Inactivation of dehydratase [4Fe-4S] clusters and disruption of iron homeostasis upon cell exposure to peroxynitrite

Distinct iron binding property of two putative iron donors for the iron-sulfur cluster assembly: IscA and the bacterial frataxin ortholog CyaY under physiological and oxidative stress conditions

Frataxin, a small mitochondrial protein linked to the neurodegenerative disease Friedreich ataxia, has recently been proposed as an iron donor for the iron-sulfur cluster assembly. An analogous function has also been attributed to IscA, a key member of the iron-sulfur cluster assembly machinery found in bacteria, yeast, and humans. Here we have compared the iron binding property of IscA and the frataxin ortholog CyaY from Escherichia coli under physiological and oxidative stress conditions. In the presence of the thioredoxin reductase system, which emulates the intracellular redox potential, CyaY fails to bind any iron even at a 10-fold excess of iron in the incubation solution. Under the same physiologically relevant conditions, IscA efficiently recruits iron and transfers the iron for the iron-sulfur cluster assembly in a proposed scaffold IscU. In the presence of hydrogen peroxide, however, IscA completely loses its iron binding activity, whereas CyaY becomes a competent iron-binding protein and attenuates the iron-mediated production of hydroxyl free radicals. Hydrogen peroxide appears to oxidize the iron binding thiol groups in IscA, thus blocking the iron binding in the protein. Once the oxidized thiol groups in IscA are re-reduced with the thioredoxin reductase system, the iron binding activity of IscA is fully restored. On the other hand, hydrogen peroxide has no effect on the iron binding carboxyl groups in CyaY, allowing the protein to bind iron under oxidative stress conditions. The results suggest that IscA is capable of recruiting intracellular iron for the iron-sulfur cluster assembly under normal physiological conditions, whereas CyaY may serve as an iron chaperon to sequester redox active free iron and alleviate cellular oxidative damage under oxidative stress conditions.

Nitric oxide and peroxynitrite in health and disease

The discovery that mammalian cells have the ability to synthesize the free radical nitric oxide (NO) has stimulated an extraordinary impetus for scientific research in all the fields of biology and medicine. Since its early description as an endothelial-derived relaxing factor, NO has emerged as a fundamental signaling device regulating virtually every critical cellular function, as well as a potent mediator of cellular damage in a wide range of conditions. Recent evidence indicates that most of the cytotoxicity attributed to NO is rather due to peroxynitrite, produced from the diffusion-controlled reaction between NO and another free radical, the superoxide anion. Peroxynitrite interacts with lipids, DNA, and proteins via direct oxidative reactions or via indirect, radical-mediated mechanisms. These reactions trigger cellular responses ranging from subtle modulations of cell signaling to overwhelming oxidative injury, committing cells to necrosis or apoptosis. In vivo, peroxynitrite generation represents a crucial pathogenic mechanism in conditions such as stroke, myocardial infarction, chronic heart failure, diabetes, circulatory shock, chronic inflammatory diseases, cancer, and neurodegenerative disorders. Hence, novel pharmacological strategies aimed at removing peroxynitrite might represent powerful therapeutic tools in the future. Evidence supporting these novel roles of NO and peroxynitrite is presented in detail in this review.

Glutathione reductase and a mitochondrial thioredoxin play overlapping roles in maintaining iron-sulfur enzymes in fission yeast

In the fission yeast Schizosaccharomyces pombe, the pgr1+ gene encoding glutathione (GSH) reductase (GR) is essentially required for cell survival. Depletion of GR caused proliferation arrest at the G1 phase of the cell cycle under aerobic conditions. Multicopy suppressors that restore growth were screened, and one effective suppressor was found to be the trx2+ gene, encoding a mitochondrial thioredoxin. This suggests that GR is critically required for some mitochondrial function(s). We found that GR resides in both cytosolic and organellar fractions of the cell. Depletion of GR lowered the respiration rate and the activity of oxidation-labile Fe-S enzymes such as mitochondrial aconitase and cytosolic sulfite reductase. Trx2 did not reverse the high ratio of oxidized glutathione to GSH or the low respiration rate observed in GR-depleted cells. However, it brought the activity of oxidation-labile Fe-S enzymes to a normal level, suggesting that the maintenance of Fe-S enzymes is a critical factor in the survival of S. pombe. The activity of succinate dehydrogenase, an oxidation-insensitive Fe-S enzyme, however, was not affected by GR depletion, suggesting that GR is not required for the biogenesis of the Fe-S cluster. The total iron content was greatly increased by GR depletion and was brought to a nearly normal level by Trx2. These results indicate that the essentiality of GR in the aerobic growth of S. pombe is derived from its role in maintaining oxidation-labile Fe-S enzymes and iron homeostasis.

How Escherichia coli and Saccharomyces cerevisiae build Fe/S proteins

Owing to the versatile electronic properties of iron and sulfur, iron sulfur (Fe/S) clusters are perfectly suited for sensing changes in environmental conditions and regulating protein properties accordingly. Fe/S proteins have been recruited in a wide array of diverse biological processes, including electron transfer chains, metabolic pathways and gene regulatory circuits. Chemistry has revealed the great diversity of Fe/S clusters occurring in proteins. The question now is to understand how iron and sulfur come together to form Fe/S clusters and how these clusters are subsequently inserted into apoproteins. Iron, sulfide and reducing conditions were found to be sufficient for successful maturation of many apoproteins in vitro, opening the possibility that insertion might be a spontaneous event. However, as in many other biological pathways such as protein folding, genetic analyses revealed that Fe/S cluster biogenesis and insertion depend in vivo upon auxiliary proteins. This was brought to light by studies on Azotobacter vinelandii nitrogenase, which, in particular, led to the concept of scaffold proteins, the role of which would be to allow transient assembly of Fe/S cluster. These studies paved the way toward the identification of the ISC and SUF systems, subjects of the present review that allow Fe/S cluster assembly into apoproteins of most organisms. Despite the recent discovery of the SUF and ISC systems, remarkable progress has been made in our understanding of their molecular composition and biochemical mechanisms. Such a rapid increase in our knowledge arose from a convergent interest from researchers engaged in unrelated fields and whose complementary expertise covered most experimental approaches used in biology. Also, the high conservation of ISC and SUF systems throughout a wide array of organisms helped cross-feeding between studies. The ISC system is conserved in eubacteria and most eukaryotes, while the SUF system arises in eubacteria, archaea, plants and parasites. ISC and SUF systems share a common core function made of a cysteine desulfurase, which acts as a sulfur donor, and scaffold proteins, which act as sulfur and iron acceptors. The ISC and SUF systems also exhibit important differences. In particular, the ISC system includes an Hsp70/Hsp40-like pair of chaperones, while the SUF system involves an unorthodox ATP-binding cassette (ABC)-like component. The role of these two sets of ATP-hydrolyzing proteins in Fe/S cluster biogenesis remains unclear. Both systems are likely to target overlapping sets of apoproteins. However, regulation and phenotypic studies in E. coli, which synthesizes both types of systems, leads us to envisage ISC as the house-keeping one that functions under normal laboratory conditions, while the SUF system appears to be required in harsh environmental conditions such as oxidative stress and iron starvation. In Saccharomyces cerevisiae, the ISC system is located in the mitochondria and its function is necessary for maturation of both mitochondrial and cytosolic Fe/S proteins. Here, we attempt to provide the first comprehensive review of the ISC and SUF systems since their discovery in the mid and late 1990s. Most emphasis is put on E. coli and S. cerevisiae models with reference to other organisms when their analysis provided us with information of particular significance. We aim at covering information made available on each Isc and Suf component by the different experimental approaches, including physiology, gene regulation, genetics, enzymology, biophysics and structural biology. It is our hope that this parallel coverage will facilitate the identification of both similarities and specificities of ISC and SUF systems.

Global transcriptome analysis of Shewanella oneidensis MR-1 exposed to different terminal electron acceptors

To gain insight into the complex structure of the energy-generating networks in the dissimilatory metal reducer Shewanella oneidensis MR-1, global mRNA patterns were examined in cells exposed to a wide range of metal and non-metal electron acceptors. Gene expression patterns were similar irrespective of which metal ion was used as electron acceptor, with 60% of the differentially expressed genes showing similar induction or repression relative to fumarate-respiring conditions. Several groups of genes exhibited elevated expression levels in the presence of metals, including those encoding putative multidrug efflux transporters, detoxification proteins, extracytoplasmic sigma factors and PAS-domain regulators. Only one of the 42 predicted c-type cytochromes in MR-1, SO3300, displayed significantly elevated transcript levels across all metal-reducing conditions. Genes encoding decaheme cytochromes MtrC and MtrA that were previously linked to the reduction of different forms of Fe(III) and Mn(IV), exhibited only slight decreases in relative mRNA abundances under metal-reducing conditions. In contrast, specific transcriptome responses were displayed to individual non-metal electron acceptors resulting in the identification of unique groups of nitrate-, thiosulfate- and TMAO-induced genes including previously uncharacterized multi-cytochrome gene clusters. Collectively, the gene expression results reflect the fundamental differences between metal and non-metal respiratory pathways of S. oneidensis MR-1, where the coordinate induction of detoxification and stress response genes play a key role in adaptation of this organism under metal-reducing conditions. Moreover, the relative paucity and/or the constitutive nature of genes involved in electron transfer to metals is likely due to the low-specificity and the opportunistic nature of the metal-reducing electron transport pathways.

Nitric Oxide in Salmonella and Escherichia coli Infections

This review discusses the role that nitric oxide (NO) and its congeners play on various stages in the pathophysiology of Escherichia coli and Salmonella infections, with special emphasis on the regulatory pathways that lead to high NO synthesis, the role of reactive nitrogen species (RNS) in host resistance, and the bacterial molecular targets and defense mechanisms that protect enteric bacteria against the nitrosative stress encountered in diverse host anatomical sites. In general, NO can react directly with prosthetic groups containing transition metal centers, with other radicals, or with sulfhydryl groups in the presence of an electron acceptor. Binding to iron complexes is probably the best characterized direct reaction of NO in biological systems. The targets of RNS are numerous. RNS can facilitate oxidative modifications including lipid peroxidation, hydroxylation, and DNA base and protein oxidation. In addition, RNS can inflict nitrosative stress through the nitrosation of amines and sulfhydryls. Numerous vital bacterial molecules can be targeted by NO. It is therefore not surprising that enteropathogenic bacteria are armed with a number of sensors to coordinate the protective response to nitrosative stress, along with an assortment of antinitrosative defenses that detoxify, repair, or avoid the deleterious effects of RNS encountered within the host. NO and NO-derived RNS play important roles in innate immunity to Salmonella and E. coli. Enzymatic NO production by NO synthases can be enhanced by microbial and other inflammatory stimuli and it exerts direct antimicrobial actions as well as immunomodulatory and vasoregulatory effects.

Genome-wide transcriptional profiling of the steady-state response of Pseudomonas aeruginosa to hydrogen peroxide

The steady-state response of Pseudomonas aeruginosa to the oxidative-stress-generating agent hydrogen peroxide was analyzed by PAO1 transcriptome profiling. In total, 694, 411, and 237 genes were upregulated and 668, 576, and 468 genes were downregulated in P. aeruginosa strains TB, 892, and PAO1, respectively. The expression profiles of the two variants of the TB clone were significantly more related to each other than the expression profile of either strain was to that of PAO1. Exposure to H(2)O(2) activated by more than 10-fold the expression of the cyoABCD operon, which is key for aerobic respiration, and of oxidative-stress response elements such as the catalase KatB, the alkyl hydroperoxide reductase AhpF, and the thioredoxin reductase 2 operon. Genes for iron and sulfur homeostasis were upregulated. Most enzymes necessary for the conversion of amino acids into the citric acid cycle were globally downregulated at the transcriptional level. Nitrate respiration and arginine fermentation were shut off in the clone TB strains and attenuated in the PAO strain. The transcriptional profiles indicate that the two clone TB strains are more proficient in coping with H(2)O(2)-mediated oxidative stress than the reference strain PAO. According to this data, we recommend study of the transcriptome of strain PAO1 in parallel with those of at least two strains of another clone in order to differentiate common responses from clone- and strain-specific responses and to minimize overinterpretations of microarray data.

Genes required for rapid expression of nitrogenase activity in Azotobacter vinelandii

Rnf proteins are proposed to form membrane-protein complexes involved in the reduction of target proteins such as the transcriptional regulator SoxR or the dinitrogenase reductase component of nitrogenase. In this work, we investigate the role of rnf genes in the nitrogen-fixing bacterium Azotobacter vinelandii. We show that A. vinelandii has two clusters of rnf-like genes: rnf1, whose expression is nif-regulated, and rnf2, which is expressed independently of the nitrogen source in the medium. Deletion of each of these gene clusters produces a time delay in nitrogen-fixing capacity and, consequently, in diazotrophic growth. Deltarnf mutations cause two distinguishable effects on the nitrogenase system: (i), slower nifHDK gene expression and (ii), impairment of nitrogenase function. In these mutants, dinitrogenase reductase activity is lowered, whereas dinitrogenase activity remains essentially unaltered. Further analysis indicates that deltarnf mutants accumulate an inactive and iron-deficient form of NifH because they have lower rates of incorporation of [4Fe-4S] into NifH. Deltarnf mutations also cause a noticeable decrease in aconitase activity; however, they do not produce general oxidative stress or modification of Fe metabolism in A. vinelandii. Our results suggest the existence of a redox regulatory mechanism in A. vinelandii that controls the rate of expression and maturation of nitrogenase by the activity of the Rnf protein complexes. rnf1 plays a major and more specific role in this scheme, but the additive effects of mutations in rnf1 and rnf2 indicate the existence of functional complementation between the two homologous systems.

Possible Reasons for Difference in Sensitivity to Oxygen of Two Escherichia coli Strains

In preliminary experiments it was found that Escherichia coli strains AB1157 and KS400 are different in their abilities to grow under various oxygen levels in cultivation medium: the first strain does not grow under high oxygen conditions, unlike the second one. To investigate whether the damage to cellular components due to production of reactive oxygen species (ROS) was responsible for this difference, the intensity of free radical oxidation of proteins and lipids as well as the activities of selected antioxidant and associated enzymes (superoxide dismutase, catalase, peroxidase, glutathione reductase, glucose-6-phosphate dehydrogenase) were compared in the two strains. The level of thiobarbituric acid-reactive substances was 1.8-2.5-fold higher in AB1157 than in KS400, but the concentration of carbonyl proteins was lower in the AB1157 strain. In both strains growth under higher oxygen levels resulted in higher superoxide dismutase and peroxidase activities in both exponential and stationary phases. Overall, the activities of antioxidant enzymes were always higher in the KS400 strain than in AB1157. The results for both lipid and protein oxidative damage and antioxidant enzyme activities suggest that the differences in oxygen tolerance between these two strains may be due to their different abilities to cope with ROS.

Cross-compartment protection by SOD1

The absence of SOD1 in yeast has been found to result in inactivation of Lys4p. This [4Fe-4S]-containing dehydratase is in the pathway of biosynthesis of lysine, hence the oxygen-dependent lysine auxotrophy seen in this case. O(2)(-) is known to oxidize and thus destabilize the [Fe-4S] clusters of dehydratases; hence, this would make perfect sense were it not for the fact that SOD1 localizes to the cytosol and the intermembrane space of mitochondria, whereas Lys4p localizes to the mitochondrial matrix. How could SOD1 in one compartment protect against O(2)(-) attack in a different compartment? We suggest that the relatively high levels of O(2)(-) in the cytosol and intermembrane space of the SOD1 mutant may react with endogenous NO, forming HOONO that can diffuse into the mitochondrial matrix and there inactivate Lys4p and other [4Fe-4S]-containing dehydratases.

Lipoprotein PsaA in virulence of Streptococcus pneumoniae: surface accessibility and role in protection from superoxide

PsaA of Streptococcus pneumoniae, originally believed to be an adhesin, is the lipoprotein component of an Mn2+ transporter. Mutations in psaA cause deficiencies in growth, virulence, adherence, and the oxidative stress response. Immunofluorescence microscopy shows that PsaA is hidden beneath the cell wall and the polysaccharide capsule and only exposed to antibodies upon cell wall removal. A psaBC deletion mutant, expressing PsaA normally, was as deficient in adherence to Detroit 562 cells as were strains lacking PsaA. Thus, PsaA does not appear to act directly as an adhesin, but rather, psaA mutations indirectly affect this process through the disruption of Mn2+ transport. The deficiency in Mn2+ transport also causes hypersensitivity to oxidative stress from H2O2 and superoxide. In a chemically defined medium, growth of the wild-type strain was possible in the absence of Fe2+ and Mn2+ cations after a lag of about 15 h. Addition of Mn2+ alone or together with Fe2+ allowed prompt and rapid growth. In the absence of Mn2+, the addition of Fe2+ alone extended the 15-h lag phase to 25 h. Thus, while Fe2+ adversely affects the transition from lag phase to log phase, perhaps through increasing oxidative stress, this effect is relieved by the presence of Mn2+. A scavenger specific for superoxides but not those specific for hydroxyl radicals or H2O2 was able to eliminate the inhibition of growth caused by iron supplementation in the absence of Mn2+. This implies that superoxides are a key player in oxidative stress generated in the presence of iron.

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 suf operon requirement for Fe-S cluster assembly during iron starvation in Escherichia coli

The suf and isc operons of Escherichia coli have been implicated in Fe-S cluster assembly. However, it has been unclear why E. coli has two systems for Fe-S cluster biosynthesis. We have examined the regulatory characteristics and mutant phenotypes of both operons to discern if the two operons have redundant functions or if their cellular roles are divergent. Both operons are similarly induced by hydrogen peroxide and the iron chelator 2,2'-dipyridyl, although by different mechanisms. Regulation of the isc operon is mediated by IscR, whereas the suf operon requires OxyR and IHF for the response to oxidative stress and Fur for induction by iron starvation. Simultaneous deletion of iscS and most suf genes is synthetically lethal. However, although the suf and isc operons have overlapping functions, they act as distinct complexes because the SufS desulphurase alone cannot substitute for the IscS enzyme. In addition, suf deletion mutants are more sensitive to iron starvation than isc mutants, and the activity of the Fe-S enzyme gluconate dehydratase is diminished in the suf mutant during iron starvation. These findings are consistent with the model that the isc operon encodes the housekeeping Fe-S cluster assembly system in E. coli, whereas the suf operon is specifically adapted to synthesize Fe-S clusters when iron or sulphur metabolism is disrupted by iron starvation or oxidative stress.

The role of glutathione in nitric oxide donor toxicity to SN56 cholinergic neuron-like cells

Our study was designed to determine if compounds used experimentally to generate nitric oxide excess differ in ability to elicit degenerative stress to cholinergic neurons and, if so, what mechanisms account for their differences. Nitric oxide donors are often used experimentally in attempts to emulate the bioactivities of endogenous NO, but the pharmacological actions of NO donors can vary dramatically according to the species of NO (NOx) and other agents (e.g., iron cations, cyanide anion, superoxide anion) released, and as affected by the state of the cellular redox environment. To determine whether different types of NO donors exert differential toxicity in a cholinergic neuronal model, we measured cell viability markers, indicators of NOx formation, levels of intracellular-reduced glutathione (GSH), protein nitrosothiols, and the activation of the transcription factor NF-kappaB in a mouse medial septal cholinergic cell line (clone SN56) following exposure to the NO donors S-nitroso-N-acetyl-dl-penicillamine (SNAP), 3-morpholinosydnonimine (SIN-1), or sodium nitroprusside (SNP). SNAP and SIN-1, but not SNP, elicited dramatic increases in media nitrite and intracellular NOx-related fluorescence from cells preloaded with a NOx indicator. Nevertheless, SN56 cells were readily killed by SNP (IC(50) approximately 0.5 mM), while even higher levels (up to 2 mM) of SNAP or SIN-1 were essentially ineffective. SNAP (an NO(+) generator) and SIN-1 (a peroxynitrite generator) both caused increases in SN56 GSH levels; in contrast, SNP caused an immediate and rapid decline in GSH. The increase in GSH in response to SNAP and SIN-1 probably indicates augmentation of intracellular defense mechanisms, because prior depletion of GSH rendered the cells vulnerable to these two donors. GSH depletion did not change the potency of SNP, but GSH depletion made SNAP about twice as potent as SNP. SNAP and SNP, but not SIN-1, activated the transcription factor NF-kappaB, as indicated by increases in p65 nuclear immunoreactivity. Treatment with SNAP, but not SNP or SIN-1, increased levels of S-nitrosothiols in SN56 proteins, consistent with the transfer of an NO(+) equivalent to intracellular thiols. Our experiments show that these three NO donors differ dramatically in their ability to intoxicate SN56 cells, probably because of the different species of NOx and other agents they release, and as reflected in their differing modes of interaction with cellular antioxidant and survival systems.

Reactive nitrogen species in the chemical biology of inflammation

The preponderance of epidemiological evidence now points to a strong association between chronic inflammation and cancers of several organs, including the gastrointestinal tract, liver, and lungs. The strongest evidence for a mechanistic link here involves the generation of reactive oxygen and nitrogen species by macrophages and neutrophils that respond to cytokines and other signaling processes arising at sites of inflammation. These reactive species cause oxidation, nitration, halogenation, and deamination of biomolecules of all types, including lipids, proteins, carbohydrates, and nucleic acids, with the formation of toxic and mutagenic products. This review, in honor of Bruce Ames, will focus on recent advances in our understanding of the protein and DNA damage caused by reactive nitrogen species produced by macrophages and neutrophils, with emphasis on nitric oxide, nitrous anhydride, peroxynitrite, and nitrogen dioxide radical.

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.

A mechanism by which nitric oxide accelerates the rate of oxidative DNA damage in Escherichia coli

The presence of nitric oxide (NO) greatly accelerates the rate at which hydrogen peroxide (H2O2) kills Escherichia coli. Workers have suggested that this effect may be important in the process of bacteriocide by phagocytes. The goal of this study was to determine the mechanism of this synergism. The filamentation of the dead cells, and their protection by cell-permeable iron chelators, indicated that NO/H2O2 killed cells by damaging their DNA through the Fenton reaction. Indeed, the number of DNA lesions was far greater when NO was present during H2O2 exposure. In the Fenton reaction, free intracellular iron transfers electrons from adventitious donors to H2O2, producing hydroxyl radicals. Although NO damaged the [Fe-S] clusters of dehydratases, this did not increase the amount of free iron and was therefore not the reason for acceleration of Fenton chemistry. However, NO also blocked respiration, an event that previous studies have shown can stimulate oxidative DNA damage. The resultant accumulation of NADH accelerates the reduction of free flavins by flavin reductase, and these reduced flavins drive Fenton chemistry by transferring electrons to free iron. Indeed, mutants lacking the respiratory quinol oxidases were sensitive to H2O2, and NO did not have any further effect. Further, mutants that lack flavin reductase were resistant to NO/H2O2, and overproducing strains were hypersensitive. We discuss the possibility that H2O2 and NO synergize when macrophages attack captive bacteria.

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.

Why does SOD overexpression sometimes enhance, sometimes decrease, hydrogen peroxide production? A minimalist explanation

Toxic effects of superoxide dismutase (SOD) overexpression are commonly attributed to increased hydrogen peroxide (H(2)O(2)) production. Still, published experiments yield contradictory evidence on whether SOD overexpression increases or decreases H(2)O(2) production. We analyzed this issue using a minimal mathematical model. The most relevant mechanisms of superoxide consumption are treated as pseudo first-order processes, and both superoxide production and the activity of enzymes other than SOD were considered constant. Even within this simple framework, SOD overexpression may increase, hold constant, or decrease H(2)O(2) production. At normal SOD levels, the outcome depends on the ratio between the rate of processes that consume superoxide without forming H(2)O(2) and the rate of processes that consume superoxide with high (>/= 1) H(2)O(2) yield. In cells or cellular compartments where this ratio is exceptionally low (< 1), a modest decrease in H(2)O(2) production upon SOD overexpression is expected. Where the ratio is higher than unity, H(2)O(2) production should increase, but at most linearly, with SOD activity. The results are consistent with the available experimental observations. According to the minimal model, only where most superoxide is eliminated through H(2)O(2)-free processes does SOD activity have the moderately large influence on H(2)O(2) production observed in some experiments.

A potential role for periplasmic superoxide dismutase in blocking the penetration of external superoxide into the cytosol of Gram-negative bacteria

Superoxide is a key component of the antibacterial weaponry of phagocytes. Presumably, for this reason, strains of Salmonella typhimurium express a periplasmic superoxide dismutase (SOD) that is essential for full virulence. Because most anions cannot easily penetrate lipid membranes, it is thought that the phagosomal superoxide either damages an unknown target on the bacterial surface or reacts with nitric oxide to form peroxynitrite (HOONO), a toxic oxidant that can freely enter bacteria. However, in this study, we tested whether superoxide itself could penetrate membranes. Superoxide that was generated at high pH (>7.5) very slowly reduced cytochrome c that was encapsulated inside lipid vesicles. It did so much more quickly at lower pH (<7). Under the latter conditions, more superoxide was protonated and uncharged (HO2*), and the penetrance of superoxide was proportional to the concentration of this species. The permeability coefficient of HO2* was determined to be 9 x 10(-4) cm sec(-1), just slightly lower than that of water and far higher than the value of the anionic form (O2-, <10(-7) cm sec(-1). When Escherichia coli mutants that lack periplasmic SOD were exposed to super-oxide at pH 6.5, cytosolic fumarase B was damaged. Damage was minimal at higher pH or in strains that contained periplasmic SOD. Thus, in the acidic phagolysosome, superoxide may be able to penetrate and attack cytosolic targets of captive bacteria. This process may contribute to the potency of the oxidative burst. One role of periplasmic SOD may be to avert this damage. In contrast, periplasmic SOD was ineffective at lowering the extracellular super-oxide concentration and, therefore, may have little impact upon HOONO formation.

Killing of spores of Bacillus subtilis by peroxynitrite appears to be caused by membrane damage

During an infection of a higher eukaryote, dormant spores of a Bacillus species have been previously shown to be present in cells that can generate the toxic agent peroxynitrite (PON). Dormant spores of Bacillus subtilis were much more resistant to killing by PON than were growing cells, and spore-coat alteration or removal greatly decreased PON resistance. Spores were not killed by PON through DNA damage and lost no dipicolinic acid (DPA) during PON treatment. However, PON-killed spores lost DPA during subsequent heat treatments that caused much less DPA release from untreated spores. Although dead, the PON-killed spores germinated and initiated metabolism but never went through outgrowth; the great majority of germinated PON-killed spores also took up propidium iodide, indicating that they had suffered significant membrane damage and were dead. Together these data suggest that spore killing by PON is through some type of damage to the spore's inner membrane.

Mechanisms of Peroxynitrite Decomposition Catalyzed by FeTMPS, a Bioactive Sulfonated Iron Porphyrin

Peroxynitrite is a known cytotoxic agent that plays a role in many pathological conditions. Various peroxynitrite decomposition catalysts and pathways are being explored to develop efficient therapeutic agents that can safely remove peroxynitrite from cells and tissues. Water-soluble porphyrins, such as iron(III) meso-tetra(2,4,6-trimethyl-3,5-disulfonato)porphine chloride (FeTMPS) and iron(III) meso-tetra(N-methyl4-pyridyl)porphine chloride (FeTMPyP), have been shown to react catalytically with peroxynitrite (ONOO-). However, their mechanisms are yet to be fully understood. In this study, we have explored the reactivity of FeTMPS in the catalytic decomposition of peroxynitrite. The mechanism of this complex process has been determined. According to this mechanism, Fe(III)TMPS is oxidized by peroxynitrite to produce oxoFe(lV)TMPS and NO2 (k1 = 1.3 x 10(5) M(-1)(s(-1). The porphyrin is then reduced back to Fe(III)TMPS by nitrite, but this rate (k2 = 1.4 x 10(4) M(-1)s(-1)) is not sufficient to maintain the catalytic process at the observed rate. The overall rate of peroxynitrite decomposition catalysis, kcat, was determined to be 6 x 10(4) M(-1)s(-1), under typical conditions. We have postulated that an additional reduction pathway must exist. Kinetic simulations showed that a reaction of oxoFe(IV)TMPS with NO2 (k3 = 1.7 x 10(7) M((-1)s(-1)) could explain the behavior of this system and account for the fast reduction of oxoFe(IV)TMPS to Fe(III). Using the kinetic simulation analysis, we have also shown that two other rearrangement reactions, involving FeTMPS and peroxynitrite, are plausible pathways for peroxynitrite decay. A "cage-return" reaction between the generated oxoFe(IV)TMPS and NO2 (k8 = 5.4 x 10(4) M(-1)s(-1)), affording Fe(III)TMPS and nitrate, and a reaction between oxoFe(IV)TMPS and peroxynitrite (k7 = 2.4 x 10(4) M(-1)s(-1)) that affords oxoFe(IV)TMPS and nitrate are presented. The mechanism of FeTMPS-catalyzed peroxynitrite decay differs markedly from that of FeTMPyP, providing some insight into the reactivity of metal centers with peroxynitrite and biologically important radicals such as NO2.

How does oxygen inhibit central metabolism in the obligate anaerobe Bacteroides thetaiotaomicron

The molecular basis of obligate anaerobiosis is not well established. Bacteroides thetaiotaomicron is an opportunistic pathogen that cannot grow in fully aerobic habitats. Because microbial niches reflect features of energy-producing strategies, we suspected that aeration would interfere with its central metabolism. In anaerobic medium, this bacterium fermented carbohydrates to a mixture of succinate, propionate and acetate. When cultures were exposed to air, the formation of succinate and propionate ceased abruptly. In vitro analysis demonstrated that the fumarase of the succinate-propionate pathway contains an iron-sulphur cluster that is sensitive to superoxide. In vivo, fumarase activity fell to < 5% when cells were aerated; virtually all activity was recovered after extracts were chemically treated to rebuild iron-sulphur clusters. Aeration minimally affected the remainder of this pathway. However, aeration reduced pyruvate:ferredoxin oxidoreductase (PFOR), the first enzyme in the acetate fermentation branch, to 3% of its anaerobic activity. This cluster-containing enzyme was damaged in vitro by molecular oxygen but not by superoxide. Thus, aerobic growth is precluded by the vulnerability of these iron-sulphur cluster enzymes to oxidation. Importantly, both enzymes were maintained in a stable, inactive form for long periods in aerobic cells; they were then rapidly repaired when the bacterium was returned to anaerobic medium. This result explains how this pathogen can easily recover from occasional exposure to oxygen.

Yeast lacking superoxide dismutase(s) show elevated levels of "free iron" as measured by whole cell electron paramagnetic resonance

A current hypothesis explaining the toxicity of superoxide anion in vivo is that it oxidizes exposed [4Fe-4S] clusters in certain vulnerable enzymes causing release of iron and enzyme inactivation. The resulting increased levels of "free iron" catalyze deleterious oxidative reactions in the cell. In this study, we used low temperature Fe(III) electron paramagnetic resonance (EPR) spectroscopy to monitor iron status in whole cells of the unicellular eukaryote, Saccharomyces cerevisiae. The experimental protocol involved treatment of the cells with desferrioxamine, a cell-permeant, Fe(III)-specific chelator, to promote oxidation of all of the "free iron" to the Fe(III) state wherein it is EPR-detectable. Using this method, a small amount of EPR-detectable iron was detected in the wild-type strain, whereas significantly elevated levels were found in strains lacking CuZn-superoxide dismutase (CuZn-SOD) (sod1 delta), Mn-SOD (sod2 delta), or both SODs, throughout their growth but particularly in stationary phase. The accumulation was suppressed by expression of wild-type human CuZn-SOD (in the sod1 delta mutant), by pmr1, a genetic suppressor of the sod delta mutant phenotype (in the sod1 delta sod2 delta double knockout strain), and by anaerobic growth. In wild-type cells, an increase in the EPR-detectable iron pool could be induced by treatment with paraquat, a redox-cycling drug that generates superoxide. Cells that were not pretreated with desferrioxamine had Fe(III) EPR signals that were equally as strong as those from treated cells, indicating that "free iron" accumulated in the ferric form in our strains in vivo. Our results indicate a relationship between superoxide stress and iron handling and support the above hypothesis for superoxide-related oxidative damage.

The cysteine desulfurase, IscS, has a major role in in vivo Fe-S cluster formation in Escherichia coli

The cysteine desulfurase, IscS, provides sulfur for Fe-S cluster synthesis in vitro, but a role for IscS in in vivo Fe-S cluster formation has yet to be established. To study the in vivo function of IscS in Escherichia coli, a strain lacking IscS was constructed and characterized. Using this iscS deletion strain, we have observed decreased specific activities for proteins containing [4Fe-4S] clusters from soluble (aconitase B, 6-phosphogluconate dehydratase, glutamate synthase, fumarase A, and FNR) and membrane-bound proteins (NADH dehydrogenase I and succinate dehydrogenase). A specific role for IscS in in vivo Fe-S cluster assembly was demonstrated by showing that an Fe-S cluster independent mutant of FNR is unaffected by the lack of IscS. These data support the conclusion that, via its cysteine desulfurase activity, IscS provides the sulfur that subsequently becomes incorporated during in vivo Fe-S cluster synthesis. We also have characterized a growth phenotype associated with the loss of IscS. Under aerobic conditions the deletion of IscS caused an auxotrophy for thiamine and nicotinic acid, whereas under anaerobic conditions, only nicotinic acid was required. The lack of IscS also had a general effect on the growth of E. coli because the iscS deletion strain grew at half the rate of wild type in many types of media even when the auxotrophies were satisfied.

Role of quinones in toxicology

Quinones represent a class of toxicological intermediates which can create a variety of hazardous effects in vivo, including acute cytotoxicity, immunotoxicity, and carcinogenesis. The mechanisms by which quinones cause these effects can be quite complex. Quinones are Michael acceptors, and cellular damage can occur through alkylation of crucial cellular proteins and/or DNA. Alternatively, quinones are highly redox active molecules which can redox cycle with their semiquinone radicals, leading to formation of reactive oxygen species (ROS), including superoxide, hydrogen peroxide, and ultimately the hydroxyl radical. Production of ROS can cause severe oxidative stress within cells through the formation of oxidized cellular macromolecules, including lipids, proteins, and DNA. Formation of oxidatively damaged bases such as 8-oxodeoxyguanosine has been associated with aging and carcinogenesis. Furthermore, ROS can activate a number of signaling pathways, including protein kinase C and RAS. This review explores the varied cytotoxic effects of quinones using specific examples, including quinones produced from benzene, polycyclic aromatic hydrocarbons, estrogens, and catecholamines. The evidence strongly suggests that the numerous mechanisms of quinone toxicity (i.e., alkylation vs oxidative stress) can be correlated with the known pathology of the parent compound(s).

Sensitivity of FRDA lymphoblasts to salts of transition metal ions

Friedreich's ataxia (FRDA) is an autosomal recessive neurodegenerative disease resulting from decreased expression of the nuclear-encoded mitochondrial protein, frataxin. FRDA patients have characteristic iron deposits and dysfunction of mitochondrial enzymes in the heart. Inactivation of the frataxin homologue in yeast causes dysregulation of both mitochondrial iron levels and iron export. Previously, we have observed sensitivity of FRDA fibroblasts to FeCl3 and hydrogen peroxide, results consistent with the hypothesis that FRDA cells may experience increased Fenton chemistry. To determine whether the sensitivity of FRDA cells to transition metal ions is a general or specific property, we have compared the sensitivity of lymphoblasts from FRDA patients and healthy controls to the transition metal salts CoCl2, CuSO4 FeCl3 FeSO4, MnCl2, and ZnCl2. FRDA lymphoblasts were significantly more sensitive to FeCl3 and MnCl2 than control cells. However, there were no significant differences observed in sensitivity to CoCl2, CuSO4, FeSO4 and ZnCl2 in the concentration ranges studied. Thus, the sensitivity of FRDA lymphoblasts exposed to transition metals appears to be specific, and could be relevant to the pathophysiological mechanism, which is discussed.

Saccharomyces cerevisiae ISU1 and ISU2: members of a well-conserved gene family for iron-sulfur cluster assembly

Recent studies in bacteria and eukaryotes have led to the identification of several new genes implicated in the biogenesis of iron-sulfur (Fe/S) cluster-containing proteins. This report focuses on two genes of bakers yeast Saccharomyces cerevisiae, ISU1 and ISU2, which encode homologues to bacterial IscU and NifU, potential iron-binding or cluster-assembly proteins. As with other yeast genes implicated in Fe/S protein assembly, deletion of either ISU1 or ISU2 results in increased accumulation of iron within the mitochondria, loss of activity of the [4Fe-4S] aconitase enzyme, and suppression of oxidative damage in cells lacking cytosolic copper/zinc superoxide dismutase. Both genes are induced in strains expressing an activated allele of Aft1p, the iron-sensing transcription factor, suggesting that they are regulated by the iron status of the cell. Immunoblotting studies using an antibody directed against Escherichia coli IscU reveal that both Isu1p and Isu2p are localized primarily in the mitochondria and that Isu1p is the predominant form expressed under all growth conditions tested. The possible role of the Isu proteins in the assembly and/or repair of Fe/S clusters is discussed.

An intracellular iron chelator pleiotropically suppresses enzymatic and growth defects of superoxide dismutase-deficient Escherichia coli

Mutants of Escherichia coli that lack cytoplasmic superoxide dismutase (SOD) exhibit auxotrophies for sulfur-containing, branched-chain, and aromatic amino acids and cannot catabolize nonfermentable carbon sources. A secondary-site mutation substantially relieved all of these growth defects. The requirement for fermentable carbon and the branched-chain auxotrophy occur because superoxide (O2-) leaches iron from the [4Fe-4S] clusters of a family of dehydratases, thereby inactivating them; the suppression of these phenotypes was mediated by the restoration of activity to these dehydratases, evidently without changing the intracellular concentration of O2-. Cloning, complementation, and sequence analysis identified the suppressor mutation to be in dapD, which encodes tetrahydrodipicolinate succinylase, an enzyme involved in diaminopimelate and lysine biosynthesis. A block in dapB, which encodes dihydrodipicolinate reductase in the same pathway, conferred similar protection. Genetic analysis indicated that the protection stems from the intracellular accumulation of tetrahydro- or dihydrodipicolinate. Heterologous expression in the SOD mutants of the dipicolinate synthase of Bacillus subtilis generated dipicolinate and similarly protected them. Dipicolinates are excellent iron chelators, and their accumulation in the cell triggered derepression of the Fur regulon and a large increase in the intracellular pool of free iron, presumably as a dipicolinate chelate. A fur mutation only partially relieved the auxotrophies, indicating that Fur derepression assists but is not sufficient for suppression. It seems plausible that the abundant internal iron permits efficient reactivation of superoxide-damaged iron-sulfur clusters. This result provides circumstantial evidence that the sulfur and aromatic auxotrophies of SOD mutants are also directly or indirectly linked to iron metabolism.

Bacterioferritin A modulates catalase A (KatA) activity and resistance to hydrogen peroxide in Pseudomonas aeruginosa

We have cloned a 3.6-kb genomic DNA fragment from Pseudomonas aeruginosa harboring the rpoA, rplQ, katA, and bfrA genes. These loci are predicted to encode, respectively, (i) the alpha subunit of RNA polymerase; (ii) the L17 ribosomal protein; (iii) the major catalase, KatA; and (iv) one of two iron storage proteins called bacterioferritin A (BfrA; cytochrome b1 or b557). Our goal was to determine the contributions of KatA and BfrA to the resistance of P. aeruginosa to hydrogen peroxide (H2O2). When provided on a multicopy plasmid, the P. aeruginosa katA gene complemented a catalase-deficient strain of Escherichia coli. The katA gene was found to contain two translational start codons encoding a heteromultimer of approximately 160 to 170 kDa and having an apparent Km for H2O2 of 44.7 mM. Isogenic katA and bfrA mutants were hypersusceptible to H2O2, while a katA bfrA double mutant demonstrated the greatest sensitivity. The katA and katA bfrA mutants possessed no detectable catalase activity. Interestingly, a bfrA mutant expressed only approximately 47% the KatA activity of wild-type organisms, despite possessing wild-type katA transcription and translation. Plasmids harboring bfrA genes encoding BfrA altered at critical amino acids essential for ferroxidase activity could not restore wild-type catalase activity in the bfrA mutant. RNase protection assays revealed that katA and bfrA are on different transcripts, the levels of which are increased by both iron and H2O2. Mass spectrometry analysis of whole cells revealed no significant difference in total cellular iron levels in the bfrA, katA, and katA bfrA mutants relative to wild-type bacteria. Our results suggest that P. aeruginosa BfrA may be required as one source of iron for the heme prosthetic group of KatA and thus for protection against H2O2.

Oxidative stress

Much has been learnt about oxidative stress from studies of Escherichia coli. Key regulators of the adaptive responses in this organism are the SoxRS and OxyR transcription factors, which induce the expression of antioxidant activities in response to O2*- and H2O2 stress, respectively. Recently, a variety of biochemical assays together with the characterization of strains carrying mutations affecting the antioxidant activities and the regulators have given general insights into the sources of oxidative stress, the damage caused by oxidative stress, defenses against the oxidative stress, and the mechanisms by which the stress is perceived. These studies have also shown that the oxidative stress responses are intimately coupled to other regulatory networks in the cell.

Glyceraldehyde-3-phosphate dehydrogenase inactivation by peroxynitrite

Rabbit muscle glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was inactivated by peroxynitrite under biologically relevant conditions. The decrease of enzymatic activity followed an exponential function, and the concentration of peroxynitrite needed to inactivate 50% of 7 microM GAPDH (IC50) was 17 microM. Hydroxyl radical scavengers did not protect GAPDH from inactivation, but molecules that react directly with peroxynitrite such as cysteine, glutathione, or methionine and the substrate, glyceraldehyde 3-phosphate, afforded significant protection. Assuming simple competition kinetics between scavengers and the enzyme, we estimated a second-order rate constant of (2.5 +/- 0.5) x 10(5) M-1 s-1 at 25 degreesC and pH 7.4 for the GAPDH tetramer. The loss of enzyme activity was accompanied by protein thiol oxidation (two thiols oxidized per subunit) with only one critical thiol responsible of enzyme inactivation. Indeed, the pH profile of inactivation was consistent with the reaction of GAPDH sulfhydryls (GAPDH-SH) with peroxynitrite. Peroxynitrite-inactivated GAPDH was resistant to arsenite reduction and only 15% recovered by 20 mM dithiothreitol, suggesting that GAPDH-SH has been mainly oxidized to sulfinic or sulfonic acid, with a minor proportion yielding a disulfide. On the other hand, under anaerobic conditions the peroxynitrite precursor, nitric oxide (*NO), only slowly inactivated GAPDH with a rate constant of 11 M-1 s-1. The remarkable reactivity of the critical thiol group in GAPDH (Cys-149) toward peroxynitrite, which is one order of magnitude higher than that of previously studied sulfhydryls, indicate that it may constitute a preferential intracellular target for peroxynitrite.

Suppressors of superoxide dismutase (SOD1) deficiency in Saccharomyces cerevisiae. Identification of proteins predicted to mediate iron-sulfur cluster assembly

Yeast deficient in the cytosolic copper/zinc superoxide dismutase (SOD1) exhibit metabolic defects indicative of oxidative damage even under non-stress conditions. To help identify the endogenous sources of this oxidative damage, we isolated mutant strains of S. cerevisiae that suppressed metabolic defects associated with loss of SOD1. Six complementation groups were isolated and three of the corresponding genes have been identified. One sod1Delta suppressor represents SSQ1 which encodes a hsp70-type molecular chaperone found in the mitochondria. A second sod1Delta suppressor gene, designated JAC1, represents a new member of the 20-kDa J-protein family of co-chaperones. Jac1p contains a mitochondrial targeting consensus sequence and may serve as the partner for Ssq1p. Homologues of Ssq1p and Jac1p are found in bacteria in close association with genes proposed to be involved in iron-sulfur protein biosynthesis. The third suppressor gene identified was NFS1. Nfs1p is homologous to cysteine desulfurase enzymes that function in iron-sulfur cluster assembly and is also predicted to be mitochondrial. Each of the suppressor mutants identified exhibited diminished rates of respiratory oxygen consumption and was found to have reduced mitochondrial aconitase and succinate dehydrogenase activities. Taken together these results suggest a role for Ssq1p, Jac1p, and Nfs1p in assembly/maturation of mitochondrial iron-sulfur proteins and that one or more of the target Fe/S proteins contribute to oxidative damage in cells lacking copper/zinc SOD.

Nitric oxide and peroxynitrite-dependent aconitase inactivation and iron-regulatory protein-1 activation in mammalian fibroblasts

The reaction of reactive oxygen and nitrogen species with the [4Fe-4S]2+ cluster of mitochondrial (m-) and cytosolic (c-) aconitases leads to loss of catalytic activity and, in the case of the c-aconitase, triggers total cluster disruption to yield the iron-regulatory protein-1 (IRP-1). Herein we have studied the relative contribution and interplay of reactive oxygen species (O and H2O2), nitric oxide (*NO), and peroxynitrite in the modulation of m- and c-aconitase and IRP-1 activities in V79-M8 mammalian fibroblasts, identifying key variables that control the various reactivities at the cellular level. Extracellular production of H2O2 led to inactivation of both m- and c-aconitase and IRP-1 activation, while extracellular had no effect. However, increased intracellular production of caused a loss in m- and c-aconitase activity and IRP-1 activation. Nitric oxide released from NOC-12 had a more complex effect on aconitase and IRP-1 activities. Mitochondrial aconitase was more sensitive than c-aconitase to *NO-mediated inactivation and minimal activation of IRP-1 was observed during a 30-min exposure to the *NO donor. The action of *NO was down- or upregulated by the presence of extra- or intracelular, respectively. Extracellular decreased the *NO-mediated inactivation of aconitases, due to the preferential extracellular decomposition and the lower diffusivity of peroxynitrite compared to *NO. On the other hand, *NO exposure concomitant with enhanced intracellular fluxes lead to intracellular peroxynitrite formation as evidenced by Western blot analysis of nitrated proteins, which increased the effects observed with *NO alone. Peroxynitrite-mediated aconitase inactivation, IRP-1 activation, and cellular protein nitration were more pronounced in cells with low GSH content such as V79-M8 glutathione-depleted cells as well as in pGSOD4 cells which contain 32% of the GSH of the parental strain. Mechanistically, our results imply that the differential actions of the studied reactive species toward cellular aconitases depend on at least three critical factors: (i) their reaction rates with aconitases, (ii) the cellular compartment where they are formed, and (iii) the intracellular status of glutathione.

Balance between endogenous superoxide stress and antioxidant defenses

Cells devoid of cytosolic superoxide dismutase (SOD) suffer enzyme inactivation, growth deficiencies, and DNA damage. It has been proposed that the scant superoxide (O2-) generated by aerobic metabolism harms even cells that contain abundant SOD. However, this idea has been difficult to test. To determine the amount of O2- that is needed to cause these defects, we modulated the O2- concentration inside Escherichia coli by controlling the expression of SOD. An increase in O2- of more than twofold above wild-type levels substantially diminished the activity of labile dehydratases, an increase in O2- of any more than fourfold measurably impaired growth, and a fivefold increase in O2- sensitized cells to DNA damage. These results indicate that E. coli constitutively synthesizes just enough SOD to defend biomolecules against endogenous O2- so that modest increases in O2- concentration diminish cell fitness. This conclusion is in excellent agreement with quantitative predictions based upon previously determined rates of intracellular O2- production, O2- dismutation, dehydratase inactivation, and enzyme repair. The vulnerability of bacteria to increased intracellular O2- explains the widespread use of superoxide-producing drugs as bactericidal weapons in nature. E. coli responds to such drugs by inducing the SoxRS regulon, which positively regulates synthesis of SOD and other defensive proteins. However, even toxic amounts of endogenous O2- did not activate SoxR, and SoxR activation by paraquat was not at all inhibited by excess SOD. Therefore, in responding to redox-cycling drugs, SoxR senses some signal other than O2-.