microm H(2) O(2) or 0.5 mm DA, respectively, suggesting a dissociation between PrSSG formation and effects on respiration. The lack of inhibition of respiration by GSSG could not be accounted for by inadequate delivery of GSSG into mitochondria as increases in PrSSG levels in both membrane-bound (2-fold) and intramatrix (3.5-fold) protein compartments were observed. Furthermore, GSSG was without effect on electron transport chain activities in freeze-thawed brain mitochondria or in pig heart electron transport particles (ETP). In contrast, H(2) O(2) showed differential effects on inhibition of respiration supported by different substrates with a sensitivity of succinate > pyruvate/malate > glutamate/malate. NADH oxidase and succinate oxidase activities in freeze-thawed mitochondria were inhibited with IC(50) approximately 2-3-fold higher than in intact mitochondria. ETPs, however, were relatively insensitive to H(2) O(2). Co-administration of desferrioxamine with H(2) O(2) had no effect on complex I-associated inhibition in intact mitochondria, but attenuated inhibition of rotenone-sensitive NADH oxidase activity by 70% in freeze-thawed mitochondria. The results show that DA-associated inhibition of respiration is dependent on MAO and that H(2) O(2) and its downstream hydroxyl radical rather than increased GSSG and subsequent PrSSG formation mediate the effects.

Protein glutathionylation: coupling and uncoupling of glutathione to protein thiol groups in lymphocytes under oxidative stress and HIV infection

We show here that exposure to oxidative stress induces glutathione (GSH) modification of protein cysteinyl residues (glutathionylation) in T cell blasts. Treating the cells with the oxidant diamide induces thiolation of a series of proteins that can be detected by 2D electrophoresis when 35S-cysteine is used to label the intracellular GSH pool. This thiolation is reversible, proteins are rapidly dethiolated and GSH is released from proteins once the oxidants are washed and the cells are allowed to recover. Dethiolation is dependent on the availability of GSH and thiols, since it is inhibited by GSH-depleting agents and improved by N-acetyl-L-cysteine (NAC). The capacity of these agents to reverse glutathionylation is diminished in T cell blasts infected in vitro with HIV, which is known to cause oxidative stress. Consistent with these findings, the activity of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), an enzyme known to be inhibited by glutathionylation, is inhibited in diamide-treated cells and recovers rapidly when cells are allowed to dethiolate. Further, GAPDH activity is diminished by GSH-depleting agents and augmented by NAC. Thus, reversible glutathionylation of proteins can rapidly shift the activity of a key metabolic enzyme and thereby result in dramatic, reversible changes in cellular metabolism.

Hydrogen peroxide removal and glutathione mixed disulfide formation during metabolic inhibition in mesencephalic cultures

Compromised mitochondrial energy metabolism and oxidative stress have been associated with the pathophysiology of Parkinson's disease. Our previous experiments exemplified the importance of GSH in the protection of neurons exposed to malonate, a reversible inhibitor of mitochondrial succinate dehydrogenase/complex II. This study further defines the role of oxidative stress during energy inhibition and begins to unravel the mechanisms by which GSH and other antioxidants may contribute to cell survival. Treatment of mesencephalic cultures with 10 microM buthionine sulfoximine for 24 h depleted total GSH by 60%, whereas 3 h exposure to 5 mM 3-amino-1,2,4-triazole irreversibly inactivated catalase activity by 90%. Treatment of GSH-depleted cells with malonate (40 mM) for 6, 12 or 24 h both potentiated and accelerated the time course of malonate toxicity, however, inhibition of catalase had no effect. In contrast, concomitant treatment with buthionine sulfoximine plus 3-amino-1,2,4-triazole in the presence of malonate significantly potentiated toxicity over that observed with malonate plus either inhibitor alone. Consistent with these findings, GSH depletion enhanced malonate-induced reactive oxygen species generation prior to the onset of toxicity. These findings demonstrate that early generation of reactive oxygen species during mitochondrial inhibition contributes to cell damage and that GSH serves as a first line of defense in its removal. Pre-treatment of cultures with 400 microM ascorbate protected completely against malonate toxicity (50 mM, 12 h), whereas treatment with 1 mM Trolox provided partial protection. Protein-GSH mixed disulfide formation during oxidative stress has been suggested to either protect vulnerable protein thiols or conversely to contribute to toxicity. Malonate exposure (50 mM) for 12 h resulted in a modest increase in mixed disulfide formation. However, exposure to the protective combination of ascorbate plus malonate increased membrane bound protein-GSH mixed disulfides three-fold. Mixed disulfide levels returned to baseline by 72 h of recovery indicating the reversible nature of this formation. These results demonstrate an early role for oxidative events during mitochondrial impairment and stress the importance of the glutathione system for removal of reactive oxygen species. Catalase may serve as a secondary defense as the glutathione system becomes limiting. These findings also suggest that protein-GSH mixed disulfide formation under these circumstances may play a protective role.

The role of cysteine in the regulation of blood glutathione-protein mixed disulfides in rats treated with diamide

The kinetics of GSH, GSSG, and thiol-protein mixed disulfides (RS-SP) of GSH (GS-SP) and cysteine (CYS-SP) were studied in rat blood and liver in the time range 0-120 min after treatment with 100 and 200 mg/kg i.p. of diamide. Total consumption (10 min) and regeneration (120 min) of blood GSH, matched by parallel increases and decreases in RS-SP, were observed. GSSG did not change appreciably. No dose-effect relationship was obtained with either treatment. On the contrary, in vitro treatment of blood with 0.75 mM diamide provoked the same trends of GSH and RS-SP as in vivo (e.g., reversible modifications), whereas treatment with 1.5 mM caused drops and rises in GSH and RS-SP, respectively, without any subsequent return to control values. The presence of a hematic factor responsible for RS-SP regulation is hypothesized in the in vivo experiment. Successive experiments involving in vitro pretreatment with 2 mM diamide and treatment with 0.5 mM of various thiols indicated that cysteine (CYS), but not GSH or N-acetylcysteine, rapidly restored erythrocyte GSH and RS-SP to their basal levels. No evident sign of hemolysis was observed in these experiments. These results indicate that CYS is a diffusible thiol important for RS-SP regulation. Analysis of whole blood of rats treated with 100 mg/kg i.p. diamide and the presence of two reversible peaks (about 10 times the corresponding control level) of CYS-SP and free CYS confirmed the plausible role of CYS in maintaining the reversibility of the process. Preliminary results in liver of rats treated with 100 mg/kg diamide indicated that CYS may act by metabolic cooperation between organs. We suggest that CYS may have a role in the regulation of the intracellular redox state of rat erythrocytes during oxidative stress.

Regulation of ubiquitin-conjugating enzymes by glutathione following oxidative stress

Upon oxidative stress cells show an increase in the oxidized glutathione (GSSG) to reduced glutathione (GSH) ratio with a concomitant decrease in activity of the ubiquitinylation pathway. Because most of the enzymes involved in the attachment of ubiquitin to substrate proteins contain active site sulfhydryls that might be covalently modified (thiolated) upon enhancement of GSSG levels (glutathiolation), it appeared plausible that glutathiolation might alter ubiquitinylation rates upon cellular oxidative stress. This hypothesis was explored using intact retina and retinal pigment epithelial (RPE) cell models. Exposure of intact bovine retina and RPE cells to H2O2 (0.1-1.7 micromol/mg) resulted in a dose-dependent increase in the GSSG:GSH ratio and coincident dose-dependent reductions in the levels of endogenous ubiquitin-activating enzyme (E1)-ubiquitin thiol esters and endogenous protein-ubiquitin conjugates and in the ability to form de novo retinal protein-125I-labeled ubiquitin conjugates. Oxidant-induced decrements in ubiquitin conjugates were associated with 60-80% reductions in E1 and ubiquitin-conjugating enzyme (E2) activities as measured by formation of ubiquitin thiol esters. When GSH levels in RPE cells recovered to preoxidation levels following H2O2 removal, endogenous E1 activity and protein-ubiquitin conjugates were restored. Evidence that S thiolation of E1 and E2 enzymes is the biochemical link between cellular redox state and E1/E2 activities includes: (i) 5-fold increases in levels of immunoprecipitable, dithiothreitol-labile 35S-E1 adducts in metabolically labeled, H2O2-treated, RPE cells; (ii) diminished formation of E1- and E2-125I-labeled ubiquitin thiol esters, oligomerization of E225K, and coincident reductions in protein-125I-labeled ubiquitin conjugates in supernatants from nonstressed retinas upon addition of levels of GSSG equivalent to levels measured in oxidatively stressed retinas; and (iii) partial restoration of E1 and E2 activities and levels of protein-125I-labeled ubiquitin conjugates in supernatants from H2O2-treated retinas when GSSG:GSH ratios were restored to preoxidation levels by the addition of physiological levels of GSH. These data suggest that the cellular redox status modulates protein ubiquitinylation via reversible S thiolation of E1 and E2 enzymes, presumably by glutathione.

Protein thiol modifications of human red blood cells treated with t-butyl hydroperoxide

Oxidative stress causes modification of cellular macromolecules and leads to cell damage. The objective of this study was to identify protein modifications that relate to thiol groups in human red blood cells under oxidative stress. With t-butyl hydroperoxide (t-BH) treatment, results of isoelectric focusing (IEF) analysis showed that two dithiothreitol-reversible modifications are observed, one toward the cathode and the other to the anode. Protein change toward the cathode was demonstrated to be hemoglobin oxidation, which gains a net positive charge, based on the same focus on IEF gels as hemoglobin and methemoglobin and molecular weight analysis by SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Otherwise, the change toward the anode was the result of mixed disulfide formation between GSH and protein thiols. Based on the results of molecular weight analysis and its reversion from methemoglobin, protein formed mixed disulfides with GSH were also regarded as hemoglobin. As red blood samples were treated with diamide or GSSG, in addition to the mixed disulfides observed in t-BH-treated cells, additional hemoglobin-GSH mixed disulfide appeared. But the disappearance of this diamide-induced additional mixed disulfide by treating cells with t-BH after diamide treatment suggests that the increase of negative charges from GSH are offset by ferrohemoglobin oxidation to ferrihemoglobin. Additionally, other dithiothreitol-reversible modifications of one cell membrane protein, spectrin, were also observed from the formation of high molecular weight molecules as detected by SDS-PAGE. Results indicate that protein thiols in human red blood cells are susceptible to modification under oxidative stress. IEF analysis provides a useful tool to measure methemoglobin and hemoglobin GSH mixed disulfide formation.

Loss of GSH, oxidative stress, and decrease of intracellular pH as sequential steps in viral infection

Madin-Darby canine kidney cells infected with Sendai virus rapidly lose GSH without increase in the oxidized products. The reduced tripeptide was quantitatively recovered in the culture medium of the cells. Since the GSH loss in infected cells was not blocked by methionine, a known inhibitor of hepatocyte GSH transport, a nonspecific leakage through the plasma membrane is proposed. UV-irradiated Sendai virus gave the same results, confirming that the major loss of GSH was due to membrane perturbation upon virus fusion. Consequent to the loss of the tripeptide, an intracellular pH decrease occurred, which was due to a reversible impairment of the Na+/H+ antiporter, the main system responsible for maintaining unaltered pHi in those cells. At the end of the infection period, a rise in both pHi value and GSH content was observed, with a complete recovery in the activity of the antiporter. However, a secondary set up of oxidative stress was observed after 24 h from infection, which is the time necessary for virus budding from cells. In this case, the GSH decrease was partly due to preferential incorporation of the cysteine residue in the viral proteins and partly engaged in mixed disulfides with intracellular proteins. In conclusion, under our conditions of viral infection, oxidative stress is imposed by GSH depletion, occurring in two steps and following direct virus challenge of the cell membrane without the intervention of reactive oxygen species. These results provide a rationale for the reported, and often contradictory, mutual effects of GSH and viral infection.

Differential distribution of free and bound glutathione and cyst(e)ine in human blood

The redox status of free and bound glutathione (GSH) and cyst(e)ine (Cys) is altered by oxidative stress, drugs, and disease. Most studies measure only their free forms and not the bound forms, which may have a crucial protective role. For this reason, we determined free and bound, reduced and oxidized GSH and Cys in whole blood, red cells, and plasma of human blood from healthy adults. Distinct compartments of GSH and Cys were found. In whole blood, > 99% GSH was in red cells, of which 16% was bound. GSH values were the same for red cells in whole blood or in cells isolated from the same samples. Only 0.5% of GSH was in plasma, all of which was bound. In contrast, 97% of Cys was in plasma and only 3% in red cells. This was a remarkable separation of these closely related metabolites in the same tissue. In plasma, 60% of Cys was bound. Also, strong correlations were shown of bound vs free Cys and also vs free plus bound Cys. The bound Cys was more constant and suggested that it is a metabolic reserve. Our findings demonstrate the occurrence of significant bound forms of GSH and Cys and have implications for future studies in disease and toxicology.

Paraquat toxicity and oxidative damage. Reduction by melatonin

The ability of melatonin to protect against paraquat-induced oxidative damage in rat lung, liver, and serum was examined. Changes in the levels of malondialdehyde (MDA) plus 4-hydroxyalkenals (4-HDA) and reduced and oxidized glutathione concentrations were measured. Paraquat (50 mg/kg) was injected i.p. into either Sprague-Dawley or Wistar rats with or without the co-administration of 5 mg/kg melatonin. Paraquat alone increased MDA + 4-HDA levels in serum and lungs of both rat strains, with these increases being abolished by melatonin co-treatment. Paraquat also decreased reduced glutathione levels and increased oxidized glutathione concentrations in lung and liver; these changes were negated by melatonin. The effect of melatonin on paraquat-induced mortality was also studied. Paraquat at a dose of 79 mg/kg was lethal for 50% of animals within 24 hr; when administered together with melatonin, the LD50 for paraquat increased to 251 mg/kg.

Different mechanisms of formation of glutathione-protein mixed disulfides of diamide and tert-butyl hydroperoxide in rat blood

The mechanisms of glutathione-protein mixed disulfide (GSSP) formation caused by diamide and tert-butyl hydroperoxide were studied in rat blood after in vitro treatment in the 0.3-4 mM dose range. tert-Butyl hydroperoxide formed GSSP, via GSSG, according to the reaction, GSSG + PSH --> GSSP + GSH, whereas diamide reacted first with protein SH groups, giving PS-diamide adducts and then, after reaction with GSH, GSSP. Moreover, after diamide treatment, GSSP patterns were characterized by a much slower or irreversible dose-related return to basal levels in comparison with those observed with tert-butyl hydroperoxide, always reversible. Experiments with purified hemoglobin revealed the existence of a large fraction of protein SH groups which formed GSSP and had a higher reactivity than GSH. Experiments on glucose consumption and role of various erythrocyte enzymes, carried out to explain the inertness of GSSP to reduction after treatment of blood with diamide, were substantially negative. Other tests carried out to confirm the efficiency of the enzymatic machinery of blood samples successively treated with diamide and tert-butyl hydroperoxide, indicated that GSSP performed by diamide was difficult to reduce, whereas those generated by tert-butyl hydroperoxide were reversible as normal. Our results suggest that a fraction of GSSP generated by diamide is different and less susceptible to reduction than that obtained with tert-butyl hydroperoxide.

The detection of S-glutathionation of hepatic carbonic anhydrase III in rats treated with paraquat or diquat

Protein S-glutathionation has been demonstrated to be one of the cellular responses under oxidative stress and may be involved in many cellular metabolisms. In this study, the effect of redox cycling bipyridylium compounds, paraquat and diquat, on this protein modification was investigated. Male Sprague-Dawley rats were administered i.p. either paraquat at 20 or 40 mg/kg body wt. or diquat at 85 or 170 mg/kg body wt., respectively. The liver was examined at different time points for taking the measurement of the S-glutathionation of carbonic anhydrase III (CA III), thiobarbituric acid-reactive substances (TBARS), vitamin E depletion, glutathione (GSH) and glutathione disulfide (GSSG) contents. The extent of S-glutathionation of CA III was chosen as a marker and was determined by a method combining isoelectric focusing analysis with immunoblotting. Those results indicated that paraquat and diquat significantly increased the generation of TBARS and showed a time-dependent response. The significant effect on vitamin E depletion was only obtained in rats treated with a high dose of diquat for 2 h. Hepatic cellular GSSG contents did not increase but tended to decrease all of the treatments. Although oxidative damage was actually generated in liver, based on the increase of TBARS generation and vitamin E depletion, no increase of CA III S-glutathionation was observed. We propose that the reason for this observation under this circumstance is probably due to the reversible characteristic of CA III S-glutathionation, which has been demonstrated in our previous study (Chai et al., 1991) Arch. Biochem. Biophys. 384, 270-278) and named as dethiolation.

Skin burn injury and oxidative stress in liver and lung tissues of rabbit models

The effects of burn injury (30% of total body surface area) on the levels of oxidized and reduced glutathione, malondialdehyde, and on the activities of certain glutathione-dependent enzymes, have been determined in tissues of rabbit models. Thus, the malondialdehyde, glutathione (GSH), glutathione disulfide (GSSG) concentrations and the specific activities of glutathione peroxidase, glutathione S-transferase, and glutathione reductase were measured in liver and lung of 24-h burn rabbit models and compared to the corresponding values in 24-h sham burn (medicated, anesthetic/analgesic) rabbit models. It was found that the concentrations of malondialdehyde in liver and lung of burn models were increased by 17% and 29% respectively. Glutathione concentrations were decreased by 29% in liver and 13% in lung, and glutathione disulfide concentrations were increased by 35% in liver and 33% in lung, in burn versus sham burn models. It was also found that the specific activities of glutathione peroxidase decreased significantly, resultant to burn injury, by an average of 35% and 27% in liver and lung, respectively. Burn injury also decreased glutathione S-transferase specific activities by 14% in liver and 23% in lung tissues. In contrast, glutathione reductase specific activity was increased in liver tissues (22%), but was decreased (19%), as with the other enzymes studied, in lung tissues of burn models. Control model studies (no medication, no sham burn) show that these effects of burn injury are additional to effects elicited by medication associated with sham burn models. The data of this study are indicative of a major oxidative stress in liver and lung tissues due to burn injury at a remote site.

Mitochondrial bioenergetics is affected by the herbicide paraquat

The potential toxicity of the herbicide paraquat (1,1-dimethyl-4,4'-bipyridylium dichloride) was tested in bioenergetic functions of isolated rat liver mitochondria. Paraquat increases the rate of State 4 respiration, doubling at 10 mM, indicating uncoupling effects. Additionally, State 3 respiration is depressed by about 15%, at 10 mM paraquat, whereas uncoupled respiration in the presence of CCCP is depressed by about 30%. Furthermore, paraquat partially inhibits the ATPase activity through a direct effect on this enzyme complex. However, at high concentrations (5-10 mM), the ATPase activity is stimulated, probably as consequence of the described uncoupling effect. Depression of respiratory activity is mediated through partial inhibitions of mitochondrial complexes III and IV. Paraquat depresses delta psi as a function of herbicide concentration. In addition, the depolarization induced by ADP is decreased and repolarization is biphasic suggesting a double effect. Repolarization resumes at a level consistently higher than the initial level before ADP addition, for paraquat concentrations up to 10 mM. This particular effect is clear at 1 mM paraquat and tends to fade out with increasing concentrations of the herbicide.

Oxidative stress in isolated rat hepatocytes during naproxen metabolism

Naproxen, a non-steroidal anti-inflammatory drug, induced lipid peroxidation in isolated hepatocytes of rats. The viability of the hepatocytes decreased upon lipid peroxidation, and this effect was accompanied by the formation of high molecular weight protein aggregates in the hepatocytes. Protein aggregation occurred slowly compared with the formation of thiobarbituric acid reactive substances (TBARS). The increase of TBARS was strongly correlated with the decrease of intracellular glutathione. Chemiluminescence was produced from the hepatocyte suspension during naproxen metabolism, and was correlated with the formation of TBARS. These results indicate that lipid peroxidation in the hepatocytes was provoked by reactive oxygens produced in the process of naproxen metabolism.

The time-course of mixed disulfide formation between GSH and proteins in rat blood after oxidative stress with tert-butyl hydroperoxide

Variations in time of GSH, GSSG and glutathione-protein mixed disulfides (GSSP) were studied in rat blood in vitro experiments of oxidative stress with tert-butyl hydroperoxide (t-BOOH, dose range 0.3-2 mM; time range 15 sec-60 min). The aim was to elucidate the potential for GSSG reduction of protein-bound SH groups (PSH). GSSP was estimated by two methods, indirectly from GSHt (GSH + 2 GSSG) variations and directly from precipitated and washed proteins. After treatment with t-BOOH, GSH and GSSG concentrations showed an immediate (15-30 sec) drop and a peak respectively and returned to control levels (time zero values) between 30 and 60 min. A t-BOOH dose-dependent minimum of GSHt and a corresponding GSSP maximum were obtained within 1-6 min and subsequently returned to control values. Basal GSH, GSSG and GSSP levels were similar in aged and fresh blood. In contrast, after treatment with 1 mM t-BOOH substantial differences in kinetic patterns were observed: for instance GSSP concentrations were higher in aged than in fresh blood with no return to the initial values. The pretreatment of aged blood with 10 mM glucose decreased GSSP formation and produced a reversible pattern similar to that observed in fresh blood. The role of glucose in regulating GSSP generation is discussed.

Metabolic alterations in hepatocytes promoted by the herbicides paraquat, dinoseb and 2,4-D

The cytotoxic effects of the herbicides paraquat (1,1'-dimethyl-4,4'-bipyridylium dichloride), dinoseb (2-sec-butyl-4,6-dinitrophenol) and 2,4-D (2,4-dichlorophenoxyacetic acid) on freshly isolated rat hepatocytes were investigated. Paraquat and 2,4-D (1-10 mM) caused a dose and time dependent cell death accompanied by depletion of intracellular glutathione (GSH) and mirroring increase of oxidized glutathione (GSSG). Dinoseb, the most effective cytotoxic compound under study (used in concentrations 1000 fold lower than paraquat and 2,4-D), exhibited moderate effects upon the level of GSH and GSSG. These limited effects are at variance with significant effects upon the adenine and pyridine nucleotide contents. ATP and NADH levels are rapidly depleted by herbicide metabolism. This depletion is observed in the millimolar range for paraquat and 2,4-D and in the micromolar range for dinoseb. 2,4-D completely depletes cellular ATP, with subsequent cell death, as detected by LDH leakage. Paraquat rapidly depletes NADH, according to the redox cycling of the herbicide metabolism. The most effective compound is dinoseb since it exerts similar effects as described for paraquat and 2,4-D at concentrations 1000 fold lower. Simultaneously with NADH and ATP depletion, the levels of ADP, AMP and NAD+ increase in hepatocytes incubated in the presence of the herbicides. In contrast to NADH, the time course and extent of ATP depletion and fall in energy charge correlate reasonably with the time of onset and rate of cell death. It is concluded that the herbicides, paraquat and 2,4-D are hepatotoxic and initiate the process of cell death by decreasing cellular GSH.(ABSTRACT TRUNCATED AT 250 WORDS)

Transduction of reducing power across the plasma membrane by reduced glutathione. A 1H-NMR spin-echo study of intact human erythrocytes

The NMR signal of reduced glutathione (GSH) was monitored in intact human erythrocytes by the 1H spin-echo Carr-Purcell-Meiboom-Gill pulse sequence. Addition of GSH, which was unable to cross the erythrocyte membrane, produced an approximate twofold increase of the GSH signal in glucose-depleted cells. Addition of oxidised glutathione (GSSG), did not affect the signal, and addition of GSH to hemolysates gave a much smaller increase. Reduction of internal GSSG by NADPH-dependent enzymes was excluded by experiments with glucose-supplied or glucose-6-phosphate dehydrogenase deficient cells. Involvement of external thiol groups of the erythrocyte membrane was shown by the lack of effect in cells treated with an impermeable thiol-blocking compound. Involvement of spectrin was indicated by the proportional loss of the effect in erythrocytes with variable genetic deficiency of spectrin. Protein-glutathione mixed disulfides appeared to be the source of the NMR response since an increase of their content, by diamide treatment or aging procedures, produced a higher GSH signal, while their reduction by permeable reductants gave the opposite effect. It is concluded that GSH can transduce its reducing power by a thiol/disulfide exchange mechanism that sequentially involves sulfur-rich proteins spanning across the erythrocyte membrane.

Abnormal hepatic mitochondrial respiration and cytochrome C oxidase activity in rats with long-term copper overload

BACKGROUND

Dietary copper overload in the rat is associated with morphological abnormalities and lipid peroxidation of hepatic mitochondria. This study was designed to determine if copper hepatotoxicity was associated with functional alterations in mitochondrial respiration in conjunction with lipid peroxidation.

METHODS

Weanling male rats were pair-fed for 8 weeks on diets containing normal or high levels of copper in combination with sufficient vitamin E. Serum and liver samples were obtained, and hepatic mitochondria were isolated by differential centrifugation.

RESULTS

Oxidant injury (decreased levels of hepatic glutathione and alpha tocopherol and increased levels of mitochondrial thiobarbituric acid-reacting substances) was present in the copper-overloaded rats. Serum aminotransferase levels correlated with concentrations of mitochondrial copper and thiobarbituric acid-reacting substances. Copper overload caused a decrease in state 3 respiration and the respiratory control ratio in hepatic mitochondria when several electron donors were used. Analysis of the oxidoreductase activities of the four mitochondrial electron transport protein complexes showed that complex IV (cytochrome C oxidase) activity was reduced by 60% in copper overload.

CONCLUSIONS

Functional abnormalities of mitochondria accompany lipid peroxidation and the morphological alterations caused by copper overload, supporting the hypothesis that the mitochondrion is one of the major intracellular targets in copper hepatotoxicity.

Protein-specific S-thiolation in human endothelial cells during oxidative stress

Confluent human umbilical vein endothelial cells were treated with diamide, t-butyl hydroperoxide (t-BH) or the hydrogen peroxide generating system glucose/glucose oxidase and the effects on glutathione oxidation and protein S-thiolation were examined. In the presence of all three oxidants glutathione was rapidly oxidized to a similar extent and S-thiolation of a limited number of proteins occurred. Diamide caused considerable S-thiolation of proteins with molecular masses of 44, 34, 24 and 14 kDa, of which the protein with molecular mass of 44 kDa was most extensively modified. t-BH caused extensive modification of proteins with molecular masses of 24 and 14 kDa whilst hydrogen peroxide caused S-thiolation of proteins of 39, 24 and 14 kDa. This study shows that S-thiolation of proteins is an important metabolic response to oxidant insult in human endothelial cells and that the specificity of the response depends on the chemical nature of the oxidant.

A comparison of proteins S-thiolated by glutathione to those arylated by acetaminophen

This study was designed to evaluate whether the same proteins that irreversibly bind reactive electrophiles of drugs also bind glutathione (GSH) under oxidative conditions. Specifically, proteins that can be arylated by acetaminophen were compared to those that form glutathione-protein mixed disulfides (PSSG) after incubation with diamide. Data are presented which suggest that both GSH and acetaminophen bind to a subset of N-ethylmaleimide (NEM)-reactive protein thiols. To evaluate the pattern of proteins that bind GSH, PSSGs were formed in vitro by incubating cytosolic proteins with GSH and diamide. A sensitive procedure was developed in which PSSGs were first reduced with 0.1 mM dithiothreitol (DTT), and the newly exposed protein thiols were labeled with either [3H]NEM (for quantitative analysis) or with fluorescein-5-maleimide (for visual detection). Acetaminophen binding was achieved by incubating cytosolic proteins in vitro with the reactive acetaminophen metabolite, N-acetyl-p-benzoquinoneimine (NAPQI). Proteins from both assays were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred to nitrocellulose for Western blot analysis. Acetaminophen binding was detected by immunoblotting with an affinity-purified antibody against acetaminophen, and PSSGs were visualized using anti-fluorescein antibodies. In both instances, binding to proteins was observed to be selective. A comparison of the proteins modified by GSH binding with those that bind acetaminophen indicates that the major cytosolic acetaminophen-binding protein of 58 kDa may also be modified by glutathiolation under oxidative conditions.

Lipid peroxidation, protein thiol oxidation and DNA damage in hydrogen peroxide-induced injury to endothelial cells: role of activation of poly(ADP-ribose)polymerase

These experiments are a continuation of work investigating the mechanism of oxidant-induced damage to cultured bovine pulmonary artery endothelial cells (BPEC). Earlier experiments implicated DNA strand breakage and activation of poly(ADP-ribose)polymerase as critical steps in cell injury. In the current report, a better defined model of oxidant stress was used to investigate DNA damage, lipid peroxidation and protein thiol oxidation in BPEC following oxidant stress. The dose and time response of LDH release following exposure to H2O2 were established. H2O2 was metabolized rapidly by BPEC (t1/2 = 20 min). Hydrogen peroxide-induced increases in thiobarbituric acid (TBA) reactive material were prevented by pretreatment with the lipophilic antioxidant diphenylphenylinediamine (DPPD). However, DPPD did not decrease LDH release. Conversely, pretreatment with 5 mM 3-aminobenzamide (3AB), a competitive inhibitor of poly(ADP-ribose)polymerase, prevented LDH release from BPEC following H2O2 treatment. Dithiothreitol (DTT), a sulfhydryl reducing agent, also prevented LDH release. The effects of 3AB and DTT on H2O2-induced changes in DNA strand breaks and NAD+ and ATP levels were investigated as well as the effect of H2O2 on soluble and protein-bound thiols. As DPPD inhibited peroxidation without preventing LDH release, lipid peroxidation does not appear to play a role in the loss of BPEC viability in response to oxidant stress. As protein thiol oxidation was not caused by H2O2, it does not appear to play a causative role in cytotoxicity, although DTT may protect via maintenance of soluble thiols. H2O2 induces DNA strand breaks, which activate poly(ADP-ribose)polymerase, leading to depletion of cellular NAD+ and ATP and loss in cell viability. This supports earlier studies implicating the activation of poly(ADP-ribose)polymerase in oxidant injury to cultured endothelial cells.