Showing papers on "Oxidative stress published in 1993"

The role of active oxygen in the response of plants to water deficit and desiccation.

Abstract: Water deficits cause a reduction in the rate of photosynthesis. Exposure to mild water deficits, when relative water content (RWC) remains above 70%, primarily causes limitation to carbon dioxide uptake because of stomatal closure. With greater water deficits, direct inhibition of photosynthesis occurs. In both cases limitation of carbon dioxide fixation results in exposure of chloroplasts to excess excitation energy. Much of this can be dissipated by various photoprotective mechanisms. These include dissipation as heat via carotenoids, photorespiration, CAM idling and, in some species, leaf movements and other morphological features which minimize light absorption. The active oxygen species superoxide and singlet oxygen are produced in chloroplasts by photoreduction of Oxygen and energy transfer from triplet excited chlorophyll to oxygen, respectively. Hydrogen peroxide and hydroxyl radicals can form as a result of the reactions of superoxide. All these species are reactive and potentially damaging, causing lipid peroxidation and inactivation of enzymes. They are normally scavenged by a range of antioxidants and enzymes which are present in the chloroplast and other subcellular compartments. When carbon dioxide fixation is limited by water deficit, the rate of active oxygen formation increases in chloroplasts as excess excitation energy, not dissipated fay the photoprotective mechanisms, is used to form superoxide and singlet oxygen. However, photorespiratory hydrogen peroxide production in peroxisomes decreases. Increased superoxide can be detected by EPR (electron paramagnetic resonance) in chloroplasts from droughted plants. Stiperoxide formation leads to changes suggestive of oxidative damage including lipid peroxidation and a decrease in ascorbate. These changes are not, however, apparent until severe water deficits develop, and they could also be interpreted as secondary effects of water deficit-induced senescence or wounding. Non-lethal water deficits often result in increased activity of superoxide dismutase, glutathione reductase and monodehydroascorbate reductase. Increased capacity of these protective enzymes may be part of a general antioxidative response in plants involving regulation of protein synthesis or gene expression. Since the capacity of these enzymes is also increased by other treatments which cause oxidative damage, and which alter the balance between excitation energy input and carbon dioxide fixation such as low temperature and high irradiance, it is suggested that water deficit has the same effect. Light levels that are not normally excessive do become excessive and photoprotective/antioxidative systems are activated. Some of the photoprotective mechanisms themselves could result in active oxygen formation. Photoinhibitory damage also includes a component of oxidative damage. During normally-encountered degrees of water deficit the capacity of the antioxidant systems and their ability to respond to increased active oxygen generation may be sufficient to prevent overt expression of damage. Desiccation-tolerant tissues such as bryophytes, lichens, spores, seeds, some algae and a few vascular plant leaves can survive desiccation to below 30-40% RWC, A component of desiccation damage in seeds and bacteria is oxygen-dependent. Desiccation causes oxidation of glutathione, a major antioxidant, and appearance of a free radical signal detected by EPR in a number of tissues suggesting that oxidative damage has occurred. In photosynthetic cells damage may arise from photooxidation. Disruption of membrane-bound electron tranport systems in partially hydrated tissue could lead to reduction of oxygen to superoxide. Oxidation of lipids and sulphydryl groups may also occur in dry tissue. Tolerant cells recover upon rehydration and arc able to reduce their glutathione pool. Non-tolerant species go on to show further oxidative damage including lipid peroxidation. It is difficult to attribute this subsequent damage to the cause or effect of death. Embryos in seeds lose desiccation tolerance soon after imbibition. This is associated with membrane damage that has been attributed to superoxide-mediated deesterification of phospholipids and loss of lipophilic antioxidants. These effects are discussed in relation to other mechanisms involved in desiccation tolerance. Contents Summary 27 I. Introduction 28 II. Generation of active oxygen and defence mechanisms in plant cells 29 III. The effect of water deficit on photosynthesis 31 IV. Mechanisms for active oxygen generation during water deficit 36 V. Evidence for oxidative damage during water deficit 39 VI. Desiccation 47 VII. Conclusions 52 Acknowledgements 53 References 53.

Nitric oxide protects against cellular damage and cytotoxicity from reactive oxygen species

Abstract: Nitric oxide, NO, which is generated by various components of the immune system, has been presumed to be cytotoxic. However, NO has been proposed to be protective against cellular damage resulting during ischemia reperfusion. Along with NO there is often concomitant formation of superoxide/hydrogen peroxide, and hence a synergistic relationship between the cytotoxic effects of nitric oxide and these active oxygen species is frequently assumed. To study more carefully the potential synergy between NO and active oxygen species in mammalian cell cytotoxicity, we utilized either hypoxanthine/xanthine cell cytotoxicity, we utilized either hypoxanthine/xanthine oxidase (a system that generates superoxide/hydrogen peroxide) or hydrogen peroxide itself. NO generation was accomplished by the use of a class of compounds known as "NONOates," which release NO at ambient temperatures without the requirement of enzyme activation or biotransformation. When Chinese hamster lung fibroblasts (V79 cells) were exposed to hypoxanthine/xanthine oxidase for various times or increasing amounts of hydrogen peroxide, there was a dose-dependent decrease in survival of V79 cells as measured by clonogenic assays. However, in the presence of NO released from (C2H5)2N[N(O)NO]-Na+ (DEA/NO), the cytotoxicity resulting from superoxide or hydrogen peroxide was markedly abrogated. Similarly, primary cultures of rat mesencephalic dopaminergic cells exposed either to hydrogen peroxide or to hypoxanthine/xanthine oxidase resulted in the degradation of the dopamine uptake and release mechanism. As was observed in the case of the V79 cells, the presence of NO essentially abrogated this peroxide-mediated cytotoxic effect on mesencephalic cells.

Aging and resistance to oxidative damage in Caenorhabditis elegans.

Abstract: The dauer larva state and the age-1 mutation, both of which extend life-span in Caenorhabditis elegans, were tested for hyperresistance to cellular damage that may be relevant to aging. The age-1 strain TJ401 displayed hyperresistance to oxidative stress relative to its parental strain. The activities of two enzymes that protect cells from oxidative damage, superoxide dismutase (SOD) and catalase, showed an age-dependent increase in mutant animals, which was not seen in the parental strain. These increases in activities paralleled the time course of the hyperresistance. The results are consistent with the age-1 gene product functioning as a negative regulator of SOD and catalase activities. In wild-type and age-1 dauer larvae, elevated levels of SOD activity, but not of catalase activity, were present when compared with young adults. The common increase in SOD activity prompted cloning the C. elegans Cu/Zn SOD gene. Its position on the physical map of the genome was in the region to which the age-1 gene has been genetically mapped, but it is unlikely that a mutation at the SOD locus confers the Age phenotype. Results support the free radical theory of aging by suggesting that the increased resistance to oxidative stress may be among the causes of increased longevity in both strain TJ401 and in the dauer larva.

Diabetes mellitus and free radicals. Free radicals, transition metals and oxidative stress in the aetiology of diabetes mellitus and complications.

Abstract: Diabetes mellitus is a syndrome initially characterized by a loss of glucose homeostasis. The disease is progressive and is associated with high risk of atherosclerosis, kidney and nerve damage as well as blindness. Abnormalities in the regulation of peroxide and transition metal metabolism are postulated to result in establishment of the disease as well as its longer term complications. Diabetes mellitus is associated with oxidative reactions, particularly those which are catalyzed by decompartmentalized transition metals, but their causative significance in diabetic tissue damage remains to be established.

Exercise-induced oxidative stress

Abstract: The role of exercise in free radical processes is not clear; however, recent evidence suggests that elevated oxygen consumption may increase free radical activity. Direct measurement of free radical signals can be made by electron spin resonance and indirect measures include mitochondrial membrane damage, conjugated dienes, hydroperoxides, thiobarbituric acid reactive substances, short chain hydrocarbons, and oxidized nucleosides. Although exact levels are not known, the type, duration, and intensity of exercise affect biomarkers of free radical activity, as does one's training status. Oxidative stress associated with exercise-induced free radical activity seems to be better tolerated by trained subjects exercising at moderate intensity.

Melatonin, hydroxyl radical‐mediated oxidative damage, and aging: A hypothesis

Abstract: Melatonin is a very potent and efficient endogenous radical scavenger. The pineal indolamine reacts with the highly toxic hydroxyl radical and provides on-site protection against oxidative damage to biomolecules within every cellular compartment. Melatonin acts as a primary non-enzymatic antioxidative defense against the devastating actions of the extremely reactive hydroxyl radical. Melatonin and structurally related tryptophan metabolites are evolutionary conservative molecules principally involved in the prevention of oxidative stress in organisms as different as algae and rats. The rate of aging and the time of onset of age-related diseases in rodents can be retarded by the administration of melatonin or treatments that preserve the endogenous rhythm of melatonin formation. The release of excitatory amino acids such as glutamate enhances endogenous hydroxyl radical formation. The activation of central excitatory amino acid receptors suppress melatonin synthesis and is therefore accompanied by a reduced detoxification rate of hydroxyl radicals. Aged animals and humans are melatonin-deficient and more sensitive to oxidative stress. Experiments investigating the effects of endogenous excitatory amino acid antagonists and stimulants of melatonin biosynthesis such as magnesium may finally lead to novel therapeutic approaches for the prevention of degeneration and dysdifferentiation associated with diseases related to premature aging.

Oxidative Stimulation of Glutathione Synthesis in Arabidopsis thaliana Suspension Cultures.

Abstract: A system based on Arabidopsis thaliana suspension cultures was established for the analysis of glutathione (GSH) synthesis in the presence of hydrogen peroxide. Mild oxidative stress was induced by use of the catalase inhibitor, aminotriazole, and its development was monitored by measurement of the oxidative inactivation of aconitase. Addition of 2 mM aminotriazole resulted in a 25% decrease in activity of aconitase over 4 h. During the subsequent 10 h, no further decrease in aconitase activity was measured despite a sustained inhibition of catalase. In combination with our failure to detect significant increases in the level of lipid peroxidation, another marker indicative of oxidative injury, these data suggest that although hydrogen peroxide initially leaked into the cytosol, its accumulation was limited by a cytosolic catalase-independent mechanism. A 4-fold increase in the level of GSH, which was almost exclusively in the reduced form, was observed under the same treatment. To determine to what extent this increase in reduced GSH played a role in limiting the accumulation of hydrogen peroxide in the cytosol, we inhibited GSH synthesis with buthionine sulfoximine (BSO), a specific inhibitor of [gamma]-glutamylcysteine synthetase. No significant oxidative injury was detected as a result of treatment with 50 [mu]M BSO alone, and furthermore, this treatment had no effect on cell viability, However, addition of 2 mM aminotriazole to cells preincubated with 50 [mu]M BSO for 15 h led to a rapid loss of aconitase activity (75% in 4 h), and significant accumulation of products of lipid peroxidation. Within 72 h, cell viability was lost completely. After removal of BSO from the growth medium, GSH levels recovered to normal over a period of 20 h. Addition of 2 mM aminotriazole to cells at different time points during this recovery period demonstrated a strong correlation between the level of reduced GSH and the degree of protection against oxidative injury. These data strongly suggest that the induction of GSH synthesis by an oxidative stimulus plays a crucial role in determining the susceptibility of cells to oxidative stress.

Antioxidant enzyme response to exercise and aging.

Abstract: Antioxidant enzymes play an important role in defending the cells against free radical-mediated oxidative damage. The present investigations, using rats as models, indicate that antioxidant enzyme systems undergo significant alteration during aging and in response to acute and chronic exercise. Hepatic and myocardial antioxidant enzymes show a general decline at older age, whereas activity of glutathione-related enzymes in the liver and mitochondrial enzymes in the heart increase significantly. Skeletal muscle antioxidant enzymes are uniformly elevated during aging. An acute bout of exercise can increase activity of certain antioxidant enzymes in various tissues. The mechanism for this activation is unclear. Exercise training has little effect on hepatic or myocardial enzyme systems but can cause adaptive responses in skeletal muscle antioxidant enzymes, particularly glutathione peroxidase. These findings suggest that both aging and exercise may impose an oxidative stress to the body.

Oxidative stress and ageing in Caenorhabditis elegans.

Abstract: Mutations in the age-1 gene double both the mean and maximum life span of Caenorhabditis elegans. They also result in an age-specific increase of catalase and Cu/Zn superoxide dismutase activity levels. The higher superoxide dismutase activity levels in age-1 mutants confer hyperresistance to the superoxide-anion-generating drug paraquat. The rate of superoxide anion production by microsome fractions declines linearly with age in age-1(+) worms, but, after an initial decline, is stabilized at a higher level in senescent age-1 mutant nematodes. These results clearly show that oxidative stress resistance and potential life span are correlated in this organism, and they suggest that the natural product of age-1 either directly or indirectly downregulates the activities of several other genes as a function of age.

Salt-induced oxidative stress mediated by activated oxygen species in pea leaf mitochondria

Abstract: The effect in vivo of salt stress on the activated oxygen metabolism of mitochondria, was studied in leaves from two NaCl-treated cultivars of Pisum sativum L. with different sensitivity to NaCl. In mitochondria from NaCl-sensitive plants, salinity brought about a significant decrease of Mn-SOD (EC 1. 15. 1. 1) Cu, Zn-SOD I (EC 1. 15. 1. 1) and fumarase (EC 4. 2. 1. 2) activities. Conversely, in salt-tolerant plants NaCl treatment produced an increase in the mitochondrial Mn-SOD activity and, to a lesser extent, in fumarase activity. In mitochondria from both salt-treated cultivars, the internal H2O2 concentration remained unchanged. The NADH- and succinate-dependent generation of O2.−radicals by submitochondrial particles and the lipid peroxidation of mitochondrial membranes, increased as a result of salt treatment, and these changes were higher in NaCl-sensitive than in NaCl-tolerant plants. Accordingly, the enhanced rates of superoxide production by mitochondria from salt-sensitive plants were concomitant with a strong decrease in the mitochondrial Mn-SOD activity, whereas NaCl-tolerant plants appear to have a protection mechanism against salt-induced increased O2.− production by means of the induction of the mitochondrial Mn-SOD activity. These results indicate that in the subcellular toxicity of NaCl in pea plants, at the level of mitochondria, an oxidative stress mechanism mediated by superoxide radicals is involved, and also imply a function for mitochondrial Mn-SOD in the molecular mechanisms of plant tolerance to NaCl.

The Role of Oxygen Radicals in Human Disease, with Particular Reference to the Vascular System

Abstract: Free radicals, such as superoxide, hydroxyl and nitric oxide, and other reactive oxygen species (ROS), such as hydrogen peroxide, are formed in vivo. Imbalance between production of ROS and anti-oxidant defence can result in oxidative stress, which may arise either from deficiencies of anti-oxidants (such as glutathione, ascorbate or alpha-tocopherol) and/or from increased formation of ROS. Oxidative stress can result in glutathione depletion, lipid peroxidation, membrane damage and DNA strand breaks as well as activation of proteases, nucleases and protein kinases. Some degree of oxidative stress occurs in most, if not all, human diseases, and the major question to be answered is whether it makes a significant contribution to the disease pathology. In the case of atherosclerosis, evidence from studies with the chain-breaking anti-oxidant probucol and from epidemiological work suggests that oxidative damage does indeed make an important contribution to plaque development.

Protein oxidative damage is associated with life expectancy of houseflies.

Abstract: The objective of this study was to test some of the predictions of the oxidative-stress hypothesis of aging, which postulates that aging is causally associated with the molecular damage inflicted by reactive oxygen species. Protein carbonyl content was used as an index of molecular oxidative modifications. The carbonyl content was found to be associated with the physiological age or life expectancy of flies rather than with their chronological age. Exposure of flies to sublethal hyperoxia (100% oxygen) irreversibly enhanced the carbonyl content of the flies and decreased their rate of oxygen consumption. Results of this study indicate that protein carbonyl content may be a biomarker of aging and support the general concept that oxidative stress may be a causal factor in the aging process.

DNA and Free Radicals

Abstract: Oxidants are major contributors to cancer and aging. Part 1 Chemistry and measurement of damage to DNA by reactive oxygen species: chemistry of free radical damage to DNA and nucleoproteins DNA damage induced by photosensitization oxidative damage - meaning and measurement. Part 2 Mechanism of DNA damage by oxidative stress: hydrogen peroxide and DNA damage oxidative stress and calcium homeostasis lipid peroxidation and cancer the role of organic peroxyl radicals in carcinogenesis. Part 3 Consequences of oxidative DNA damage: enzymes that repair oxidative damage to DNA sex as a response to oxidative DNA damage oxidative stress and cell proliferation "in vitro" ornithine decarboxylase and tumour promotion - a role for oxidants. Part 4 Pro-oxidants and DNA damage: redox cycling drugs and DNA damage role of reactive oxygen species in the mutagenicity of complex mixtures of plant origin mineral fibres, cigarette smoke and oxidative DNA damage use of DNA damage as a measure of pro-oxidant actions of antioxidant food additives and nutrient components.

Genetic control of inflammatory gene induction and NF-kappa B-like transcription factor activation in response to an atherogenic diet in mice.

Abstract: A high fat, high cholesterol "atherogenic" diet induced considerably greater hepatic levels of conjugated dienes and expression of several inflammatory and oxidative stress responsive genes (JE, the mouse homologue of monocyte chemotactic protein-1, colony-stimulating factors, heme oxygenase, and members of the serum amyloid A family) in fatty streak susceptible C57BL/6 mice compared to fatty streak resistant C3H/HeJ mice. Since serum amyloid A proteins bind exclusively to HDL and influence the properties of HDL, serum amyloid A expression may contribute to the decrease in HDL levels seen in the susceptible strains. Induction of a similar set of genes was observed upon injection of minimally oxidized low density lipoprotein. The transcription factor NF-kappa B is known to be activated by oxidative stress and is involved in the transcriptional regulation of several of these genes. On the atherogenic diet the susceptible C57BL/6 mice exhibited significant NF-kappa B-like activation whereas the resistant C3H/HeJ mice exhibited little or no activation. These results are consistent with the hypothesis that the atherogenic diet resulted in the accumulation of oxidized lipids in certain tissues (e.g., liver and arteries) and the resulting inflammatory response to this oxidative stress was genetically determined.

Time course and mechanism of oxidative stress and tissue damage in rat liver subjected to in vivo ischemia-reperfusion.

Abstract: The time course of oxidative stress and tissue damage in zonal liver ischemia-reperfusion in rat liver in vivo was evaluated. After 180 min of ischemia, surface chemiluminescence decreased to zero, state 3 mitochondrial respiration decreased by 70-80%, and xanthine oxidase activity increased by 26% without change in the water content and in the activities of superoxide dismutase, catalase, and glutathione peroxidase. After reperfusion, marked increases in oxyradical production and tissue damage were detected. Mitochondrial oxygen uptake in state 3 and respiratory control as well as the activities of superoxide dismutase, catalase, and glutathione peroxidase and the level of nonenzymatic antioxidants (evaluated by the hydroperoxide-initiated chemiluminescence) were decreased. The severity of the post-reperfusion changes correlated with the time of ischemia. Morphologically, hepatocytes appeared swollen with zonal cord disarrangement which ranged from mild to severe for the tissue reperfused after 60-180 min of ischemia. Neutrophil infiltration was observed after 180 min of ischemia and 30 min of reperfusion. Mitochondria appear as the major source of hydrogen peroxide in control and in reperfused liver, as indicated by the almost complete inhibition of hydrogen peroxide production exerted by the uncoupler carbonylcyanide p-(trifluoromethoxy) phenylhydrazone. Additionally, inhibition of mitochondrial electron transfer by antimycin in liver slices reproduced the inhibition of state 3 mitochondrial respiration and the increase in hydrogen peroxide steady-state concentration found in reperfused liver. Increased rates of oxyradical production by inhibited mitochondria appear as the initial cause of oxidative stress and liver damage during early reperfusion in rat liver.

The possible role of iron in the etiopathology of parkinson's disease

Abstract: The identification of 6-hydroxydopamine (6-OHDA) and N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) as dopaminergic neurotoxins that can induce parkinsonism in humans and animals has contributed to a better understanding of Parkinson's disease (PD). Although the involvement of similar neurotoxins has been implicated in PD, the etiology of the disease remains obscure. However, the recently described pathology of PD supports the view for a state of oxidative stress in the substantia nigra (SN), resulting as a consequence of the selective accumulation of iron in SN zona compacta and within the melanized dopamine neurons. Whether iron is directly involved cannot be ascertained. Nevertheless, the biochemical changes due to oxidative stress resulting from tissue iron overload (siderosis) are similar to those now being identified in parkinsonian SN. These include the reduction of mitochondrial electron transport, complex I and III activities, glutathione peroxidase activity, glutathione (GSH) ascorbate, calcium-binding protein, and superoxide dismutase and increase of basal lipid peroxidation and deposition of iron. The participation of iron-induced oxygen free radicals in the process of nigrostriatal dopamine neuron degeneration is strengthened by recent studies in which the neurotoxicity of 6-OHDA has been linked to the release of iron from its binding sites in ferritin. This is further supported by experiments with the prototype iron chelator, desferrioxamine (Desferal), a free-radical inhibitor, which protects against 6-OHDA-induced lesions in the rat. Indeed, intranigral iron injection in rats produces a selective lesioning of dopamine neurons, resulting in a behavioral and biochemical parkinsonism.

Tissue glutathione, nutrition, and oxidative stress.

Abstract: Glutathione, a cysteine-containing tripeptide, is the most abundant nonprotein thiol in mammalian cells. Glutathione plays an important role in the detoxification of xenobiotic compounds and in the...

Metallothionein protects DNA from oxidative damage.

Abstract: Metallothionein (MT) is a potent hydroxyl radical scavenger but its antioxidant properties in vivo have not been defined. Most of the recent results indicate that it does not afford protection to cells against the lethal action of oxidative stress. However, the possibility that MT confers protection against oxidative damage to a specific cellular target, such as DNA, had not been considered. We compared V79 Chinese hamster cells enriched in and depleted of MT in terms of DNA-strand scission. Zinc induces an increase in MT content of V79 Chinese hamster cells, without concomitant increase in the GSH level. These induced cells are more resistant to the production of DNA-strand scission by H2O2 than the parental cells. Conversely, cells rendered partially deprived of MT, by transfection with a plasmid vector in which the MT-I cDNA is antisense oriented in relation to a simian virus 40 promoter, became more susceptible to the DNA-damaging action of H2O2. The transfected cells did not exhibit alterations of GSH, superoxide dismutase- and H2O2-destroying enzymes. Indirect immunofluorescence indicated that most of the MT was concentrated in the cell nucleus. Neither overexpression nor lower expression of MT resulted in differential resistance to the killing action of H2O2. However, the combined high nuclear concentration of MT and its excellent hydroxyl scavenger properties confer protection to DNA from hydroxyl radical attack.

Oxidative DNA damage and cellular sensitivity to oxidative stress in human autoimmune diseases.

Abstract: OBJECTIVES--To estimate the extent of genomic DNA damage and killing of lymphocytes by reactive oxygen intermediates in autoimmune diseases. METHODS--8-Oxo-7-hydrodeoxyguanosine (8-oxodG), a promutagenic DNA lesion induced by reactive oxygen intermediates, was measured by high performance liquid chromatography, coupled with electrochemical detection, in hydrolysates of DNA which had been extracted from lymphocyte and polymorphonuclear leucocyte fractions of human blood. In addition, human primary blood lymphocytes stimulated by concanavalin A were assayed for cytotoxicity induced by hydrogen peroxide on day 0, by assessing cell proliferation during seven days of culture. RESULTS--Constitutive 8-oxodG was detectable (mean (2 SEM) moles 8-oxodG/10(6) moles deoxyguanosine) in DNA isolated from normal human blood lymphocytes (68 (8), n = 26) and polymorphonuclear leucocytes (118 (24), n = 24). Lymphocyte DNA from donors with the following inflammatory autoimmune diseases contained significantly higher levels of 8-oxodG than that from healthy donors: rheumatoid arthritis (98 (16)), systemic lupus erythematosus (137 (28)), vasculitis (100 (32)), and Behcet's disease (92 (19)). Lymphocyte 8-oxodG levels in non-autoimmune controls and patients with scleroderma were not significantly different from those of healthy controls. The levels of 8-oxodG were significantly higher in the DNA from normal polymorphonuclear leucocytes than in paired DNA samples from normal lymphocytes, but there were no differences between levels of 8-oxodG in polymorphonuclear leucocytes from normal subjects and the patients studied. Levels of 8-oxodG did not correlate with disease duration, disease severity, or age. Lymphocytes from patients with systemic lupus erythematosus and rheumatoid arthritis, but not those with scleroderma, also showed cellular hypersensitivity to the toxic effects of hydrogen peroxide. CONCLUSION--There was increased genomic DNA damage, and increased susceptibility to cytotoxic killing by hydrogen peroxide, in lymphocytes from patients with certain autoimmune diseases. These results might be explained by defective repair of DNA damage or by increased production of reactive oxygen intermediates in inflammation. Although more direct studies are needed, the evidence available favours the former explanation.