Some properties of mitochondrial glutathione

Interactions between zinc and NRF2 in vascular redox signalling

Recent evidence highlights the importance of trace metal micronutrients such as zinc (Zn) in coronary and vascular diseases. Zn2+ plays a signalling role in modulating endothelial nitric oxide synthase and protects the endothelium against oxidative stress by up-regulation of glutathione synthesis. Excessive accumulation of Zn2+ in endothelial cells leads to apoptotic cell death resulting from dysregulation of glutathione and mitochondrial ATP synthesis, whereas zinc deficiency induces an inflammatory phenotype, associated with increased monocyte adhesion. Nuclear factor-E2-related factor 2 (NRF2) is a transcription factor known to target hundreds of different genes. Activation of NRF2 affects redox metabolism, autophagy, cell proliferation, remodelling of the extracellular matrix and wound healing. As a redox-inert metal ion, Zn has emerged as a biomarker in diagnosis and as a therapeutic approach for oxidative-related diseases due to its close link to NRF2 signalling. In non-vascular cell types, Zn has been shown to modify conformations of the NRF2 negative regulators Kelch-like ECH-associated Protein 1 (KEAP1) and glycogen synthase kinase 3β (GSK3β) and to promote degradation of BACH1, a transcriptional suppressor of select NRF2 genes. Zn can affect phosphorylation signalling, including mitogen-activated protein kinases (MAPK), phosphoinositide 3-kinases and protein kinase C, which facilitate NRF2 phosphorylation and nuclear translocation. Notably, several NRF2-targeted proteins have been suggested to modify cellular Zn concentration via Zn exporters (ZnTs) and importers (ZIPs) and the Zn buffering protein metallothionein. This review summarises the cross-talk between reactive oxygen species, Zn and NRF2 in antioxidant responses of vascular cells against oxidative stress and hypoxia/reoxygenation.

Development of a mitochondria targetable ratiometric time-gated luminescence probe for biothiols based on lanthanide complexes

A mitochondria targetable ratiometric luminescence probe based on a mixture of Eu³⁺ and Tb³⁺ complexes has been developed for the specific recognition and ratiometric time-gated luminescence detection of biothiols in aqueous and living samples.

A mitochondria-targeted turn-on fluorescent probe for the detection of glutathione in living cells

A novel turn-on red fluorescent BODIPY-based probe (Probe 1) for the detection of glutathione was developed. Such a probe carries a para-dinitrophenoxy benzyl pyridinium moiety at the meso position of a BODIPY dye as self-immolative linker. Probe 1 responds selectively to glutathione with the detection limit of 109nM over other amino acids, common metal ions, reactive oxygen species, reactive nitrogen species, and reactive sulfur species. A novel electrostatic interaction to modulate the SNAr attack of glutathione was believed to play significant role for the observed selective response to glutathione. The cleavage of dinitrophenyl ether by glutathione leads to the production of para-hydroxybenzyl moiety which is able to self-immolate through an intramolecular 1,4-elimination reaction to release the fluorescent BODIPY dye. The low toxic probe has been successfully used to detect mitochondrial glutathione in living cells.

Tunable heptamethine-azo dye conjugate as an NIR fluorescent probe for the selective detection of mitochondrial glutathione over cysteine and homocysteine

Although a lot of mitochondria-targeting biothiol probes have been developed and applied to cellular imaging through thiol-induced disulfide cleavage or Michael addition reactions, relatively few probes assess mitochondrial GSH with high selectivity over Cys and Hcy and with NIR fluorescence capable of noninvasive imaging in biological samples. In order to monitor mitochondrial GSH with low background autofluorescence, we designed a heptamethine-azo conjugate as an NIR fluorescent probe by introducing a tunable lipophilic cation unit as the biomarker for mitochondria and a nitroazo group as the GSH-selective reaction site as well as the fluorescence quencher. The probe exhibited a dramatic off-on NIR fluorescence response toward GSH with high selectivity over other amino acids including Cys and Hcy. Further application to cellular imaging indicated that the probe was highly responsive to the changes of mitochondrial GSH in cells.

Mitochondrial thiols in the regulation of cell death pathways

SIGNIFICANCE

Regulation of mitochondrial H(2)O(2) homeostasis and its involvement in the regulation of redox-sensitive signaling and transcriptional pathways is the consequence of the concerted activities of the mitochondrial energy- and redox systems.

RECENT ADVANCES

The energy component of this mitochondrial energy-redox axis entails the formation of reducing equivalents and their flow through the respiratory chain with the consequent electron leak to generate [Formula: see text] and H(2)O(2). The mitochondrial redox component entails the thiol-based antioxidant system, largely accounted for by glutathione- and thioredoxin-based systems that support the activities of glutathione peroxidases, peroxiredoxins, and methionine sulfoxide reductase. The ultimate reductant for these systems is NADPH: mitochondrial sources of NADPH are the nicotinamide nucleotide transhydrogenase, isocitrate dehydrogenase-2, and malic enzyme. NADPH also supports the glutaredoxin activity that regulates the extent of S-glutathionylation of mitochondrial proteins in response to altered redox status.

CRITICAL ISSUES

The integrated network of these mitochondrial thiols constitute a regulatory device involved in the maintenance of steady-state levels of H(2)O(2), mitochondrial and cellular redox and metabolic homeostasis, as well as the modulation of cytosolic redox-sensitive signaling; disturbances of this regulatory device affects transcription, growth, and ultimately influences cell survival/death.

FUTURE DIRECTIONS

The modulation of key mitochondrial thiol proteins, which participate in redox signaling, maintenance of the bioenergetic machinery, oxidative stress responses, and cell death programming, provides a pivotal direction in developing new therapies towards the prevention and treatment of several diseases.

Mitochondrial compartmentalization of redox processes

Knowledge of location and intracellular subcompartmentalization is essential for the understanding of redox processes, because oxidants, owing to their reactive nature, must be generated close to the molecules modified in both signaling and damaging processes. Here we discuss known redox characteristics of various mitochondrial microenvironments. Points covered are the locations of mitochondrial oxidant generation, characteristics of antioxidant systems in various mitochondrial compartments, and diffusion characteristics of oxidants in mitochondria. We also review techniques used to measure redox state in mitochondrial subcompartments, antioxidants targeted to mitochondrial subcompartments, and methodological concerns that must be addressed when using these tools.

Mitochondrial glutathione in toxicology and disease of the kidneys

The tripeptide glutathione (GSH), comprised of the amino acids l-cysteine, glycine, and l-glutamate, is found in all cells of aerobic organisms and plays numerous, critical roles as an antioxidant and nucleophile in regulating cellular homeostasis and drug metabolism. GSH is synthesized exclusively in the cytoplasm of most cells by two ATP-dependent reactions. Despite this compartmentation, GSH is found in other subcellular compartments, including mitochondria. As the GSH molecule has a net negative charge at physiological pH, it cannot cross cellular membranes by diffusion. Rather, GSH is a substrate for a variety of anion and amino acid transporters. Two organic anion carriers in the inner membrane of renal mitochondria, the dicarboxylate carrier (DIC; Slc25a10) and the 2-oxoglutarate carrier (OGC; Slc25a11), are responsible for most of the transport of GSH from cytoplasm into mitochondrial matrix. Genetic manipulation of DIC and/or OGC expression in renal cell lines demonstrated the ability to produce sustained increases in mitochondrial GSH content, which then protected these cells from cytotoxicity due to several oxidants and mitochondrial toxicants. Several diseases and pathological states are associated with mitochondrial dysfunction and oxidative stress, suggesting that the mitochondrial GSH pool may be a therapeutic target. One such disease that is of particular concern for public health is diabetic nephropathy. Another chronic, pathological state that is associated with bioenergetic and redox changes is compensatory renal hypertrophy that results from reductions in functional renal mass. This review summarizes pathways of mitochondrial GSH transport and discusses studies on its manipulation in toxicological and pathological states.

Redox signaling and protein phosphorylation in mitochondria: progress and prospects

As we learn more about the factors that govern cardiac mitochondrial bioenergetics, fission and fusion, as well as the triggers of apoptotic and necrotic cell death, there is growing appreciation that these dynamic processes are finely-tuned by equally dynamic post-translational modification of proteins in and around the mitochondrion. In this minireview, we discuss the evidence that S-nitrosylation, glutathionylation and phosphorylation of mitochondrial proteins have important bioenergetic consequences. A full accounting of these targets, and the functional impact of their modifications, will be necessary to determine the extent to which these processes underlie ischemia/reperfusion injury, cardioprotection by pre/post-conditioning, and the pathogenesis of heart failure.

Mitochondrial glutathione transport: physiological, pathological and toxicological implications

Although most cellular glutathione (GSH) is in the cytoplasm, a distinctly regulated pool is present in mitochondria. Inasmuch as GSH synthesis is primarily restricted to the cytoplasm, the mitochondrial pool must derive from transport of cytoplasmic GSH across the mitochondrial inner membrane. Early studies in liver mitochondria primarily focused on the relationship between GSH status and membrane permeability and energetics. Because GSH is an anion at physiological pH, this suggested that some of the organic anion carriers present in the inner membrane could function in GSH transport. Indeed, studies by Lash and colleagues in isolated mitochondria from rat kidney showed that most of the transport (>80%) in that tissue could be accounted for by function of the dicarboxylate carrier (DIC, Slc25a10) and the oxoglutarate carrier (OGC, Slc25a11), which mediate electroneutral exchange of dicarboxylates for inorganic phosphate and 2-oxoglutarate for other dicarboxylates, respectively. The identity and function of specific carrier proteins in other tissues is less certain, although the OGC is expressed in heart, liver, and brain and the DIC is expressed in liver and kidney. An additional carrier that transports 2-oxoglutarate, the oxodicarboxylate or oxoadipate carrier (ODC; Slc25a21), has been described in rat and human liver and its expression has a wide tissue distribution, although its potential function in GSH transport has not been investigated. Overexpression of the cDNA for the DIC and OGC in a renal proximal tubule-derived cell line, NRK-52E cells, showed that enhanced carrier expression and activity protects against oxidative stress and chemically induced apoptosis. This has implications for development of novel therapeutic approaches for treatment of human diseases and pathological states. Several conditions, such as alcoholic liver disease, cirrhosis or other chronic biliary obstructive diseases, and diabetic nephropathy, are associated with depletion or oxidation of the mitochondrial GSH pool in liver or kidney.

Nuclear and mitochondrial compartmentation of oxidative stress and redox signaling

New methods to measure thiol oxidation show that redox compartmentation functions as a mechanism for specificity in redox signaling and oxidative stress. Redox Western analysis and redox-sensitive green fluorescent proteins provide means to quantify thiol/disulfide redox changes in specific subcellular compartments. Analyses using these techniques show that the relative redox states from most reducing to most oxidizing are mitochondria > nuclei > cytoplasm > endoplasmic reticulum > extracellular space. Mitochondrial thiols are an important target of oxidant-induced apoptosis and necrosis and are especially vulnerable to oxidation because of the relatively alkaline pH. Maintenance of a relatively reduced nuclear redox state is critical for transcription factor binding in transcriptional activation in response to oxidative stress. The new methods are applicable to a broad range of experimental systems and their use will provide improved understanding of the pharmacologic and toxicologic actions of drugs and toxicants.

Disulphide formation on mitochondrial protein thiols

A large number of proteins contain free thiols that can be modified by the formation of internal disulphides or by mixed disulphides with low-molecular-mass thiols. The majority of these latter modifications result from the interaction of protein thiols with the endogenous glutathione pool. Protein glutathionylation and disulphide formation are of significance both for defence against oxidative damage and in redox signalling. As mitochondria are central to both oxidative damage and redox signalling within the cell, these modifications of mitochondrial proteins are of particular importance. In the present study, we review the mechanisms and physiological significance of these processes.

Glutathionylation of mitochondrial proteins

Many proteins contain free thiols that can be modified by the reversible formation of mixed disulfides with low-molecular-weight thiols through a process called S-thiolation. As the majority of these modifications result from the interaction of protein thiols with the endogenous glutathione pool, protein glutathionylation is the predominant alteration. Protein glutathionylation is of significance both for defense against oxidative damage and in redox signaling. As mitochondria are at the heart of both oxidative damage and redox signaling within the cell, the glutathionylation of mitochondrial proteins is of particular importance. Here we review the mechanisms and physiological significance of the glutathionylation of mitochondrial thiol proteins.

Polyamine depletion switches the form of 2-deoxy-D-ribose-induced cell death from apoptosis to necrosis in HL-60 cells

Our previous studies demonstrated that intracellular polyamine depletion blocked HL-60 cell apoptosis triggered by exposure to 2-deoxy-d-ribose (dRib). Here, we have characterized the intracellular events underlying the apoptotic effects of dRib and the involvement of polyamines in these effects. Treatment of HL-60 cells with dRib induces loss of mitochondrial transmembrane potential, radical oxygen species production, intracellular glutathione depletion and translocation of Bax from cytosol to membranes. These effects are followed by cell death. However, the mode of cell death caused by dRib depends on intracellular levels of polyamines. d-Rib-treated cells with normal polyamine levels, progressing through the G(1) into the S and G(2)/M phases, undergo apoptosis, while in polyamine-depleted cells, being blocked at the G(1) phase, cell death mechanisms are switched to necrosis. The present study points to a relationship between the cell cycle distribution and the mode of cell death, and suggests that the level of intracellular spermidine, essential to cell cycle progression, may determine whether a cell dies by apoptosis or necrosis in response to a death stimulus.

Mitochondrial Glutathione and Chemically Induced Stress Including Ethanol

The mechanisms of alcohol toxicity as related to mitochondrial dysfunction and the glutathione-dependent protective systems are reviewed. The pathophysiology of ethanol-induced liver damage is defined in terms of an early phase and a late phase. CYP2E1 dependent toxicity appears closely related to oxidative stress injury with possible roles of peroxynitrite, TNFalpha, protein adducts, and enhanced protein expression. Modulation of mitochondrial glutathione affects mitochondrial function and cell survival with superoxide and hydrogen peroxide generation being crucial to mitochondrial membrane permeability transition and apoptosis.

Mitochondrial theory of aging: importance to explain why females live longer than males

Females live longer than males in many species, including humans. This can be explained on the basis of the mitochondrial theory of aging. Mitochondria from females produce significantly less hydrogen peroxide than those from males and have higher levels of mitochondrial reduced glutathione, manganese superoxide dismutase, and glutathione peroxidase than males. Oxidative damage to mitochondrial DNA is also fourfold higher in males than in females. These differences may be explained by estrogens. Ovariectomy abolishes the gender differences between males and females and estrogen replacement rescues the ovariectomy effect. The challenge for the future is to find molecules that have the beneficial effects of estradiol, but without its feminizing effects. Phytoestrogens or phytoestrogen-related molecules may be good candidates to meet this challenge.

Interactions of mitochondrial thiols with nitric oxide

The interaction of nitric oxide (NO) with mitochondria is of pathological significance and is also a potential mechanism for the regulation of mitochondrial function. Some of the ways in which NO may affect mitochondria are by reacting with low-molecular-weight thiols such as glutathione and with protein thiols. However, the detailed mechanisms and the consequences of these interactions for mitochondria are uncertain. Here we review mitochondrial thiol metabolism, outline how NO and its metabolites interact with thiols, and discuss the implications of these reactions for mitochondrial and cell function.

In vivo reversal of glutathione deficiency and susceptibility to in vivo dexamethasone-induced apoptosis by N-acetylcysteine and L-2-oxothiazolidine-4-carboxylic acid, but not ascorbic acid, in thymocytes from gamma-glutamyltranspeptidase-deficient knockout mice

Cellular glutathione is released during apoptosis and may play a role in the regulation of the mitochondrial permeability transition pore. The question of whether only cytosolic glutathione is important in apoptosis, or whether mitochondrial glutathione also plays a role, was investigated using gamma-glutamyltranspeptidase-deficient knockout mice. Thymocytes from these mice were found to have both glutathione pools diminished and they were more susceptible to dexamethasone (DEX)-induced apoptosis. Supplementation with N-acetylcysteine (NAC) and L-2-oxothiazolidine-4-carboxylic acid replenished both glutathione pools and provided protection from apoptosis. Ascorbate supplementation was beneficial to the mitochondrial glutathione pool, but apoptosis was not prevented. NAC supplementation caused an increase in reactive oxygen species formation and cardiolipin oxidation, but had no adverse affect on the amount of apoptotic cells. Our results suggest that the glutathione status is an important factor in apoptosis and indirect evidence indicates that the cytosolic pool of glutathione may be important in DEX-induced apoptosis, with mitochondrial events being secondary, and may reflect the execution phase.

Alterations in glutathione and amino acid concentrations after hypoxia-ischemia in the immature rat brain

Hypoxic-ischemic brain injury involves an increased formation of reactive oxygen species. Key factors in the cellular protection against such agents are the GSH-associated reactions. In the present study we examined alterations in total glutathione and GSSG concentrations in mitochondria-enriched fractions and tissue homogenates from the cerebral cortex of 7-day-old rats at 0, 1, 3, 8, 14, 24 and 72 h after hypoxia-ischemia. The concentration of total glutathione was transiently decreased immediately after hypoxia-ischemia in the mitochondrial fraction, but not in the tissue, recovered, and then decreased both in mitochondrial fraction and homogenate after 14 h, reaching a minimum at 24 h after hypoxia-ischemia. The level of GSSG was approximately 4% of total glutathione and increased selectively in the mitochondrial fraction immediately after hypoxia-ischemia. The decrease in glutathione may be important in the development of cell death via impaired free radical inactivation and/or redox related changes. The effects of hypoxia-ischemia on the concentrations of selected amino acids varied. The levels of phosphoethanolamine, an amine previously reported to be released in ischemia, mirrored the changes in glutathione. GABA concentrations initially increased (0-3 h) followed by a decrease at 72 h. Glutamine levels increased, whereas glutamate and aspartate were unchanged up to 24 h after the insult. The results on total glutathione and GSSG are discussed in relation to changes in mitochondrial respiration and microtubule associated protein-2 (MAP2) which are reported on in accompanying paper [64].

gamma-glutamyltranspeptidase-deficient knockout mice as a model to study the relationship between glutathione status, mitochondrial function, and cellular function

gamma-Glutamyltranspeptidase (GGT)-deficient mice (GGT(-/-)) display chronic glutathione (GSH) deficiency, growth retardation, and die at a young age (<20 weeks). Using livers from these mice, we investigated the relationship between GSH content, especially mitochondrial, and mitochondrial and cellular function. We found that the GSH content of isolated liver mitochondria was diminished by >/=50% in GGT(-/-) mice when compared with wild-type mice. Respiratory control ratios (RCRs) of GGT(-/-) mice liver mitochondria were </=60% those of wild-type mice primarily as a result of impaired state 3 respiration. Mitochondrial adenine nucleotide content was decreased by >/=40% in mitochondria obtained from GGT(-/-) mice. We observed a strong correlation between mitochondrial GSH content and RCRs. Even moderate decreases (<50%) correlated with adverse effects with respect to respiration. Electron microscopy revealed that livers from GGT(-/-) knockout mice were deprived of fat and glycogen, and swollen mitochondria were observed in animals that were severely deprived of GSH. Thus, GGT(-/-) mice exhibit a loss of GSH homeostasis and impaired oxidative phosphorylation, which may be related to the rate of adenosine triphosphate (ATP) formation and subsequently leads to progressive liver injury, which characterizes the diseased state. We also found that supplementation of GGT(-/-) mice with N-acetylcysteine (NAC) partially restored liver GSH, but fully restored mitochondrial GSH and respiratory function. Electron microscopy revealed that the livers of NAC-supplemented GGT(-/-) mice contained fat and glycogen; however, slightly enlarged mitochondria were found in some livers. NAC supplementation did not have any beneficial effect on the parameters examined in wild-type mice.

The regulation of reactive oxygen species production during programmed cell death

Reactive oxygen species (ROS) are thought to be involved in many forms of programmed cell death. The role of ROS in cell death caused by oxidative glutamate toxicity was studied in an immortalized mouse hippocampal cell line (HT22). The causal relationship between ROS production and glutathione (GSH) levels, gene expression, caspase activity, and cytosolic Ca2+ concentration was examined. An initial 5-10-fold increase in ROS after glutamate addition is temporally correlated with GSH depletion. This early increase is followed by an explosive burst of ROS production to 200-400-fold above control values. The source of this burst is the mitochondrial electron transport chain, while only 5-10% of the maximum ROS production is caused by GSH depletion. Macromolecular synthesis inhibitors as well as Ac-YVAD-cmk, an interleukin 1beta-converting enzyme protease inhibitor, block the late burst of ROS production and protect HT22 cells from glutamate toxicity when added early in the death program. Inhibition of intracellular Ca2+ cycling and the influx of extracellular Ca2+ also blocks maximum ROS production and protects the cells. The conclusion is that GSH depletion is not sufficient to cause the maximal mitochondrial ROS production, and that there is an early requirement for protease activation, changes in gene expression, and a late requirement for Ca2+ mobilization.

The reduction of glutathione disulfide produced by t-butyl hydroperoxide in respiring mitochondria

Factors affecting the reduction of GSSG by rat liver mitochondria after a t-butyl hydroperoxide-induced (t-BOOH) oxidative stress were studied. The amounts of ADP and mitochondrial protein were adjusted to consume less than 50% of the available oxygen during the 8-min experimental period. A 4-min treatment of mitochondria with 24 nmol t-BOOH/mg protein (60 microM) oxidized 91% of total glutathione. In the presence of glutamate/malate, succinate or ascorbate/N,N,N',N'-tetramethyl-p- phenylenediamine (TMPD) (state 4 respiration), 84, 84, and 28% of the GSSG formed during t-BOOH treatment was reduced after 4 min, respectively. A similar extent of reduction was seen during state 3 respiration (1.5 mM ADP) with glutamate/malate, but no reduction occurred during state 3 respiration with either succinate or ascorbate/TMPD. The succinate-supported reduction of GSSG was completely blocked by rotenone, antimycin A, carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), or 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). In contrast, oligomycin potentiated GSSG reduction using either glutamate/malate or succinate as substrates. Rotenone partially blocked glutamate/malate-supported GSSG reduction. NADPH levels were altered in direct proportion to the effects on GSSG reduction. The current data indicate that the reduction of GSSG in oxidatively-stressed isolated rat liver mitochondria occurs most efficiently during state 4 respiration and is independent of ATP synthesis. Both transhydrogenation and the transmembrane proton gradient appear to be important in NADPH regeneration and consequent GSSG reduction.

Age- and peroxidative stress-related modifications of the cerebral enzymatic activities linked to mitochondria and the glutathione system

The aging brain undergoes a process of enhanced peroxidative stress, as shown by reports of altered membrane lipids, oxidized proteins, and damaged DNA. The aims of this review are to examine: (1) the possible contribution of mitochondrial processes to the formation and release of reactive oxygen species (ROS) in the aging brain; and (2) the age-related changes of antioxidant defenses, both enzymatic and nonenzymatic. It will focus on studies investigating the role of the electron transfer chain as the site of ROS formation in brain aging and the alterations of the glutathione system, also in relation to the effects of exogenous pro-oxidant agents. The possible role of peroxidative stress in age-related neurodegenerative diseases will also be discussed.

Mitochondrial changes associated with glutathione deficiency

Glutathione deficiency produced by giving buthionine sulfoximine (an inhibitor of gamma-glutamylcysteine synthetase) to animals, leads to biphasic decline in cellular glutathione levels associated with sequestration of glutathione in mitochondria. Liver mitochondria lack the enzymes needed for glutathione synthesis. Mitochondrial glutathione arises from the cytosol. Rat liver mitochondria have a multicomponent system (with Kms of approx. 60 microM and 5.4 mM) that underlies their remarkable ability to transport and retain glutathione. Mitochondria produce substantial quantities of reactive oxygen species; this is opposed by reactions involving glutathione. Glutathione deficiency leads to widespread mitochondrial damage which is lethal in newborn rats and guinea pigs, animals that do not synthesize ascorbate. Glutathione esters and ascorbate protect against the lethal and other effects of glutathione deficiency. Ascorbate spares glutathione; it increases mitochondrial glutathione in glutathione-deficient animals. Glutathione esters delay onset of scurvy in ascorbate-deficient guinea pigs; thus, glutathione spares ascorbate. Glutathione and ascorbate function together in protecting mitochondria from oxidative damage.

Influence of metabolic inhibitors on mitochondrial permeability transition and glutathione status

Treatment of isolated mitochondria with Ca2+ and inorganic phosphate (Pi) induces an inner membrane permeability that appears to be mediated through a cyclosporin A (CsA)-inhibitable Ca(2+)-dependent pore. Isolated mitochondria during inner membrane permeability undergo rapid efflux of matrix solutes such as glutathione as GSH and Ca2+, loss of coupled functions, and large amplitude swelling. Permeability transition without large amplitude swelling, a parameter often used to assess inner membrane permeability, has been observed. The addition of either oligomycin, antimycin, or sulfide to incubation buffer containing Ca2+ and Pi abolished large amplitude swelling of mitochondria. The GSH status during a Ca(2+)- and Pi-dependent mechanism of mitochondrial GSH release in isolated mitochondria was influenced significantly by metabolic inhibitors of the respiratory chain but did not prevent inner membrane permeability as demonstrated by the release of mitochondrial GSH and Ca2+. The release of GSH was inhibited by the addition of CsA, a potent inhibitor of permeability transition. Under these conditions we did not find GSSG; however, rapid oxidation of pyridine nucleotides and depletion of ATP and ADP with conversion to AMP occurred. The addition of CsA, prevented the oxidation of pyridine nucleotides and depletion of ATP and ADP. Since NADH and NADPH were extensively oxidized, protection against oxidative stress is reflected in maintenance of GSH and not observable lipid peroxidation. Evidence from transmission electron microscopy analysis, combined with the GSH release data, indicate that permeability transition can be observed in the absence of large amplitude swelling.

Cellular reducing equivalents and oxidative stress

Energy has been proposed to play a role in the ability of cells and tissues to defend against oxidative stress, even though the ultimate antioxidant capacity of a tissue is determined by the supply of reducing equivalents. The pathways involved in supplying reducing equivalents in response to an oxidative stress remain unclear, particularly if competing reactions such as ATP synthesis are active. Glutathione (GSH), a major component of cellular antioxidant systems, is maintained in the reduced form by glutathione reductase. Although this enzyme is specific for NADPH, the ability of intact cells, isolated mitochondria (which are a major source of free radicals and contain antioxidant systems independent of the rest of the cell), and whole tissues to supply reducing equivalents and maintain normal levels of GSH appears to involve NADH. This article reviews available data regarding the source and pathways by which reducing equivalents are made available to reduce exogenous oxidants, and suggests energy is not a factor. An improved understanding of the mechanism by which reducing equivalents are supplied by tissues to respond to an oxidative stress may direct future research toward designing strategies for augmenting the ability of tissues to defend themselves against oxidative stress induced by reperfusion or xenobiotics.

Depletion of a discrete nuclear glutathione pool by oxidative stress, but not by buthionine sulfoximine. Correlation with enhanced alkylating agent cytotoxicity to human melanoma cells in vitro

The existence of a distinct pool of glutathione in the nucleus of cultured human melanoma cells was demonstrated. Melanoma cell nuclei contained 13-35 pmol of glutathione/10(6) nuclei, or approximately 0.4-1.3% of the total cellular glutathione. This nuclear glutathione pool resisted depletion by buthionine sulfoximine, an agent that inhibits glutathione synthesis, but was rapidly and reversibly depleted by subtoxic concentrations of Adriamycin plus carmustine, two agents that promote oxidation of glutathione without permitting its regeneration through enzymatic reduction of glutathione disulfide. The ability of Adriamycin plus carmustine to deplete this small but significant pool of glutathione in the cell nucleus may explain why these agents potentiate the cytotoxic effects of the DNA-alkylating agent melphalan to a much higher degree than does buthionine sulfoximine at concentrations that are equipotent in depleting cytosolic glutathione.

Lung mitochondrial function following oxygen exposure and diethyl maleate-induced depletion of glutathione

Diethyl maleate (DEM) pretreatment has previously been shown to result in a transient depletion of lung glutathione and an associated decrease of the time to the onset of rat mortality resulting from exposures to 100% oxygen in vivo. The effects of oxygen exposure on mitochondrial energy metabolism were assessed by measurements of ADP-stimulated rates of O2 utilization by lung homogenates prepared from untreated and DEM-treated rats following 4 and 24 hr of exposure to either air or 100% oxygen. Twenty-four hours of oxygen exposure of untreated rats resulted in significant decreases in lung homogenate ADP-stimulated rates of respiration supported by the substrates, pyruvate, isocitrate, and alpha-ketoglutarate. No changes were observed in succinate-supported respiration, indicating that oxygen exposure appears to adversely affect NAD-linked rather than FAD-linked pathways of mitochondrial energy metabolism. The decreased lung mitochondrial glutathione, observed 4 hr following DEM treatment, returned to normal levels following 24 hr of air and oxygen exposure. No effects of glutathione depletion were observed on ADP-stimulated rates of respiratory activity 4 hr following DEM treatment. The DEM-induced transient depletion of glutathione also did not result in any additional detrimental effects on mitochondrial respiratory activity following 24 hr of oxygen exposure in vivo. These results suggested that transient mitochondrial depletion of glutathione does not accelerate the oxygen-induced impairment of mitochondrial energy metabolism. The onset of mortality associated with DEM-pretreatment might therefore result from a failure of glutathione-dependent cytosolic protective mechanisms, rather than from an increased rate of oxygen-induced mitochondrial damage.

Mitochondrial inner membrane permeability changes induced by octadecadienoic acid hydroperoxide. Role of mitochondrial GSH pool

The effect of exogenous octadecadienoic acid hydroperoxide (HPODE) on the functional properties of inner membrane of isolated rat liver mitochondria, as evaluated by the measurement of the membrane potential (delta psi) has been studied. Very low concentrations of HPODE (1.5-4.5 nmol/mg prot.) do not modify the delta psi of control mitochondria appreciably while bringing about the drop of delta psi, in a concentration-dependent mode, in mitochondria with a GSH level diminished by approx. 60%. Mitochondrial GSH depletion was obtained by intraperitoneal administration of buthionine sulfoximine, a specific inhibitor of GSH synthesis, to rats. The presence in the incubation system of GSH-methyl ester which normalizes mitochondrial GSH, fully prevents any drop in levels of delta psi induced by HPODE. The same protective effect has been presented by EGTA, which chelates the available Ca2+. Neither an antioxidant nor a specific inhibitor of mitochondrial phospholipase A2 are able to prevent the HPODE effect. From the results obtained we can assume that HPODE itself, at the concentrations used here, induces permeability changes in the inner membrane, with the loss of coupled functions, when the GSH mitochondrial level is below a critical value.

Role of glutathione in repair of free radical damage in hippocampus in vitro

Depletion of glutathione (GSH), an intrinsic antioxidant, increases vulnerability to free radical damage in a number of cell systems. This study investigates the role of GSH in limiting electrophysiological damage and/or recovery from free radical exposure in slices of guinea pig hippocampus. Synaptic potentials (PSPs) and population spikes (PSs) were recorded from field CA1. Free radicals were generated from 0.006% peroxide through the Fenton reaction. Analysis of the input-output curves showed that peroxide treatment decreased PSPs and impaired ability of the PSPs to generate PSs as previously reported. Recovery was nearly total within a half hour. Treatment with 5 mM buthionine sulfoximine (BSO) for 2 h depleted hippocampal GSH to 79.2% of control values. The extent of free radical damage was not increased. Recovery, however, was only partial. GSH was further depleted by oxidation with diamide or covalent bonding with dimethyl fumarate (DMF) immediately before and during the peroxide treatment. Neither diamide nor DMF treatment in BSO-incubated tissue enhanced peroxide-induced electrophysiological deficits. Following these treatments, however, tissue showed little recovery from free radical damage. We conclude that glutathione is essential for repair processes in hippocampal neurons exposed to oxidative damage.

Involvement of glutathione in 1-naphthylisothiocyanate (ANIT) metabolism and toxicity to isolated hepatocytes

1-Naphthylisothiocyanate (ANIT) is a model compound which causes cholestasis in laboratory animals. Various biochemical and morphological changes including biliary epithelial and parenchymal cell necrosis occur in the liver of animals treated with ANIT. Although the mechanism(s) for these effects is not understood, a role for glutathione (GSH) in toxicity has been implicated. The possible role of GSH in hepatocellular toxicity caused by ANIT was investigated in this study. Treatment of freshly isolated rat hepatocytes with ANIT caused a concentration- and time-dependent depletion of cellular GSH that preceded lactate dehydrogenase (LDH) leakage. Analysis of the incubation medium indicated that the majority of the cellular GSH which was lost was present extracellularly as GSH or as a GSH-releasing compound. Mixing ANIT with GSH at pH 7.5 yielded a compound that was characterized by HPLC and fast atom bombardment-mass spectrometry (FAB-MS) S-(N-naphthyl-thiocarbamoyl)-L-glutathione (GS-ANIT). When dissolved in aqueous solutions at neutral pH, 95% of GS-ANIT dissociated to yield free ANIT and GSH. Under conditions designed to maximize formation and stability of GS-ANIT, GS-ANIT was found in the extracellular medium of hepatocytes treated with ANIT. Treatment of hepatocytes with the GS-ANIT caused GSH depletion and LDH leakage similar to that observed with equimolar amounts of ANIT. These data suggest that ANIT depletes hepatocytes of GSH through a reversible conjugation process. Such a process may play a role in the toxicity of ANIT.

Calcium- and phosphate-dependent release and loading of glutathione by liver mitochondria

The status of glutathione (GSH) was studied in isolated rat liver mitochondria under conditions which induce a permeability transition. This transition, which is inhibited by cyclosporin A (CyA), requires the presence of Ca2+ and an inducing agent such as near physiological levels (3 mM) of inorganic phosphate (Pi). The transition is characterized by an increased inner membrane permeability to some low molecular weight solutes and by large amplitude swelling under some experimental conditions. Addition of 70 microM Ca2+ and 3 mM Pi to mitochondria resulted in mitochondrial swelling and extensive release of GSH that was recovered in the extramitochondrial medium as GSH. Both swelling and the efflux of mitochondrial GSH were prevented by CyA. Incubation of mitochondria in the presence of Ca2+, Pi, and GSH followed by addition of CyA provided a mechanism to load mitochondria with exogenous GSH that was greater than the rate of uptake by untreated mitochondria. Thus, GSH efflux from mitochondria may occur under toxicological and pathological conditions in which mitochondria are exposed to elevated Ca2+ in the presence of near physiological concentrations of Pi through a nonspecific pore. Cyclical opening and closing of the pore could also provide a mechanism for uptake of GSH by mitochondria.

Uptake of glutathione by renal cortical mitochondria

Transport of GSH into renal cortical mitochondria was studied. Mitochondria were highly enriched with little contamination from other subcellular organelles (as assessed by marker enzymes), they exhibited coupled respiration (respiratory control ratio greater than 3.0), and they had initial GSH concentrations of 5.71 +/- 0.65 nmol/mg protein (n = 47). Incubation of mitochondria with GSH in a triethanolamine, pH 7.4, buffer containing sucrose, potassium phosphate, MgCl2, and KCl, produced time- and concentration-dependent increases in intramitochondrial GSH content. Uptake was linear versus time for at least 2 min and exhibited kinetics consistent with one low-affinity, high-capacity process (Km = 1.3 mM, Vmax = 5.59 nmol/min per mg protein), although the results cannot exclude the presence of other, less quantitatively significant pathways. The initial rate of uptake of 5 mM GSH was not significantly altered by uncouplers (0.1 mM 2,4-dinitrophenol and 25 microM carbonyl cyanide m-chlorophenylhydrazone) or by 1 mM ADP. In contrast, incubation with 1 mM ATP, 1 mM KCN, 0.1 mM or 1 mM CaCl2 inhibited uptake by 41, 39, 43, or 55%, respectively. GSH uptake was markedly inhibited by gamma-glutamylglutamate and by a series of S-alkyl GSH derivatives. Strong interactions (i.e., both cis and trans effects) were observed with other dicarboxylates (i.e., succinate, malate, glutamate) but not with monocarboxylates (i.e., lactate, pyruvate). Preincubation of mitochondria with GSH protected against tert-butyl hydroperoxide- or methyl vinyl ketone-induced inhibition of state 3 respiration. These results demonstrate uptake of GSH into renal cortical mitochondria that appears to involve electroneutral countertransport (exchange) with other dicarboxylates. Functionally, GSH uptake into mitochondria can protect these organelles from various forms of injury, such as oxidative stress.

Glutathione deficiency produced by inhibition of its synthesis, and its reversal; applications in research and therapy

Glutathione, which is synthesized within cells, is a component of a pathway that uses NADPH to provide cells with their reducing milieu. This is essential for (a) maintenance of the thiols of proteins (and other compounds) and of antioxidants (e.g. ascorbate, alpha-tocopherol), (b) reduction of ribonucleotides to form the deoxyribonucleotide precursors of DNA, and (c) protection against oxidative damage, free radical damage, and other types of toxicity. Glutathione interacts with a wide variety of drugs. Despite its many and varied cellular functions, it is possible to achieve therapeutically useful modulations of glutathione metabolism. This article emphasizes an approach in which the synthesis of glutathione is selectively inhibited in vivo leading to glutathione deficiency. This is achieved through use of transition-state inactivators of gamma-glutamylcysteine synthetase, the enzyme that catalyzes the first and rate-limiting step of glutathione synthesis. The effects of marked glutathione deficiency, thus produced in the absence of applied stress, include cellular damage associated with severe mitochondrial degeneration in a number of tissues. Such glutathione deficiency is not prevented or reversed by giving glutathione. The cellular utilization of GSH involves its extracellular degradation, uptake of products, and intracellular synthesis of GSH. This is a normal pathway by which cysteine moieties are taken up by cells. Glutathione deficiency induced by inhibition of its synthesis may be prevented or reversed by administration of glutathione esters which, in contrast to glutathione, are readily transported into cells and hydrolyzed to form glutathione intracellularly. Research derived from this model has led to several potentially useful therapeutic approaches, one of which is currently in clinical trial. Thus, certain tumors, including those that exhibit resistance to several drugs and to radiation, are sensitized to these modalities by selective inhibition of glutathione synthesis. An alternative interpretation is suggested which is based on the concept that some resistant tumors have high capacity for glutathione synthesis and that such increased capacity may be as significant or more significant in promoting the resistance of some tumors than the cellular levels of glutathione. Therapeutic approaches are proposed in which normal cells may be selectively protected against toxic antitumor agents and radiation by cysteine- and glutathione-delivery compounds. Current studies suggest that research on other modulations of glutathione metabolism and transport would be of interest.

Glutathione depletion and formation of glutathione-protein mixed disulfide following exposure of brain mitochondria to oxidative stress

t-Butyl hydroperoxide was utilized to alter the thiol homeostasis in rat brain mitochondria. Following exposure to t-butyl hydroperoxide (50-500 microM), intramitochondrial GSH content decreased rapidly and irreversibly with a major portion of the depleted GSH being accounted for as protein-SS-Glutathione mixed disulfide. Formation of GSSG was not observed nor was efflux of GSSG or GSH from the mitochondria detected in the incubation medium. The loss of intramitochondrial GSH was accompanied by loss of protein thiols. Unlike liver mitochondria, which can reverse t-butyl hydroperoxide induced formation of GSSG, addition of 50 microM t-butyl hydroperoxide resulted in irreversible loss; indicating greater susceptibility of brain mitochondria to oxidative stress than liver mitochondria.

The effect of the radioprotector WR-2721 and WR-1065 on mitochondrial lipid peroxidation

The radioprotective agent WR-2721 is dephosphorylated to the free thiol form WR-1065 in vivo. The effects of WR-2721, WR-1065 and reduced glutathione on a mitochondrial lipid peroxidation system were compared. WR-2721 had no effect on mitochondrial lipid peroxidation in vitro, and could not prevent the inactivation of mitochondrial enzymes. Both WR-1065 and glutathione were effective inhibitors of mitochondrial lipid peroxidation induced by ADP/Fe/NADPH or by ADP/Fe/ascorbate. Both thiols correspondingly delayed the free radical-mediated inactivation of succinate dehydrogenase and isocitrate dehydrogenase. WR-1065 was able to reduce cumene hydroperoxide non-enzymatically, and proved to be weak substrate for glutathione peroxidase. The disulfide formed from WR-1065 could be reduced by glutathione without the participation of glutathione reductase. A redox cycle is proposed between WR-1065, glutathione and glutathione reductase to explain the inhibitory effect of WR-1065 on lipid peroxidation.

Transport of glutathione across the mitochondrial membranes

Transport of glutathione (GSH) into mitochondria was observed when mitochondria in state 4 respiration were incubated with high concentrations of GSH. This transport was suppressed by antimycin A or dicyclohexyl-carbodiimide, or in state 3 respiration. Upon dissipation of the proton gradient by a proton ionophore, mitochondrial GSH was released into the medium. GSH moved freely across the proton-permeated mitochondrial membrane, its movement depending only on the GSH gradient across the inner membrane. These results indicate that there is a transport system for GSH in the mitochondrial membrane, and that a proton gradient is necessary to maintain GSH in the matrix, and to transport GSH into mitochondria.

Effective depletion of glutathione in rat striatum and substantia nigra by L-buthionine sulfoximine in combination with 2-cyclohexene-1-one

The effects of L-buthionine sulfoximine (L-BSO), 2-cyclohexene-1-one and diethylmaleate (DEM) on the concentration of rat brain glutathione (GSH) were investigated. Both DEM and 2-cyclohexene-1-one, administered subcutaneously, produced marked and rapid reduction of brain GSH, but 2-cyclohexene-1-one appeared less toxic than DEM. Six hours after 2-cyclohexene-1-one (100 microliters/kg) the striatal GSH concentration was 35% of control values, whereas the level was 55% of controls at 24 h and 80% of controls at 48 h. Similar results were obtained with DEM (800 microliters/kg). L-BSO (3.2 mg), administered intracerebroventricularly, produced a slower depletion of brain GSH. A 55% reduction of striatal GSH was obtained 24 h after the administration, and the level was approximately 50% of control at 48 h. Thus, the effect of 2-cyclohexene-1-one and DEM is rapid in onset but relatively short lasting, whereas the disappearance of brain GSH after L-BSO is slower but the effect is more long-lasting. By combining L-BSO with either 2-cyclohexene-1-one or DEM both a rapid and long-lasting GSH depletion was obtained that was more profound than after any of the drugs alone. The combination of L-BSO and 2-cyclohexene-1-one was well tolerated, but the combination of L-BSO and DEM led to death in half of the rats the second day after injection. The disappearance rate of GSH after L-BSO alone gives an estimate of the turn-over of GSH. We found the turn-over of GSH to be higher in the substantia nigra pars compacta than in the striatum. The present work suggest that L-BSO and 2-cyclohexene-1-one would be very useful for evaluation of the biological role of GSH in the central nervous system.

Depletion of mitochondrial coenzyme A and glutathione by 4-dimethylaminophenol and formation of mixed thioethers

4-Dimethylaminophenol (DMAP), an antidote in cyanide poisoning, has been shown to produce kidney lesions in rats, to damage isolated rat kidney tubules and to impair mitochondrial functions as already described for 4-aminophenol. Since DMAP upon oxidation forms bis- and tris-substituted thioethers with GSH, it was anticipated that mitochondrial toxicity of DMAP might result from CoA depletion. In a model reaction DMAP was oxidized by oxyhemoglobin in the presence of CoA and GSH resulting in formation of tris-(CoA-S-yl)-DMAP, tris-(GSH-S-yl)-DMAP and two mixed thioethers, namely, (CoA-S-yl)-bis-(GSH-S-yl)-DMAP and (GSH-S-yl)-bis-(CoA-S-yl)-DMAP. The compounds were isolated by HPLC and identified spectroscopically, by amino acid analysis and Raney-Nickel desulfuration. Rat liver mitochondria (5 mg protein/ml) incubated under state IV conditions with 20 and 50 microM DMAP were depleted of GSH and total coenzyme A with formation of GSSG and the above-mentioned thioethers which were quantified by isotope dilution techniques using [14C]-labelled DMAP and the isolated, inactive thioethers. The results confirm earlier suggestions that part of the cytotoxicity of DMAP may result from depletion of vital mitochondrial thiols, particularly CoA. Since 4-aminophenol reacts analogously, similar cytotoxic effects can be expected from compounds which on (aut)oxidation form quinoid systems capable of 1.4-addition reactions with nucleophilic thiols.

Relationships between the NAD(P) redox state, fatty acid oxidation, and inner membrane permeability in rat liver mitochondria

Dysfunction of mitochondria after oxidation of endogenous NAD(P)H, especially after calcium accumulation, has been abundantly reported, but the causes of membrane perturbations did not receive a full explanation. In light of several additional observations reported in this study, we propose a general scheme which shows the sequential processes that are likely involved in the appearance of calcium-induced membrane leakiness. Addition of acetoacetate, oxaloacetate, or ketomalonate to rotenone-treated mitochondria led to a massive oxidation of both NADH and NADPH. Under these conditions, stimulation of fatty acid oxidation could be observed. This process was shown to be accompanied by a reduction of intramitochondrial NADP+. The reduction of NADP+ was inhibited by uncouplers, electron transfer inhibitors and N,N'-dicyclohexylcarbodiimide. It was thus probably catalyzed by the mitochondrial transhydrogenase. Oxidation of pyridine nucleotides in the presence of acetoacetate induced (i) a slight decrease in the number of sulfhydryl groups reactive with N-ethylmaleimide (but no change in the amount of intramitochondrial reduced glutathione) and (ii) modifications of the kinetics and the orientation of the ADP/ATP carrier. In the presence of calcium ions, acetoacetate-stimulated fatty acid oxidation promoted an extensive swelling of mitochondria. Uptake of calcium ions into the matrix was a critical factor for triggering the swelling. Thiols, if they were added at a sufficiently high concentration, suppressed the swelling. Also ligands of the ADP/ATP carrier which stabilized the m-state conformation of the protein, exerted an efficient protective action. Three essential interacting factors emerge from this study: (i) The crucial role of the ADP/ATP carrier orientation in promoting the calcium-induced membrane destabilization. More precisely, it has been shown that the ADP/ATP carrier adopts the c-state conformation (i.e., nucleotide binding site facing the cytoplasm) during fatty acid oxidation. (ii) The modification of a very small number of sulfhydryl groups of mitochondrial protein. These groups are probably in an oxidized state when the level of reduced pyridine nucleotides is low. (iii) The prevailing role of the transhydrogenase, the function of which is also intimately associated with fatty acid oxidation. After energization, transhydrogenase can hinder thiol oxidation and therefore partially protect the membrane structure.

Rat kidney function related to tissue glutathione levels. Effects of different glutathione depletors

1. Rat renal function was evaluated during acute depletion of glutathione (GSH) produced by different doses of diethyl-maleate (DEM) or buthionine-sulfoximine (BSO). 2. Similar alterations in renal function were observed when similar GSH levels were obtained independently of the GSH depletor employed. 3. These results confirm the relationship between GSH levels and renal function.

Effect of As2O3 on gluconeogenesis

1) The effect of As2O3 and As2O5 on gluconeogenesis from various substrates in the liver and kidney of rats was investigated. 2) A concentration-dependent inhibition by As2O3 was found. The effect was not dependent on the amount of investigated material (hepatocytes or kidney tubules). For either hepatocytes or kidney tubules the extent of inhibition depended strongly on the substrate used. The highest degree of inhibition was observed in incubations with pyruvate. The inhibition of glucose formation was accompanied to a lesser extent by a diminution in O2 consumption and ATP content. The effect was also dependent on the substrate used. Maximum effect was found in incubations with pyruvate. 3) Oleate, 0.5 mmol/l, increased gluconeogenesis from pyruvate. The effect was not abolished by As2O3. 4) A decrease in the content of acetyl-CoA, 3-hydroxybutyrate, and reduced glutathione was found in suspensions of isolated rat kidney tubules or hepatocytes incubated with As2O3. 5) About 10 times higher concentrations of As2O5 were necessary to induce a similar extent of inhibition of gluconeogenesis, decrease in O2 consumption, and in ATP content as compared with As2O3. The extent of the As2O5 effect depended on the concentration of the toxicant and on the substrate used. Gluconeogenesis from pyruvate exhibited the highest sensitivity to As2O5. 6) All findings can be largely explained by inhibition of pyruvate dehydrogenase as the central target for arsenicals. The subsequent depletion of acetyl CoA results in impaired formation of reducing equivalents in the citric acid cycle, decrease in high energy phosphates and, acetyl CoA being a strong positive modulator of pyruvate carboxylase, in gluconeogenesis inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitochondrial glutathione status during Ca2+ ionophore-induced injury to isolated hepatocytes

In this study the Ca2+ ionophore, A23187, was used to determine the effects of disrupted Ca2+ homeostasis on cellular thiols. Isolated rat hepatocytes were incubated with varying concentrations of extracellular Ca2+ and A23187 to induce accumulation or loss of cellular Ca2+. These treatments resulted in loss of mitochondrial and cytosolic glutathione (GSH), loss of protein-thiols, and cell injury. This injury was dependent on the concentrations of ionophore and extracellular Ca2+. A correlation was found between cell injury and the loss of mitochondrial GSH, while the loss of cytosolic glutathione preceded both these events. The time course of protein-thiol loss appeared secondary to the loss of non-protein thiols. In the absence of extracellular Ca2+, the antioxidants alpha-tocopherol and diphenyl-p-phenylenediamine both totally prevented A23187-induced cell injury and loss of mitochondrial GSH, and thus protected the cells from the effects of mobilization of intracellular Ca2+. In the presence of extracellular Ca2+, cell injury as well as the loss of mitochondrial GSH were only partially prevented by antioxidant treatment. The mitochondrial Ca2+ channel blocker, ruthenium red, protected hepatocytes from A23187-induced injury in the absence of extracellular Ca2+. Leupeptin, an inhibitor of Ca2+-activated proteases, and dibucaine, a phospholipase inhibitor, did not affect cytotoxicity. Our results indicate that the level of mitochondrial GSH may be important for cell survival during ionophore-induced perturbation of cellular Ca2+ homeostasis.

Retention of oxidized glutathione by isolated rat liver mitochondria during hydroperoxide treatment

The addition of tert-butyl hydroperoxide (t-BuOOH) to isolated mitochondria resulted in oxidation of approximately 80% of the mitochondrial reduced glutathione (GSH) independently of the dose of t-BuOOH (1-5 mM). Concomitant with the oxidation of GSH inside the mitochondria was the formation of GSH-protein mixed disulfides (protein-SSG), with approximately 1% of the mitochondrial protein thiols involved. A dose-dependent rate of GSH recovery was observed, via the reduction of oxidized GSH (GSSG) and a slower reduction of protein-SSG. Although t-BuOOH administration affected the respiratory control ratio, the mitochondria remained coupled and loss of the matrix enzyme, citrate synthase, was not increased over the control and was less than 3% over 60 min. A slow loss of GSH out of the coupled non-treated mitochondria was not increased by t-BuOOH treatment, in fact, a dose-dependent drop of GSH levels occurred in the medium. However, no GSSG was found outside the mitochondria, indicating the necessary involvement of enzymes in the t-BuOOH-induced conversion of GSH to GSSG. The absence of GSSG in the medium also suggests that, unlike the plasma membrane, the mitochondrial membranes do not have the ability to export GSSG as a response to oxidative stress. Our results demonstrate the inability of mitochondria to export GSSG during oxidative stress and may explain the protective role of mitochondrial GSH in cytotoxicity.

Glutathione depleting agents and lipid peroxidation

The mechanisms by which glutathione (GSH) depleting agents produce cellular injury, particularly liver cell injury have been reviewed. Among the model molecules most thoroughly investigated are bromobenzene and acetaminophen. The metabolism of these compounds leads to the formation of electrophilic reactants that easily conjugate with GSH. After substantial depletion of GSH, covalent binding of reactive metabolites to cellular macromolecules occurs. When the hepatic GSH depletion reaches a threshold level, lipid peroxidation develops and severe cellular damage is produced. According to experimental evidence, the cell death seems to be more strictly related to lipid peroxidation rather than to covalent binding. Loss of protein sulfhydryl groups may be an important factor in the disturbance of calcium homeostasis which, according to several authors, leads to irreversible cell injury. In the bromobenzene-induced liver injury loss of protein thiols as well as impairment of mitochondrial and microsomal Ca2+ sequestration activities are related to lipid peroxidation. However, some redox active compounds such as menadione and t-butylhydroperoxide produce direct oxidation of protein thiols.

The reduction of dithiobis(2-nitrobenzoate) by rat liver mitochondria

5,5'-Dithiobis(2-nitrobenzoate) (DTNB) is reduced in mitochondrial suspensions to 5-mercapto-2-nitrobenzoate (MNB) by 3-hydroxybutyrate and isocitrate. Although most of the MNB produced is found in the suspension medium, there is also some within the particles. The amount of MNB found in these fraction varies with the DTNB concentration used and is much lower if mitochondrial glutathione (GSH) is depleted with 1-chloro-2,4-dinitrobenzene. If hydroxybutyrate is present, the reduction of DTNB is increased by ATP and oligomycin. The pellet contains only a little MNB and GSH but these are considerably elevated by antimycin and rotenone as well as by ATP and oligomycin. If isocitrate is present, the reduction of DTNB is greatly stimulated by valinomycin, triethyltin and, to a lesser extent, oligomycin. MNB in the pellet falls and GSH concentrations are unchanged. The results suggest that with hydroxybutyrate (an NAD reducing substrate), the rate of reduction of DTNB is limited by the rate of regeneration of GSH while with isocitrate (an NADP reducing substrate) it is limited by the rate of export of MNB from the matrix.