Superoxide sensitivity of the Escherichia coli aconitase

Substantial DNA damage from submicromolar intracellular hydrogen peroxide detected in Hpx- mutants of Escherichia coli

Since the discovery of catalase, it has been postulated that aerobic organisms generate enough oxidants to threaten their own fitness and, in particular, their genetic stability. An alternative is that these enzymes exist to defend the cell against more-abundant oxidants imposed by external sources. These hypotheses were tested directly through study of Hpx- (katG katE ahpCF) mutants of Escherichia coli, which lack enzymes to scavenge hydrogen peroxide (H2O2). These strains grew well in anaerobic medium but poorly when they were aerated. The Hpx- bacteria formed filaments and exhibited high rates of mutagenesis, both indicators of DNA damage. An additional recA mutation caused Hpx- cells to die rapidly upon aeration, even though the intracellular H2O2 was <1 microM. Spin-trap experiments detected substantial hydroxyl radicals, and cell-permeable iron chelators eliminated both the phenotypic defects and hydroxyl-radical formation, confirming that the Fenton reaction was responsible. An Hpx- oxyR strain exhibited even more DNA lesions than did the Hpx- mutant, indicating that the OxyR stress response induced protein(s) that suppressed DNA damage. One critical protein was Dps, an iron-sequestration protein, because Hpx- dps mutants exhibited sensitivity similar to that of the Hpx- oxyR mutant. These results reveal that aerobic E. coli generates sufficient H2O2 to create toxic levels of DNA damage. Scavenging enzymes and controls on free iron are required to avoid that fate. The rate constant of the Fenton reaction measured at physiological pH was much higher than under the acidic conditions that were used to determine the commonly cited value.

Overexpression of manganese superoxide dismutase promotes the survival of prostate cancer cells exposed to hyperthermia

It has been hypothesized that exposure of cells to hyperthermia results in an increased flux of reactive oxygen species (ROS), primarily superoxide anion radicals, and that increasing antioxidant enzyme levels will result in protection of cells from the toxicity of these ROS. In this study, the prostate cancer cell line, PC-3, and its manganese superoxide dismutase (MnSOD)-overexpressing clones were subjected to hyperthermia (43 degrees C, 1 h). Increased expression of MnSOD increased the mitochondrial membrane potential (MMP). Hyperthermic exposure of PC-3 cells resulted in increased ROS production, as determined by aconitase inactivation, lipid peroxidation, and H2O2 formation with a reduction in cell survival. In contrast, PC-3 cells overexpressing MnSOD had less ROS production, less lipid peroxidation, and greater cell survival compared to PC-3 Wt cells. Since MnSOD removes superoxide, these results suggest that superoxide free radical or its reaction products are responsible for part of the cytotoxicity associated with hyperthermia and that MnSOD can reduce cellular injury and thereby enhance heat tolerance.

Selective neuronal vulnerability and inadequate stress response in superoxide dismutase mutant mice

To understand the role of oxidative stress and mitochondrial defects in the development of neurodegeneration, we examined the age-related pathological changes and corresponding gene expression profiles in homozygous mutant mice deficient in the mitochondrial form of superoxide dismutase (MnSOD, SOD2). These Sod2-/- mice, generated on a B6D2F1 background, developed ataxia at Postnatal Day (P) 11 and progressively deteriorated with frequent seizures by P14. Histopathological examination revealed neurodegenerative changes consistent with the neurological signs. Vacuolar degeneration was observed in neurons and neuropil throughout the brainstem and rostral cortex. The motor trigeminal nucleus in brainstem and the deeper layers of the motor cortex were the earliest regions to degenerate, with the thalamus and hippocampus affected at later stages. Oligonucleotide microarrays were used to compare gene expression profiles in the brainstem and thalamus of Sod2+/+ and -/- mice from birth to P18. Notably, a large set of heat-shock protein genes was transcriptionally down regulated, and this was most likely due to a reduction in the heat-shock transcription factor 1 (HSF1). Other major classes of differentially expressed genes include lipid biosynthesis and ROS metabolism.

CuZnSOD deficiency leads to persistent and widespread oxidative damage and hepatocarcinogenesis later in life

Mice deficient in CuZn superoxide dismutase (CuZnSOD) showed no overt abnormalities during development and early adulthood, but had a reduced lifespan and increased incidence of neoplastic changes in the liver. Greater than 70% of Sod1-/- mice developed liver nodules that were either nodular hyperplasia or hepatocellular carcinoma (HCC). Cross-sectional studies with livers collected from Sod1-/- and age-matched +/+ controls revealed extensive oxidative damage in the cytoplasm and, to a lesser extent, in the nucleus and mitochondria from as early as 3 months of age. A marked reduction in cytosolic aconitase, increased levels of 8-oxo dG and F2-isoprostanes, and a moderate reduction in glutathione peroxidase activities and porin levels were observed in all age groups of Sod1-/- mice examined. There were also age-related reductions in Mn superoxide dismutase activities and carbonic anhydrase III. Parallel to the biochemical changes, there were progressive increases in the DNA repair enzyme APEX1, the cell cycle control proteins cyclin D1 and D3, and the hepatocyte growth factor receptor Met. Increased cell proliferation in the presence of persistent oxidative damage to macromolecules likely contributes to hepatocarcinogenesis later in life.

Nickel decreases cellular iron level and converts cytosolic aconitase to iron-regulatory protein 1 in A549 cells

Nickel (Ni) compounds are well-established carcinogens and are known to initiate a hypoxic response in cells via the stabilization and transactivation of hypoxia-inducible factor-1 alpha (HIF-1alpha). This change may be the consequence of nickel's interference with the function of several Fe(II)-dependent enzymes. In this study, the effects of soluble nickel exposure on cellular iron homeostasis were investigated. Nickel treatment decreased both mitochondrial and cytosolic aconitase (c-aconitase) activity in A549 cells. Cytosolic aconitase was converted to iron-regulatory protein 1, a form critical for the regulation of cellular iron homeostasis. The increased activity of iron-regulatory protein 1 after nickel exposure stabilized and increased transferrin receptor (Tfr) mRNA and antagonized the iron-induced ferritin light chain protein synthesis. The decrease of aconitase activity after nickel treatment reflected neither direct interference with aconitase function nor obstruction of [4Fe-4S] cluster reconstitution by nickel. Exposure of A549 cells to soluble nickel decreased total cellular iron by about 40%, a decrease that likely caused the observed decrease in aconitase activity and the increase of iron-regulatory protein 1 activity. Iron treatment reversed the effect of nickel on cytosolic aconitase and iron-regulatory protein 1. To assess the mechanism for the observed effects, human embryonic kidney (HEK) cells over expressing divalent metal transporter-1 (DMT1) were compared to A549 cells expressing only endogenous transporters for inhibition of iron uptake by nickel. The inhibition data suggest that nickel can enter via DMT1 and compete with iron for entry into the cell. This disturbance of cellular iron homeostasis by nickel may have a great impact on the ability of the cell to regulate a variety of cell functions, as well as create a state of hypoxia in cells under normal oxygen tension. These effects may be very important in how nickel exerts phenotypic selection pressure to convert a normal initiated cell into a cancer cell.

Bacterial redox sensors

Redox reactions pervade living cells. They are central to both anabolic and catabolic metabolism. The ability to maintain redox balance is therefore vital to all organisms. Various regulatory sensors continually monitor the redox state of the internal and external environments and control the processes that work to maintain redox homeostasis. In response to redox imbalance, new metabolic pathways are initiated, the repair or bypassing of damaged cellular components is coordinated and systems that protect the cell from further damage are induced. Advances in biochemical analyses are revealing a range of elegant solutions that have evolved to allow bacteria to sense different redox signals.

Oxidants, antioxidants and the ischemic brain

Despite numerous defenses, the brain is vulnerable to oxidative stress resulting from ischemia/reperfusion. Excitotoxic stimulation of superoxide and nitric oxide production leads to formation of highly reactive products, including peroxynitrite and hydroxyl radical, which are capable of damaging lipids, proteins and DNA. Use of transgenic mutants and selective pharmacological antioxidants has greatly increased understanding of the complex interplay between substrate deprivation and ischemic outcome. Recent evidence that reactive oxygen/nitrogen species play a critical role in initiation of apoptosis, mitochondrial permeability transition and poly(ADP-ribose) polymerase activation provides additional mechanisms for oxidative damage and new targets for post-ischemic therapeutic intervention. Because oxidative stress involves multiple post-ischemic cascades leading to cell death, effective prevention/treatment of ischemic brain injury is likely to require intervention at multiple effect sites.

Mitochondrial dysfunction and oxidative stress: cause and consequence of epileptic seizures

Mitochondrial dysfunction has been implicated as a contributing factor in diverse acute and chronic neurological disorders. However, its role in the epilepsies has only recently emerged. Animal studies show that epileptic seizures result in free radical production and oxidative damage to cellular proteins, lipids, and DNA. Mitochondria contribute to the majority of seizure-induced free radical production. Seizure-induced mitochondrial superoxide production, consequent inactivation of susceptible iron-sulfur enzymes, e.g., aconitase, and resultant iron-mediated toxicity may mediate seizure-induced neuronal death. Epileptic seizures are a common feature of mitochondrial dysfunction associated with mitochondrial encephalopathies. Recent work suggests that chronic mitochondrial oxidative stress and resultant dysfunction can render the brain more susceptible to epileptic seizures. This review focuses on the emerging role of oxidative stress and mitochondrial dysfunction both as a consequence and as a cause of epileptic seizures.

Isolation and characterization of Arabidopsis thaliana ISU1 gene

We describe the isolation of a cDNA encoding Arabidopsis thaliana ISU1 (AtISU1), which regulates iron homeostasis in the mitochondria. The AtISU1 gene contained an open reading frame that encoded 167 amino acid residues. Northern blot analysis demonstrated that AtISU1 gene was ubiquitously expressed in plant tissues examined. The yeast seo5-1, which harbors a single base-pair deletion in ScISU1, is a suppressor of oxidative damage in sod1-deficient mutant. Based on comparative expression analyses using yeast ISU1 gene (ScISU1) in seo5-1 mutant, we found that AtISU1 acts as a counterpart of ScISU1.

Oxygen Tolerance and Coupling of Mitochondrial Electron Transport

Oxygen is critical to aerobic metabolism, but excessive oxygen (hyperoxia) causes cell injury and death. An oxygen-tolerant strain of HeLa cells, which proliferates even under 80% O2, termed "HeLa-80," was derived from wild-type HeLa cells ("HeLa-20") by selection for resistance to stepwise increases of oxygen partial pressure. Surprisingly, antioxidant defenses and susceptibility to oxidant-mediated killing do not differ between these two strains of HeLa cells. However, under both 20 and 80% O2, intracellular reactive oxygen species (ROS) production is significantly (approximately 2-fold) less in HeLa-80 cells. In both cell lines the source of ROS is evidently mitochondrial. Although HeLa-80 cells consume oxygen at the same rate as HeLa-20 cells, they consume less glucose and produce less lactic acid. Most importantly, the oxygen-tolerant HeLa-80 cells have significantly higher cytochrome c oxidase activity (approximately 2-fold), which may act to deplete upstream electron-rich intermediates responsible for ROS generation. Indeed, preferential inhibition of cytochrome c oxidase by treatment with n-methyl protoporphyrin (which selectively diminishes synthesis of heme a in cytochrome c oxidase) enhances ROS production and abrogates the oxygen tolerance of the HeLa-80 cells. Thus, it appears that the remarkable oxygen tolerance of these cells derives from tighter coupling of the electron transport chain.

Role of oxidative modifications in atherosclerosis

This review focuses on the role of oxidative processes in atherosclerosis and its resultant cardiovascular events. There is now a consensus that atherosclerosis represents a state of heightened oxidative stress characterized by lipid and protein oxidation in the vascular wall. The oxidative modification hypothesis of atherosclerosis predicts that low-density lipoprotein (LDL) oxidation is an early event in atherosclerosis and that oxidized LDL contributes to atherogenesis. In support of this hypothesis, oxidized LDL can support foam cell formation in vitro, the lipid in human lesions is substantially oxidized, there is evidence for the presence of oxidized LDL in vivo, oxidized LDL has a number of potentially proatherogenic activities, and several structurally unrelated antioxidants inhibit atherosclerosis in animals. An emerging consensus also underscores the importance in vascular disease of oxidative events in addition to LDL oxidation. These include the production of reactive oxygen and nitrogen species by vascular cells, as well as oxidative modifications contributing to important clinical manifestations of coronary artery disease such as endothelial dysfunction and plaque disruption. Despite these abundant data however, fundamental problems remain with implicating oxidative modification as a (requisite) pathophysiologically important cause for atherosclerosis. These include the poor performance of antioxidant strategies in limiting either atherosclerosis or cardiovascular events from atherosclerosis, and observations in animals that suggest dissociation between atherosclerosis and lipoprotein oxidation. Indeed, it remains to be established that oxidative events are a cause rather than an injurious response to atherogenesis. In this context, inflammation needs to be considered as a primary process of atherosclerosis, and oxidative stress as a secondary event. To address this issue, we have proposed an "oxidative response to inflammation" model as a means of reconciling the response-to-injury and oxidative modification hypotheses of atherosclerosis.

Differences in enzymatic properties allow SodCI but not SodCII to contribute to virulence in Salmonella enterica serovar Typhimurium strain 14028

Salmonella enterica serovar Typhimurium produces two Cu/Zn cofactored periplasmic superoxide dismutases, SodCI and SodCII. While mutations in sodCI attenuate virulence eightfold, loss of SodCII does not confer a virulence phenotype, nor does it enhance the defect observed in a sodCI background. Despite this in vivo phenotype, SodCI and SodCII are expressed at similar levels in vitro during the stationary phase of growth. By exchanging the open reading frames of sodCI and sodCII, we found that SodCI contributes to virulence when placed under the control of the sodCII promoter. In contrast, SodCII does not contribute to virulence even when expressed from the sodCI promoter. Thus, the disparity in virulence phenotypes is due primarily to some physical difference between the two enzymes. In an attempt to identify the unique property of SodCI, we have tested factors that might affect enzyme activity inside a phagosome. We found no significant difference between SodCI and SodCII in their resistance to acid, resistance to hydrogen peroxide, or ability to obtain copper in a copper-limiting environment. Both enzymes are synthesized as apoenzymes in the absence of copper and can be fully remetallated when copper is added. The one striking difference that we noted is that, whereas SodCII is released normally by an osmotic shock, SodCI is "tethered" within the periplasm by an apparently noncovalent interaction. We propose that this novel property of SodCI is crucial to its ability to contribute to virulence in serovar Typhimurium.

The redox regulation of intermediary metabolism by a superoxide-aconitase rheostat

In this article, we discuss a hypothesis to explain the preferential synthesis of the superoxide sensitive form of aconitase in mitochondria and the phenotype observed in manganese superoxide dismutase mutant mice, which show a gross over accumulation of stored fat in liver. The model proposes that intermediary metabolism is redox regulated by mitochondrial superoxide generated during mitochondrial respiration. This regulates the level of reducing equivalents (NADH) entering the electron transport chain (ETC) through the reversible inactivation of mitochondrial aconitase. This control mechanism has a dual function; firstly, it regulates levels of superoxide generated by the ETC and, secondly, it fine-tunes metabolism by channeling citrate either for the production of NADH for energy metabolism or diverting it for the synthesis of fats. In this setting, the mitochondrial redox state influences metabolic decisions via a superoxide-aconitase rheostat.

Are respiratory enzymes the primary sources of intracellular hydrogen peroxide?

Endogenous H2O2 is believed to be a source of chronic damage in aerobic organisms. To quantify H2O2 formation, we have generated strains of Escherichia coli that lack intracellular scavenging enzymes. The H2O2 that is formed within these mutants diffuses out into the medium, where it can be measured. We sought to test the prevailing hypothesis that this H2O2 is primarily generated by the autoxidation of redox enzymes within the respiratory chain. The rate of H2O2 production increased when oxygen levels were raised, confirming that H2O2 is formed by an adventitious chemical process. However, mutants that lacked NADH dehydrogenase II and fumarate reductase, the most oxidizable components of the respiratory chain in vitro, continued to form H2O2 at normal rates. NADH dehydrogenase II did generate substantial H2O2 when it was when overproduced or quinones were absent, forcing electrons to accumulate on the enzyme. Mutants that lacked both NADH dehydrogenases respired very slowly, as expected; however, these mutants showed no diminution of H2O2 excretion, suggesting that H2O2 is primarily formed by a source outside the respiratory chain. That source has not yet been identified. In respiring cells the rate of H2O2 production was approximately 0.5% the rate of total oxygen consumption, with only modest changes when cells used different carbon sources.

NADPH oxidase

NADPH oxidase is an enzyme that catalyzes the production of superoxide from oxygen and NADPH. It is a complex enzyme consisting of two membrane-bound components and three components in the cytosol, plus rac 1 or rac 2. Activation of the oxidase involves the phosphorylation of one of the cytosolic components. Recent crystallography data indicate that the tail of this cytosolic component lies in a groove between two Src homology 3 domains and, when phosphorylated, the tail leaves the groove and is replaced by the tail of one of the membrane-bound components. Chronic granulomatous disease is an inherited immune deficiency caused by the absence of one of the components of the oxidase. The most important recent advances in the field have been the crystallographic analysis of the oxidase and the use of antifungal agents in the prophylaxis of chronic granulomatous disease.

Complex III releases superoxide to both sides of the inner mitochondrial membrane

Mechanisms of mitochondrial superoxide formation remain poorly understood despite considerable medical interest in oxidative stress. Superoxide is produced from both Complexes I and III of the electron transport chain, and once in its anionic form it is too strongly charged to readily cross the inner mitochondrial membrane. Thus, superoxide production exhibits a distinct membrane sidedness or "topology." In the present work, using measurements of hydrogen peroxide (Amplex red) as well as superoxide (modified Cypridina luciferin analog and aconitase), we demonstrate that Complex I-dependent superoxide is exclusively released into the matrix and that no detectable levels escape from intact mitochondria. This finding fits well with the proposed site of electron leak at Complex I, namely the iron-sulfur clusters of the (matrix-protruding) hydrophilic arm. Our data on Complex III show direct extramitochondrial release of superoxide, but measurements of hydrogen peroxide production revealed that this could only account for approximately 50% of the total electron leak even in mitochondria lacking CuZn-superoxide dismutase. We posit that the remaining approximately 50% of the electron leak must be due to superoxide released to the matrix. Measurements of (mitochondrial matrix) aconitase inhibition, performed in the presence of exogenous superoxide dismutase and catalase, confirmed this hypothesis. Our data indicate that Complex III can release superoxide to both sides of the inner mitochondrial membrane. The locus of superoxide production in Complex III, the ubiquinol oxidation site, is situated immediately next to the intermembrane space. This explains extramitochondrial release of superoxide but raises the question of how superoxide could reach the matrix. We discuss two models explaining this result.

Repair of oxidized iron-sulfur clusters in Escherichia coli

The [4Fe-4S]2+ clusters of dehydratases are rapidly damaged by univalent oxidants, including hydrogen peroxide, superoxide, and peroxynitrite. The loss of an electron destabilizes the cluster, causing it to release its catalytic iron atom and converting the cluster initially to an inactive [3Fe-4S]1+ form. Continued exposure to oxidants in vitro leads to further iron release. Experiments have shown that these clusters are repaired in vivo. We sought to determine whether repair is mediated by either the Isc or Suf cluster-assembly systems that have been identified in Escherichia coli. We found that all the proteins encoded by the isc operon were critical for de novo assembly, but most of these were unnecessary for cluster repair. IscS, a cysteine desulfurase, appeared to be an exception: although iscS mutants repaired damaged clusters, they did so substantially more slowly than did wild-type cells. Because sulfur mobilization should be required only if clusters degrade beyond the [3Fe-4S]1+ state, we used whole cell EPR to visualize the fate of oxidized enzymes in vivo. Fumarase A was overproduced. Brief exposure of cells to hydrogen peroxide resulted in the appearance of the characteristic [3Fe-4S]1+ signal of the oxidized enzyme. When hydrogen peroxide was then scavenged, the enzyme activity reappeared within minutes, in concert with the disappearance of the EPR signal. Thus it is unclear why IscS is required for efficient repair. The iscS mutants grew poorly, allowing the possibility that metabolic defects indirectly slow the repair process. Our data did indicate that damaged clusters decompose beyond the [3Fe-4S]1+ state in vivo when stress is prolonged. Under the conditions of our experiments, mutants that lacked other repair candidates--Suf proteins, glutathione, and NADPH: ferredoxin reductase--all repaired clusters at normal rates. We conclude that the mechanism of cluster repair is distinct from that of de novo assembly and that this is true because mild oxidative stress does not degrade clusters in vivo to the point of presenting an apoenzyme to the de novo cluster-assembly systems.

A protective role for zinc on intestinal peroxidative damage during oral iron repletion

Oral iron-supplementation is a general practice to correct iron deficiency anemia. Exposure of iron-deficient intestine to large doses of iron is known to induce oxidative damage, leading to loss of functional integrity, and reduced mucosal cell turnover. Conditioning of intestine with anti-oxidants during iron administration was shown to suppress iron-induced oxidative damage. Zinc is known to protect cells from peroxidative damage by inducing metallothionein and maintaining the sulfhydryl group stability. Nevertheless, co-administration of iron and zinc may antagonize each other with respect to absorption. In the present study, we show that combined supplementation of iron and zinc though marginally inhibits iron uptake significantly attenuates the oxidative stress by induction of metallothionein and elevating the levels of GSH. Further, presence of zinc in situ reduced the iron-induced hydroxyl radical production in the intestinal mucosa, as assessed by EPR spectroscopy. These results strongly suggest a protective role for zinc on iron-induced oxidative stress, which might have implications in anemia control programs.

Respiration metabolism reduces oxidative and acid stress to improve long-term survival of Lactococcus lactis

The impact of oxygen on a cell is strongly dependent on its metabolic state: survival in oxygen of free-living Lactococcus lactis, best known as a fermenting, acidifying bacterium, is generally poor. In contrast, if haem is present, L. lactis uses oxygen to switch from fermentation to respiration metabolism late in growth, resulting in spectacularly improved long-term survival. Oxygen is thus beneficial rather than detrimental for survival if haem is provided. We examined the effects of respiration on oxygen toxicity by comparing integrity of stationary phase cells after aerated growth without and with added haem. Aeration (no haem) growth caused considerable cellular protein and chromosomal DNA damage, increased spontaneous mutation frequencies and poor survival of recA mutants. These phenotypes were greatly diminished when haem was present, indicating that respiration constitutes an efficient barrier against oxidative stress. Using the green fluorescent protein as an indicator of intracellular oxidation state, we showed that aeration growth provokes significantly greater oxidation than respiration growth. Iron was identified as a main contributor to mortality and DNA degradation in aeration growth. Our results point to two features of respiration growth in lactococci that are responsible for maintaining low oxidative damage: One is a more reduced intracellular state, which is because of efficient oxygen elimination by respiration. The other is a higher pH resulting from the shift from acid-forming fermentation to respiration metabolism. These results have relevance to other bacteria whose respiration capacity depends on addition of exogenous haem.

Glutamine utilization by Rhizobium etli

We undertook the study of the use of glutamine (Gln) as the source of carbon and energy by Rhizobium etli. Tn5-induced mutagenesis allowed us to identify several genes required for Gln utilization, including those coding for two broad-range amino acid transporters and a glutamate dehydrogenase. The isolated mutants were characterized by the analysis of their capacity i) to grow on different media, ii) to transport Gln (uptake assays), and iii) to utilize Gln as the C energy source (CO2 production from Gln). We show that Gln is degraded through the citric acid cycle and that its utilization as the sole C source is related to a change in the bacterial cell shape (from bacillary to coccoid form) and a high susceptibility to a thiol oxidative insult. Both these data and the analysis of ntr-dependent promoters suggested that Gln-grown bacteria are under a condition of C starvation and N sufficiency, and as expected, the addition of glucose counteracted the morphological change and increased both the bacterial growth rate and their resistance to oxidative stress. Finally, a nodulation analysis indicates that the genes involved in Gln transport and degradation are dispensable for the bacterial ability to induce and invade developing nodules, whereas those involved in gluconeogenesis and nucleotide biosynthesis are strictly required.

Mitochondrial metabolism underlies hyperoxic cell damage

Exposure of mammals to hyperoxia causes pulmonary and ocular pathology. Hyperoxic damage and cell death may derive from enhanced intracellular formation of reactive oxygen species (ROS), probably of mitochondrial origin. There is, however, controversy on this point. When wild-type and respiration-deficient (rho(o)) HeLa cells were cultured in 80% O2, wild-type cells stopped growing after 5 days and died thereafter whereas rho(o) cells survived and grew to confluence. This tolerance of rho(o) cells for hyperoxia was not associated with greater resistance to oxidants such as hydrogen peroxide and t-butyl hydroperoxide. Under both 20% and 80% O2, rho(o) cells exhibited substantially decreased ROS production, and, under 80% O2, rho(o) cells showed no suppression of aconitase activity or mitochondrial protein carbonyl formation. Replacement of normal mitochondria in rho(o) cells restored ROS production and susceptibility to hyperoxia. Two other approaches that diminished mitochondrial ROS generation also increased tolerance for hyperoxia. HeLa cells constantly exposed to the protonophoric uncoupler carbonyl cyanide m-chlorophenylhydrazone, which enhances respiration but decreases ROS production, showed preferential survival under 80% O2, as did HeLa cells treated with chloramphenicol, which suppresses both respiration and mitochondrial ROS production. We conclude that interactions between respiring mitochondria and O2 are primarily responsible for hyperoxic cell damage.

Superoxide inhibits 4Fe-4S cluster enzymes involved in amino acid biosynthesis. Cross-compartment protection by CuZn-superoxide dismutase

Among the phenotypes of Saccharomyces cerevisiae mutants lacking CuZn-superoxide dismutase (Sod1p) is an aerobic lysine auxotrophy; in the current work we show an additional leaky auxotrophy for leucine. The lysine and leucine biosynthetic pathways each contain a 4Fe-4S cluster enzyme homologous to aconitase and likely to be superoxide-sensitive, homoaconitase (Lys4p) and isopropylmalate dehydratase (Leu1p), respectively. We present evidence that direct aerobic inactivation of these enzymes in sod1 Delta yeast results in the auxotrophies. Located in the cytosol and intermembrane space of the mitochondria, Sod1p likely provides direct protection of the cytosolic enzyme Leu1p. Surprisingly, Lys4p does not share a compartment with Sod1p but is located in the mitochondrial matrix. The activity of a second matrix protein, the tricarboxylic acid cycle enzyme aconitase, was similarly lowered in sod1 Delta mutants. We measured only slight changes in total mitochondrial iron and found no detectable difference in mitochondrial "free" (EPR-detectable) iron making it unlikely that a gross defect in mitochondrial iron metabolism is the cause of the decreased enzyme activities. Thus, we conclude that when Sod1p is absent a lysine auxotrophy is induced because Lys4p is inactivated in the matrix by superoxide that originates in the intermembrane space and diffuses across the inner membrane.

Transcriptome and proteome analysis of Bacillus subtilis gene expression in response to superoxide and peroxide stress

The Gram-positive soil bacterium Bacillus subtilis responds to oxidative stress by the activation of different cellular defence mechanisms. These are composed of scavenging enzymes as well as protection and repair systems organized in highly sophisticated networks. In this study, the peroxide and the superoxide stress stimulons of B. subtilis were characterized by means of transcriptomics and proteomics. The results demonstrate that oxidative-stress-responsive genes can be classified into two groups. One group encompasses genes which show similar expression patterns in the presence of both reactive oxygen species. Examples are members of the PerR and the Fur regulon which were induced by peroxide and superoxide stress. Similarly, both kinds of stress stimulated the activation of the stringent response. The second group is composed of genes primarily responding to one stimulus, like the members of the SOS regulon which were particularly upregulated in the presence of peroxide, and many genes involved in sulfate assimilation and methionine biosynthesis which were only induced by superoxide. Several genes encoding proteins of unknown function could be assigned to one of these groups.

The protistan parasite Perkinsus marinus is resistant to selected reactive oxygen species

The parasite Perkinsus marinus has devastated natural and farmed oyster populations along the Atlantic and Gulf coasts of North America. When viable P. marinus trophozoites are engulfed by oyster hemocytes, the typical accumulation of reactive oxygen species (ROS) normally associated with phagocyte activity is not observed. One hypothesis to explain this is that the parasite rapidly removes ROS. A manifestation of efficient ROS removal should be a high level of resistance to exogenous ROS. We investigated the in vitro susceptibility of P. marinus to ROS as compared to the estuarine bacterium Vibrio splendidus. We find that P. marinus is markedly less susceptible than V. splendidus to superoxide and hydrogen peroxide (H(2)O(2)), but equally sensitive to hypochlorite. Viable P. marinus trophozoites degrade H(2)O(2) in vitro, but lack detectable catalase activity. However, extracts contain an ascorbate dependent peroxidase activity that may contribute to H(2)O(2) removal in vitro and in vivo.

Post-transcriptional regulation of bacterial motility by aconitase proteins

Escherichia coli and Bacillus subtilis aconitases can act as iron and oxidative stress-responsive post-transcriptional regulators. Here, it is shown that a Salmonella enterica serovar Typhimurium LT2 acnB mutant exhibits impaired binding to the surface of J774 macrophage-like cells. Proteomic analyses were used to investigate further the binding defect of the acnB mutant. These revealed that the levels of the flagellum protein FliC were much lower for the acnB mutant. This strain was correspondingly less motile and possessed fewer flagella than either the parental strain or the acnA and acnAB mutants. The acnB lesion did not alter fliC transcription, nor did apo-AcnB select the fliC transcript from a library of S. enterica transcripts; thus, the effect of AcnB on FliC is indirect. Evidence is presented to show that apo-AcnB regulates FliC synthesis via interaction with the ftsH transcript to decrease the intracellular levels of FtsH. The lower levels of FtsH protease activity then influence sigma32, DnaK and, ultimately, FliC production.

Pathways of oxidative damage

The phenomenon of oxygen toxicity is universal, but only recently have we begun to understand its basis in molecular terms. Redox enzymes are notoriously nonspecific, transferring electrons to any good acceptor with which they make electronic contact. This poses a problem for aerobic organisms, since molecular oxygen is small enough to penetrate all but the most shielded active sites of redox enzymes. Adventitious electron transfers to oxygen create superoxide and hydrogen peroxide, which are partially reduced species that can oxidize biomolecules with which oxygen itself reacts poorly. This review attempts to present our still-incomplete understanding of how reactive oxygen species are formed inside cells and the mechanisms by which they damage specific target molecules. The vulnerability of cells to oxidation lies at the root of obligate anaerobiosis, spontaneous mutagenesis, and the use of oxidative stress as a biological weapon.

Mitochondria: are they the seat of senescence?

The frequently quoted figure for the fractional univalent reduction of oxygen to superoxide in mitochondria is certainly too high by at least one order of magnitude. This is so because the higher number (2%) was derived from mitochondria whose cytochrome c oxidase was blocked with cyanide. Nevertheless, even the more correct number (0.1%) means that the production of O(2)(-) and H(2)O(2) in mitochondria is large and apt to result in damage to macromolecules in spite of such defensive enzymes as superoxide dismutases and glutathione peroxidase. The data available for nematodes and flies provide a compelling case for the view that the accumulation of oxidative damage to specific mitochondrial proteins leads to the progressive dysfunction that we see as senescence. The data available from work with mammals are much weaker and do not yet allow a strong position to be taken.

Alpha-synuclein up-regulation and aggregation during MPP+-induced apoptosis in neuroblastoma cells: intermediacy of transferrin receptor iron and hydrogen peroxide

1-Methyl-4-phenylpyridinium (MPP(+)) is a neurotoxin that causes Parkinson's disease in experimental animals and humans. Despite the fact that intracellular iron was shown to be crucial for MPP(+)-induced apoptotic cell death, the molecular mechanisms for the iron requirement remain unclear. We investigated the role of transferrin receptor (TfR) and iron in modulating the expression of alpha-synuclein (alpha-syn) in MPP(+)-induced oxidative stress and apoptosis. Results show that MPP(+) inhibits mitochondrial complex-1 and aconitase activities leading to enhanced H(2)O(2) generation, TfR expression and alpha-syn expression/aggregation. Pretreatment with cell-permeable iron chelators, TfR antibody (that inhibits TfR-mediated iron uptake), or transfection with glutathione peroxidase (GPx1) enzyme inhibits intracellular oxidant generation, alpha-syn expression/aggregation, and apoptotic signaling as measured by caspase-3 activation. Cells overexpressing alpha-syn exacerbated MPP(+) toxicity, whereas antisense alpha-syn treatment totally abrogated MPP(+)-induced apoptosis in neuroblastoma cells without affecting oxidant generation. The increased cytotoxic effects of alpha-syn in MPP(+)-treated cells were attributed to inhibition of mitogen-activated protein kinase and proteasomal function. We conclude that MPP(+)-induced iron signaling is responsible for intracellular oxidant generation, alpha-syn expression, proteasomal dysfunction, and apoptosis. Relevance to Parkinson's disease is discussed.

Compartment-specific protection of iron-sulfur proteins by superoxide dismutase

Iron and oxygen are essential but potentially toxic constituents of most organisms, and their transport is meticulously regulated both at the cellular and systemic levels. Compartmentalization may be a homeostatic mechanism for isolating these biological reactants in cells. To investigate this hypothesis, we have undertaken a genetic analysis of the interaction between iron and oxygen metabolism in Drosophila. We show that Drosophila iron regulatory protein-1 (IRP1) registers cytosolic iron and oxidative stress through its labile iron sulfur cluster by switching between cytosolic aconitase and RNA-binding functions. IRP1 is strongly activated by silencing and genetic mutation of the cytosolic superoxide dismutase (Sod1), but is unaffected by silencing of mitochondrial Sod2. Conversely, mitochondrial aconitase activity is relatively insensitive to loss of Sod1 function, but drops dramatically if Sod2 activity is impaired. This strongly suggests that the mitochondrial boundary limits the range of superoxide reactivity in vivo. We also find that exposure of adults to paraquat converts cytosolic aconitase to IRP1 but has no affect on mitochondrial aconitase, indicating that paraquat generates superoxide in the cytosol but not in mitochondria. Accordingly, we find that transgene-mediated overexpression of Sod2 neither enhances paraquat resistance in Sod1+ flies nor compensates for lack of SOD1 activity in Sod1-null mutants. We conclude that in vivo, superoxide is confined to the subcellular compartment in which it is formed, and that the mitochondrial and cytosolic SODs provide independent protection to compartment-specific protein iron-sulfur clusters against attack by superoxide generated under oxidative stress within those compartments.

The Geobacillus stearothermophilus V iscS gene, encoding cysteine desulfurase, confers resistance to potassium tellurite in Escherichia coli K-12

Many eubacteria are resistant to the toxic oxidizing agent potassium tellurite, and tellurite resistance involves diverse biochemical mechanisms. Expression of the iscS gene from Geobacillus stearothermophilus V, which is naturally resistant to tellurite, confers tellurite resistance in Escherichia coli K-12, which is naturally sensitive to tellurite. The G. stearothermophilus iscS gene encodes a cysteine desulfurase. A site-directed mutation in iscS that prevents binding of its pyridoxal phosphate cofactor abolishes both enzyme activity and its ability to confer tellurite resistance in E. coli. Expression of the G. stearothermophilus iscS gene confers tellurite resistance in tellurite-hypersensitive E. coli iscS and sodA sodB mutants (deficient in superoxide dismutase) and complements the auxotrophic requirement of an E. coli iscS mutant for thiamine but not for nicotinic acid. These and other results support the hypothesis that the reduction of tellurite generates superoxide anions and that the primary targets of superoxide damage in E. coli are enzymes with iron-sulfur clusters.

Novel antioxidant role of alcohol dehydrogenase E from Escherichia coli

Alcohol dehydrogenase E (AdhE) is an Fe-enzyme that, under anaerobic conditions, is involved in dissimilation of glucose. The enzyme is also present under aerobic conditions, its amount is about one-third and its activity is only one-tenth of the values observed under anaerobic conditions. Nevertheless, its function in the presence of oxygen remained ignored. The data presented in this paper led us to propose that the enzyme has a protective role against oxidative stress. Our results indicated that cells deleted in adhE gene could not grow aerobically in minimal media, were extremely sensitive to oxidative stress and showed division defects. In addition, compared with wild type, mutant cells displayed increased levels of internal peroxides (even higher than those found in a Delta katG strain) and increased protein carbonyl content. This pleiotropic phenotype disappeared when the adhE gene was reintroduced into the defective strain. The purified enzyme was highly reactive with hydrogen peroxide (with a Ki of 5 microM), causing inactivation due to a metal-catalyzed oxidation reaction. It is possible to prevent this reactivity to hydrogen peroxide by zinc, which can replace the iron atom at the catalytic site of AdhE. This can also be achieved by addition of ZnSO4 to cell cultures. In such conditions, addition of hydrogen peroxide resulted in reduced cell viability compared with that obtained without the Zn treatment. We therefore propose that AdhE acts as a H2O2 scavenger in Escherichia coli cells grown under aerobic conditions.

Subcellular localization of Pseudomonas pyocyanin cytotoxicity in human lung epithelial cells

The Pseudomonas aeruginosa secretory product pyocyanin damages lung epithelium, likely due to redox cycling of pyocyanin and resultant superoxide and H(2)O(2) generation. Subcellular site(s) of pyocyanin redox cycling and toxicity have not been well studied. Therefore, pyocyanin's effects on subcellular parameters in the A549 human type II alveolar epithelial cell line were examined. Confocal and electron microscopy studies suggested mitochondrial redox cycling of pyocyanin and extracellular H(2)O(2) release, respectively. Pyocyanin decreased mitochondrial and cytoplasmic aconitase activity, ATP levels, cellular reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide, and mitochondrial membrane potential. These effects were transient at low pyocyanin concentrations and were linked to apparent cell-mediated metabolism of pyocyanin. Overexpression of MnSOD, but not CuZnSOD or catalase, protected cellular aconitase, but not ATP, from pyocyanin-mediated depletion. This suggests that loss of aconitase activity is not responsible for ATP depletion. How pyocyanin leads to ATP depletion, the mechanism of cellular metabolism of pyocyanin, and the impact of mitochondrial pyocyanin redox cycling on other cellular events are important areas for future study.

The importance of glutathione in human disease

Reduced glutathione (GSH) is the most prevalent non-protein thiol in animal cells. Its de novo and salvage synthesis serves to maintain a reduced cellular environment and the tripeptide is a co-factor for many cytoplasmic enzymes and may also act as an important post-translational modification in a number of cellular proteins. The cysteine thiol acts as a nucleophile in reactions with both exogenous and endogenous electrophilic species. As a consequence, reactive oxygen species (ROS) are frequently targeted by GSH in both spontaneous and catalytic reactions. Since ROS have defined roles in cell signaling events as well as in human disease pathologies, an imbalance in expression of GSH and associated enzymes has been implicated in a variety of circumstances. Cause and effect links between GSH metabolism and diseases such as cancer, neurodegenerative diseases, cystic fibrosis (CF), HIV, and aging have been shown. Polymorphic expression of enzymes involved in GSH homeostasis influences susceptibility and progression of these conditions. This review provides an overview of the biological importance of GSH at the level of the cell and organism.

Contrasting sensitivities of Escherichia coli aconitases A and B to oxidation and iron depletion

Superoxide damages dehydratases that contain catalytic [4Fe-4S](2+) clusters. Aconitases are members of that enzyme family, and previous work showed that most aconitase activity is lost when Escherichia coli is exposed to superoxide stress. More recently it was determined that E. coli synthesizes at least two isozymes of aconitase, AcnA and AcnB. Synthesis of AcnA, the less-abundant enzyme, is positively controlled by SoxS, a protein that is activated in the presence of superoxide-generating chemicals. We have determined that this arrangement exists because AcnA is resistant to superoxide in vivo. Surprisingly, purified AcnA is extremely sensitive to superoxide and other chemical oxidants unless it is combined with an uncharacterized factor that is present in cell extracts. In contrast, AcnB is highly sensitive to a variety of chemical oxidants in vivo, in extracts, and in its purified form. Thus, the induction of AcnA during oxidative stress provides a mechanism to circumvent a block in the tricarboxylic acid cycle. AcnA appears to be as catalytically competent as AcnB, so the retention of the latter as the primary housekeeping enzyme must provide some other advantage. We observed that the [4Fe-4S] cluster of AcnB is in dynamic equilibrium with the surrounding iron pool, so that AcnB is rapidly demetallated when intracellular iron pools drop. AcnA and other dehydratases do not show this trait. Demetallated AcnB is known to bind its cognate mRNA. The absence of AcnB activity also causes the accumulation and excretion of citrate, an iron chelator for which E. coli synthesizes a transport system. Thus, AcnB may be retained as the primary aconitase because the lability of its exposed cluster allows E. coli to sense and respond to iron depletion.

How oxygen damages microbes: oxygen tolerance and obligate anaerobiosis

The orbital structure of molecular oxygen constrains it to accept electrons one at a time, and its unfavourable univalent reduction potential ensures that it can do so only with low-potential redox partners. In E. coli, this restriction prevents oxygen from oxidizing structural molecules. Instead, it primarily oxidizes reduced flavins, a reaction that is harmful only in that it generates superoxide and hydrogen peroxide as products. These species are stronger oxidants than is oxygen itself. They can oxidize dehydratase iron-sulphur clusters and sulphydryls, respectively, and thereby inactivate enzymes that are dependent upon these functional groups. Hydrogen peroxide also oxidizes free iron, generating hydroxyl radicals. Because hydroxyl radicals react with virtually any biomolecules they encounter, their reactivity is broadly dissipated, and only their reactions with DNA are known to have an important physiological impact. E. coli elaborates scavenging and repair systems to minimize the impact of this adventitious chemistry; mutants that lack these defences grow poorly in aerobic habitats. Some of the growth deficits of these mutants cannot be easily ascribed to sulphydryl, cluster, or DNA damage, indicating that important aspects of oxidative stress still lack a biochemical explanation. Obligate anaerobes cannot tolerate oxygen because they utilize metabolic schemes built around enzymes that react with oxidants. The reliance upon low-potential flavoproteins for anaerobic respiration probably causes substantial superoxide and hydrogen peroxide to be produced when anaerobes are exposed to air. These species then generate damage of the same type that they produce in aerotolerant bacteria. However, obligate anaerobes also utilize several classes of dioxygen-sensitive enzymes that are not needed by aerobes. These enzymes are used for processes that help maintain the redox balance during anaerobic fermentations. They catalyse reactions that are chemically difficult, and the reaction mechanisms require the solvent exposure of radicals or low-potential metal clusters that can react rapidly with oxygen. Recent work has uncovered adaptive strategies by which obligate anaerobes seek to minimize the damage done by superoxide and hydrogen peroxide. Their failure to divest themselves of enzymes that can be directly damaged by molecular oxygen suggests that evolution has not yet provided economical options to them.

Global adjustment of microbial physiology during free radical stress

Oxidation can damage all biological macromolecules, and the survival of a cell therefore depends on its ability to control the level of oxidants. Microbes possess an astonishing variety of antioxidant defences, ranging from small, oxidant-scavenging molecules to self-regulating, homeostatic gene networks. Most often these antioxidant defences are activated by exposure to specific classes of oxidants. Interestingly, the isolation of pleiotropic mutations that impair or exacerbate the expression of subsets of oxidant-responsive genes led to the identification of global regulators. In a few, well-characterized cases, these regulators can transduce oxidative damage into gene regulation. Recently, the application of genomic tools to study the antioxidant responses of E. coli has both confirmed previous observations and provided evidence for a wealth of putative new anti-oxidant functions. Here, we review the remarkable diversity of antioxidant defence mechanisms, with emphasis on signal transduction by global regulator proteins and the corresponding genetic networks that protect the microbial cell against oxidative stress.

RNA interference-mediated silencing of Sod2 in Drosophila leads to early adult-onset mortality and elevated endogenous oxidative stress

Oxidative stress has been widely implicated as an important factor in the aging process. Because mitochondrial respiration is the principal source of reactive oxygen within cells, the mitochondrially localized superoxide dismutase (SOD) 2 is thought to play an important front-line defensive role against aging-related oxidative stress. Although genetic studies with mutants deficient in SOD1, the predominantly cytosolic isoform of SOD, have been instrumental in elucidating the role of reactive oxygen metabolism in aging in Drosophila, the lack of available mutations in the Sod2 gene has hampered an equivalent analysis of the participation of this important antioxidant enzyme in the Drosophila aging model. Here we report that ablation of mitochondrial SOD2 through expression of a GAL4-regulated, inverted-repeat Sod2 RNA-interference transgene in an otherwise normal animal causes increased endogenous oxidative stress, resulting in loss of essential enzymatic components of the mitochondrial respiratory chain and the tricarboxylic acid cycle, enhances sensitivity to applied oxidative stress, and causes early-onset mortality in young adults. In sharp contrast, ablation of SOD2 has no overt effect on the development of larvae and pupae, which may reflect a fundamental transition in oxygen utilization andor reactive oxygen metabolism that occurs during metamorphosis from larval to adult life.

Peroxynitrite reactions and formation in mitochondria

Mitochondria constitute a primary locus for the intracellular formation and reactions of peroxynitrite, and these interactions are recognized to contribute to the biological and pathological effects of both nitric oxide ((*)NO) and peroxynitrite. Extra- or intramitochondrially formed peroxynitrite can diffuse through mitochondrial compartments and undergo fast direct and free radical-dependent target molecule reactions. These processes result in oxidation, nitration, and nitrosation of critical components in the matrix, inner and outer membrane, and intermembrane space. Mitochondrial scavenging and repair systems for peroxynitrite-dependent oxidative modifications operate but they can be overwhelmed under enhanced cellular (*)NO formation as well as under conditions that lead to augmented superoxide formation by the electron transport chain. Peroxynitrite can lead to alterations in mitochondrial energy and calcium homeostasis and promote the opening of the permeability transition pore. The effects of peroxynitrite in mitochondrial physiology can be largely rationalized based on the reactivities of peroxynitrite and peroxynitrite-derived carbonate, nitrogen dioxide, and hydroxyl radicals with critical protein amino acids and transition metal centers of key mitochondrial proteins. In this review we analyze (i) the existing evidence for the intramitochondrial formation and reactions of peroxynitrite, (ii) the key reactions and fate of peroxynitrite in mitochondria, and (iii) their impact in mitochondrial physiology and signaling of cell death.

Mechanism of superoxide and hydrogen peroxide formation by fumarate reductase, succinate dehydrogenase, and aspartate oxidase

Oxidative stress is created in aerobic organisms when molecular oxygen chemically oxidizes redox enzymes, forming superoxide (O2*-) and hydrogen peroxide (H2O2). Prior work identified several flavoenzymes from Escherichia coli that tend to autoxidize. Of these, fumarate reductase (Frd) is notable both for its high turnover number and for its production of substantial O2*- in addition to H2O2. We have sought to identify characteristics of Frd that predispose it to this behavior. The ability of excess succinate to block autoxidation and the inhibitory effect of lowering the flavin potential indicate that all detectable autoxidation occurs from its FAD site, rather than from iron-sulfur clusters or bound quinones. The flavin adenine dinucleotide (FAD) moiety of Frd is unusually solvent-exposed, as evidenced by its ability to bind sulfite, and this may make it more likely to react adventitiously with O2*-. The autoxidizing species is apparently fully reduced flavin rather than flavosemiquinone, since treatments that more fully reduce the enzyme do not slow its turnover number. They do, however, switch the major product from O2*- to H2O2. A similar effect is achieved by lowering the potential of the proximal [2Fe-2S] cluster. These data suggest that Frd releases O2*- into bulk solution if this cluster is available to sequester the semiquinone electron; otherwise, that electron is rapidly transferred to the nascent superoxide, and H2O2 is the product that leaves the active site. This model is supported by the behavior of "aspartate oxidase" (aspartate:fumarate oxidoreductase), an Frd homologue that lacks Fe-S clusters. Its dihydroflavin also reacts avidly with oxygen, and H2O2 is the predominant product. In contrast, succinate dehydrogenase, with high potential clusters, generates O2*- exclusively. The identities of enzyme autoxidation products are significant because O2*- and H2O2 damage cells in different ways.

Pseudomonas aeruginosa anaerobic respiration in biofilms: relationships to cystic fibrosis pathogenesis

Recent data indicate that cystic fibrosis (CF) airway mucus is anaerobic. This suggests that Pseudomonas aeruginosa infection in CF reflects biofilm formation and persistence in an anaerobic environment. P. aeruginosa formed robust anaerobic biofilms, the viability of which requires rhl quorum sensing and nitric oxide (NO) reductase to modulate or prevent accumulation of toxic NO, a byproduct of anaerobic respiration. Proteomic analyses identified an outer membrane protein, OprF, that was upregulated approximately 40-fold under anaerobic versus aerobic conditions. Further, OprF exists in CF mucus, and CF patients raise antisera to OprF. An oprF mutant formed poor anaerobic biofilms, due, in part, to defects in anaerobic respiration. Thus, future investigations of CF pathogenesis and therapy should include a better understanding of anaerobic metabolism and biofilm development by P. aeruginosa.

Lon protease preferentially degrades oxidized mitochondrial aconitase by an ATP-stimulated mechanism

Mitochondrial aconitase is sensitive to oxidative inactivation and can aggregate and accumulate in many age-related disorders. Here we report that Lon protease, an ATP-stimulated mitochondrial matrix protein, selectively recognizes and degrades the oxidized, hydrophobic form of aconitase after mild oxidative modification, but that severe oxidation results in aconitase aggregation, which makes it a poor substrate for Lon. Similarly, a morpholino oligodeoxynucleotide directed against the lon gene markedly decreases the amount of Lon protein, Lon activity and aconitase degradation in WI-38 VA-13 human lung fibroblasts and causes accumulation of oxidatively modified aconitase. The ATP-stimulated Lon protease may be an essential defence against the stress of life in an oxygen environment. By recognizing minor oxidative changes to protein structure and rapidly degrading the mildly modified protein, Lon protease may prevent extensive oxidation, aggregation and accumulation of aconitase, which could otherwise compromise mitochondrial function and cellular viability. Aconitase is probably only one of many mitochondrial matrix proteins that are preferentially degraded by Lon protease after oxidative modification.

Ageing is reversed, and metabolism is reset to young levels in recovering dauer larvae of C. elegans

The nematode Caenorhabditis elegans responds to unfavourable environmental conditions by arresting development and entering diapause as a dauer larva. Dauers can survive several times the normal life span and the duration of the dauer state has no effect on postdauer life span. This led to the suggestion that dauers are non-ageing, and that dauers eventually perish as the consequence of depletion of stored nutrients. We have investigated physiological changes associated with long-term diapause survival, and found that dauer larvae slowly develop senescence-like symptoms, including decrease of metabolic capacity, aconitase enzyme activity, and ATP stores, and increase of lipofuscin- and oxidised flavin-specific fluorescence. However, these changes are reversed when the dauers recover. Thus senescence can occur before attainment of reproductive maturity, and furthermore, is reversible. Other life processes, including respiration rate and heat output, remain unaltered over four weeks of diapause at 24 degrees C. Possible determinants of the enhanced life maintenance include increased resistance to oxidative stress provided by enhanced superoxide dismutase and catalase activities, and a shift to a highly reducing redox status.

Antioxidant responses to oxidant-mediated lung diseases

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are generated throughout the human body. Enzymatic and nonenzymatic antioxidants detoxify ROS and RNS and minimize damage to biomolecules. An imbalance between the production of ROS and RNS and antioxidant capacity leads to a state of "oxidative stress" that contributes to the pathogenesis of a number of human diseases by damaging lipids, protein, and DNA. In general, lung diseases are related to inflammatory processes that generate increased ROS and RNS. The susceptibility of the lung to oxidative injury depends largely on its ability to upregulate protective ROS and RNS scavenging systems. Unfortunately, the primary intracellular antioxidants are expressed at low levels in the human lung and are not acutely induced when exposed to oxidative stresses such as cigarette smoke and hyperoxia. However, the response of extracellular antioxidant enzymes, the critical primary defense against exogenous oxidative stress, increases rapidly and in proportion to oxidative stress. In this paper, we review how antioxidants in the lung respond to oxidative stress in several lung diseases and focus on the mechanisms that upregulate extracellular glutathione peroxidase.

Manganese supplementation relieves the phenotypic deficits seen in superoxide-dismutase-null Escherichia coli

Escherichia coli, lacking cytoplasmic superoxide dismutases, exhibits a variety of oxygen-dependent phenotypic deficits. Enrichment of the growth medium with Mn(II) relieved those deficits. Extracts of cells grown on Mn(II)-rich medium exhibited superoxide dismutase-like activity that was due partially to low-molecular-weight and partially to high-molecular-weight complexes. The high-molecular-weight activity was sensitive to proteolysis. Hence this activity is likely associated with low-affinity binding of Mn to proteins.

Transferrin receptor-dependent iron uptake is responsible for doxorubicin-mediated apoptosis in endothelial cells: role of oxidant-induced iron signaling in apoptosis

In the past, investigators have successfully used iron chelators to mitigate the cardiotoxicity of doxorubicin (DOX), a widely used anticancer drug that induces reactive oxygen species (ROS), oxidative damage, and apoptosis. Although intracellular iron plays a critical role in initiating DOX-induced apoptosis, the molecular mechanism(s) that link iron, ROS, and apoptosis are still unknown. In this study, we demonstrate that apoptosis results from the exposure of bovine aortic endothelial cells to DOX and that the apoptotic cell death is accompanied by a significant increase in cellular iron ((55)Fe) uptake and activation of iron regulatory protein-1. Furthermore, DOX-induced iron uptake was shown to be mediated by the transferrin receptor (TfR)-dependent mechanism. Treatment with the anti-TfR antibody (IgA class) dramatically inhibited DOX-induced apoptosis, iron uptake, and intracellular oxidant formation as measured by fluorescence using dichlorodihydrofluorescein. Treatment with cell-permeable iron chelators and ROS scavengers inhibited DOX-induced cellular (55)Fe uptake, ROS formation, and apoptosis. Based on these findings, we conclude that DOX-induced iron signaling is regulated by the cell surface TfR expression, intracellular oxidant levels, and iron regulatory proteins. The implications of TfR-dependent iron transport in oxidant-induced apoptosis in endothelial cells are discussed.

Superoxide modification and inactivation of a neuronal receptor-like complex

Excessive superoxide (O(-)(2)) formation is toxic to cells and organisms. O(-)(2) reacts with either iron-sulfur centers or cysteines (Cys) of cytoplasmic proteins. Reactions with membrane proteins, however, have not been fully characterized. In the present studies, the reaction of O(-)(2) with a protein complex that has glutamate/N-methyl-D-aspartate (NMDA) receptor characteristics and with one of the subunits of this complex was examined. Exposure of the complex purified from neuronal membranes and the recombinant glutamate-binding protein (GBP) subunit of this complex to the O(-)(2)-generating system of xanthine (X) plus xanthine oxidase (XO) caused strong inhibition of L-[3H]glutamate binding. Inhibition of glutamate binding to the complex and GBP by O(-)(2) was greater than that produced by H(2)O(2), another product of the X plus XO reaction. Mutation of two cysteine (Cys) residues in recombinant GBP (Cys(190,191)) eliminated the effect of O(-)(2) on L-[3H]glutamate binding. Both S-thiolation reaction of GBP in synaptic membranes with [35S]cystine and reaction of Cys residues in GBP with [3H]NEM were significantly decreased after exposure of membranes to O(-)(2). Inhibition of cysteylation of membrane GBP by O(-)(2) was still observed after iron chelation by desferrioxamine, albeit diminished, and was not altered by the presence of catalase. Overall, the results indicated that GBP exposure to O(-)(2) modified Cys residues in this protein. The modification was not characterized but it was probably that of disulfide formation.