Superoxide sensitivity of the Escherichia coli aconitase

NF kappa B-independent transcriptional induction of the human manganous superoxide dismutase gene

Numerous conditions induce expression of manganese-containing superoxide dismutase (MnSOD) in mammalian cells. The reported inducers of MnSOD are all agents that activate two transcription factors, AP-1 and NF kappa B, but several reports have suggested that MnSOD induction relies solely on NF kappa B. We investigated the contribution of the individual transcription factors by using antioxidants and metal chelators to modulate MnSOD transcriptional activation in response to phorbol esters or hydrogen peroxide. The results indicate substantial transcriptional induction of the MnSOD gene independent of NF kappa B. The metal chelator and antioxidant pyrrolidine dithiocarbamate (PDTC) at 60 or 100 microM induced the MnSOD transcript in HeLa cells while diminishing expression of the NF kappa B-responsive transcript I kappa B-alpha. Induction of the MnSOD mRNA by phorbol-12-myristate-13-acetate (PMA) was only slightly diminished in the presence of PDTC, which in contrast virtually eliminated induction of the NF kappa B-dependent transcript I kappa B-alpha by PMA. MnSOD RNA induction by H2O2 was only approximately 1.5-fold, compared to a ca. 3-fold activation of I kappa B-alpha expression. Two other antioxidants, N-acetyl-L-cysteine and butylated hydroxyanisole, failed to block induction of the MnSOD transcript by PMA, which is consistent with a role for AP-1. In vitro DNA binding studies confirmed strong AP-1 activation under conditions where NF kappa B is blocked but the MnSOD transcript is strongly induced (e.g., PMA treatment in the presence of PDTC).

Elevation of manganese superoxide dismutase gene expression by thioredoxin

Manganese superoxide dismutase (MnSOD) is a mitochondrial enzyme that dismutates potentially toxic superoxide radical into hydrogen peroxide and dioxygen. This enzyme is critical for protection against cellular injury due to elevated partial pressures of oxygen. Thioredoxin (TRX) is a potent protein disulfide reductase found in most organisms that participates in many thiol-dependent cellular reductive processes and plays an important role in antioxidant defense, signal transduction, and regulation of cell growth and proliferation. Here we describe induction of manganese superoxide dismutase by thioredoxin. MnSOD mRNA and activity were increased dramatically by low concentrations of TRX (28 microM). Elevation of MnSOD mRNA by TRX was inhibited by actinomycin D, but not cycloheximide, occurring both in cell lines and primary human lung microvascular endothelial cells. mRNAs for other antioxidant enzymes including copper-zinc superoxide dismutase and catalase were not elevated, demonstrating specificity of induction of MnSOD by TRX. Thiol oxidation by diamide or alkylation by chlorodinitrobenzene inhibited MnSOD induction, further indicating a requirement for reduced TRX. Because both oxidized and reduced thioredoxin (28 microM) induced MnSOD mRNA, the intracellular redox status of externally added Escherichia coli oxidized TRX was determined. About 45% of internalized E. coli TRX was reduced, with 8% in fully reduced form and about 37% in partially reduced form. However, when TRX reductase and nicotinamide adenine dinucleotide (NADPH) were added to the extracellular medium with TRX, more than 80% of E. coli TRX was found to be in a fully reduced state in human adenocarcinoma (A549) cells. Although lower concentrations of oxidized TRX (7 microM) did not induce MnSOD mRNA, this concentration of TRX, when reduced by NADPH and TRX reductase, increased MnSOD mRNA six-fold. In additional studies, MCF-7 cells stably transfected with the human TRX gene had elevated expression of MnSOD mRNA relative to vector-transfected controls. Thus, both endogenously produced and exogenously added TRX elevate MnSOD gene expression. These findings suggest a novel mechanism involving reduced TRX in regulation of MnSOD.

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

Phagocytes produce both nitric oxide and superoxide as components of the oxidative defense against pathogens. Neither molecule is likely at physiological concentrations to kill cells. However, two of their reaction products, hydrogen peroxide and peroxynitrite, are strong oxidants, cell-permeant, and toxic. Hydrogen peroxide generates oxidative DNA damage, while the primary mechanism of toxicity of peroxynitrite has not yet been determined. Recent in vitro studies indicated that peroxynitrite is capable of oxidizing the [4Fe-4S] clusters of a family of dehydratases (Hausladen, A., and Fridovich, I. (1994) J. Biol. Chem. 269, 29405-29408; Castro, L., Rodriguez, M., and Radi, R. (1994) J. Biol. Chem. 269, 29409-29415). We demonstrate here that peroxynitrite at 1% of its lethal dose almost fully inactivated the labile dehydratases in Escherichia coli. The rate at which peroxynitrite inactivated the clusters substantially exceeded the rate at which it oxidized thiols or spontaneously decomposed. These results suggest that these dehydratases may be primary targets of peroxynitrite in vivo. Another consequence of the cluster damage was the release of 100 microM iron into the cytosol. During phagocytosis, this intracellular free iron could increase lethal DNA damage by hydrogen peroxide or protein modification by additional peroxynitrite. In response to peroxynitrite challenges, E. coli rapidly sequestered the intracellular free iron using an undefined scavenging system. The iron-sulfur clusters were more gradually repaired by a process that drew iron from its iron-storage proteins. These are likely to be critical events in the struggle between phagocyte and pathogen.

Oxidative damage during aging targets mitochondrial aconitase

The mechanisms that cause aging are not well understood. The oxidative stress hypothesis proposes that the changes associated with aging are a consequence of random oxidative damage to biomolecules. We hypothesized that oxidation of specific proteins is critical in controlling the rate of the aging process. Utilizing an immunochemical probe for oxidatively modified proteins, we show that mitochondrial aconitase, an enzyme in the citric acid cycle, is a specific target during aging of the housefly. The oxidative damage detected immunochemically was paralleled by a loss of catalytic activity of aconitase, an enzyme activity that is critical in energy metabolism. Experimental manipulations which decrease aconitase activity should therefore cause a decrease in life-span. This expected decrease was observed when flies were exposed to hyperoxia, which oxidizes aconitase, and when they were given fluoroacetate, an inhibitor of aconitase. The identification of a specific target of oxidative damage during aging allows for the assessment of the physiological age of a specific individual and provides a method for the evaluation of treatments designed to affect the aging process.

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.

Redox signal transduction via iron-sulfur clusters in the SoxR transcription activator

Protein iron-sulfur (FeS) centers have recently been implicated in the regulation of gene expression. In the redox-sensing SoxR protein, the oxidation state of [2Fe-2S] centers controls its activity as a transcription activator independent of DNA-binding ability. Thus, FeS centers allosterically link cellular oxidative stress to the expression of defense genes.

Lucigenin luminescence as a measure of intracellular superoxide dismutase activity in Escherichia coli

Lucigenin and paraquat are similar in that each can be taken into Escherichia coli and can then mediate O2.- production by cycles of univalent reduction, to the corresponding monocation radical, followed by autoxidation. Thus, both compounds caused induction of enzymes that are regulated by the soxRS regulon. The lucigenin cation radical has the added property of reacting with O2.-, in a radical-radical addition, to yield an unstable dioxetane, whose decomposition yields light. Superoxide dismutases (SOD), by decreasing [O2.-], inhibit light production and to the same degree inhibit other O2.(-)-dependent reactions in the cell. Lucigenin luminescence was used to show that the levels of SOD in the parental strain provide approximately 95% protection of all O2.(-)-sensitive targets in E. coli. This degree of protection was so close to the limit of 100% that halving the parental level of [SOD], or increasing it 5-fold, had only marginal effects on the intensity of lucigenin-dependent luminescence.

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.

Impairment of mitochondrial respiration and electron transport chain enzymes during cocaine-induced hepatic injury

Morphological and biochemical changes in mitochondrial have been reported early in the course of cocaine-induced hepatotoxicity. This study was designed to examine the effects of repeated cocaine exposure in vivo on mitochondrial respiration, activities of respiratory chain enzymes, and lipid peroxide measures in liver. Male Sprague-Dawley rats were exposed to cocaine (5 i.p. injections of 25 mg/kg; 3-day period). Blood and liver samples were taken, and hepatic mitochondria were isolated by differential centrifugation. The cocaine-treated rats developed oxidative stress in hepatic mitochondria as evidenced by a significant increase in malonaldialdehyde (MDA; 52%; p < 0.0001) and a decreased glutathione (GSH; 22%; p < 0.0003). Blood aspartate aminotransferase (AST) and glutathione s-transferase (GST) levels in cocaine groups were significantly elevated (2.6 and 3.2 fold, respectively; p < 0.0001 for both). Cocaine caused a decrease in state-3 respiration and respiratory control ratio (RCR) ratio when exposed to site I and II substrates; these changes were parallelled by a decrease in complex I (22%; p < 0.003), succinate cytochrome c reductase (27%; p < 0.004), and complex IV (24%; p < 0.003). In conclusion, functional abnormalities of hepatic mitochondria accompany lipid peroxidation caused by cocaine, supporting the hypothesis that the mitochondria is one of the major intracellular targets of cocaine hepatotoxicity.

Overproduction of the rbo gene product from Desulfovibrio species suppresses all deleterious effects of lack of superoxide dismutase in Escherichia coli

In an attempt to isolate the superoxide dismutase (SOD) gene from the anaerobic sulfate-reducing bacterium Desulfoarculus baarsii, a DNA fragment was isolated which functionally complemented an Escherichia coli mutant (sodA sodB) deficient in cytoplasmic SODs. This region carries two open reading frames with sequences which are very similar to that of the rbo-rub operon from Desulfovibrio vulgaris. Independent expression of the rbo and rub genes from ptac showed that expression of rbo was responsible for the observed phenotype. rbo overexpression suppressed all deleterious effects of SOD deficiency in E. coli, including inactivation by superoxide of enzymes containing 4Fe-4S clusters and DNA damage produced via the superoxide-enhanced Fenton reaction. Thus, rbo restored to the sodA sodB mutant the ability to grow on minimal medium without the addition of branched amino acids, and growth on gluconate and succinate carbon sources was no longer impaired. The spontaneous mutation rate, which is elevated in SOD-deficient mutants, returned to the wild-type level in the presence of Rbo, which also restored aerobic viability of sodA sodB recA mutants. Rbo from Desulfovibrio vulgaris, but not Desulfovibrio gigas desulforedoxin, which corresponds to the NH2-terminal domain of Rbo, complemented sod mutants. The physiological role of Rbo in sulfate-reducing bacteria is unknown. In E. coli, Rbo may permit the bacterium to avoid superoxide stress by maintaining functional (reduced) superoxide sensitive 4Fe-4S clusters. It would thereby restore enzyme activities and prevent the release of iron that occurs after cluster degradation and presumably leads to DNA damage.

Superoxide accelerates DNA damage by elevating free-iron levels

Superoxide promotes hydroxyl-radical formation and consequent DNA damage in cells of all types. The long-standing hypothesis that it primarily does so by delivering electrons to adventitious iron on DNA was refuted by recent studies in Escherichia coli. Alternative proposals have suggested that superoxide may accelerate oxidative DNA damage by leaching iron from storage proteins or enzymic [4Fe-4S] clusters. The released iron might then deposit on the surface of the DNA, where it could catalyze the formation of DNA oxidants using other electron donors. The latter model is affirmed by the experiments described here. Whole-cell electron paramagnetic resonance demonstrated that the level of loose iron in superoxide-stressed cells greatly exceeds that of unstressed cells. Bacterial iron storage proteins were not the major source for free iron, since superoxide also increased iron levels in mutants lacking these iron storage proteins. However, overproduction of an enzyme containing a labile [4Fe-4S] cluster dramatically increased the free iron content of cells when they were growing in air. The rates of spontaneous mutagenesis and DNA damage from exogenous H2O2 increased commensurately. It is striking that both growth defects and DNA damage caused by superoxide ensue from its ability to damage a subset of iron-sulfur clusters.

Role of nitric oxide and its interaction with superoxide in the suppression of cardiac muscle mitochondrial respiration. Involvement in response to hypoxia/reoxygenation

BACKGROUND

Nitric oxide (NO); superoxide anion (O2.d-); the reaction product of NO with O2.d-, peroxynitrite (ONOO-); and ischemia/reperfusion have all been reported to inhibit respiration in isolated mitochondria. However, the specific species involved in the inhibition of respiration in intact tissues are poorly understood.

METHODS AND RESULTS

O2 consumption in isolated cardiac muscle from bovine calf hearts was quantified by use of a Clark-type electrode. Exogenous and endogenous sources of NO, from S-nitroso-N-acetylpenicillamine (SNAP) and bradykinin or carbachol, reversibly inhibited respiration, whereas the O2.- releasing agent, pyrogallol (PG), inhibited respiration in a manner that was only partially reversed when examined 15 minutes after the removal of PG. The generation of ONOO- with SNAP + PG caused a potentiation of the O2(-)-elicited inhibition of respiration when examined 15 minutes after the removal of the ONOO- generating system. Tiron (a scavenger of O2.-) did not alter the actions of SNAP, but it attenuated the direct inhibitory effects of PG +/- SNAP and essentially eliminated the suppression of respiration observed 15 minutes after removal of the O2.- or ONOO- generating system. Urate (a scavenger of ONOO-) antagonized only the actions of PG + SNAP. After exposure of muscle slices to a model of hypoxia (15 minutes) and reoxygenation (10 minutes), respiratory inhibition was observed. This reoxygenation-induced inhibition was potentiated by L-arginine, the substrate for NO biosynthesis, and was markedly blocked by nitro-L-arginine (an NO synthase inhibitor), Tiron, or urate.

CONCLUSIONS

The potentially physiological reversible regulation of respiration in cardiac muscle by NO is converted to an effect that does not show rapid reversibility under conditions in which ONOO- forms, and this could contribute to cardiac dysfunction in situations such as hypoxia/reoxygenation.

The yeast copper/zinc superoxide dismutase and the pentose phosphate pathway play overlapping roles in oxidative stress protection

In Saccharomyces cerevisiae, loss of cytosolic superoxide dismutase (Sod1) results in several air-dependent mutant phenotypes, including methionine auxotrophy and oxygen sensitivity. Here we report that these two sod1Delta phenotypes were specifically suppressed by elevated expression of the TKL1 gene, encoding transketolase of the pentose phosphate pathway. The apparent connection between Sod1 and the pentose phosphate pathway prompted an investigation of mutants defective in glucose-6-phosphate dehydrogenase (Zwf1), which catalyzes the rate-limiting NADPH-producing step of this pathway. We confirmed that zwf1Delta mutants are methionine auxotrophs and report that they also are oxygen-sensitive. We determined that a functional ZWF1 gene product was required for TKL1 to suppress sod1Delta, leading us to propose that increased flux through the oxidative reactions of the pentose phosphate pathway can rescue sod1 methionine auxotrophy. To better understand this methionine growth requirement, we examined the sulfur compound requirements of sod1Delta and zwf1Delta mutants, and noted that these mutants exhibit the same apparent defect in sulfur assimilation. Our studies suggest that this defect results from the impaired redox status of aerobically grown sod1 and zwf1 mutants, implicating Sod1 and the pentose phosphate pathway as being critical for maintenance of the cellular redox state.

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.

Interruption of the gpxA gene increases the sensitivity of Neisseria meningitidis to paraquat

Antioxidant enzymes are thought to be important for the survival of pathogenic Neisseria species. We have further characterized the glutathione peroxidase homolog gene (gpxA), which we recently isolated from Neisseria meningitidis FAM20 (T.D.E. Moore and P.F. Sparling, Infect. Immun. 63:1603-1607, 1995). GpxA was found to be produced constitutively in vivo. An isogenic, omega insertion mutant in the gpxA gene was constructed and characterized. The gpxA insertion mutant was much more sensitive to the oxidative stress caused by paraquat and slightly more sensitive to hydrogen peroxide. This is the first demonstration of a phenotype arising from a mutation of a glutathione peroxidase homolog gene in a prokaryotic organism. Protection of the cell by GpxA from the effects of oxidative stress caused by aerobic metabolism may contribute to the ability of Neisseria meningitidis to cause disease in the human host.

Ferric uptake regulator (Fur) mutants of Pseudomonas aeruginosa demonstrate defective siderophore-mediated iron uptake, altered aerobic growth, and decreased superoxide dismutase and catalase activities

Pseudomonas aeruginosa is considered a strict aerobe that possesses several enzymes important in the disposal of toxic oxygen reduction products including iron- and manganese-cofactored superoxide dismutase and catalase. At present, the nature of the regulation of these enzymes in P. aeruginosa Is not understood. To address these issues, we used two mutants called A4 and C6 which express altered Fur (named for ferric uptake regulation) proteins and constitutively produce the siderophores pyochelin and pyoverdin. Both mutants required a significant lag phase prior to log-phase aerobic growth, but this lag was not as apparent when the organisms were grown under microaerobic conditions. The addition of iron salts to mutant A4 and, to a greater extent, C6 cultures allowed for an increased growth rate under both conditions relative to that of bacteria without added iron. Increased manganese superoxide dismutase (Mn-SOD) and decreased catalase activities were also apparent in the mutants, although the second catalase, KatB, was detected in cell extracts of each fur mutant. Iron deprivation by the addition of the iron chelator 2,2'-dipyridyl to wild-type bacteria produced an increase in Mn-SOD activity and a decrease in total catalase activity, similar to the fur mutant phenotype. Purified wild-type Fur bound more avidly than mutant Fur to a PCR product containing two palindromic 19-bp "iron box" regions controlling expression of an operon containing the sodA gene that encodes Mn-SOD. All mutants were defective in both ferripyochelin- and ferripyoverdin-mediated iron uptake. Two mutants of strain PAO1, defective in pyoverdin but not pyochelin biosynthesis, produced increased Mn-SOD activity. Sensitivity to both the redox-cycling agent paraquat and hydrogen peroxide was greater in each mutant than in the wild-type strain. In summary, the results indicate that mutations in the P. aeruginosa fur locus affect aerobic growth and SOD and catalase activities in P. aeruginosa. We postulate that reduced siderophore-mediated iron uptake, especially that by pyoverdin, may be one possible mechanism contributing to such effect.

The irreversible inactivation of ribonucleotide reductase from Escherichia coli by superoxide radicals

The expression of superoxide dismutase in all aerobic living organisms supports the concept that superoxide radicals are toxic species. However, because of the limited chemical reactivity of superoxide, the mechanisms of this toxicity are still uncertain. Protein R2, the small component of ribonucleotide reductase, a key enzyme for DNA synthesis, is shown here to be irreversibly inactivated during incubation with an enzymatic generator of superoxide radicals, at neutral pH. During inactivation the essential tyrosyl radical of protein R2 is irreversibly destroyed. Full protection is afforded by superoxide dismutase. It is proposed that coupling between superoxide radicals and the radical protein R2 generates oxidized forms of tyrosine, tyrosine peroxide and 3,4-dihydroxyphenylalanine.

Superoxide dismutase activity is essential for stationary phase survival in Saccharomyces cerevisiae. Mitochondrial production of toxic oxygen species in vivo

Yeast lacking copper-zinc superoxide dismutase (CuZnSOD), manganese superoxide dismutase (SOD), catalase T, or metallothionein were studied using long-term stationary phase (10-45 days) as a simple model system to study the roles of antioxidant enzymes in aging. In well aerated cultures, the lack of either SOD resulted in dramatic loss of viability over the first few weeks of culture, with the CuZnSOD mutant showing the more severe defect. The double SOD mutant died within a few days. The severity reversed in low aeration; the CuZnSOD mutant remained viable longer than the manganese SOD mutant. To test whether reactive oxygen species generated during respiration play an important role in the observed cellular death, growth in nonfermentable carbon sources was measured. All strains grew under low aeration, indicating respiratory competence. High aeration caused much reduced growth in single SOD mutants, and the double mutant failed to grow. However, removal of respiration via another mutation dramatically increased short term survival and reversed the known air-dependent methionine and lysine auxotrophies. Our results suggest strongly that mitochondrial respiration is a major source of reactive oxygen species in vivo, as has been shown in vitro, and that these species are produced even under low aeration.

Cloning and analysis of sodC, encoding the copper-zinc superoxide dismutase of Escherichia coli

Benov and Fridovich recently reported the existence of a copper- and zinc-containing superoxide dismutase (CuZnSOD) in Escherichia coli (L. T. Benov and I. Fridovich, J. Biol. Chem. 269:25310-25314,1994). We have used the N-terminal protein sequence to isolate the gene encoding this enzyme. The gene, denoted sodC, is located at 37.1 min on the chromosome, adjacent to lhr and sodB. A monocistronic transcript of sodC accumulates only in stationary phase. The presence of a conventional leader sequence is consistent with physical data indicating that the E. coli enzyme, like other bacterial CuZnSODs, is secreted into the periplasm. Because superoxide cannot cross membranes, this localization indicates that the enzyme has evolved to defend periplasmic biomolecules against an extracytoplasmic superoxide source. Neither the source nor the target of the superoxide is known. Although once considered an exclusively eukaryotic enzyme, CuZnSOD has now been found in species that span three subdivisions of the purple bacteria. The bacterial CuZnSODs are more homologous to one another than to the eukaryotic enzymes, but active-site residues and structural motifs are clearly shared by both families of enzymes. The use of copper and an invariant disulfide bond suggest that the ancestral gene of present-day CuZnSODs evolved in an aerobic environment, long after the evolutionary split between the eukaryotes and the eubacteria. If so, a CuZnSOD gene must have been transferred laterally between members of these domains. The eukaryotic SODs most closely resemble that of Caulobacter crescentus, a relatively close descendant of the mitochondrial ancestor, suggesting that sodC may have entered the eukaryotes during the establishment of mitochondria.

Periplasmic copper-zinc superoxide dismutase of Legionella pneumophila: role in stationary-phase survival

Copper-zinc superoxide dismutases (CuZnSODs) are infrequently found in bacteria although widespread in eukaryotes. Legionella pneumophila, the causative organism of Legionnaires' disease, is one of a small number of bacterial species that contain a CuZnSOD, residing in the periplasm, in addition to an iron SOD (FeSOD) in their cytoplasm. To investigate CuZnSOD function, we purified the enzyme from wild-type L. pneumophila, obtained amino acid sequence data from isolated peptides, cloned and sequenced the gene from a L. pneumophila library, and then constructed and characterized a CuZnSOD null mutant. In contrast to the cytoplasmic FeSOD, the CuZnSOD of L. pneumophila is not essential for viability. However, CuZnSOD is critical for survival during the stationary phase of growth. The CuZnSOD null mutant survived 10(4)- to 10(6)-fold less than wild-type L. pneumophila. In wild-type L. pneumophila, the specific activity of CuZnSOD increased during the transition from exponential to stationary-phase growth while the FeSOD activity was constant. These data support a role of periplasmic CuZnSOD in survival of L. pneumophila during stationary phase. Since L. pneumophila survives extensive periods of dormancy between growth within hosts. CuZnSOD may contribute to the ability of this bacterium to be a pathogen. In exponential phase, wild-type and CuZnSOD null strains grew with comparable doubling times. In cultured HL-60 and THP-1 macrophage-like cell lines and in primary cultures of human monocytes, multiplication of the CuZnSOD null mutant was comparable to that of wild type. This indicated that CuZnSOD is not essential for intracellular growth within macrophages or for killing of macrophages in those systems.

The adaptation of Rhizobium leguminosarum bv. phaseoli to oxidative stress and its overlap with other environmental stress responses

Summary: This paper reports the adaptation of Rhizobium leguminosarum bv. phaseoli to oxidative stress and the investigation of its overlap with other environmental stress responses. Treatment of R. leguminosarum bv. phaseoli cells with low concentrations of either menadione (MD, a superoxide generating agent) or 1-chloro-2,4-dinitrobenzene (CDNB, which depletes GSH levels) induced an adaptive response which resulted in cells becoming resistant to subsequent treatment with high concentrations of these oxidative stress compounds. There was overlap between the adaptive response to MD-generated superoxide stress and the response previously demonstrated in this organism to H2O2 (A. J. Crockford, G. A. Davis & H. D. Williams, 1995, Microbiology 141, 843-851); pretreatment with H2O2 was protective against cell killing by MD and vice versa. In contrast, similar experiments indicated only a limited overlap between the responses to H2O2 and CDNB-mediated GSH depletion. It was also found that H2O2, but not MD or CDNB, adaptation protected cells against subsequent osmotic challenge and heat shock. Carbon-starved cells were more resistant to H2O2 and MD killing than exponentially growing cultures, but were more sensitive to CDNB-mediated GSH depletion. Therefore, this work shows that there is a substantial, but incomplete overlap between the responses of R. leguminosarum to different forms of oxidative and other environmental stresses.

Requirement for superoxide in excitotoxic cell death

We tested the pathogenic role of O2-) radicals in excitotoxic injury. Inactivation of the TCA cycle enzyme, aconitase, was used as a marker of intracellular O2- levels, and a porphyrin SOD mimetic was used to scavenge O2-. The selective, reversible, and SOD-sensitive inactivation of aconitase by known O2- generators was used to validate aconitase activity as a marker of O2- generation. Treatment of rat cortical cultures with NMDA, KA, or the intracellular O2- generator PQ2+ produced a selective and reversible inactivation of aconitase, which closely correlated with subsequent cell death produced by these agents. The SOD mimetic, but not its less active congener, attenuated both aconitase inactivation and cell death produced by NMDA, KA, and PQ2+. These results provide direct evidence implicating O2(-) generation in the pathway to excitotoxic injury.

Modulation of iron regulatory protein functions. Further insights into the role of nitrogen- and oxygen-derived reactive species

Iron regulatory protein (IRP) is a cytosolic bifunctional [Fe-S] protein which exhibits aconitase activity or binds iron responsive elements (IREs) in untranslated regions of specific mRNA. The modulators of these activities are the intracellular concentration of iron and, as recently described, NO synthase activity. In this study, we attempted to establish in in vitro experiments whether peroxynitrite (ONOO-, the product of the reaction between NO and O2-.), as well as oxygen-derived radicals (O2-. and H2O2) and various NO donors, allow IRP to bind IREs using cytosol extract of macrophagelike RAW 264.7 cells. Neither the addition of a bolus of ONOO- or H2O2 nor O2-. generation significantly affected IRE binding even though they inhibited its aconitase activity. Moreover, we show that 3-morpholinosydnonimine (SIN-1), a chemical which releases both NO and O2-. enhanced IRE binding activity of IRP only in the presence of superoxide dismutase (SOD). S-Nitrosothiols and the NONOate sper/NO plus gluthathione (GSH) activated IRE binding by IRP whereas oxyhemoglobin prevented enhancement of this binding by SIN-1/SOD and sper/NO plus GSH. cis-Aconitate, substrate, also abolished the effect of SIN-1/SOD on IRE binding by IRP. These results imply that neither O2-. nor ONOO- can convert [4Fe-4S] IRP into IRE-binding protein but rather suggest that an active redox form of NO converts IRP into its IRE binding form by targeting the [Fe-S] cluster.

The impact of oxidative stress on eukaryotic iron metabolism

The processes of iron uptake and distribution are highly regulated in mammalian cells. Expression of the transferrin receptor is increased when cells are iron-depleted, while expression of the iron sequestration protein ferritin is increased in cells that are iron-replete. Regulation of expression of proteins of iron uptake (transferrin receptor) and iron sequestration (ferritin) presumably ensures that levels of reactive free iron are not high in cells. Formation of reactive oxygen species occurs when free iron reacts with oxygen, and tight regulation of iron metabolism may enable cells to avoid engaging in destructive chemical reactions. Levels of intracellular iron are directly sensed by two iron sensing proteins. Iron regulatory protein 1 (IRP1) is a bifunctional protein; in cells that are iron-replete, IRP1 contains an iron-sulfur cluster and functions as cytosolic aconitase. In cells that are iron-depleted, IRP1 binds stem-loop structures in RNA transcripts known as iron responsive elements (IREs). Iron regulatory protein 2 (IRP2) binds similar stem-loop structures, but the mode of regulation of IRP2 is different in that IRP2 is rapidly degraded in iron-replete cells. The post-transcriptional regulation of genes of iron metabolism in mammalian cells ensures that cells have an adequate supply of iron, and also ensures that cells do not generate excess reactive oxygen species through the interaction of free iron and oxygen.

Toxicity of iron and hydrogen peroxide: the Fenton reaction

Iron and hydrogen peroxide are capable of oxidizing a wide range of substrates and causing biological damage. The reaction, referred to as the Fenton reaction, is complex and capable of generating both hydroxyl radicals and higher oxidation states of the iron. The mechanism and how it is affected by different chelators, and the interpretation of results obtained in biological systems, are discussed.

Pseudomonas aeruginosa sodA and sodB mutants defective in manganese- and iron-cofactored superoxide dismutase activity demonstrate the importance of the iron-cofactored form in aerobic metabolism

The consumption of molecular oxygen by Pseudomonas aeruginosa can lead to the production of reduced oxygen species, including superoxide, hydrogen peroxide, and the hydroxyl radical. As a first line of defense against potentially toxic levels of endogenous superoxide, P. aeruginosa possesses an iron- and manganese-cofactored superoxide dismutase (SOD) to limit the damage evoked by this radical. In this study, we have generated mutants which possess an interrupted sodA (encoding manganese SOD) or sodB (encoding iron SOD) gene and a sodA sodB double mutant. Mutagenesis of sodA did not significantly alter the aerobic growth rate in rich medium (Luria broth) or in glucose minimal medium in comparison with that of wild-type bacteria. In addition, total SOD activity in the sodA mutant was decreased only 15% relative to that of wild-type bacteria. In contrast, sodB mutants grew much more slowly than the sodA mutant or wild-type bacteria in both media, and sodB mutants possessed only 13% of the SOD activity of wild-type bacteria. There was also a progressive decrease in catalase activity in each of the mutants, with the sodA sodB double mutant possessing only 40% of the activity of wild-type bacteria. The sodA sodB double mutant grew very slowly in rich medium and required approximately 48 h to attain saturated growth in minimal medium. There was no difference in growth of either strain under anaerobic conditions. Accordingly, the sodB but not the sodA mutant demonstrated marked sensitivity to paraquat, a superoxide-generating agent. P. aeuroginosa synthesizes a blue, superoxide-generating antibiotic similar to paraquat in redox properties which is called pyocyanin, the synthesis of which is accompanied by increased iron SOD and catalase activities (D.J. Hassett, L. Charniga, K. A. Bean, D. E. Ohman, and M. S. Cohen, Infect. Immun. 60:328-336, 1992). Pyocyanin production was completely abolished in the sodB and sodA sodB mutants and was decreased approximately 57% in sodA mutants relative to that of the wild-type organism. Furthermore, the addition of sublethal concentrations of paraquat to wild-type bacteria caused a concentration-dependent decrease in pyocyanin production, suggesting that part of the pyocyanin biosynthetic cascade is inhibited by superoxide. These results suggest that iron SOD is more important than manganese SOD for aerobic growth, resistance to paraquat, and optimal pyocyanin biosynthesis in P. aeruginosa.

Binuclear [2Fe-2S] clusters in the Escherichia coli SoxR protein and role of the metal centers in transcription

SoxR protein of Escherichia coli is activated by superoxide-generating agents or nitric oxide as a powerful transcription activator of the soxS gene, whose product activates approximately 10 other promoters. SoxR contains non-heme iron essential for abortive initiation of transcription in vitro. Here we show that this metal dependence extends to full-length transcription in vitro. In the presence of E. coli sigma 70 RNA polymerase, iron-containing SoxR mediates open complex formation at the soxS promoter, as determined using footprinting with Cu-5-phenyl-1,10-phenanthroline. We investigated the nature of the SoxR iron center by chemical analyses and electron paramagnetic resonance spectroscopy. Dithionite-reduced Fe-SoxR exhibited an almost axial paramagnetic signature with g values of 2.01 and 1.93 observable up to 100 K. These features, together with quantitation of spin, iron, and S2-, and hydrodynamic evidence that SoxR is a homodimer in solution, indicate that (SoxR)2 contains two [2Fe-2S] clusters. Treatment of Fe-SoxR with high concentrations of dithiothreitol caused subtle changes in the visible absorption spectrum and blocked transcriptional activity without generating reduced [2Fe-2S] centers, but was also associated with the loss of iron from the protein. However, lowering the thiol concentration by dilution allowed spontaneous regeneration of active Fe-SoxR.

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-..

The role of free radicals and antioxidants: how do we know that they are working?

This review briefly discusses how free radicals are formed and the possible participation of free radicals in disease. The review describes the basic radical reactions and the types of products that are formed from the free-radical reactions of cellular constituents. In many cases, in vivo free-radical oxidation can be detected by measuring products that were derived from radical reactions. Since aerobic organisms generate oxygen-containing free radicals during oxygen metabolism, they carry chemicals and enzymes that reduce the threat posed by these radicals. The more common sources of in vivo free radicals are described in the article as well as the methods used by cells to protect themselves from free-radical damage. Generation of free radicals in vivo also may be the result of exposure to certain chemical agents present in the environment. Many of these agents cause pathologic changes to the exposed tissues and organs by initiating free-radical reactions.

The NADPH: sulfite reductase of Escherichia coli is a paraquat reductase

The NADPH:sulfite reductase of Escherichia coli is a soluble enzyme that has a subunit structure alpha 8 beta 4, where the alpha subunit is a flavoprotein and the beta subunit is a metalloprotein. Overexpression of the holoenzyme in E. coli has greatly simplified the purification of this enzyme. Under aerobic conditions, recombinant sulfite reductase catalyzes the reduction of 1,1'-dimethyl-4,4'-bipyridinium dichloride (paraquat) by NADPH, with Km values for paraquat and NADPH of approximately 70 microM and 80 microM, respectively. Since pure flavoprotein alpha subunit, encoded by the cysJ gene, has similar catalytic activities, it is suggested that paraquat receives electrons directly from the alpha subunit. A mutant strain lacking an active cysJ gene is resistant to paraquat. The NADPH:ferredoxin reductase of E. coli is also a paraquat reductase but with much higher Km values for paraquat and lower enzyme activities. These results suggest that the sulfite reductase is a major paraquat reductase in E. coli and is responsible for the toxic activation of the drug.

Importance of anaerobic superoxide dismutase synthesis in facilitating outgrowth of Escherichia coli upon entry into an aerobic habitat

The manganese-containing isozyme of superoxide dismutase (MnSOD) is synthesized by Escherichia coli only during aerobiosis, in accordance with the fact that superoxide can be formed only in aerobic environments. In contrast, E. coli continues to synthesize the iron-containing isozyme (FeSOD) even in the absence of oxygen. A strain devoid of FeSOD exhibited no deficits during either anaerobic or continuously aerobic growth, but its growth lagged for 2 h during the transition from anaerobiosis to aerobiosis. Complementation of this defect with heterologous SODs established that anaerobic SOD synthesis per se is necessary to permit a smooth transition to aerobiosis. The growth deficit was eliminated by supplementation of the medium with branched-chain amino acids, indicating that the growth interruption was due to the established sensitivity of dihydroxyacid dehydratase to endogenous superoxide. Components of the anaerobic respiratory chain rapidly generated superoxide when exposed to oxygen in vitro, suggesting that this transition may be a period of acute oxidative stress. These results show that facultative bacteria must preemptively synthesize SOD during anaerobiosis in preparation for reaeration. The data suggest that evolution has chosen FeSOD for this function because of the relative availability of iron, in comparison to manganese, during anaerobiosis.

Paraquat diaphorases in Escherichia coli

Extracts of E. coli contain at least three easily separable NAD(P)H:paraquat diaphorases. One of these is identified as thioredoxin reductase, which accounts for most of the PQ++ diaphorase in a thioredoxin reductase overproducer but is only 25% of this activity in a wild type. NADP+, but not NAD+, inhibited the diaphorase activity of thioredoxin reductase. All of the soluble PQ++ diaphorases of E. coli are stable during fractionation by HPLC and none depend upon the cooperative action of components separable by this technique. GSSG reductase is inhibited by PQ++ and is not, to any significant degree, a contributor to the diaphorase activity of E. coli.

Mathematical model supporting the superoxide theory of oxygen toxicity

The discovery of superoxide dismutase was followed by a proposal that superoxide anion radical (O2.-) is a major factor in oxygen toxicity. The knowledge of superoxide chemistry, however, led some chemists to conclude that since O2.- is not very reactive in aqueous solution, the more reactive hydroxyl radical (HO.) was most likely to be the major damage causing species. Some have defended the superoxide theory by emphasizing that nonindiscriminate and selective reactivity could provide more toxicity than would high, indiscriminate reactivity. In the present study, network thermodynamic simulation was used to create a situation in which O2.- would selectively react with a substrate in a hypothetical sequence of subreactions supporting biological processes. In this situation, when the simulation of the chemical reactions was carried out using reasonable parametric values found in the literature, the selective reaction of O2.- to one molecule in the sequence caused a 95% disruption of the observable process, whereas indiscriminately targeted HO. attack caused only 0 to 35% inhibition. The major cause of the weak effect of HO. was found, in this particular model, to be a lack of sufficient availability of HO. due to both its slow generation by the Fenton reaction and a large demand for reactions with inconsequential targets. This model supports the superoxide theory of oxygen toxicity by demonstrating that a simple set of circumstances can quantitatively lead to the proposed selective superoxide toxicity. The present study also advocates the use of novel network thermodynamic simulation techniques for solving problems concerning biological oxidants and antioxidants.

The role of O2.- in the production of HO.: in vitro and in vivo

In vitro O2.- reduces Fe(III) to Fe(II), which, in turn, reduces the H2O2, yielding Fe(II)O or HO.. In vivo O2.- increases the supply of free iron by oxidatively attacking the [4Fe-4S] clusters of dehydratases such that they release Fe(II), which can then reduce H2O2. In vivo, O2.- also increases the production of H2O2 by acting as an oxidant toward the dehydratases and toward other cellular reductants.

Superoxide dismutase activity, oxidative damage, and mitochondrial energy metabolism in familial and sporadic amyotrophic lateral sclerosis

The cause of neuronal death in amyotrophic lateral sclerosis (ALS) is unknown. Recently, it was found that some patients with autosomal-dominant familial ALS (FALS) have point mutations in the gene that encodes Cu/Zn superoxide dismutase (SOD1). In this study of postmortem brain tissue, we examined SOD activity and quantified protein carbonyl groups, a marker of oxidative damage, in samples of frontal cortex (Brodmann area 6) from 10 control patients, three FALS patients with known SOD1 mutations (FALS-1), one autosomal-dominant FALS patient with no identifiable SOD1 mutations (FALS-O), and 11 sporadic ALS (SALS) patients. Also, we determined the activities of components of the electron transport chain (complexes I, II-III, and IV) in these samples. The cytosolic SOD activity, which is primarily SOD1 activity, was reduced by 38.8% (p < 0.05) in the FALS-1 patients and not significantly altered in the SALS patients or the FALS-O patient relative to the control patients. The mitochondrial SOD activity, which is primarily SOD2 activity, was not significantly altered in the FALS-1, FALS-O, or SALS patients. The protein carbonyl content was elevated by 84.8% (p < 0.01) in the SALS patients relative to the control patients. Finally, the complex I activity was increased by 55.3% (p < 0.001) in the FALS-1 patients relative to the control patients. These results from cortical tissue demonstrate that SOD1 activity is reduced and complex I activity is increased in FALS-1 patients and that oxidative damage to proteins is increased in SALS patients.

Cloning and characterization of the Pseudomonas aeruginosa sodA and sodB genes encoding manganese- and iron-cofactored superoxide dismutase: demonstration of increased manganese superoxide dismutase activity in alginate-producing bacteria

Pseudomonas aeruginosa is a strict aerobe which is likely exposed to oxygen reduction products including superoxide and hydrogen peroxide during the metabolism of molecular oxygen. To counterbalance the potentially hazardous effects of elevated endogenous levels of superoxide, most aerobic organisms possess one or more superoxide dismutases or compounds capable of scavenging superoxide. We have previously shown that P. aeruginosa possesses both an iron- and a manganese-cofactored superoxide dismutase (D. J. Hassett, L. Charniga, K. A. Bean, D. E. Ohman, and M. S. Cohen, Infect. Immun. 60:328-336, 1992). In this study, the genes encoding manganese (sodA)- and iron (sodB)- cofactored superoxide dismutase were cloned by using a cosmid library of P. aeruginosa FRD which complemented an Escherichia coli (JI132) strain devoid of superoxide dismutase activity. The sodA and sodB genes of P. aeruginosa, when cloned into a high-copy-number vector (pKS-), partially restored the aerobic growth rate defect, characteristic of the Sod- strain, to that of the wild type (AB1157) when grown in Luria broth. The nucleotide sequences of sodA and sodB have open reading frames of 612 and 579 bp that encode dimeric proteins of 22.9 and 21.2 kDa, respectively. These data were also supported by the results of in vitro expression studies. The deduced amino acid sequence of the P. aeruginosa manganese and iron superoxide dismutase revealed approximately 50 and 67% similarity with manganese and iron superoxide dismutases from E. coli, respectively. There was also remarkable similarity with iron and manganese superoxide dismutases from other phyla. The mRNA start site of sodB was mapped to 174 bp upstream of the ATG codon. A likely promoter with similarity to the -10 and -35 consensus sequence of E. coli was observed upstream of the ATG start codon of sodB. Regions sequenced 519 bp upstream of the sodA electrophoresis, sodA gene revealed no such promoter, suggesting an alternative mode of control for sodA. By transverse field electrophoresis, sodA and sodB were mapped to the 71- to 75-min region on the P. aeruginosa PAO1 chromosome. Strikingly, mucoid alginate-producing bacteria generated greater levels of manganese superoxide dismutase than nonmucoid revertants, suggesting that mucoid P. aeruginosa is responding to oxidative stress and/or changes in the redox status of the cell.