Glutathione disulfide reduction in tumor mitochondria after t-butyl hydroperoxide treatment

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.

Mechanisms of injury in the central nervous system

Neurotoxicants with similar structural features or common mechanisms of chemical action frequently produce widely divergent neuropathologic outcomes. Methylmercury (MeHg) produces marked cerebellar dysmorphogenesis during critical periods of development. The pathologic picture is characterized by complete architectural disruption of neuronal elements within the cerebellum. MeHg binds strongly to protein and soluble sulphydryl groups. Binding to microtubular -SH groups results in catastrophic depolymerization of immature tyrosinated microtubules. However, more mature acetylated microtubules are resistant to MeHg-induced depolymerization. In contrast to MeHg, the structurally similar organotin trimethyltin (TMT) elicits specific apoptotic destruction of pyramidal neurons in the CA3 region of the hippocampus and in other limbic structures. Expression of the phylogenetically conserved protein stannin is required for development of TMT-induced lesions. Inhibition of expression using antisense oligonucleotides against stannin protects neurons from the effects of TMT, suggesting that this protein is required for expression of neurotoxicity. However, expression of stannin alone is insufficient for induction of apoptotic pathways in neuronal populations. The aromatic nitrocompound 1,3-dinitrobenzene (DNB) has 2 independent nitro groups that can redox cycle in the presence of molecular oxygen. Despite its ability to deplete neural glutathione stores, DNB produces edematous gliovascular lesions in the brain stem of rats. Glial cells are susceptible despite high concentrations of reduced glutathione compared with neuronal somata in the central nervous system (CNS). The severity of lesions produced by DNB is modulated by the activity of neurons in the affected pathways. The inherent discrepancy between susceptibility of neuronal and glial cell populations is likely mediated by differential control of the mitochondrial permeability transition in astrocytes and neurons. Lessons learned in the mechanistic investigation of neurotoxicants suggest caution in the evaluation and interpretation of structure-activity relationships, eg, TMT, MeHg, and DNB all induce oxidative stress, whereas TMT and triethyltin produce neuronal damage and myelin edema, respectively. The precise CNS molecular targets of cell-specific lipophilic neurotoxicants remain to be determined.

Mitochondrial oxidative alterations following partial hepatectomy

Mitochondria, isolated from rat livers during the early phase of liver regeneration (7-24 h after partial hepatectomy), show: (i) decrease in the rate of ATP synthesis; (ii) increase of malondialdehyde and of oxidized protein production; (iii) decrease of the content of intramitochondrial glutathione and of protein thiols on mitochondrial proteins; (iv) increase of the glutathione bound to mitochondrial proteins by disulfide bonds. These observations suggest an increase of production of oxygen radicals in liver mitochondria, following partial hepatectomy, which can alter the function of the enzymes involved in the oxidative phosphorylation. Blue-native gel electrophoresis of rat liver mitochondria, isolated after partial hepatectomy, shows, during the early phase of liver regeneration (0-24 h after partial hepatectomy), a progressive decrease of the content of F0F1-ATP synthase complex. The amount of glutathione bound to the F0F1-ATP synthase, electroeluted from the blue-native gels, progressively increased during the early phase of liver regeneration. It is concluded that partial hepatectomy causes mitochondrial oxidative stress that, in turn, modifies proteins (such as F0F1-ATP synthase) involved in the mitochondrial oxidative phosphorylation.

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.

Reduction of glutathione disulfide and the maintenance of reducing equivalents in hypoxic hearts after the infusion of diamide

A tissue's response to an oxidative stress is related to its capacity to supply reducing equivalents and may be affected by energy levels. The ability of intact rat heart tissue to supply NADPH and reduce glutathione disulfide (GSSG) produced by diamide was determined under normoxic or hypoxic conditions with and without glycolytic energy production. Cardiac ATP and phosphocreatine (PCr) levels remained relatively constant (approximately 20 nmol/mg dry weight) during a 60 min perfusion with oxygenated Krebs-Henseleit buffer containing glucose. Levels of ATP and PCr were depleted 85-92% following 60 min of hypoxia. A 5 min infusion of 800 microM diamide, after 60 min of normoxia or hypoxia, oxidized 70-80% of cardiac glutathione (GSH), but had no effect on total glutathione. After a subsequent 25 min diamidefree perfusion, 75-85% of the GSSG formed was reduced in both normoxic and hypoxic hearts. The removal of glucose, or the inhibition of glycolysis with 2-deoxy-D-glucose, did not affect GSSG reduction. Cardiac NADH levels were increased from 0.05 to 0.5 nmol/mg dry weight after 60 min hypoxia in hearts perfused with or without glucose. A 5 min infusion of diamide in hypoxic hearts slightly decreased NADH levels, but there was no further change after a subsequent 25 min diamide-free period. Inhibition of glutathione reductase with 1,3-bis(2-chloroethyl)-1-nitrosourea prevented GSSG reduction, showing NADPH was required. However, NADPH levels were not affected by hypoxia or diamide infusion and remained constant at 0.2 nmol/mg dry weight in hearts perfused with or without glucose. Inhibition of glycolysis with 2-deoxy-D-glucose also did not affect NADPH levels. These results demonstrate that hypoxia did not affect the ability of oxidatively stressed, intact heart tissue to supply NADPH for the reduction of GSSG. In addition, GSSG reduction was independent of energy levels and appeared to be unaffected by glucose availability. NADH may be involved in maintaining NADPH levels through interconversion pathways.

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.