The effect of oxidative stress onSaccharomyces cerevisiae

Impact of silver ions and silver nanoparticles on biochemical parameters and antioxidant enzyme modulations in Saccharomyces cerevisiae under co-exposure to static magnetic field: a comparative investigation

The increase in simultaneous exposure to magnetic fields and other hazardous compounds released from industrial applications poses multiple stress conditions on the ecosystems and public human health. In this work, we investigated the effects of co-exposure to a static magnetic field (SMF) and silver ions (AgNO3) on biochemical parameters and antioxidant enzyme activities in the yeast Saccharomyces cerevisiae. Sub-chronic exposure to AgNO3 (0.5 mM) for 9 h resulted in a significant decrease in antioxidant enzyme activity, including glutathione peroxidase (GPx), catalase (CAT), superoxide dismutase (SOD), and glutathione transferase (GST). The total glutathione (GSH) level increased in yeast cells exposed to Ag. Additionally, a notable elevation in malondialdehyde (MDA) levels and protein carbonyl content was observed in both the AgNP and AgNO3 groups compared to the control group. Interestingly, the SMF alleviated the oxidative stress induced by silver nitrate, normalizing antioxidant enzyme activities by reducing cellular ROS formation, MDA levels, and protein carbonylation (PCO) concentrations.

Quantification of oxidative stress in Saccharomyces pastorianus propagation: Gene expression analysis using quantitative reverse transcription polymerase chain reaction and flow cytometry

When confronted with environmental stress, yeast cell reacts, among others, by modifying the expression of specific genes. In this study, gene expression was analyzed via RT-qPCR to quantify the oxidative stress of Saccharomyces pastorianus during yeast propagation as a reaction to different aeration levels. Target genes were identified, and a reference gene system was developed. Fermentation experiments were conducted in shaking flasks, applying different shaking speeds to generate various aeration efficiencies. The cells were sampled at different propagation stages and, additionally to the expression study, analyzed by flow cytometry after staining with dihydroethidium (DHE) to quantify reactive oxygen species (ROS) inside the cells. The results indicate that high oxygen fermentation conditions led to an increased expression of the catalase-A gene CTA1 during propagation. Furthermore, the determination of cell internal ROS shows increasing oxidative stress over the process in accordance with the RT-qPCR measurements.

A mechanistic study on electro-Fenton system cooperating with phangerochate chrysosporium to degrade lignin

The combined catalytic system of Electro-Fenton (E-Fenton) and Phanerochaete chrysosporium (P. chrysosporium) was constructed in liquid medium with additional potential to overcome the limitations of lignin degradation by white rot fungi alone. To further understand the mechanism of synergistic catalysis, we optimized the optimum potential for lignin catalysis by P. chrysosporium and built synergistic versus separate catalyses. After 48 h of incubation, the optimum growth environment and the highest lignin degradation rate (43.8%) of P. chrysosporium were achieved when 4 V was applied. After 96 h, the lignin degradation rate of the cocatalytic system was 62% (E-Fenton catalysis alone 22% and P. chrysosporium catalysis alone 19%), the pH of the growth maintenance system of P. chrysosporium was approximately 3.5, and the lignin peroxidase (LiP) and manganese-dependent peroxidase (MnP) enzyme activities, were significantly better than those of the control. The qPCR results indicated that the expression of both MnP and LiP genes was higher in the cocatalytic system. Meanwhile, FTIR and 2D-HSQC NMR confirmed that the synergistic catalysis was effective in breaking the aromatic functional groups and the side chains of the aliphatic region of lignin. This study showed that the synergistic catalytic process of electro-Fenton and P. chrysosporium was highly efficient in the degradation of lignin. In addition, the synergetic system is simple to operate, economical and green, and has good prospects for industrial application.

Reconfiguration of metabolic fluxes in Pseudomonas putida as a response to sub-lethal oxidative stress

As a frequent inhabitant of sites polluted with toxic chemicals, the soil bacterium and plant-root colonizer Pseudomonas putida can tolerate high levels of endogenous and exogenous oxidative stress. Yet, the ultimate reason of such phenotypic property remains largely unknown. To shed light on this question, metabolic network-wide routes for NADPH generation-the metabolic currency that fuels redox-stress quenching mechanisms-were inspected when P. putida KT2440 was challenged with a sub-lethal H₂O₂ dose as a proxy of oxidative conditions. ¹³C-tracer experiments, metabolomics, and flux analysis, together with the assessment of physiological parameters and measurement of enzymatic activities, revealed a substantial flux reconfiguration in oxidative environments. In particular, periplasmic glucose processing was rerouted to cytoplasmic oxidation, and the cyclic operation of the pentose phosphate pathway led to significant NADPH-forming fluxes, exceeding biosynthetic demands by ~50%. The resulting NADPH surplus, in turn, fueled the glutathione system for H₂O₂ reduction. These properties not only account for the tolerance of P. putida to environmental insults-some of which end up in the formation of reactive oxygen species-but they also highlight the value of this bacterial host as a platform for environmental bioremediation and metabolic engineering.

Trehalose Protects the Probiotic Yeast Saccharomyces boulardii against Oxidative Stress-Induced Cell Death

Saccharomyces boulardii is the only probiotic yeast with US Food and Drug Administration approval. It is routinely used to prevent or treat acute diarrhea and other gastrointestinal disorders, including the antibiotic-associated diarrhea caused by Clostridium difficile infections. The formation of reactive oxygen species (ROS), specifically H₂O₂ during normal aerobic metabolism, contributes to programmed cell death and represents a risk to the viability of the probiotic microbe. Moreover, a loss of viability reduces the efficacy of the probiotic treatment. Therefore, inhibiting the accumulation of ROS in the oxidant environment could improve the viability of the probiotic yeast and lead to more efficacious treatment. Here, we provide evidence that supplementation with a non-reducing disaccharide, namely trehalose, enhanced the viability of S. boulardii exposed to an oxidative environment by preventing metacaspase YCA1-mediated programmed cell death through inhibition of intracellular ROS production. Our results suggest that supplementation with S. boulardii together with trehalose could increase the viability of the organism, and thus improve its effectiveness as a probiotic and as a treatment for acute diarrhea and other gastrointestinal disorders.

New bioactive fungal molecules with high antioxidant and antimicrobial capacity isolated from Cerrena unicolor idiophasic cultures

Three bioactive fractions, extracellular laccase (ex-LAC), crude endopolysaccharides (c-EPL), and a low molecular subfraction of secondary metabolites (ex-LMS), were isolated from the idiophasic cultures of the white rot fungus Cerrena unicolor. For the first time, we determined the antioxidant properties of these samples by chemiluminometric measurement (a) and assessment of the scavenging effect on ABTS (b) and the DPPH reduction rate (c). The highest reducing capability was found for the ex-LMS fraction: 39-90% for (a), 20-90% for (b), and 10-59% for (c) at the concentration of 6.25-800 µg/mL. The scavenging abilities of the C. unicolor c-EPL were between 36 and 70% for (a), 2 and 60% for (b), and 28 and 32% for (c) at the concentration of 6.25-800 µg/mL. A very high prooxidative potential was observed for the ex-LAC probes. The preliminary toxicity tests were done using the Microtox system and revealed the following percentage of the toxic effect against Vibrio fischeri: 85.37% for c-EPL, 50.67% for ex-LAC, and 99.8% for ex-LMS, respectively. The ex-LAC sample showed the antibacterial activity against Escherichia coli, c-EPL against Staphylococcus aureus, and ex-LMS against both bacterial strains, respectively, but the stronger inhibitory effect was exerted on S. aureus.

Hsp90 regulates Paracoccidioides brasiliensis proliferation and ROS levels under thermal stress and cooperates with calcineurin to control yeast to mycelium dimorphism

Paracoccidioidomycosis is a systemic human mycosis in Latin America caused by Paracoccidioides brasiliensis, a dimorphic pathogenic fungus that lives as a mold in the environment and as yeast during infections of human lungs. In this work, we provide evidence that the inhibition of Hsp90 by geldanamycin (GDA) impairs the proliferation of the yeast, but has no effect on mycelial development. Treatment with cyclosporin A (CsA), an inhibitor of the Hsp90 client protein calcineurin, did not increase the effect of GDA. In contrast, GDA prevented mycelial to yeast differentiation through a mechanism partially dependent on calcineurin, whereas differentiation from yeast to mycelia occurred independent of GDA or CsA. A significant increase in reactive oxygen species (ROS) levels was detected in GDA-treated yeast at 42°C. However, the levels of ROS remained unchanged in GDA-treated yeast or mycelia incubated at 37°C, suggesting that Hsp90 plays different roles under normal and thermal stress conditions. We propose that Hsp90 strengthens the stress response of P. brasiliensis at 37°C through a mechanism that does not involve ROS. Moreover, we suggest that Hsp90 has calcineurin-dependent functions in this organism.

Can the different heat shock response thresholds found in fermenting and respiring yeast cells be attributed to their differential redox states?

In this study we used a heat-shock (HS) reporter gene to demonstrate that respiring cells are intrinsically less sensitive (by 5 degrees C) than their fermenting counterparts to a sublethal heat shock. We also used an oxidant-sensitive fluorescent probe to demonstrate that this correlates with lower levels of sublethal reactive oxygen species (ROS) accumulation in heat-stressed respiring cells. Moreover, this relationship between HS induction of the reporter gene and ROS accumulation extends to respiring cells that have had their ROS levels modified by treatment with the anti-oxidant ascorbic acid and the pro-oxidant H(2)O(2). Thus, by demonstrating that the ROS/HSR correlation previously demonstrated in fermenting cells also holds for respiring cells (despite their greater HS insensitivity and higher level of intrinsic thermotolerance), we provide evidence that the intracellular redox state may influence both the sensitivity of the heat-shock response (HSR) and stress tolerance in the yeast Saccharomyces cerevisiae.

Uth1p: a yeast mitochondrial protein at the crossroads of stress, degradation and cell death

UTH1 is a yeast aging gene that has been identified on the basis of stress resistance and longer life span of mutants. It was also shown to participate in mitochondrial biogenesis. The absence of Uth1p was found to trigger resistance to autophagy induced by rapamycin. Uth1p is therefore the first mitochondrial protein proven to be required for the autophagic degradation of mitochondria. Since this protein is also involved in yeast cell death induced by heterologous expression of the pro-apoptotic protein Bax, the results are discussed in the light of evidence suggesting a co-regulation of apoptosis and autophagy in mammalian cells.

Reactive oxygen species and induction of lignin peroxidase in Phanerochaete chrysosporium

We studied oxidative stress in lignin peroxidase (LIP)-producing cultures (cultures flushed with pure O(2)) of Phanerochaete chrysosporium by comparing levels of reactive oxygen species (ROS), cumulative oxidative damage, and antioxidant enzymes with those found in non-LIP-producing cultures (cultures grown with free exchange of atmospheric air [control cultures]). A significant increase in the intracellular peroxide concentration and the degree of oxidative damage to macromolecules, e.g., DNA, lipids, and proteins, was observed when the fungus was exposed to pure O(2) gas. The specific activities of manganese superoxide dismutase, catalase, glutathione reductase, and glutathione peroxidase and the consumption of glutathione were all higher in cultures exposed to pure O(2) (oxygenated cultures) than in cultures grown with atmospheric air. Significantly higher gene expression of the LIP-H2 isozyme occurred in the oxygenated cultures. A hydroxyl radical scavenger, dimethyl sulfoxide (50 mM), added to the culture every 12 h, completely abolished LIP expression at the mRNA and protein levels. This effect was confirmed by in situ generation of hydroxyl radicals via the Fenton reaction, which significantly enhanced LIP expression. The level of intracellular cyclic AMP (cAMP) was correlated with the starvation conditions regardless of the oxygenation regimen applied, and similar cAMP levels were obtained at high O(2) concentrations and in cultures grown with atmospheric air. These results suggest that even though cAMP is a prerequisite for LIP expression, high levels of ROS, preferentially hydroxyl radicals, are required to trigger LIP synthesis. Thus, the induction of LIP expression by O(2) is at least partially mediated by the intracellular ROS.

Aging and oxidative stress: studies of some genes involved both in aging and in response to oxidative stress

Aging is a complex physiological phenomenon and several theories have been developed about its origin. Among such theories, the 'mitochondrial theory of aging' has been supported by numerous studies and reviews. Cell oxidative damage, in particular the accumulation of mtDNA mutations, is determined by the rate of reactive oxygen species production and degradation induced by the antioxidant defense systems. In this review, data from our laboratory and from the recent literature have been examined to provide arguments that reinforce the crucial role of mitochondria in aging. Various genes that affect life span have been described in numerous organisms. Some of them encode signal transduction proteins and participate in the regulation of mitochondrial metabolism.

Cadmium-inducible expression of the yeast GSH1 gene requires a functional sulfur-amino acid regulatory network

Glutathione (gamma-l-glutamyl-l-cysteinylglycine) is an important antioxidant molecule, helping to buffer the cell against free radicals and toxic electrophiles. Expression of the yeast GSH1 gene, encoding the first enzyme involved in glutathione biosynthesis, gamma-glutamylcysteine synthetase, is regulated by oxidants and the heavy metal cadmium at the level of transcription. We present evidence that the transcription factors involved in controlling the network of sulfur amino acid metabolism genes are also responsible for regulating GSH1 expression in response to cadmium. In particular the transcription factors Met-4, Met-31, and Met-32 are essential for cadmium-mediated regulation of gene expression, whereas the DNA-binding protein Cbf1 appears to play a negative role in controlling GSH1 expression.

The Photoactivated Cercospora Toxin Cercosporin: Contributions to Plant Disease and Fundamental Biology

Plant pathogenic fungi in eight genera produce light-activated perylenequinone toxins that are toxic to plants via the generation of activated oxygen species, particularly singlet oxygen. Studies on the cercosporin toxin produced by Cercospora species have documented an important role for this toxin in pathogenesis of host plants. Cercosporin-generated active oxygen species destroy the membranes of host plants, providing nutrients to support the growth of these intercellular pathogens. Resistance of Cercospora species to the toxic effects of their own toxin has allowed these organisms to be used as a model for understanding the cellular basis of resistance to singlet oxygen and to general oxidative stress. In particular, the recent discovery that pyridoxine (vitamin B6) quenches singlet oxygen has led to the understanding of a novel role for this vitamin in cells as well as the discovery of a novel pathway of biosynthesis.

The biosynthesis of erythroascorbate in Saccharomyces cerevisiae and its role as an antioxidant

This study investigated the ability of the yeast Saccharomyces cerevisiae to synthesize ascorbate and its 5-carbon analogue erythroascorbate from a variety of precursors, and their importance as antioxidants in this organism. Studies of ascorbate and analogues in micro-organisms have been reported previously, but their function as antioxidants have been largely ignored. Ascorbate and erythroascorbate concentrations in yeast extracts were measured spectrophotometrically, and their levels and identity were checked using liquid chromatography-electrospray mass spectrometry. The yeast was readily able to synthesize ascorbate from L-galactono-1,4-lactone or erythroascorbate from D-arabinose and D-arabino-1,4-lactone, whereas L-gulono-1,4-lactone was a much poorer substrate for ascorbate biosynthesis. In untreated cells, the concentration of ascorbate-like compounds was below the level of detection of the methods of analysis used in this study (approximately 0.1 mM). Intracellular ascorbate and erythroascorbate were oxidized at high concentrations of tert-butylhydroperoxide, but not hydrogen peroxide. Their synthesis was not increased in response to low levels of stress, however, and preloading with erythroascorbate did not protect glutathione levels during oxidative stress. This study provides new information on the metabolism of ascorbate and erythroascorbate in S. cerevisiae, and suggests that erythroascorbate is of limited importance as an antioxidant in S. cerevisiae.

Aggregation of recombinant hepatitis B surface antigen induced in vitro by oxidative stress

In order to examine whether oxygen radicals could be responsible for aggregation of recombinant hepatitis B surface antigen (HBsAg) during its assembly in yeast, purified HBsAg was oxidized with ammonium peroxodisulphate (AP) and analyzed by non-denaturing and denaturing size exclusion chromatography, immunoassay and immunoelectron microscopy. As a result, peroxodisulphate radicals induced a reproducible aggregation of HBsAg. At 44 mM AP, the aggregation process took a few hours and the resulting structures were large, branched and non-antigenic. During more gentle oxidation with 9 mM AP and 20-80 microM Cu2+, a continuous structural modification to HBsAg delaying for tens of hours preceded the aggregation event. During this pre-aggregation period, peroxidation of HBsAg lipids and covalent cross-linking of S protein chains occurred that led a complete loss of antigenicity of oxidized particles. In contrast, yeast-derived HBsAg aggregate is decomposed to S monomers under reducing conditions and recognized by anti-HBsAg polyclonal and monoclonal antibodies, suggesting that is has been assembled in vivo from antigenic and reversibly cross-linked particles. Based on these observations, we conclude that oxidation, at least with respect to the specific molecular sites oxidized by AP, is not a primary event in HBsAg aggregate formation in vivo. Since oxidized HBsAg was shown to be irreversibly cross-linked and non-antigenic, there are no suitable techniques for detection HBsAg oxidation in biological samples. Hence, at present, the magnitude of the in-vivo oxidative damage to HBsAg cannot be evaluated and thus, whether the plasma-derived HBsAg undergoes radical-induced oxidation in the course of viral hepatitis remains to be established. If this occurs, this process is expected to contribute to low HBsAg levels in chronic hepatitis B carriers, failure of the currently available immunoassays to identify HBsAg-positive blood donors and inconsistency in the results provided by HBsAg- and anti-HBsAg-based tests in several recent reports.

Effect of Cu,Zn superoxide dismutase disruption mutation on replicative senescence in Saccharomyces cerevisiae

The role of oxidative damage in determining the replicative lifespan of Saccharomyces cerevisiae was investigated using a wild-type haploid laboratory yeast and a Cu,Zn superoxide dismutase (sod1) mutant derivative on glucose, ethanol, glycerol and galactose media. SOD1 expression was necessary to ensure longevity on all carbon sources tested. Whilst carbon source and SOD1 gene expression do influence yeast lifespan, the relationship between the two factors is complex.

A highly conserved sequence is a novel gene involved in de novo vitamin B6 biosynthesis

The Cercospora nicotianae SOR1 (singlet oxygen resistance) gene was identified previously as a gene involved in resistance of this fungus to singlet-oxygen-generating phototoxins. Although homologues to SOR1 occur in organisms in four kingdoms and encode one of the most highly conserved proteins yet identified, the precise function of this protein has, until now, remained unknown. We show that SOR1 is essential in pyridoxine (vitamin B6) synthesis in C. nicotianae and Aspergillus flavus, although it shows no homology to previously identified pyridoxine synthesis genes identified in Escherichia coli. Sequence database analysis demonstrated that organisms encode either SOR1 or E. coli pyridoxine biosynthesis genes, but not both, suggesting that there are two divergent pathways for de novo pyridoxine biosynthesis in nature. Pathway divergence appears to have occurred during the evolution of the eubacteria. We also present data showing that pyridoxine quenches singlet oxygen at a rate comparable to that of vitamins C and E, two of the most highly efficient biological antioxidants, suggesting a previously unknown role for pyridoxine in active oxygen resistance.

Flow cytometric investigation of heterogeneous copper-sensitivity in asynchronously grown Saccharomyces cerevisiae

The variable stress-sensitivity of individual cells within pure cultures is widely noted but generally unexplained. Here, factors determining the heterogeneous susceptibility to copper toxicity in Saccharomyces cerevisiae were examined with a rapid non-perturbing approach based on flow cytometry. By determination of the DNA content (with propidium iodide) in cell fractions gated by forward angle light scatter (an indicator of the cell volume), it was shown that forward angle light scatter measurements gave an approximation of the cell cycle stage. Thus, our observation that cells in different forward angle light scatter fractions displayed differing Cu-sensitivities indicated that heterogeneous Cu-sensitivity is a function of the cell cycle stage. Furthermore, cells sorted by their Cu-sensitivity and-resistance and subsequently analyzed for DNA content were found predominantly to occupy G1/S and G2/M cell cycle stages, respectively. The oxidant-sensitive probe 2',7'-dichlorodihydrofluorescein diacetate was used to show that the Cu-sensitivity of G2/M phase S. cerevisiae was correlated with greater levels of pre-existing reactive oxygen species in these cells. The results indicate that differential Cu-sensitivity in a S. cerevisiae culture is linked to the cell cycle stage and this link may be determined partly by cell cycle-dependent fluctuations in basal reactive oxygen species generation.

Analysis of the oxidative stress response of Penicillium chrysogenum to menadione

The intracellular superoxide and glutathione disulphide concentrations increased in Penicillium chrysogeum treated with 50, 250 or 500 microM menadione (MQ). A significant increase in the intracellular peroxide concentration was also observed when mycelia were exposed to 250 or 500 microM MQ. The specific activity of Cu,Zn and Mn superoxide dismutases, glutathione reductase and glutathione S-transferase as well as the glutathione producing activity increased in the presence of MQ while glutathione peroxidase and gamma-glutamyltranspeptidase were only induced by high intracellular peroxide levels. The glucose-6-phosphate dehydrogenase and catalase activities did not respond to the oxidative stress caused by MQ.

Oxidative stress in microorganisms--I. Microbial vs. higher cells--damage and defenses in relation to cell aging and death

Oxidative stress in microbial cells shares many similarities with other cell types but it has its specific features which may differ in prokaryotic and eukaryotic cells. We survey here the properties and actions of primary sources of oxidative stress, the role of transition metals in oxidative stress and cell protective machinery of microbial cells, and compare them with analogous features of other cell types. Other features to be compared are the action of Reactive Oxygen Species (ROS) on cell constituents, secondary lipid- or protein-based radicals and other stress products. Repair of oxidative injury by microorganisms and proteolytic removal of irreparable cell constituents are briefly described. Oxidative damage of aerobically growing microbial cells by endogenously formed ROS mostly does not induce changes similar to the aging of multiplying mammalian cells. Rapid growth of bacteria and yeast prevents accumulation of impaired macromolecules which are repaired, diluted or eliminated. During growth some simple fungi, such as yeast or Podospora spp., exhibit aging whose primary cause seems to be fragmentation of the nucleolus or impairment of mitochondrial DNA integrity. Yeast cell aging seems to be accelerated by endogenous oxidative stress. Unlike most growing microbial cells, stationary-phase cells gradually lose their viability because of a continuous oxidative stress, in spite of an increased synthesis of antioxidant enzymes. Unlike in most microorganisms, in plant and animal cells a severe oxidative stress induces a specific programmed death pathway--apoptosis. The scant data on the microbial death mechanisms induced by oxidative stress indicate that in bacteria cell death can result from activation of autolytic enzymes (similarly to the programmed mother-cell death at the end of bacillary sporulation). Yeast and other simple eukaryotes contain components of a proapoptotic pathway which are silent under normal conditions but can be activated by oxidative stress or by manifestation of mammalian death genes, such as bak or bax. Other aspects, such as regulation of oxidative-stress response, role of defense enzymes and their control, acquisition of stress tolerance, stress signaling and its role in stress response, as well as cross-talk between different stress factors, will be the subject of a subsequent review.

Flavohemoglobin expression and function in Saccharomyces cerevisiae. No relationship with respiration and complex response to oxidative stress

The yeast Saccharomyces cerevisiae contains a flavohemoglobin, encoded by the gene YHB1, whose function is unclear. Previous reports presented evidence that its maximal expression requires disruption of mitochondrial respiration and that it plays a role in the response to oxidative stress. We have studied the expression of YHB1 in respiratory deficient cells and in cells exposed to various compounds causing oxidative stress. Several different strains and approaches (spectroscopic detection of the oxygenated form of Yhb1p, beta-galactosidase activity of a YHB1-lacZ fusion, and Northern blot analysis) were used to demonstrate that YHB1 expression and Yhb1p production are not increased by respiration deficiency. YHB1 expression was unchanged in cells challenged with antimycin A or menadione, while it decreased in cells exposed to H2O2, diamide, dithiothreitol, and Cu2+. Transcription of YHB1 is not under the control of the transcriptional factor Yap1p. These results do not support a participation of YHB1 in the genetic response to oxidative stress. We also analyzed the growth phenotypes associated with altered Yhb1p production using YHB1-deleted strains and strains that greatly overproduced Yhb1p. Yhb1p appears to protect cells against the damage caused by Cu2+ and dithiothreitol, while sensitizing them to H2O2. Yhb1p overproduction in a glucose-6-phosphate dehydrogenase-deficient mutant decreased its growth rate. These data indicate that there is a complex relationship(s) between Yhb1p function(s) and cell defense reactions against various stresses.

Mitochondrial function is required for resistance to oxidative stress in the yeast Saccharomyces cerevisiae

Yeast strains that lack mitochondrial function are sensitive to oxidative stress caused by reactive oxygen species (ROS). Specifically, rho0 mutants that lack mitochondrial DNA, and strains deleted for the nuclear genes COX6 and COQ3 that are required for function of the respiratory electron transport chain, were sensitive to H2O2. In addition, treatment with mitochondrial inhibitors including antimycin A, oligomycin, potassium cyanide and sodium azide increased sensitivity to H2O2. The mechanism does not appear to depend on the antioxidant status of the cell since respiratory-deficient strains were able to mount an inducible adaptive response to H2O2. We suggest that the oxidant sensitivity is due to a defect in an energy-requiring process that is needed for detoxification of ROS or for the repair of oxidatively damaged molecules.

Saccharomyces cerevisiae exhibits a yAP-1-mediated adaptive response to malondialdehyde

Malondialdehyde (MDA) is a highly reactive aldehyde generally formed as a consequence of lipid peroxidation. MDA has been inferred to have mutagenic and cytotoxic roles and possibly to be a participant in the onset of atherosclerosis. Wild-type Saccharomyces cerevisiae acquires resistance to a lethal dose (5 mM) of MDA following prior exposure to a nonlethal concentration (1 mM). This response was completely inhibited by cycloheximide (50 microg ml(-1)), indicating a requirement for protein synthesis for adaptation. Furthermore, we have examined the roles of glutathione (GSH), mitochondrial function, and yAP-1-mediated transcription in conferring resistance and adaptation to MDA. A yap1 disruption mutant exhibited the greatest sensitivity and was unable to adapt to MDA, implicating yAP-1 in both the adaptive response and constitutive survival. The effect of MDA on GSH mutants indicated a role for GSH in initial resistance, whereas resistance acquired through adaptation was independent of GSH. Likewise, respiratory mutants (petite mutants) were sensitive to MDA but were still able to mount an adaptive response similar to that of the wild type, excluding mitochondria from any role in adaptation. MDA was detected in yeast cells by the thiobarbituric acid test and subsequent high-pressure liquid chromatography separation. Elevated levels were detected following treatment with hydrogen peroxide. However, the MDA-adaptive response was independent of that to H2O2.

Glutathione metabolism and protection against oxidative stress caused by peroxides in Penicillium chrysogenum

The filamentous fungus Penicillium chrysogenum showed remarkable resistance to the oxidative stress caused by high concentrations of either hydrogen peroxide (0.35-0.70 M) or tert-butyl hydroperoxide (tert-BOOH, 0.5-2.0 mM), which could be explained well with high levels of glutathione (GSH) peroxidase and catalase activities. The majority of exogenous H2O2 was likely removed by catalase from the cells while tert-BOOH was likely eliminated mainly by the GSH-dependent pathways. The GSH pool decreased considerably at high tert-BOOH concentrations, the glutathione disulphide (GSSG) pool increased at high H2O2 and tert-BOOH concentrations, meanwhile all the peroxide concentrations tested increased markedly the intracellular peroxide concentration. All the enzyme activities taking part in the glutathione metabolism (glutathione peroxidase, glutathione reductase, gamma-glutamyltranspeptidase and glutathione producing activities) except glutathione S-transferase increased significantly after exposing mycelia to both peroxides while the specific glucose-6-phosphate dehydrogenase and catalase activities remained unchanged. In the presence of 0.5 mM diamide both GSSG and GSH concentrations as well as the glutathione reductase and glutathione producing activities were elevated but no significant changes were found in the intracellular peroxide concentration or in any of the other enzyme activities examined.

Saccharomyces cerevisiae mutants defective in heme biosynthesis as a tool for studying the mechanism of phototoxicity of porphyrins

Mutants of Saccharomyces cerevisiae accumulating uroporphyrin (UP) or protoporphyrin (PP) were used as a model for the in vivo phototoxic effect of porphyrins observed in the human skin photosensitivity associated with porphyrias (porphyria cutanea tarda and erythropoietic protoporphyria). We have found that UP is localized in vacuoles and PP is present in all compartments except vacuoles in yeast cells. Endogenous PP is much more effective as a photosensitizer of yeast cells than UP. Protoporphyrin action is strictly dependent on the presence of oxygen. In contrast, UP displays a phototoxic effect even if oxygen is not present in the suspension, implicating a free radical mechanism that operates in anaerobiosis upon photosensitization by UP. Catalase or superoxide dismutase deficiency affects photosensitization by UP. A possible mechanism of UP photosensitizing activity is discussed.

Glutathione is an important antioxidant molecule in the yeast Saccharomyces cerevisiae

The tripeptide gamma-L-glutamyl-L-cystinylglycine (glutathione) is one of the major antioxidant molecules of cells and is thought to play a vital role in buffering the cell against reactive oxygen species and toxic electrophiles. We wished to determine the role of glutathione in the protection of the yeast Saccharomyces cerevisiae against oxidative stress. This study shows that glutathione is an important antioxidant molecule in yeast, with gamma-glutamylcysteine synthetase (gsh1) mutants, deficient in glutathione synthesis, being hypersensitive to H2O2 and superoxide anions in both exponential- and stationary-phase cultures. Despite this, these mutants are still able to induce adaptive stress responses to oxidants.

Analysis of the adaptive oxidative stress response of Candida albicans

Treatment of Candida albicans with low concentrations of either hydrogen peroxide or menadione (a superoxide generating agent) induces an adaptive response which protects cells from the lethal effects of a subsequent challenge with higher concentrations of these oxidants. Pre-treatment with either menadione or hydrogen peroxide is protective against cell killing by either oxidant. This suggests that the pathogenic yeast C. albicans (unlike the budding yeast Saccharomyces cerevisiae which has separate responses) possesses an adaptive response that responds to both these oxidants. In addition, we found that C. albicans showed a greater level of resistance to oxidants, both H2O2 and redox-cycling agents, compared to that observed with S. cerevisiae. In an attempt to characterise the oxidative stress response in more detail we have analysed the effect of oxidants on the activities of a number of enzymes with known antioxidant activity.

Transcriptional regulators of oxidative stress-inducible genes in prokaryotes and eukaryotes

It appears that redox regulation is an important mechanism for the control of transcription factor activation. The role of oxidation-reduction is probably determined in part by the structure of the transcription factors. For example, the presence of cysteine residues within the DNA binding sites may sensitize a transcription factor to ROS. The ROS-mediated regulation of transcription factors is specific, some ROS are more efficient than other ROS in activating defined regulators. While the protective antioxidant responses induced by ROS in prokaryotes and eukaryotes are rather conserved (for example, SOD, HSP...), the regulators for these genes do not appear to be conserved. Further studies designed to fully characterize these regulators and understand the subtle mechanisms involved in redox gene regulation are ongoing, and should provide the theoretical basis for clinical approaches using antioxidant therapies in human diseases in which oxidative stress is implicated.

The role of the YAP1 and YAP2 genes in the regulation of the adaptive oxidative stress responses of Saccharomyces cerevisiae

The YAP1 and YAP2 genes encode yeast transcription factors of the c-jun family. We show that yeast mutants deleted for either the YAP1 or the YAP2 genes are hypersensitive to oxidants, particularly H2O2, and that these genes play a role in regulating the induction of the H2O2 adaptive stress response in Saccharomyces cerevisiae. They do not significantly affect the regulation of the superoxide adaptive stress response. The intrinsic resistance of stationary-phase and respiring yeast cells towards superoxide anions is unaffected by deletion of the YAP1 and YAP2 genes. However, resistance towards H2O2 under these conditions is significantly reduced. We show that expression of the yeast GSH1 gene (encoding gamma-glutamylcysteine synthetase) and the SSA1 gene (encoding an HSP70 isoform) are induced by oxidants. Unlike the SSA1 and thioredoxin (TRX2) genes, expression of the GSH1 gene is more strongly induced by superoxide anions than by H2O2. In the absence of added oxidants, transcription of the GSH1 gene is reduced in strains carrying the yap1 deletion. However, we show that Yap1 is not required for the superoxide anion-mediated induction of GSH1 gene expression. Furthermore, while the H2O2-mediated induction of SSA1 expression is shown to by YAP1 dependent, the heat-shock-mediated induction of the SSA1 gene does not require YAP1. We also present evidence to show that the YAP2 gene does not regulate the expression of the TRX2, SSA1 or GSH1 genes.