Effect of glutathione on aconitase in Escherichia coli

Bacterial Approaches for Assembling Iron-Sulfur Proteins

Building iron-sulfur (Fe-S) clusters and assembling Fe-S proteins are essential actions for life on Earth. The three processes that sustain life, photosynthesis, nitrogen fixation, and respiration, require Fe-S proteins. Genes coding for Fe-S proteins can be found in nearly every sequenced genome. Fe-S proteins have a wide variety of functions, and therefore, defective assembly of Fe-S proteins results in cell death or global metabolic defects. Compared to alternative essential cellular processes, there is less known about Fe-S cluster synthesis and Fe-S protein maturation. Moreover, new factors involved in Fe-S protein assembly continue to be discovered. These facts highlight the growing need to develop a deeper biological understanding of Fe-S cluster synthesis, holo-protein maturation, and Fe-S cluster repair. Here, we outline bacterial strategies used to assemble Fe-S proteins and the genetic regulation of these processes. We focus on recent and relevant findings and discuss future directions, including the proposal of using Fe-S protein assembly as an antipathogen target.

Live cell imaging of mitochondrial redox state in mammalian cells and yeast

The redox state of mitochondria is determined by the levels of reducing and oxidizing species in the organelle, which reflects mitochondrial metabolic activity and overall fitness. Mitochondria are also the primary endogenous source of reactive oxygen species (ROS). This chapter describes methods to measure the mitochondrial superoxide levels and the redox state of the organelle in mammalian cells and yeast. We describe the use of dihydroethidium (DHE) and MitoSOX (a derivative of dihydroethidium bound to a lipophilic cation) to detect mitochondrial superoxide in yeast and mammalian cells, respectively. We also describe the use of genetically encoded fluorescent biosensors for quantitative analysis of mitochondrial NADPH levels (iNap) in mammalian cells and mitochondrial redox state (mito-roGFP) in yeast.

How Is Fe-S Cluster Formation Regulated?

Iron-sulfur (Fe-S) clusters are fundamental to numerous biological processes in most organisms, but these protein cofactors can be prone to damage by various oxidants (e.g., O2, reactive oxygen species, and reactive nitrogen species) and toxic levels of certain metals (e.g., cobalt and copper). Furthermore, their synthesis can also be directly influenced by the level of available iron in the environment. Consequently, the cellular need for Fe-S cluster biogenesis varies with fluctuating growth conditions. To accommodate changes in Fe-S demand, microorganisms employ diverse regulatory strategies to tailor Fe-S cluster biogenesis according to their surroundings. Here, we review the mechanisms that regulate Fe-S cluster formation in bacteria, primarily focusing on control of the Isc and Suf Fe-S cluster biogenesis systems in the model bacterium Escherichia coli.

Bacillithiol has a role in Fe-S cluster biogenesis in Staphylococcus aureus

Staphylococcus aureus does not produce the low-molecular-weight (LMW) thiol glutathione, but it does produce the LMW thiol bacillithiol (BSH). To better understand the roles that BSH plays in staphylococcal metabolism, we constructed and examined strains lacking BSH. Phenotypic analysis found that the BSH-deficient strains cultured either aerobically or anaerobically had growth defects that were alleviated by the addition of exogenous iron (Fe) or the amino acids leucine and isoleucine. The activities of the iron-sulfur (Fe-S) cluster-dependent enzymes LeuCD and IlvD, which are required for the biosynthesis of leucine and isoleucine, were decreased in strains lacking BSH. The BSH-deficient cells also had decreased aconitase and glutamate synthase activities, suggesting a general defect in Fe-S cluster biogenesis. The phenotypes of the BSH-deficient strains were exacerbated in strains lacking the Fe-S cluster carrier Nfu and partially suppressed by multicopy expression of either sufA or nfu, suggesting functional overlap between BSH and Fe-S carrier proteins. Biochemical analysis found that SufA bound and transferred Fe-S clusters to apo-aconitase, verifying that it serves as an Fe-S cluster carrier. The results presented are consistent with the hypothesis that BSH has roles in Fe homeostasis and the carriage of Fe-S clusters to apo-proteins in S. aureus.

Protective role of bacillithiol in superoxide stress and Fe-S metabolism in Bacillus subtilis

Glutathione (GSH) serves as the prime thiol in most organisms as its depletion increases antibiotic and metal toxicity, impairs oxidative stress responses, and affects Fe and Fe–S cluster metabolism. Many gram-positive bacteria lack GSH, but instead produce other structurally unrelated yet functionally equivalent thiols. Among those, bacillithiol (BSH) has been recently identified in several low G+C gram-positive bacteria. In this work, we have explored the link between BSH and Fe–S metabolism in Bacillus subtilis. We have identified that B. subtilis lacking BSH is more sensitive to oxidative stress (paraquat), and metal toxicity (Cu(I) and Cd(II)), but not H₂O₂. Furthermore, a slow growth phenotype of BSH null strain in minimal medium was observed, which could be recovered upon the addition of selected amino acids (Leu/Ile and Glu/Gln), supplementation of iron, or chemical complementation with BSH disulfide (BSSB) to the growth medium. Interestingly, Fe–S cluster containing isopropylmalate isomerase (LeuCD) and glutamate synthase (GOGAT) showed decreased activities in BSH null strain. Deficiency of BSH also resulted in decreased levels of intracellular Fe accompanied by increased levels of manganese and altered expression levels of Fe–S cluster biosynthetic SUF components. Together, this study is the first to establish a link between BSH and Fe–S metabolism in B. subtilis.

Analysis of spatial and temporal protein expression in the cerebral cortex after ischemia-reperfusion injury

Background and Purpose

Hypoxia, or ischemia, is a common cause of neurological deficits in the elderly. This study elucidated the mechanisms underlying ischemia-induced brain injury that results in neurological sequelae.

Methods

Cerebral ischemia was induced in male Sprague-Dawley rats by transient ligation of the left carotid artery followed by 60 min of hypoxia. A two-dimensional differential proteome analysis was performed using matrix-assisted laser desorption ionization-time-of-flight mass spectrometry to compare changes in protein expression on the lesioned side of the cortex relative to that on the contralateral side at 0, 6, and 24 h after ischemia.

Results

The expressions of the following five proteins were up-regulated in the ipsilateral cortex at 24 h after ischemia-reperfusion injury compared to the contralateral (i.e., control) side: aconitase 2, neurotensin-related peptide, hypothetical protein XP-212759, 60-kDa heat-shock protein, and aldolase A. The expression of one protein, dynamin-1, was up-regulated only at the 6-h time point. The level of 78-kDa glucose-regulated protein precursor on the lesioned side of the cerebral cortex was found to be high initially, but then down-regulated by 24 h after the induction of ischemia-reperfusion injury. The expressions of several metabolic enzymes and translational factors were also perturbed soon after brain ischemia.

Conclusions

These findings provide insights into the mechanisms underlying the neurodegenerative events that occur following cerebral ischemia.

Ratiometric biosensors that measure mitochondrial redox state and ATP in living yeast cells

Mitochondria have roles in many cellular processes, from energy metabolism and calcium homeostasis to control of cellular lifespan and programmed cell death. These processes affect and are affected by the redox status of and ATP production by mitochondria. Here, we describe the use of two ratiometric, genetically encoded biosensors that can detect mitochondrial redox state and ATP levels at subcellular resolution in living yeast cells. Mitochondrial redox state is measured using redox-sensitive Green Fluorescent Protein (roGFP) that is targeted to the mitochondrial matrix. Mito-roGFP contains cysteines at positions 147 and 204 of GFP, which undergo reversible and environment-dependent oxidation and reduction, which in turn alter the excitation spectrum of the protein. MitGO-ATeam is a Förster resonance energy transfer (FRET) probe in which the ε subunit of the FoF1-ATP synthase is sandwiched between FRET donor and acceptor fluorescent proteins. Binding of ATP to the ε subunit results in conformation changes in the protein that bring the FRET donor and acceptor in close proximity and allow for fluorescence resonance energy transfer from the donor to acceptor.

Mitochondrial response to oxidative and nitrosative stress in early stages of diabetes

Diabetes mellitus (DM) is associated with increased production of reactive oxygen and nitrogen species; consequently, an increase in lipid peroxidation and a decrease in antioxidants resulting in mitochondrial dysfunction. Using a rat model of DM induced by streptozotocin, we show the opposite: an increase in NO levels, S-nitrosylation, aconitase activity, and total glutathione and a decrease in lipid peroxidation at early stages of diabetes. These data imply that the decrease in lipid peroxidation is a vital early response to hyperglycemia to prevent escalation of ROS generation in mitochondria. These results also suggest a need for novel therapeutic targets to prevent the neurological consequences of diabetes.

Large scale physiological readjustment during growth enables rapid, comprehensive and inexpensive systems analysis

Background

Rapidly characterizing the operational interrelationships among all genes in a given organism is a critical bottleneck to significantly advancing our understanding of thousands of newly sequenced microbial and eukaryotic species. While evolving technologies for global profiling of transcripts, proteins, and metabolites are making it possible to comprehensively survey cellular physiology in newly sequenced organisms, these experimental techniques have not kept pace with sequencing efforts. Compounding these technological challenges is the fact that individual experiments typically only stimulate relatively small-scale cellular responses, thus requiring numerous expensive experiments to survey the operational relationships among nearly all genetic elements. Therefore, a relatively quick and inexpensive strategy for observing changes in large fractions of the genetic elements is highly desirable.

Results

We have discovered in the model organism Halobacterium salinarum NRC-1 that batch culturing in complex medium stimulates meaningful changes in the expression of approximately two thirds of all genes. While the majority of these changes occur during transition from rapid exponential growth to the stationary phase, several transient physiological states were detected beyond what has been previously observed. In sum, integrated analysis of transcript and metabolite changes has helped uncover growth phase-associated physiologies, operational interrelationships among two thirds of all genes, specialized functions for gene family members, waves of transcription factor activities, and growth phase associated cell morphology control.

Conclusions

Simple laboratory culturing in complex medium can be enormously informative regarding the activities of and interrelationships among a large fraction of all genes in an organism. This also yields important baseline physiological context for designing specific perturbation experiments at different phases of growth. The integration of such growth and perturbation studies with measurements of associated environmental factor changes is a practical and economical route for the elucidation of comprehensive systems-level models of biological systems.

Analysis of yggX and gshA mutants provides insights into the labile iron pool in Salmonella enterica

Strains of Salmonella enterica lacking YggX and the cellular reductant glutathione exhibit defects similar to those resulting from iron deficiency and oxidative stress. Mutant strains are sensitive to hydrogen peroxide and superoxide, deregulate the expression of the Fur-regulated gene entB, and fail to grow on succinate medium. Suppression of some yggX gshA mutant phenotypes by the cell-permeable iron chelator deferoxamine allowed the conclusion that increased levels of cellular Fenton chemistry played a role in the growth defects. The data presented are consistent with a scenario in which glutathione acts as a physiological chelator of the labile iron pool and in which YggX acts upstream of the labile iron pool by preventing superoxide toxicity.

Enhanced mitochondrial superoxide in hyperglycemic endothelial cells: direct measurements and formation of hydrogen peroxide and peroxynitrite

Hyperglycemic challenge to bovine aortic endothelial cells (BAECs) increases oxidant formation and cell damage that are abolished by MnSOD overexpression, implying mitochondrial superoxide (O2•−) as a central mediator. However, mitochondrial O2•−and its steady-state concentrations have not been measured directly yet. Therefore, we aimed to detect and quantify O2•−through different techniques, along with the oxidants derived from it. Mitochondrial aconitase, a sensitive target of O2•−, was inactivated 60% in BAECs incubated in 30 mM glucose (hyperglycemic condition) with respect to cells incubated in 5 mM glucose (normoglycemic condition). Under hyperglycemic conditions, increased oxidation of the mitochondrially targeted hydroethidine derivative (MitoSOX) to hydroxyethidium, the product of the reaction with O2•−, could be specifically detected. An 8.8-fold increase in mitochondrial O2•−steady-state concentration (to 250 pM) and formation rate (to 6 μM/s) was estimated. Superoxide formation increased the intracellular concentration of both hydrogen peroxide, measured as 3-amino-2,4,5-triazole-mediated inactivation of catalase, and nitric oxide-derived oxidants (i.e., peroxynitrite), evidenced by immunochemical detection of 3-nitrotyrosine. Oxidant formation was further evaluated by chloromethyl dichlorodihydrofluorescein (CM-H2DCF) oxidation. Exposure to hyperglycemic conditions triggered the oxidation of CM-H2DCF and was significantly reduced by pharmacological agents that lower the mitochondrial membrane potential, inhibit electron transport (i.e., myxothiazol), and scavenge mitochondrial oxidants (i.e., MitoQ). In BAECs devoid of mitochondria (rho0cells), hyperglycemic conditions did not increase CM-H2DCF oxidation. Mitochondrial O2•−formation in hyperglycemic conditions was associated with increased glucose metabolization in the Krebs cycle and hyperpolarization of the mitochondrial membrane.

Global analysis of cellular factors and responses involved in Pseudomonas aeruginosa resistance to arsenite

The impact of arsenite [As(III)] on several levels of cellular metabolism and gene regulation was examined in Pseudomonas aeruginosa. P. aeruginosa isogenic mutants devoid of antioxidant enzymes or defective in various metabolic pathways, DNA repair systems, metal storage proteins, global regulators, or quorum sensing circuitry were examined for their sensitivity to As(III). Mutants lacking the As(III) translocator (ArsB), superoxide dismutase (SOD), catabolite repression control protein (Crc), or glutathione reductase (Gor) were more sensitive to As(III) than wild-type bacteria. The MICs of As(III) under aerobic conditions were 0.2, 0.3, 0.8, and 1.9 mM for arsB, sodA sodB, crc, and gor mutants, respectively, and were 1.5- to 13-fold less than the MIC for the wild-type strain. A two-dimensional gel/matrix-assisted laser desorption ionization-time of flight analysis of As(III)-treated wild-type bacteria showed significantly (>40-fold) increased levels of a heat shock protein (IbpA) and a putative allo-threonine aldolase (GlyI). Smaller increases (up to 3.1-fold) in expression were observed for acetyl-coenzyme A acetyltransferase (AtoB), a probable aldehyde dehydrogenase (KauB), ribosomal protein L25 (RplY), and the probable DNA-binding stress protein (PA0962). In contrast, decreased levels of a heme oxygenase (HemO/PigA) were found upon As(III) treatment. Isogenic mutants were successfully constructed for six of the eight genes encoding the aforementioned proteins. When treated with sublethal concentrations of As(III), each mutant revealed a marginal to significant lag period prior to resumption of apparent normal growth compared to that observed in the wild-type strain. Our results suggest that As(III) exposure results in an oxidative stress-like response in P. aeruginosa, although activities of classic oxidative stress enzymes are not increased. Instead, relief from As(III)-based oxidative stress is accomplished from the collective activities of ArsB, glutathione reductase, and the global regulator Crc. SOD appears to be involved, but its function may be in the protection of superoxide-sensitive sulfhydryl groups.

Sites and Mechanisms of Aconitase Inactivation by Peroxynitrite: Modulation by Citrate and Glutathione

Aconitases are iron-sulfur cluster-containing proteins present both in mitochondria and cytosol of cells; the cubane iron-sulfur (Fe-S) cluster in the active site is essential for catalytic activity, but it also renders aconitase highly vulnerable to reactive oxygen and nitrogen species. This study examined the sites and mechanisms of aconitase inactivation by peroxynitrite (ONOO-), a strong oxidant and nitrating agent readily formed from superoxide anion and nitric oxide generated by mitochondria. ONOO- inactivated aconitase in a dose-dependent manner (half-maximal inhibition was observed with approximately 3 microM ONOO-). Low levels of ONOO- caused the conversion of the Fe-S cluster from the [4Fe-4S]2+ form to the inactive [3Fe-4S]1+ form with the loss of labile iron, as confirmed by low-temperature EPR analysis. In the presence of the substrate, citrate, 66-fold higher concentrations of ONOO- were required for half-maximal inhibition. The protective effects of citrate corresponded to its binding to the active site. The inactivation of aconitase in the presence of citrate was due to ONOO--mediated cysteine thiol loss and tyrosine nitration in the enzyme as shown by Western blot analyses. LC/MS/MS analyses revealed that ONOO- treatment to aconitase resulted in nitration of tyrosines 151 and 472 and oxidation to sulfonic acid of cysteines 126 and 385. The latter is one of the three cysteine residues in aconitase that binds to the Fe-S cluster. All other modified tyrosine and cysteine residues were adjacent to the binding site, thus suggesting that these modifications caused conformational changes leading to active-site disruption. Aconitase cysteine thiol modifications other than oxidation to sulfonic acid, such as S-glutathionylation, also decreased aconitase activity, thus indicating that glutathionylation may be an important means of modulating aconitase activity under oxidative and nitrative stress. Taken together, these results demonstrate that the Fe-S cluster in the active site, cysteine 385 bound to the Fe-S cluster, and tyrosine and cysteine residues in the vicinity of the active site are important targets of oxidative and/or nitrative attack, which is selectively controlled by the mitochondrial matrix citrate levels. The mechanisms inherent in aconitase inactivation by ONOO- are discussed in terms of the mitochondrial matrix metabolic and thiol redox state.

Lack of YggX results in chronic oxidative stress and uncovers subtle defects in Fe-S cluster metabolism in Salmonella enterica

As components involved in Fe-S cluster metabolism are described, the challenge becomes defining the integrated process that occurs in vivo based on the individual functions characterized in vitro. Strains lacking yggX have been used here to mimic chronic oxidative stress and uncover subtle defects in Fe-S cluster metabolism. We describe the in vivo similarities and differences between isc mutants, which have a known function in cluster assembly, and mutants disrupted in four additional loci, gshA, apbC, apbE, and rseC. The latter mutants share similarities with isc mutants: (i) a sensitivity to oxidative stress, (ii) a thiamine auxotrophy in the absence of the YggX protein, and (iii) decreased activities of Fe-S proteins, including aconitase, succinate dehydrogenase, and MiaB. However, they differ from isc mutants by displaying a phenotypic dependence on metals and a distinct defect in the SoxRS response to superoxides. Results presented herein support the proposed role of YggX in iron trafficking and protection against oxidative stress, describe additional phenotypes of isc mutants, and suggest a working model in which the ApbC, ApbE, and RseC proteins and glutathione participate in Fe-S cluster repair.

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.

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.

The YggX protein of Salmonella enterica is involved in Fe(II) trafficking and minimizes the DNA damage caused by hydroxyl radicals: residue CYS-7 is essential for YggX function

Previous work from our laboratory identified YggX as a protein whose accumulation increased the resistance of Salmonella enterica to superoxide stress, reversed defects attributed to oxidized [Fe-S] clusters, and decreased the spontaneous mutation frequency of the cells. Here we present work aimed at determining why the accumulation of YggX correlates with reduced mutation frequency. Genetic and biochemical data showed that accumulation of YggX reduced the damage to DNA by hydroxyl radicals. The ability of purified YggX to protect DNA from Fenton chemistry mediated damage in vitro and to decrease the concentration of Fe(II) ions in solution available for chelation provided a framework for the interpretation of data obtained from in vivo experiments. The interpretation of in vitro assay results, within the context of the in vivo phenotypes, was validated by a mutant variant of YggX (C7S) that was unable to function in vivo or in vitro. We propose a model, based on data presented here and reported earlier, that suggests YggX is a player in Fe(II) trafficking in bacteria.

Lack of the ApbC or ApbE protein results in a defect in Fe-S cluster metabolism in Salmonella enterica serovar Typhimurium

The isc genes function in the assembly of Fe-S clusters and are conserved in many prokaryotic and eukaryotic organisms. In most bacteria studied, the isc operon can be deleted without loss of cell viability, indicating that additional systems for Fe-S cluster assembly must exist. Several laboratories have described nutritional and biochemical defects resulting from mutations in the isc operon. Here we demonstrate that null mutations in two genes of unknown function, apbC and apbE, result in similar cellular deficiencies. Exogenous ferric chloride suppressed these deficiencies in the apbC and apbE mutants, distinguishing them from previously described isc mutants. The deficiencies caused by the apbC and isc mutations were additive, which is consistent with Isc and ApbC's having redundant functions or with Isc and ApbC's functioning in different areas of Fe-S cluster metabolism (e.g., Fe-S cluster assembly and Fe-S cluster repair). Both the ApbC and ApbE proteins are similar in sequence to proteins that function in metal cofactor assembly. Like the enzymes with sequence similarity to ApbC, purified ApbC protein was able to hydrolyze ATP. The data herein are consistent with the hypothesis that the ApbC and ApbE proteins function in Fe-S cluster metabolism in vivo.

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.

Maturation of cytosolic iron-sulfur proteins requires glutathione

Glutathione is the major protective agent against oxidative stress in Saccharomyces cerevisiae. Deletion of the GSH1 gene (strain Deltagsh1) encoding the enzyme that catalyzes the first step of glutathione biosynthesis leads to growth arrest, which can be relieved by either glutathione or reducing agents such as dithiothreitol. Because defects in the biosynthesis of cellular iron-sulfur (Fe/S) proteins are associated with increases in glutathione levels, we examined the consequences of glutathione depletion on this essential process. No significant defects were detected in the amounts, activities, and maturation of mitochondrial Fe/S proteins in glutathione-depleted Deltagsh1 cells. On the contrary, the maturation of extra-mitochondrial Fe/S proteins was decreased substantially. The defect was rectified neither by addition of dithiothreitol nor under anaerobic conditions excluding oxidative damage of Fe/S clusters. A double mutant in GSH1 and ATM1 encoding a mitochondrial ATP binding cassette (ABC) transporter involved in cytosolic Fe/S protein maturation is nonviable even in the presence of dithiothreitol. Similar to atm1 and other mutants defective in cytosolic Fe/S protein maturation, mitochondria from glutathione-depleted Deltagsh1 cells accumulated high amounts of iron. Together, our data demonstrate that glutathione, in addition to its protective role against oxidative damage, performs a novel and specific function in the maturation of cytosolic Fe/S proteins.

Protection from superoxide damage associated with an increased level of the YggX protein in Salmonella enterica

The deleterious effect of superoxide radicals on cell growth and survival is predominantly caused by rapid oxidation of labile [Fe-S] clusters in proteins. Oxidation of these clusters releases Fe(II) ions, which participate in Fenton chemistry that damages DNA. Here it is shown that elevated levels of the YggX protein increase the resistance of Salmonella enterica to superoxide stress, reverse enzymatic defects attributed to oxidized [Fe-S] clusters, and decrease the spontaneous mutation frequency. The data are consistent with a model in which YggX protects protein [Fe-S] clusters from oxidation.

In vitro conversion of propionate to pyruvate by Salmonella enterica enzymes: 2-methylcitrate dehydratase (PrpD) and aconitase Enzymes catalyze the conversion of 2-methylcitrate to 2-methylisocitrate

Salmonella enterica serovar Typhimurium LT2 catabolizes propionate through the 2-methylcitric acid cycle, but the identity of the enzymes catalyzing the conversion of 2-methylcitrate into 2-methylisocitrate is unclear. This work shows that the prpD gene of the prpBCDE operon of this bacterium encodes a protein with 2-methylcitrate dehydratase enzyme activity. Homogeneous PrpD enzyme did not contain an iron-sulfur center, displayed no requirements for metal cations or reducing agents for activity, and did not catalyze the hydration of 2-methyl-cis-aconitate to 2-methylisocitrate. It was concluded that the gene encoding the 2-methyl-cis-aconitate hydratase enzyme is encoded outside the prpBCDE operon. Computer analysis of bacterial genome databases identified the presence of orthologues of the acnA gene (encodes aconitase A) in a number of putative prp operons. Homogeneous AcnA protein of S. enterica had strong aconitase activity and catalyzed the hydration of the 2-methyl-cis-aconitate to yield 2-methylisocitrate. The purification of this enzyme allows the complete reconstitution of the 2-methylcitric acid cycle in vitro using homogeneous preparations of the PrpE, PrpC, PrpD, AcnA, and PrpB enzymes. However, inactivation of the acnA gene did not block growth of S. enterica on propionate as carbon and energy source. The existence of a redundant aconitase activity (encoded by acnB) was postulated to be responsible for the lack of a phenotype in acnA mutant strains. Consistent with this hypothesis, homogeneous AcnB protein of S. enterica also had strong aconitase activity and catalyzed the conversion of 2-methyl-cis-aconitate into 2-methylisocitrate. To address the involvement of AcnB in propionate catabolism, an acnA and acnB double mutant was constructed, and this mutant strain cannot grow on propionate even when supplemented with glutamate. The phenotype of this double mutant indicates that the aconitase enzymes are required for the 2-methylcitric acid cycle during propionate catabolism.

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.

Lesions in gshA (Encoding gamma-L-glutamyl-L-cysteine synthetase) prevent aerobic synthesis of thiamine in Salmonella enterica serovar typhimurium LT2

Thiamine pyrophosphate is an essential cofactor that is synthesized de novo in Salmonella enterica serovar Typhimurium and other bacteria. In addition to genes encoding enzymes in the biosynthetic pathway, mutations in other metabolic loci have been shown to prevent thiamine synthesis. The latter loci identify the integration of the thiamine biosynthetic pathway with other metabolic processes and can be uncovered when thiamine biosynthesis is challenged. Mutations in gshA, encoding gamma-L-glutamyl-L-cysteine synthetase, prevent the synthesis of glutathione, the major free thiol in the cell, and are shown here to result in a thiamine auxotrophy in some of the strains tested, including S. enterica LT2. Phenotypic characterization of the gshA mutants indicated they were similar enough to apbC and apbE mutants to warrant the definition of a class of mutants unified by (i) a requirement for both the hydroxymethyl pyrimidine (HMP) and thiazole (THZ) moiety of thiamine, (ii) the ability of L-tryosine to satisfy the THZ requirement, (iii) suppression of the thiamine requirement by anaerobic growth, and (iv) suppression by a second-site mutation at a single locus. Genetic data indicated that a defective ThiH generates the THZ requirement in these strains, and we suggest this defect is due to a reduced ability to repair a critical [Fe-S] cluster.

Metabolic defects caused by mutations in the isc gene cluster in Salmonella enterica serovar typhimurium: implications for thiamine synthesis

The metabolic consequences of two insertions, iscR1::MudJ and iscA2::MudJ, in the isc gene cluster of Salmonella enterica serovar Typhimurium were studied. Each of these insertions had polar effects and caused a nutritional requirement for the thiazole moiety of thiamine. Data showed that IscS was required for the synthesis of nicotinic acid and the thiazole moiety of thiamine and that one or more additional isc gene products were required for a distinct step in the thiazole biosynthetic pathway. Strains with isc lesions had reduced succinate dehydrogenase and aconitase activities. Furthermore, isc mutants accumulated increased levels of pyruvate in the growth medium in response to exogenously added iron (FeCl(3)), and this response required a functional ferric uptake regulator, Fur.

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.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Lower intracellular hydrogen peroxide levels in cells overexpressing CuZn-superoxide dismutase

Transfection of V79 Chinese hamster cells produced clones in which CuZn-superoxide dismutase (CuZn-SOD) activities were 2.2- to 3. 5-fold higher than in the parental cells. An overall reduction of antioxidant enzyme activities and both total and oxidized glutathione levels had been found in these clones. Aconitase activities in these cells were determined to indirectly measure the O2- steady-state levels. As expected, in cells overexpressing CuZn-SOD, both total and cytosolic aconitase activities have increased. Because these clones showed reduced oxidized glutathione contents, it is unlikely that they present higher H2O2 steady-state levels as a consequence of the higher SOD levels. This was confirmed by measuring H2O2 steady-state levels in cells by flow cytometric analysis of 2',7'-dichlorofluorescein diacetate-treated cells. Despite the decreased antioxidant defenses, three of the clones overexpressing CuZn-SOD showed reduced H2O2 steady-state levels. These reduced H2O2 steady-state levels were found even when the cells were treated with the O2- generator 2,3-dimethoxy-1, 4-naphthoquinone. These data provide in vivo support for the hypothesis proposed by Liochev and Fridovich [Liochev, S. I. & Fridovich, I. (1994) Free Radical Biol. Med. 16, 29-33] that O2- dismutation prevents the formation of higher H2O2 levels by other reactions.

Loss of lung mitochondrial aconitase activity due to hyperoxia in bronchopulmonary dysplasia in primates

The premature primate exposed to hyperoxia provides a useful model of bronchopulmonary dysplasia. A critical target in hyperoxic injury is the mitochondrial matrix enzyme aconitase. We hypothesized that this enzyme's activity would decline in the premature baboon lung during exposure to hyperoxia. Total aconitase activity was significantly decreased in the lungs of premature baboons of 140 days gestation with exposure to 100% oxygen for 6-10 days compared with as needed [pro re nada (PRN)] oxygen exposure and fetal controls (P = 0.0001). In activity gels, lungs from 100% oxygen-exposed animals (6-10 days) showed a nearly complete loss of mitochondrial aconitase activity relative to lungs from animals exposed only to PRN oxygen. Decreased lung aconitase activity was not a nonspecific effect of hyperoxia, causing mitochondrial damage or loss, because the activity of the mitochondrial respiratory enzyme cytochrome oxidase was not different in lungs of 100% oxygen-exposed relative to PRN oxygen-exposed newborns. In 125-day-gestation premature primates (age 6-10 days), lung total aconitase activity was correlated with inspired oxygen tension (r = 0.73 for fraction of inspired oxygen > 0.35), whereas, for animals of 140 days gestation, no such correlation was found. Thus the more premature animal's lung was more susceptible to loss of aconitase.

Nitric oxide sensitivity of the aconitases

Aconitases are important cellular targets of nitric oxide (NO.) toxicity, and NO.-derived species, rather than NO. per se, have been proposed to mediate their inactivation. NO.-mediated inactivation of the Escherichia coli aconitase and the porcine mitochondrial aconitase was investigated. In E. coli, aconitase activity decreased by approximately 70% during a 2-h exposure to an atmosphere containing 120 ppm NO. in N2. The NO.-inactivated aconitase reactivated poorly in E. coli under anaerobic or aerobic conditions. Elevated superoxide dismutase activity did not affect the aerobic inactivation of aconitase by NO., thus indicating a limited role of the NO.- and superoxide-derived species peroxynitrite. Glutathione-deficient and glutathione-containing E. coli were comparably sensitive to NO.-mediated aconitase inactivation, thus excluding the participation of S-nitrosoglutathione or more oxidizing NO.-derived species. NO. progressively decreased aconitase activity in extracts in the presence of substrates, and inactivation was greatest at an acidic pH with cis-aconitate. The porcine mitochondrial aconitase was sensitive to NO. when exposed at pH 6.5, but not at pH 7.5, and irreversible inactivation occurred during catalysis. The requirement of an acidic pH or substrates for sensitivity may explain the reported resistance of aconitases to NO. in vitro (Castro, L., Rodriguez, M., and Radi, R. (1994) J. Biol. Chem. 269, 29409-29415; Hausladen, A., and Fridovich, I. (1994) J. Biol. Chem. 269, 29405-29408). An S-nitrosation of the aconitase [4Fe-4S] center catalyzed by the solvent-exposed electron withdrawing iron atom (Fea) is proposed.

Role of oxidants in microbial pathophysiology

Reactive oxidant species (superoxide, hydrogen peroxide, hydroxyl radical, hypohalous acid, and nitric oxide) are involved in many of the complex interactions between the invading microorganism and its host. Regardless of the source of these compounds or whether they are produced under normal conditions or those of oxidative stress, these oxidants exhibit a broad range of toxic effects to biomolecules that are essential for cell survival. Production of these oxidants by microorganisms enables them to have a survival advantage in their environment. Host oxidant production, especially by phagocytes, is a counteractive mechanism aimed at microbial killing. However, this mechanism may be contribute to a deleterious consequence of oxidant exposure, i.e., inflammatory tissue injury. Both the host and the microorganism have evolved complex adaptive mechanisms to deflect oxidant-mediated damage, including enzymatic and nonenzymatic oxidant-scavenging systems. This review discusses the formation of reactive oxidant species in vivo and how they mediate many of the processes involved in the complex interplay between microbial invasion and host defense.

The role of iron-sulfur clusters in in vivo hydroxyl radical production

The in vivo production of HO- requires iron ions, H2O2 and O2- or other oxidants but probably does not occur through the Haber-Weiss reaction. Instead oxidants, such as O2-, increase free iron by releasing Fe(II) from the iron-sulfur clusters of dehydratases and by interfering with the iron-sulfur clusters reassembly. Fe(II) then reduces H2O2, and in turn Fe(III) and the oxidized cluster are re-reduced by cellular reductants such as NADPH and glutathione. In this way, SOD cooperates with cellular reductants in keeping the iron-sulfur clusters intact and the rate of HO. production to a minimum. O2- and other oxidants can release iron from Fe(II)-containing enzymes as well as copper from thionein. The released Fe(III) and Cu(II) are then reduced to Fe(II) and Cu(I) and can then participate in the Fenton reaction. In mammalian cells oxidants are able to convert cytosolic aconitase into active IRE-BP, which increases the "free" iron concentration intracellularly both by decreasing the biosynthesis of ferritin and increasing biosynthesis of transferrin receptors. The biological role of the soxRS regulon of Escherichia coli, which is involved in the adaptation toward oxidative stress, is presumably to counteract the oxidative inactivation of the iron clusters and the subsequent release of iron with consequent increased rate of production of HO.

Glutathione-mediated destabilization in vitro of [2Fe-2S] centers in the SoxR regulatory protein

SoxR is a transcription factor that governs a global defense against the oxidative stress caused by nitric oxide or excess superoxide in Escherichia coli. SoxR is a homodimer containing a pair of [2Fe-2S] clusters essential for its transcriptional activity, and changes in the stability of these metal centers could contribute to the activation or inactivation of SoxR in vivo. Herein we show that reduced glutathione (GSH) in aerobic solution disrupts the SoxR [2Fe-2S] clusters, releasing Fe from the protein and eliminating SoxR transcriptional activity. This disassembly process evidently involves oxygen-derived free radicals. The loss of [2Fe-2S] clusters does not occur in anaerobic solution and is blocked in aerobic solution by the addition of superoxide dismutase and catalase. Although H2O2 or xanthine oxidase and hypoxanthine (to generate superoxide) were insufficient on their own to cause [2Fe-2S] cluster loss, they did accelerate the rate of disassembly after GSH addition. Oxidized GSH alone was ineffective in disrupting the clusters, but the rate of [2Fe-2S] cluster disassembly was maximal when reduced and oxidized GSH were present at a ratio of approximately 1:3, which suggests the critical involvement of a GSH-based free radical in the disassembly process. Such a reaction might occur in vivo: we found that the induction by paraquat of SoxR-dependent soxS transcription was much higher in a GSH-deficient E. coli strain than in its GSH-containing parent. The results imply that GSH may play a significant role during the deactivation process of SoxR in vivo. Ironically, superoxide production seems both to activate SoxR and, in the GSH-dependent disassembly process, to switch off this transcription factor.

NO-induced oxidative stress and glutathione metabolism in rodent and human cells

Nitric oxide (NO.), a radical species produced by many types of cells, is known to play a critical role in both regulatory processes and cell defense, yet it may also participate in collateral reactions, leading to DNA damage and cell death in both NO-generating and neighboring cells. Glutathione has been shown to protect cells from the toxic effects of free radicals and reactive oxygen species. The goal of this study was to investigate whether differences in glutathione metabolism could account for the resistance or sensitivity to cell killing by NO.. The cytotoxic effect of NO. was examined in CHO-AA8 (Chinese Hamster Ovary) cells and TK6 (human lymphoblastoid) cells pretreated with L-buthionine SR-sulfoximine (BSO), a potent inhibitor of gamma-glutamylcysteine synthetase, and with 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU), an irreversible inhibitor of glutathione reductase. The consequences resulting from the depletion of glutathione levels and from the arrest of oxidoreduction allowed us to show the involvement of glutathione in protecting cells from NO. and to investigate the importance of changes in glutathione metabolism on NO-induced toxicity. In CHO-AA8 cells, we found that treatment with NO. resulted in the oxidation of reduced glutathione (GSH) to oxidized glutathione (GSSG) and to mixed glutathione disulfides (GSSR). The resulting depletion of GSH stimulated its de novo synthesis, enabling the cells to resist killing by NO.. A slight difference in GSH metabolism was observed in TK6 cells. NO. led to an increase in GSSG levels similar to that observed in CHO-AA8 cells, however, a decrease in GSH levels, no change in GSSR levels, and higher levels of toxicity were also found, suggesting that NO-treated TK6 cells are not as competent in GSH homeostasis as CHO cells. We conclude that GSH is involved in protecting cells from killing by NO. and that both de novo synthesis of GSH and GSSG reduction are important in maintaining an adequate level of protection for the cells.

Glutathione is required for maximal transcription of the cobalamin biosynthetic and 1,2-propanediol utilization (cob/pdu) regulon and for the catabolism of ethanolamine, 1,2-propanediol, and propionate in Salmonella typhimurium LT2

Transcription of the cob/pdu regulon of Salmonella typhimurium is activated by the PocR regulatory protein in response to 1,2-propanediol (1,2-PDL) in the environment. Nutritional analysis and DNA sequencing confirmed that a strain defective in expression of the cob/pdu regulon in response to 1,2-PDL lacked a functional gshA gene. gshA encodes gamma-glutamylcysteine synthetase (L-glutamate:L-cysteine gamma-ligase [ADP forming]; EC 6.3.2.2), the enzyme that catalyzes the first step in the synthesis of glutathione (GSH). The DNA sequence of gshA was partially determined, and the location of gshA in the chromosome was established by two-factor crosses. P22 cotransduction of gshA with nearby markers showed 21% linkage to srl and 1% linkage to hyd; srl was 9% cotransducible with hyd. In light of these data, the gene order gshA srl hyd is suggested. The level of reduced thiols in the gshA strain was 87% lower than the levels measured in the wild-type strain in both aerobically and anaerobically grown cells. 1,2-PDL-dependent transcription of cob/pdu was studied by using M. Casadaban's Mu-lacZ fusions. In aerobically grown cells, transcription of a cbi-lacZ fusion (the cbi genes are the subset of cob genes that encode functions needed for the synthesis of the corrin ring) was 4-fold lower and transcription of a pdu-lacZ fusion was 10-fold lower in a gshA mutant than in the wild-type strain. Expression of the cob/pdu regulon in response to 1,2-PDL was restored when GSH was included in the medium. In anaerobically grown cells, cbi-lacZ transcription was only 0.4-fold lower than in the gshA+ strain; pdu-lacZ transcription was reduced only by 0.34-fold, despite the lower thiol levels in the mutant. cobA-lacZ transcription was used as negative control of gene whose transcription is not controlled by the PocR/1,2-PDL system; under both conditions, cobA transcription remained unaffected. The gshA mutant strain was unable to utilize 1,2-PDL, ethanolamine, or propionate as a carbon and energy source. The defect in ethanolamine utilization appears to be at the level of ethanolamine ammonia-lyase activity, not at the transcriptional level. Possible roles for GSH in ethanolamine, 1,2-PDL, and propionate catabolism are proposed and discussed.

Ascorbic acid enhances ferritin mRNA translation by an IRP/aconitase switch

Replenishment of ascorbate in cultured cells, which are almost uniformly vitamin-deficient, increases ferritin mRNA translation in response to iron by 20-fold (Toth, I., Rogers, J. T., McPhee, J. A., Elliott, S. M., Abramson, S. L., and Bridges, K. R. (1995) J. Biol. Chem. 270, 2846-2852). We now demonstrate that ascorbate increases cytosolic aconitase activity. The iron-responsive element-binding protein (IRP-1) exists in three states: bound to mRNA without aconitase activity, free in the cytosol without aconitase activity, and free in the cytosol with aconitase activity. Ascorbate converts free IRP-1 to the enzymatically active form. Enhanced ferritin synthesis with subsequent iron stimulation is due to the altered equilibrium of the free IRP-1. The cellular biology of iron is closely intertwined with that of ascorbate.

Superoxide radical and iron modulate aconitase activity in mammalian cells

Aconitase is a member of a family of iron-sulfur-containing (de)hydratases whose activities are modulated in bacteria by superoxide radical (O2-.)-mediated inactivation and iron-dependent reactivation. The inactivation-reactivation of aconitase(s) in cultured mammalian cells was explored since these reactions may impact important and diverse aconitase functions in the cytoplasm and mitochondria. Conditions which increase O2-. production including exposure to the redox-cycling agent phenazine methosulfate (PMS), inhibitors of mitochondrial ubiquinol-cytochrome c oxidoreductase, or hyperoxia inactivated aconitase in mammalian cells. Overproduction of mitochondrial Mn-superoxide dismutase protected aconitase from inactivation by PMS or inhibitors of ubiquinol-cytochrome c oxidoreductase, but not from normobaric hyperoxia. Aconitase activity was reactivated (t1/2 of 12 +/- 3 min) upon removal of PMS. The iron chelator deferoxamine impaired reactivation and increased net inactivation of aconitase by O2-.. The ability of ubiquinol-cytochrome c oxidoreductase-generated O2-. to inactivate aconitase in several cell types correlated with the fraction of the aconitase activity localized in mitochondria. Extracellular O2-. generated with xanthine oxidase did not affect aconitase activity nor did exogenous superoxide dismutase decrease aconitase inactivation by PMS. The results demonstrate a dynamic and cyclical O2-.-mediated inactivation and iron-dependent reactivation of the mammalian [4Fe-4S] aconitases under normal and stress conditions and provide further evidence for the membrane compartmentalization of O2-..

Aconitase is a sensitive and critical target of oxygen poisoning in cultured mammalian cells and in rat lungs

The effect of hyperoxia on activity of the superoxide-sensitive citric acid cycle enzyme aconitase was measured in cultured human epithelial-like A549 cells and in rat lungs. Rapid and progressive loss of > 80% of the aconitase activity in A549 cells was seen during a 24-hr exposure to a PO2 of 600 mmHg (1 mmHg = 133 Pa). Inhibition of mitochondrial respiratory capacity correlated with loss of aconitase activity in A549 cells exposed to hyperoxia, and this effect could be mimicked by fluoroacetate (or fluorocitrate), a metabolic poison of aconitase. Exposure of rats to an atmospheric PO2 of 760 mmHg or 635 mmHg for 24 hr caused respective 73% and 61% decreases in total lung aconitase activity. We propose that early inactivation of aconitase and inhibition of the energy-producing and biosynthetic reactions of the citric acid cycle contribute to the sequelae of lung damage and edema seen during exposure to hyperoxia.