Continuous exposure of airway epithelial cells to hydrogen peroxide: protection by KGF

Reactive oxygen species (ROS) increase permeability in the airway epithelium. Extended periods of oxidant exposure may be experienced by those suffering from chronic inflammation of the lungs, receiving supplemental oxygen, or living in areas with high levels of air pollution. We studied the effects of long-term, continuous exposure to hydrogen peroxide (H(2)O(2)) on the trans-epithelial electrical resistance (TER) across cultured monolayers of a transformed cell line of human bronchial epithelial cells, 16HBE14o- (16HBE). A TER perfusion system was employed to continuously monitor the TER without disturbing the tissue model. The TER decreased in a dose-dependent manner with increasing concentrations of H(2)O(2) (0.1, 0.5, and 1.0 mM), regardless of pre-incubation conditions. Cell cultures pre-treated with 50 ng/ml keratinocyte growth factor (KGF) showed a significant delay in oxidant-induced TER decreases caused by 0.1 mM H(2)O(2). Exposure to 0.1 mM H(2)O(2) for 350 min led to disruption of tight junction proteins, ZO-1 and occludin, but KGF treatment prevented this damage. The recovery of epithelial barrier function after exposure to oxidants was also studied. Tissue models exposed to 0.5 mM H(2)O(2) for 25 min showed complete recovery of TER after 20 h, independent of culture pre-treatment. In contrast, KGF pre-incubation enhanced the recovery of 16HBE cultures exposed for 50 min to 0.5 mM H(2)O(2).     

Gene profile changes after Pseudomonas aeruginosa exposure in immortalized airway epithelial cells

To test the hypothesis that the excess inflammatory response in cystic fibrosis airway epithelial cells is the result of differential activation of genes in response to a laboratory strain of Pseudomonas aeruginosa (PAO1), a 48-h time course of genes expressed following PAO1 stimulation (10(9) CFU for 1 h) was studied in two pairs of airway epithelial cells: 9/HTEo- and 16HBE14o-, each with a matched normal and CF phenotype pair. cRNA was hybridized to Affymetrix HG-U95Av2 chips and pairwise comparisons against zero time (no PAO1) were calculated for each time point. PAO1 elicited profound changes in both cell pairs: for 9/HTEo-, 144 genes changed significantly in the normal pair, and 116 for the CF pair; for the 16HBE14o- pair, 57 genes changed significantly for the normal pair and 53 for the CF pair. Changes were much greater in the 9/HTEo- than in the 16HBE14o- pair, but basal levels of expression of inflammatory genes are higher in the 16HBE14o- pair, so 16HBE14o- was used mainly to corroborate the results of 9/HTEo-. Clustering analysis indicated that the pattern of gene expression is similar in the CF cells and their normal counterparts. However, there were substantial quantitative differences in gene expression. Thus, the difference between CF and normal resides in the magnitude, not the pattern, of the changes.     

Secretion of lactoferrin and lysozyme by cultures of human airway epithelium

Lactoferrin and lysozyme are important antimicrobial compounds of airway surface liquid, derived predominantly from serous cells of submucosal glands but also from surface epithelium. Here we compared release of these compounds from the following human cell cultures: primary cultures of tracheal epithelium (HTE), Calu-3 cells (a lung adenocarcinoma cell line frequently used as a model of serous gland cells), 16HBE14o- cells (an SV40 transformed line from airway surface epithelium), T84 cells (a colon carcinoma cell line), and human foreskin fibroblasts (HFF). For lysozyme, baseline secretory rates were in the order Calu-3 > 16HBE14o- > HTE T84 > HFF = 0; for lactoferrin, the only cell type showing measurable release was HTE; for mucus, HTE > Calu-3 > 16HBE14o- T84 > HFF = 0. A wide variety of neurohumoral agents and inflammatory stimuli was without effect on lactoferrin and lysozyme release from HTE or Calu-3 cells, although forskolin did stimulate secretion of water and lysozyme from Calu-3 cells. However, the concentration of lysozyme in the forskolin-induced secretions was much less than in airway gland secretions. Thus our data cast doubt on the utility of Calu-3 cells as a model of airway serous gland cells but do suggest that HTE could prove highly suitable for studies of mucin synthesis and release.     

Activation of hypoxia-inducible factor-1 protects airway epithelium against oxidant-induced barrier dysfunction

The respiratory epithelium forms an important barrier against inhaled pollutants and microorganisms, and its barrier function is often compromised during inflammatory airway diseases. Epithelial activation of hypoxia-inducible factor-1 (HIF-1) represents one feature of airway inflammation, but the functional importance of HIF-1 within the respiratory epithelium is largely unknown. Using primary mouse tracheal epithelial (MTE) cells or immortalized human bronchial epithelial cells (16HBE14o-), we evaluated the impact of HIF-1 activation on loss of epithelial barrier function during oxidative stress. Exposure of either 16HBE14o- or MTE cells to H(2)O(2) resulted in significant loss of transepithelial electrical resistance and increased permeability to fluorescein isothiocyanate-dextran (4 kDa), and this was attenuated significantly after prior activation of HIF-1 by preexposure to hypoxia (2% O(2); 6 h) or the hypoxia mimics CoCl(2) or dimethyloxalylglycine (DMOG). Oxidative barrier loss was associated with reduced levels of the tight junction protein occludin and with hyperoxidation of the antioxidant enzyme peroxiredoxin (Prx-SO(2)H), events that were also attenuated by prior activation of HIF-1. Involvement of HIF-1 in these protective effects was confirmed using the pharmacological inhibitor YC-1 and by short-hairpin RNA knockdown of HIF-1α. The protective effects of HIF-1 were associated with induction of sestrin-2, a hypoxia-inducible enzyme known to reduce oxidative stress and minimize Prx hyperoxidation. Together, our results suggest that loss of epithelial barrier integrity by oxidative stress is minimized by activation of HIF-1, in part by induction of sestrin-2.     

Influence of exogenous iron and ascorbate on H2O2-induced glutathione oxidation in red cells

The effective fall in cytosolic reduced glutathione levels in intact red cells exposed to exogenous oxidant stress in the form of Fe2+, H2O2 and ascorbate was caused by H2O2 alone. Relatively high concentrations of Fe2+ had no contributory effect on the oxidizing capacity of H2O2. Ascorbate, at physiological levels, showed no protection whereas glucose was totally protective. Since glucose, via hexose monophosphate shunt, is the only source of reducing equivalent in red cells, the NADPH/NADP+ redox role in the diminution of intracellular reduced glutathione.     

Cell culture models demonstrate that CFTR dysfunction leads to defective fatty acid composition and metabolism

Cystic fibrosis (CF) is associated with fatty acid alterations characterized by low linoleic and docosahexaenoic acid. It is not clear whether these fatty acid alterations are directly linked to cystic fibrosis transmembrane conductance regulator (CFTR) dysfunction or result from nutrient malabsorption. We hypothesized that if fatty acid alterations are a result of CFTR dysfunction, those alterations should be demonstrable in CF cell culture models. Two CF airway epithelial cell lines were used: 16HBE, sense and antisense CFTR cells, and C38/IB3-1 cells. Wild-type (WT) and CF cells were cultured in 10% fetal bovine serum (FBS) or 10% horse serum. Fatty acid levels were analyzed by GC-MS. Culture of both WT and CF cells in FBS resulted in very low linoleic acid levels. When cells were cultured in horse serum containing concentrations of linoleic acid matching those found in human plasma, physiological levels of linoleic acid were obtained and fatty acid alterations characteristic of CF tissues were then evident in CF compared with WT cells. Kinetic studies with radiolabeled linoleic acid demonstrated in CF cells increased conversion to longer and more-desaturated fatty acids such as arachidonic acid. In conclusion, these data demonstrate that CFTR dysfunction is associated with altered fatty acid metabolism in cultured airway epithelial cells.     

Antioxidative enzyme activity and metabolism of peroxide compounds in the crystalline lens during cataractogenesis

Lens antioxidative enzyme activity (catalase, superoxide dismutase, glutathione peroxidase) in cataract as well as the possibility of cataract induction by the lipid peroxidation products and their influence on the content of reduced thiols (oxy-red balance) were studied. It was shown that the rate of the H2O2 decomposition by the human cataract lenses is lowered in comparison with the normal lenses. This is not due to the lowered catalase or glutathione-peroxidase 1 activity, but depends on the deficiency of reduced glutathione in the lens. Activity of superoxide dismutase and glutathione peroxidase metabolizing organic hydroperoxides is significantly lowered in the cataract lenses. Lipid peroxidation products injected into the rabbit vitreous induce posterior subcapsular cataract, which is accompanied by depletion of reduced glutathione level in the lens. The conclusion is made that two interrelated processes: accumulation of H2O2 and of lipid peroxides induce aggregation of the soluble proteins and the fragmentation of the membrane structures in cataract lenses.     

Maintenance of higher H(2)O(2) levels, and its mechanism of action to induce growth in breast cancer cells: Important roles of bioactive catalase and PP2A

We assessed the catalase bioactivity and hydrogen peroxide (H(2)O(2)) production rate in human breast cancer (HBC) cell lines and compared these with normal human breast epithelial (HBE) cells. We observed that the bioactivity of catalase was decreased in HBC cells when compared with HBE cells. This was also accompanied by an increase in H(2)O(2) steady-state levels in HBC cells. Silencing the catalase gene led to a further increase in the steady-state level of H(2)O(2) which was also accompanied by an increase in growth rate of HBC cells. Catalase activity was up regulated on treatment with superoxide (O(2)(-)) scavengers such as pegylated SOD (PEG-SOD, indicating inhibition of catalase by the increased O(2)(-) produced by HBC cells. Transfection of either catalase or glutathione peroxidase to HBC cells decreased intracellular H(2)O(2) levels and led to apoptosis of these cells. The H(2)O(2) produced by HBC cells inhibited PP2A activity accompanied by increased phosphorylation of Akt and ERK1/2. The importance of catalase bioactivity in breast cancer was further confirmed as its bioactivity was also decreased in human breast cancer tissues when compared to normal breast tissues. We conclude that inhibition of catalase bioactivity by O(2)(-) leads to an increase in steady-state levels of H(2)O(2) in HBC cells, which in turn inhibits PP2A activity, leading to phosphorylation of ERK 1/2 and Akt and resulting in HBC cell proliferation.     

Hypochlorous acid alters bronchial epithelial cell membrane properties and prevention by extracellular glutathione.

In chronic inflammatory diseases of the airways, such as cystic fibrosis, hypochlorous acid (HOCl) generated by neutrophils is involved in airway injury. We examined the effects of HOCl on 16HBE14o- bronchial epithelial cells by bolus addition or by generation with glucose oxidase plus myeloperoxidase. HOCl produced both carbonyl formation of a discreet number of proteins and modification of surface targets that were recognized by an antibody raised against HOCl-modified protein. Bolus or enzymatically generated HOCl decreased transepithelial resistance, but surprisingly bolus HOCl also increased short-circuit current. Glutathione in lung epithelial lining fluid is an excellent scavenger of HOCl; however, glutathione content is lower in cystic fibrosis epithelial lining fluid due to deficient glutathione transport to the apical side of bronchial-tracheal epithelial cells (Gao L, Kim KJ, Yankaskas JR, and Forman HJ. Am J Physiol Lung Cell Mol Physiol 277: L113-L118, 1999). We found that alteration of the GSH content of apical fluid above 16HBE14o- cells was protective because all HOCl-induced changes were delayed or eliminated by exogenous glutathione within the physiological range. Extrapolating this to cystic fibrosis suggests that HOCl can alter cell function without destruction but that elevating glutathione could be protective.     

Biochemical interactions among intercellular adhesion molecules expressed by airway epithelial cells

Intercellular adhesion between adjacent airway epithelial cells plays a critical role in maintaining the barrier function of the respiratory mucosa. In the current study, we examined the expression and interaction of cell surface adhesion molecules (E-cadherin, ICAM-1, and MUC1) and their intracellular binding partners (alpha-catenin, beta-catenin, gamma-catenin, and ezrin) in 16HBE14o-, HBE1, 1HAEo-, BEAS-2B, A549, and NCI-H292 human airway epithelial cells. Expression of E-cadherin and MUC1, both in whole cell lysates and biotinylated surface proteins, was observed in 16HBE14o-, HBE1, A549, and NCI-H292 cells, while ICAM-1 was detected only in NCI-H292. In contrast, alpha-, beta-, and gamma-catenin and ezrin were expressed in all of the cells. E-cadherin formed coimmunoprecipitation complexes with beta- and gamma-catenin, whereas MUC1 only associated with beta-catenin. ICAM-1, but not MUC1, coimmunoprecipitated with ezrin in NCI-H292 cells. We conclude that airway epithelial cell-cell adhesion involves a complex network of protein-protein interactions mediated by a diverse array of membrane-bound and cytosolic protein partners.     

Towards an in vitro model of cystic fibrosis small airway epithelium: characterisation of the human bronchial epithelial cell line CFBE41o-

The CFBE41o- cell line was generated by transformation of cystic fibrosis (CF) tracheo-bronchial cells with SV40 and has been reported to be homozygous for the DeltaF508 mutation. A systematic characterisation of these cells, which however, is a pre-requisite for their use as an in vitro model, has not been undertaken so far. Here, we report an assessment of optimal culture conditions, the expression pattern of drug-transport-related proteins and the stability/presence of the CF transmembrane conductance regulator (CFTR) mutation in the gene and gene product over multiple passages. The CFBE41o- cell line was also compared with a wild-type airway epithelial cell line, 16HBE14o-, which served as model for bronchial epithelial cells in situ. The CFBE41o- cell line retains at least some aspects of human CF bronchial epithelial cells, such as the ability to form electrically tight cell layers with functional cell-cell contacts, when grown under immersed (but not air-interfaced) culture conditions. The cell line is homozygous for DeltaF508-CFTR over multiple passages in culture and expresses a number of proteins relevant for pulmonary drug absorption (e.g. P-gp, LRP and caveolin-1). Hence, the CFBE41o- cell line should be useful for studies of CF gene transfer or alternative treatment with small drug molecules and for the gathering of further information about the disease at the cellular level, without the need for primary culture.     

Common gene therapy viral vectors do not efficiently penetrate sputum from cystic fibrosis patients

Norwalk virus and human papilloma virus, two viruses that infect humans at mucosal surfaces, have been found capable of rapidly penetrating human mucus secretions. Viral vectors for gene therapy of Cystic Fibrosis (CF) must similarly penetrate purulent lung airway mucus (sputum) to deliver DNA to airway epithelial cells. However, surprisingly little is known about the rates at which gene delivery vehicles penetrate sputum, including viral vectors used in clinical trials for CF gene therapy. We find that sputum spontaneously expectorated by CF patients efficiently traps two viral vectors commonly used in CF gene therapy trials, adenovirus (d～80 nm) and adeno-associated virus (AAV serotype 5; d～20 nm), leading to average effective diffusivities that are ～3,000-fold and 12,000-fold slower than their theoretical speeds in water, respectively. Both viral vectors are slowed by adhesion, as engineered muco-inert nanoparticles with diameters as large as 200 nm penetrate the same sputum samples at rates only ～40-fold reduced compared to in pure water. A limited fraction of AAV exhibit sufficiently fast mobility to penetrate physiologically thick sputum layers, likely because of the lower viscous drag and smaller surface area for adhesion to sputum constituents. Nevertheless, poor penetration of CF sputum is likely a major contributor to the ineffectiveness of viral vector based gene therapy in the lungs of CF patients observed to date.     

Oxidative stress and antioxidant deficiencies in asthma: potential modification by diet

The lungs of asthmatic patients are exposed to oxidative stress due to the generation of reactive oxygen and nitrogen species as a consequence of chronic airway inflammation. Increased concentrations of NO*, H2O2 and 8-isoprostane have been measured in exhaled breath and induced sputum of asthmatic patients. O2*-, NO*, and halides interact to form highly reactive species such as peroxynitrite and HOBr, which in turn cause nitration and bromination of protein tyrosine residues. Oxidative stress may also reduce glutathione levels and cause inactivation of antioxidant enzymes such as superoxide dismutase, with a consequent increase in apoptosis, shedding of airway epithelial cells and airway remodelling. The oxidant/antioxidant equilibrium in asthmatic patients may be further perturbed by low dietary intakes of the antioxidant vitamins C and E, selenium and flavonoids, with a consequent lowering of the concentrations of these and other non-dietary antioxidants such as bilirubin and albumin in plasma and airway epithelial lining fluid. Although supplementation with vitamins C and E appears to offer protection against the adverse effects of ozone, recent randomised, placebo-controlled trials of vitamin C or E supplements for patients with mild asthma have not shown significant benefits over standard therapy. However, genetic variation in glutathione S-transferase may influence the susceptibility of asthmatic individuals to oxidative stress and the extent to which they are likely to benefit from antioxidant supplementation. Long-term prospective trials are required to determine whether modification of dietary intake will benefit asthma patients and reduce the socio-economic burden of asthma in the community.     

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.     

Activation of MMP-9 by human lung epithelial cells in response to the cystic fibrosis-associated pathogen Burkholderia cenocepacia reduced wound healing in vitro

Burkholderia cepacia complex is a group of bacterial pathogens that cause opportunistic infections in cystic fibrosis (CF). The most virulent of these is Burkholderia cenocepacia. Matrix metalloproteinases (MMPs) are upregulated in CF patients. The aim of this work was to examine the role of MMPs in the pathogenesis of B. cepacia complex, which has not been explored to date. Real-time PCR analysis showed that B. cenocepacia infection upregulated MMP-2 and MMP-9 genes in the CF lung cell line CFBE41o- within 1 h, whereas MMP-2, -7, and -9 genes were upregulated in the non-CF lung cell line 16HBE14o-. Conditioned media from both cell lines showed increased MMP-9 activation following B. cenocepacia infection. Conditioned media from B. cenocepacia-infected cells significantly reduced the rate of wound healing in confluent lung epithelia (P < 0.05), in contrast to conditioned media from Pseudomonas aeruginosa-infected cells, which showed predominant MMP-2 activation. Treatment of control conditioned media from both cell lines with the MMP activator 4-aminophenylmercuric acetate (APMA) also resulted in clear activation of MMP-9 and to a much lesser extent MMP-2. APMA treatment of control media also delayed the repair of wound healing in confluent epithelial cells. Furthermore, specific inhibition of MMP-9 in medium from cells exposed to B. cenocepacia completely reversed the delay in wound repair. These data suggest that MMP-9 plays a role in the reduced epithelial repair observed in response to B. cenocepacia infection and that its activation following B. cenocepacia infection contributes to the pathogenesis of this virulent pathogen.     

Hydrogen peroxide suppresses U937 cell death by two different mechanisms depending on its concentration

To investigate the mechanisms of H2O2 adaptation in mammalian cells, we exposed human U937 leukemia cells to 0.05 mM H2O2. This treatment significantly suppressed cell death and DNA fragmentation induced by a subsequent challenge with 1 mM H2O2. A more dramatic protection was observed when cells were pretreated with 0.25 mM H2O2. Pretreatment with either 0.05 or 0.25 mM H2O2 also imparted cells with a survival advantage against serum withdrawal and C2-ceramide treatment. H2O2 was found to be a mediator of cell death induced by serum withdrawal, but not by the addition of C2-ceramide. Interestingly, 0.25 mM H2O2 greatly induced glutathione peroxidase, a H2O2-consuming enzyme, whereas 0.05 mM H2O2 did not. Consistent with observation, pretreatment with 0.25 mM H2O2 resulted in a great reduction of cellular oxidant levels as determined by 2'7'-dichlorofluorescein fluorescence, and it also prevented elevation of oxidant levels upon subsequent challenge with 1 mM H2O2 or with serum withdrawal. These effects were not observed in cells pretreated with 0.05 mM H2O2. The sum of the data indicated that H2O2 suppresses cell death by two different mechanisms depending on its concentration: Relatively high concentrations enhance cellular antioxidant capacity, and lower concentrations block the lethal action of H2O2.     

The role of superoxide in the destruction of erythrocyte targets by human neutrophils

Human neutrophils exposed to the soluble stimulus, phorbol myristate acetate, generate a flux of O2.- which can destroy human erythrocyte targets. Under optimal conditions, each neutrophil was capable of lysing almost 10 erythrocyte targets. Hemolysis was inhibited by exogenous copper-zinc or iron superoxide dismutase while neither heat-denatured enzyme nor albumin inhibited cytotoxicity. Although neutrophils can also generate H2O2, neither catalase nor a glutathione-glutathione peroxidase system inhibited hemolysis. Hemolysis was prevented by conversion of the hemoglobin to carbon monoxyhemoglobin, suggesting an intracellular mechanism of cytotoxicity. Conversion of hemoglobin to methemoglobin by nitrite treatment did not impair neutrophil-mediated hemolysis. However, nitrite-treated targets were not protected by superoxide dismutase, while exogenous catalase inhibited cytotoxicity, suggesting a potential role for H2O2 and methemoglobin. H2O2 and methemoglobin are known to interact to form an oxidant complex whose cytotoxic potential was underlined by the marked sensitivity of nitrite-treated cells to commercial H2O2. It is proposed that neutrophil-derived O2.- oxidizes oxyhemoglobin to generate methemoglobin and H2O2 which interact to form a cytotoxic complex capable of hemolyzing the erythrocyte target.     

Novel functions of the alpha-ketoglutarate dehydrogenase complex may mediate diverse oxidant-induced changes in mitochondrial enzymes associated with Alzheimer's disease

Measures in autopsied brains from Alzheimer's Disease (AD) patients reveal a decrease in the activity of alpha-ketoglutarate dehydrogenase complex (KGDHC) and an increase in malate dehydrogenase (MDH) activity. The present experiments tested whether both changes could be caused by the common oxidant H(2)O(2) and to probe the mechanism underlying these changes. Since the response to H(2)O(2) is modified by the level of the E2k subunit of KGDHC, the interaction of MDH and KGDHC was studied in cells with varying levels of E2k. In cells with only 23% of normal E2k protein levels, one-hour treatment with H(2)O(2) decreased KGDHC and increased MDH activity as well as the mRNA level for both cytosolic and mitochondrial MDH. The increase in MDH did not occur in cells with 100% or 46% of normal E2k. Longer treatments with H(2)O(2) inhibited the activity of both enzymes. Glutathione is a major regulator of cellular redox state and can modify enzyme activities. H(2)O(2) converts reduced glutathione (GSH) to oxidized glutathione (GSSG), which reacts with protein thiols. Treatment of purified KGDHC with GSSG leads to glutathionylation of all three KGDHC subunits. Thus, cellular glutathione level was manipulated by two means to determine the effect on KGDHC and MDH activities. Both buthionine sulfoximine (BSO), which inhibits glutathione synthesis without altering redox state, and H(2)O(2) diminished glutathione to a similar level after 24 h. However, H(2)O(2), but not BSO, reduced KGDHC and MDH activities, and the reduction was greater in the E2k-23 line. These findings suggest that the E2k may mediate diverse responses of KGDHC and MDH to oxidants. In addition, the differential response of activities to BSO and H(2)O(2) together with the in vitro interaction of KGDHC with GSSG suggests that glutathionylation is one possible mechanism underlying oxidative stress-induced inhibition of the TCA cycle enzymes.