Interplay Among Nitric Oxide and Reactive Oxygen Species: A Complex Network Determining Cell Survival or Death

Programmed cell death (PCD) is an integrated cellular process occurring in plant growth, development, and defense responses to facilitate normal growth and development and better survival against various stresses as a whole. As universal toxic chemicals in plant and animal cells, reactive oxygen or nitrogen species (ROS or RNS), mainly superoxide anion (O₂^−•), hydrogen peroxide (H₂O₂) or nitric oxide (^•NO), have been studied extensively for their roles in PCD induction. Physiological and genetic studies have convincingly shown their essential roles. However, the details and mechanisms by which ROS and ^•NO interplay and induce PCD are not well understood. Our recent study on Cupressus lusitanica culture cell death revealed the elicitor-induced co-accumulation of ROS and ^•NO and interactions between ^•NO and H₂O₂ or O₂-^• in different ways to regulate cell death. ^•NO and H₂O₂ reciprocally enhanced the production of each other whereas ^•NO and O₂^−• showed reciprocal suppression on each other''s production. It was the interaction between ^•NO and O₂-^• but not between ^•NO and H₂O₂ that induced PCD, probably through peroxynitrite (ONOO⁻). In this addendum, some unsolved issues in the study were discussed based on recent studies on the complex network of ROS and ^•NO leading to PCD in animals and plants.Key Words: cell death, nitric oxide, reactive oxygen species, interaction, posttranslational modification     

Salicylic acid-induced superoxide generation catalyzed by plant peroxidase in hydrogen peroxide-independent manner

It has been reported that salicylic acid (SA) induces both immediate spike and long lasting phases of oxidative burst represented by the generation of reactive oxygen species (ROS) such as superoxide anion radical (O₂^•−). In general, in the earlier phase of oxidative burst, apoplastic peroxidase are likely involved and in the late phase of the oxidative burst, NADPH oxidase is likely involved. Key signaling events connecting the 2 phases of oxidative burst are calcium channel activation and protein phosphorylation events. To date, the known earliest signaling event in response to exogenously added SA is the cell wall peroxidase-catalyzed generation of O₂^•− in a hydrogen peroxide (H₂O₂)-dependent manner. However, this model is incomplete since the source of the initially required H₂O₂ could not be explained. Based on the recently proposed role for H₂O₂-independent mechanism for ROS production catalyzed by plant peroxidases (Kimura et al., 2014, Frontiers in Plant Science), we hereby propose a novel model for plant peroxidase-catalyzed oxidative burst fueled by SA.     

Induction of Extracellular Hydroxyl Radical Production by White-Rot Fungi through Quinone Redox Cycling

A simple strategy for the induction of extracellular hydroxyl radical (OH) production by white-rot fungi is presented. It involves the incubation of mycelium with quinones and Fe³⁺-EDTA. Succinctly, it is based on the establishment of a quinone redox cycle catalyzed by cell-bound dehydrogenase activities and the ligninolytic enzymes (laccase and peroxidases). The semiquinone intermediate produced by the ligninolytic enzymes drives OH production by a Fenton reaction (H₂O₂ + Fe²⁺ → OH + OH⁻ + Fe³⁺). H₂O₂ production, Fe³⁺ reduction, and OH generation were initially demonstrated with two Pleurotus eryngii mycelia (one producing laccase and versatile peroxidase and the other producing just laccase) and four quinones, 1,4-benzoquinone (BQ), 2-methoxy-1,4-benzoquinone (MBQ), 2,6-dimethoxy-1,4-benzoquinone (DBQ), and 2-methyl-1,4-naphthoquinone (menadione [MD]). In all cases, OH radicals were linearly produced, with the highest rate obtained with MD, followed by DBQ, MBQ, and BQ. These rates correlated with both H₂O₂ levels and Fe³⁺ reduction rates observed with the four quinones. Between the two P. eryngii mycelia used, the best results were obtained with the one producing only laccase, showing higher OH production rates with added purified enzyme. The strategy was then validated in Bjerkandera adusta, Phanerochaete chrysosporium, Phlebia radiata, Pycnoporus cinnabarinus, and Trametes versicolor, also showing good correlation between OH production rates and the kinds and levels of the ligninolytic enzymes expressed by these fungi. We propose this strategy as a useful tool to study the effects of OH radicals on lignin and organopollutant degradation, as well as to improve the bioremediation potential of white-rot fungi.White-rot fungi are unique in their ability to degrade a wide variety of organopollutants (36, 47), mainly due to the secretion of a low-specificity enzyme system whose natural function is the degradation of lignin (11). Components of this system include laccase and/or one or two types of peroxidase, such as lignin peroxidase (LiP), manganese peroxidase (MnP), and versatile peroxidase (VP) (31). Besides acting directly, the ligninolytic enzymes can bring about lignin and pollutant degradation through the generation of low-molecular-weight extracellular oxidants, including (i) Mn³⁺, (ii) free radicals from some fungal metabolites and lignin depolymerization products (7, 22), and (iii) oxygen free radicals, mainly hydroxyl radicals (OH) and lipid peroxidation radicals (21). Although OH radicals are the strongest oxidants found in cultures of white-rot fungi (1), studies of their involvement in pollutant degradation are scarce. One of the reasons is that the mechanisms proposed for OH production still await in vivo validation.Several potential sources of extracellular OH based on the Fenton reaction (H₂O₂ + Fe²⁺ → OH + OH⁻ + Fe³⁺) have been postulated for white-rot fungi. In one case, an extracellular fungal glycopeptide has been shown to reduce O₂ and Fe³⁺ to H₂O₂ and Fe²⁺ (45). Enzymatic sources include cellobiose dehydrogenase, LiP, and laccase. Among these, only cellobiose dehydrogenase is able to directly catalyze the formation of Fenton''s reagent (33). The ligninolytic enzymes, however, act as an indirect source of OH through the generation of Fe³⁺ and O₂ reductants, such as formate (CO₂⁻) and semiquinone (Q⁻) radicals. The first time evidence was provided that a ligninolytic enzyme was involved in OH production, oxalate was used to generate CO₂⁻ in a LiP reaction mediated by veratryl alcohol (4). The proposed mechanism consisted of the following cascade of reactions: production of veratryl alcohol cation radical (Valc⁺) by LiP, oxidation of oxalate to CO₂⁻ by Valc⁺, reduction of O₂ to O₂⁻ by CO₂⁻, and a superoxide-driven Fenton reaction (Haber-Weiss reaction) in which Fe³⁺ was reduced by O₂⁻. The OH production mechanism assisted by Q⁻ was inferred from the oxidation of 2-methoxy-1,4-benzohydroquinone (MBQH₂) and 2,6-dimethoxy-1,4-benzohydroquinone (DBQH₂) by Pleurotus eryngii laccase in the presence of Fe³⁺-EDTA. The ability of Q⁻ radicals to reduce both Fe³⁺ to Fe²⁺ and O₂ to O₂⁻, which dismutated to H₂O₂, was demonstrated (14). In this case, OH radicals were generated by a semiquinone-driven Fenton reaction, as Q⁻ radicals were the main agents accomplishing Fe³⁺ reduction. The first evidence of the likelihood of this OH production mechanism being operative in vivo had been obtained from incubations of P. eryngii with 2-methyl-1,4-naphthoquinone (menadione [MD]) and Fe³⁺-EDTA (15). Extracellular OH radicals were produced on a constant basis through quinone redox cycling, consisting of the reduction of MD by a cell-bound quinone reductase (QR) system, followed by the extracellular oxidation of the resulting hydroquinone (MDH₂) to its semiquinone radical (MD⁻). The production of extracellular O₂⁻ and H₂O₂ by P. eryngii via redox cycling involving laccase was subsequently confirmed using 1,4-benzoquinone (BQ), 2-methyl-1,4-benzoquinone, and 2,3,5,6-tetramethyl-1,4-benzoquinone (duroquinone), in addition to MD (16). However, the demonstration of OH production based on the redox cycling of quinones other than MD was still required.In the present paper, we describe the induction of extracellular OH production by P. eryngii upon its incubation with BQ, 2-methoxy-1,4-benzoquinone (MBQ), 2,6-dimethoxy-1,4-benzoquinone (DBQ), and MD in the presence of Fe³⁺-EDTA. The three benzoquinones were selected because they are oxidation products of p-hydroxyphenyl, guaiacyl, and syringyl units of lignin (MD was included as a positive control). Along with laccase, the involvement of P. eryngii VP in the production of O₂⁻ and H₂O₂ from hydroquinone oxidation has also been reported (13). Since hydroquinones are substrates of all known ligninolytic enzymes, quinone redox cycling catalysis could involve any of them. Here, we demonstrate OH production by P. eryngii under two different culture conditions, leading to the production of laccase or laccase and VP. We also show that quinone redox cycling is widespread among white-rot fungi by using a series of well-studied species that produce different combinations of ligninolytic enzymes.     

The potential physiological implications of polygalacturonic acid-mediated production of superoxide

PGA/OGA/PF represent apoplastic signaling molecules implicated in the control of gene expression and the activity of enzymes involved in defense regulation. However, the underlying mechanisms behind such processes are lacking. Here we unequivocally show using EPR spectroscopy with DEPMPO spin-trap capable of differentiating between ^•OH and ^•O₂⁻ that PGA and PF can produce ^•O₂⁻ by transforming ^•OH. The potential physiological implications of this unique property are discussed. We propose that PGA/OGA/PF could represent the initiators of redox signaling cascades in stress response, with H₂O₂ being a downstream secondary messenger.Key words: polygalacturonic acid, pectin, superoxide, hydrogen peroxide, apoplast, redox signalingPGA/OGA/PF represent apoplastic signaling molecules involved in defense regulation.^1–5 For example, they induce de novo enzyme synthesis in the wound-inducible defense reaction⁶ and increase the resistance of plants to pathogens.⁷ However, the underlying mechanisms behind such processes are lacking. Aldington et al.⁸ have postulated that OGAs do not act through a receptor, but rather they owe their activity to some specific physical property. Pertinent to this is the fact that there is a broad range of active OGA structures.⁵ In addition, it has been reported that methylated OGAs are not able to trigger signaling pathways that are activated by OGAs possessing ‘free’ carboxyl groups.^9,10 In contrast to this concept, several research groups have showed that PGA/OGA/PF bind to wall-associated kinases (WAK1 and WAK2).^11–15 However, potential effects of PGA/OGA/PF on the activity of WAK1 or WAK2 have not been observed to date. We propose here that the specific property proposed by Aldington and co-workers,⁸ is in fact the ability of the polymers of galacturonic acid (PGA/OGA/PF) to produce ^•O₂⁻. By taking into account previously proposed mechanisms of reaction of PGA with ^•OH,^16,17 and thermodynamic properties of species potentially involved in the reaction,¹⁸ we hypothesized that PGA could transform the ^•OH radical into ^•O₂⁻. To test our hypothesis we investigated the effects of PGA and pectin on radical production in two different ^•OH-generating systems using EPR spectroscopy with the DEPMPO spin-trap capable of differentiating between ^•OH and ^•O₂⁻.¹⁹The results presented in Figure 1 document the ability of PGA to transform ^•OH to ^•O₂⁻. In addition, our experimental approach showed that pectin shares the ^•O₂⁻-producing ability of its constituent PGA. In the control Fenton system (Fe²⁺ + H₂O₂ → ^•OH + OH⁻ + Fe³⁺) only ^•OH radical was produced (Fig. 1A). However in the presence of PGA or pectin, a significant production of ^•O₂⁻ was detected. Haber-Weiss-like reaction (^•O₂⁻ + H₂O₂ → ^•OH + OH⁻ + O₂) generated ^•OH radical, accompanied by a low level of ^•O₂⁻ (Fig. 1B). The supplementation of PGA or pectin to this system led to sole or pronounced production of ^•O₂⁻, respectively. Under the same experimental settings, no ^•O₂⁻ production was observed for other two major extracellular carbohydrates—cellulose and mannan (Fig. 2).Open in a separate windowFigure 1The ability of PGA and pectin to transform ^•OH radical into ^•O₂⁻. Presented are characteristic EPR spectra of adducts of DEPMPO with the ^•OH radical (/OH) and the ^•O₂⁻ radical (/ooh) in two ^•OH-generating systems: (A) Fenton reaction; (B) Haber-Weiss-like reaction; in the absence (control) or presence of PGA or pectin (15 mgml⁻¹ final concentration). The downward triangle represents the characteristic line of the/OH adduct. The circular symbol represents the characteristic line of the/OOH adduct. The grey lines represent the spectral simulations based on signals of DEPMPO adducts contributing to each spectrum in specific percentages [mean values from four experiments (standard deviations were <5%)].Open in a separate windowFigure 2Characteristic EPR spectra of adducts of DEPMPO with the ^•OH radical (downward triangle) and the ^•O₂⁻ radical (circular symbol) in Haber-Weiss-like ^•OH generating system in the absence (control) or presence of cellulose or mannan (15 mgml⁻¹ final concentration). No ^•O₂⁻ production can be observed in the presence of cellulose or mannan.Presented results illustrate the ability of PGA and pectin to transform ^•OH radical into ^•O₂⁻. Other carbohydrates involved in plant metabolism, such as cellulose and mannan, but also glucose and fructose,²⁰ do not show such properties. In addition, methylated PGA do not produce ^•O₂⁻ in the reaction with ^•OH, but methane,²¹ probably with ^•CH₃ radical as an intermediate.²² This implies that carboxyl groups which are characteristic for PGA play a critical role in the production of ^•O₂⁻. Zegota¹⁶ has proposed that pectin and ^•OH react to produce pectin C(5) radical, which further reacts with molecular oxygen thus forming C(5) peroxyl radical. This radical is unstable, especially at physiological pH values,¹⁷ hence it is further decomposed to carbohydrate fragment(s) and superoxide, via ^•O₂⁻-elimination.^16,17Under in vivo settings, superoxide generated in the apoplast by PGA/OGA/PF can be further dismutated by SOD to H₂O₂, which represents a crucial signaling molecule in plants.^23,24 It is very interesting that signaling properties of H₂O₂ in the plant immune response remarkably overlap with the events initiated by OGA: (1) Similarly to the inverted H₂O₂ gradient across the plant plasma membrane,²⁴ OGA has been reported to activate calcium-dependent protein kinases,²⁵ to provoke membrane depolarization with H⁺ influx and K⁺ efflux,²⁶ and to induce activation of mitogen-activated protein kinases.²⁷ (2) Both, OGA^10,28 and H₂O₂^29–31 provoke an influx of Ca²⁺ from the apoplast into the intracellular compartment. (3) It has been documented that apoplastic generation of ^•O₂⁻ and H₂O₂ follows mechanical stress and the recognition of pathogens,^4,32–34 but also the supplementation of OGAs.^35–37 In addition, it has been reported that the supplementation of OGA to plant cells leads to the increase of apoplastic and total concentration of H₂O₂.^37,38 The enlisted results obtained by others and data presented here imply that PGA/OGA/PF could represent the initiators of redox signaling cascades in stress response, with H₂O₂ being a downstream secondary messenger.Hereby-proposed mechanism of apoplastic production of H₂O₂ by PGA/OGA/PF and SOD, depends on ^•OH radicals. Hence, the central question of our hypothesis is: “Where do apoplastic ^•OH radicals come from, under in vivo conditions?”. Hydrogen peroxide is continually generated in the apoplast by NAD(P) H oxidase/SOD, cell wall peroxidase and other sources during normal metabolism, as reviewed by Neill and co-workers.²⁴ The physiological concentrations of H₂O₂ in plants are not well established,⁴⁰ but it seems that apoplastic and total (fresh weight) concentrations are similar and maintained at around 1 µM.^38–40 In the extracellular compartment, continuous generation of H₂O₂ is balanced by its degradation in ^•OH-generating Fenton reaction which involves redox active metals, such as copper and iron.⁴¹ In principle, ^•OH radicals are further removed by apoplastic ascorbate or cell wall constituents (Fig. 3A).^41–43 However, mechanical wounding (e.g., provoked by cold⁴⁴), degradation of the cell wall by pathogenic enzymes (such as polygalacturonase or pectate lyase⁴⁵) or insect chewing could release PGA/OGA/PF from the cell wall into the apoplast. The presence of PGA/OGA/PF in the apoplast related with these events could drastically change apoplastic redox poise. Positively charged redox active metals readily bind to negatively charged polymers of galacturonic acid.⁴⁶ The close proximity of PGA/OGA/PF to the site of ^•OH production could change the fate of ^•OH. Instead of being scavenged, ^•OH could react with PGA/OGA/PF, which leads to ^•O₂⁻ production and subsequent H₂O₂ re-generation (Fig. 3B). Such ‘recycling’ of H₂O₂ could result in a higher steady-state H₂O₂ concentration in the apoplast and consequent H₂O₂ influx, as H₂O₂ is capable of passing the membrane via passive diffusion and specific aquaporins.⁴⁷ In the stress response, H₂O₂ can traverse the membrane, induce Ca²⁺ influx or diffuse into surrounding healthy tissue to modulate enzyme activity⁴⁸ and initiate gene expression,^23,24,49 crucial for subsequent phases of defense and adaptation. To conclude, PGA/OGA/PF could provide the cell with information about the status of the cell wall affected by stressors, via H₂O₂ signaling.Open in a separate windowFigure 3Schematic presentation of potential effects of PGA/OGA/PF released from stressed wall on apoplastic redox poise and H₂O₂ and Ca²⁺ signaling cascades. (A) Redox processes in apoplast under physiological settings. (B) Redox processes in the apoplast of plant cell exposed to stress. PGA/OGA/PF are released from the cell wall into the apoplast changing the redox poise by transforming ^•OH to ^•O₂⁻ and H₂O₂ (‘H₂O₂ recycling’). This could lead to H₂O₂ accumulation, H₂O₂ influx (via diffusion or peroxiporins) or the activation of Ca²⁺ influx, which leads to the activation of different intracellular responses.     

Production of Superoxide Anions by Keratinocytes Initiates P. acnes-Induced Inflammation of the Skin

Acne vulgaris is a chronic inflammatory disorder of the sebaceous follicles. Propionibacterium acnes (P. acnes), a gram-positive anareobic bacterium, plays a critical role in the development of these inflammatory lesions. This study aimed at determining whether reactive oxygen species (ROS) are produced by keratinocytes upon P. acnes infection, dissecting the mechanism of this production, and investigating how this phenomenon integrates in the general inflammatory response induced by P. acnes. In our hands, ROS, and especially superoxide anions (O₂ ^•−), were rapidly produced by keratinocytes upon stimulation by P. acnes surface proteins. In P. acnes-stimulated keratinocytes, O₂ ^•− was produced by NAD(P)H oxidase through activation of the scavenger receptor CD36. O₂ ^•− was dismuted by superoxide dismutase to form hydrogen peroxide which was further detoxified into water by the GSH/GPx system. In addition, P. acnes-induced O₂ ^•− abrogated P. acnes growth and was involved in keratinocyte lysis through the combination of O₂ ^•− with nitric oxide to form peroxynitrites. Finally, retinoic acid derivates, the most efficient anti-acneic drugs, prevent O₂ ^•− production, IL-8 release and keratinocyte apoptosis, suggesting the relevance of this pathway in humans.     

Remediation of Nitrobenzene Contaminated Soil by Combining Surfactant Enhanced Soil Washing and Effluent Oxidation with Persulfate

The combination of surfactant enhanced soil washing and degradation of nitrobenzene (NB) in effluent with persulfate was investigated to remediate NB contaminated soil. Aqueous solution of sodium dodecylbenzenesulfonate (SDBS, 24.0 mmol L^-1) was used at a given mass ratio of solution to soil (20:1) to extract NB contaminated soil (47.3 mg kg^-1), resulting in NB desorption removal efficient of 76.8%. The washing effluent was treated in Fe²⁺/persulfate and Fe²⁺/H₂O₂ systems successively. The degradation removal of NB was 97.9%, being much higher than that of SDBS (51.6%) with addition of 40.0 mmol L^-1 Fe²⁺ and 40.0 mmol L^-1 persulfate after 15 min reaction. The preferential degradation was related to the lone pair electron of generated SO₄•⁻, which preferably removes electrons from aromatic parts of NB over long alkyl chains of SDBS through hydrogen abstraction reactions. No preferential degradation was observed in •OH based oxidation because of its hydrogen abstraction or addition mechanism. The sustained SDBS could be reused for washing the contaminated soil. The combination of the effective surfactant-enhanced washing and the preferential degradation of NB with Fe²⁺/persulfate provide a useful option to remediate NB contaminated soil.     

Kinetics of Nitrite Reduction and Peroxynitrite Formation by Ferrous Heme in Human Cystathionine β-Synthase

Cystathionine β-synthase (CBS) is a pyridoxal phosphate-dependent enzyme that catalyzes the condensation of homocysteine with serine or with cysteine to form cystathionine and either water or hydrogen sulfide, respectively. Human CBS possesses a noncatalytic heme cofactor with cysteine and histidine as ligands, which in its oxidized state is relatively unreactive. Ferric CBS (Fe(III)-CBS) can be reduced by strong chemical and biochemical reductants to Fe(II)-CBS, which can bind carbon monoxide (CO) or nitric oxide (NO^•), leading to inactive enzyme. Alternatively, Fe(II)-CBS can be reoxidized by O₂ to Fe(III)-CBS, forming superoxide radical anion (O₂^˙̄). In this study, we describe the kinetics of nitrite (NO₂⁻) reduction by Fe(II)-CBS to form Fe(II)NO^•-CBS. The second order rate constant for the reaction of Fe(II)-CBS with nitrite was obtained at low dithionite concentrations. Reoxidation of Fe(II)NO^•-CBS by O₂ showed complex kinetic behavior and led to peroxynitrite (ONOO⁻) formation, which was detected using the fluorescent probe, coumarin boronic acid. Thus, in addition to being a potential source of superoxide radical, CBS constitutes a previously unrecognized source of NO^• and peroxynitrite.     

The effects of dopamine on antioxidant enzymes activities and reactive oxygen species levels in soybean roots

In the current work, we investigated the effects of dopamine, an neurotransmitter found in several plant species on antioxidant enzyme activities and ROS in soybean (Glycine max L. Merrill) roots. The effects of dopamine on SOD, CAT and POD activities, as well as H₂O₂, O₂^•−, melanin contents and lipid peroxidation were evaluated. Three-day-old seedlings were cultivated in half-strength Hoagland nutrient solution (pH 6.0), without or with 0.1 to 1.0 mM dopamine, in a growth chamber (25°C, 12 h photoperiod, irradiance of 280 μmol m⁻² s⁻¹) for 24 h. Significant increases in melanin content were observed. The levels of ROS and lipid peroxidation decreased at all concentrations of dopamine tested. The SOD activity increased significantly under the action of dopamine, while CT activity was inhibited and POD activity was unaffected. The results suggest a close relationship between a possible antioxidant activity of dopamine and melanin and activation of SOD, reducing the levels of ROS and damage on membranes of soybean roots.     

Purification and Characterization of α-Amylase from Bacillus licheniformis CUMC305

α-Amylase produced by Bacillus licheniformis CUMC305 was purified 212-fold with a 42% yield through a series of four steps. The purified enzyme was homogeneous as shown by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and discontinuous gel electrophoresis. The purified enzyme showed maximal activity at 90°C and pH 9.0, and 91% of this activity remained at 100°C. The enzyme retained 91, 79, and 71% maximal activity after 3 h of treatment at 60°C, 3 h at 70°C, and 90 min at 80°C, respectively, in the absence of substrate. On the contrary, in the presence of substrate (soluble starch), the α-amylase enzyme was fully stable after a 4-h incubation at 100°C. The enzyme showed 100% stability in the pH range 7 to 9; 95% stability at pH 10; and 84, 74, 68, and 50% stability at pH values of 6, 5, 4, and 3, respectively, after 18 h of treatment. The activation energy for this enzyme was calculated as 5.1 × 10⁵ J/mol. The molecular weight was estimated to be 28,000 by sodium dodecyl sulfate-gel electrophoresis. The relative rates of hydrolysis of soluble starch, amylose, amylopectin, and glycogen were 1.27, 1.8, 1.94, and 2.28 mg/ml, respectively. V_max values for hydrolysis of these substrates were calculated as 0.738, 1.08, 0.8, and 0.5 mg of maltose/ml per min, respectively. Of the cations, Na⁺, Ca²⁺, and Mg²⁺, showed stimulatory effect, whereas Hg²⁺, Cu²⁺, Ni²⁺, Zn²⁺, Ag⁺, Fe²⁺, Co²⁺, Cd²⁺, Al³⁺, and Mn²⁺ were inhibitory. Of the anions, azide, F⁻, SO₃²⁻, SO₄³⁻, S₂O₃²⁻, MoO₄²⁻, and Wo₄²⁻ showed an excitant effect. p-Chloromercuribenzoic acid and sodium iodoacetate were inhibitory, whereas cysteine, reduced glutathione, thiourea, β-mercaptoethanol, and sodium glycerophosphate afforded protection to enzyme activity. α-Amylase was fairly resistant to EDTA treatment at 30°C, but heating at 90°C in presence of EDTA resulted in the complete loss of enzyme activity, which could be recovered partially by the addition of Cu²⁺ and Fe²⁺ but not by the addition of Ca²⁺ or any other divalent ions.     

Beneficial Effects of Nickel on Pseudomonas saccharophila under Nitrogen-Limited Chemolithotrophic Conditions

Addition of nickel stimulated growth and nitrogenase activity of Pseudomonas saccharophila under nitrogen-limited chemolithotrophic conditions, apparently because of a significant increase in expression of uptake hydrogenase activity. Inhibition of hydrogenase expression by 50 μM EDTA was relieved by nickel over a wide concentration range (1 to 200 μM). Co²⁺, Zn²⁺, Mn²⁺, and Cu²⁺ stimulated expression of hydrogenase activity, but to a much lesser degree than nickel, and Fe²⁺, Mg²⁺, SeO₄²⁻, and SeO₃²⁻ did not increase expression. Nickel in individual combination with Mg²⁺, Fe²⁺, SeO₃²⁻, and SeO₄²⁻ resulted in activities that were essentially the same as that with nickel alone. Hydrogenase synthesis required the presence of nickel, and repression by O₂ was alleviated by increasing the concentration of added nickel. Cells placed under hydrogenase derepression conditions showed progressive incorporation of radioactive nickel to a much greater extent than did cells which were not derepressed.     

Integrated Analysis of COX-2 and iNOS Derived Inflammatory Mediators in LPS-Stimulated RAW Macrophages Pre-Exposed to Echium plantagineum L. Bee Pollen Extract

Oxidative stress and inflammation play important roles in disease development. This study intended to evaluate the anti-inflammatory and antioxidant potential of Echium plantagineum L. bee pollen to support its claimed health beneficial effects. The hydromethanol extract efficiently scavenged nitric oxide (^•NO) although against superoxide (O₂ ^•−) it behaved as antioxidant at lower concentrations and as pro-oxidant at higher concentrations. The anti-inflammatory potential was evaluated in LPS-stimulated macrophages. The levels of ^•NO and L-citrulline decreased for all extract concentrations tested, while the levels of prostaglandins, their metabolites and isoprostanes, evaluated by UPLC-MS, decreased with low extract concentrations. So, E. plantagineum bee pollen extract can exert anti-inflammatory activity by reducing ^•NO and prostaglandins. The extract is able to scavenge the reactive species ^•NO and O₂ ^•− and reduce markers of oxidative stress in cells at low concentrations.     

Evidence on the Formation of Singlet Oxygen in the Donor Side Photoinhibition of Photosystem II: EPR Spin-Trapping Study

When photosystem II (PSII) is exposed to excess light, singlet oxygen (¹O₂) formed by the interaction of molecular oxygen with triplet chlorophyll. Triplet chlorophyll is formed by the charge recombination of triplet radical pair ³[P680^•+Pheo^•−] in the acceptor-side photoinhibition of PSII. Here, we provide evidence on the formation of ¹O₂ in the donor side photoinhibition of PSII. Light-induced ¹O₂ production in Tris-treated PSII membranes was studied by electron paramagnetic resonance (EPR) spin-trapping spectroscopy, as monitored by TEMPONE EPR signal. Light-induced formation of carbon-centered radicals (R^•) was observed by POBN-R adduct EPR signal. Increased oxidation of organic molecules at high pH enhanced the formation of TEMPONE and POBN-R adduct EPR signals in Tris-treated PSII membranes. Interestingly, the scavenging of R^• by propyl gallate significantly suppressed ¹O₂. Based on our results, it is concluded that ¹O₂ formation correlates with R^• formation on the donor side of PSII due to oxidation of organic molecules (lipids and proteins) by long-lived P680^•+/TyrZ^•. It is proposed here that the Russell mechanism for the recombination of two peroxyl radicals formed by the interaction of R^• with molecular oxygen is a plausible mechanism for ¹O₂ formation in the donor side photoinhibition of PSII.     

Enhancing the Production of Hydroxyl Radicals by Pleurotus eryngii via Quinone Redox Cycling for Pollutant Removal

The induction of hydroxyl radical (OH) production via quinone redox cycling in white-rot fungi was investigated to improve pollutant degradation. In particular, we examined the influence of 4-methoxybenzaldehyde (anisaldehyde), Mn²⁺, and oxalate on Pleurotus eryngii OH generation. Our standard quinone redox cycling conditions combined mycelium from laccase-producing cultures with 2,6-dimethoxy-1,4-benzoquinone (DBQ) and Fe³⁺-EDTA. The main reactions involved in OH production under these conditions have been shown to be (i) DBQ reduction to hydroquinone (DBQH₂) by cell-bound dehydrogenase activities; (ii) DBQH₂ oxidation to semiquinone (DBQ⁻) by laccase; (iii) DBQ⁻ autoxidation, catalyzed by Fe³⁺-EDTA, producing superoxide (O₂⁻) and Fe²⁺-EDTA; (iv) O₂⁻ dismutation, generating H₂O₂; and (v) the Fenton reaction. Compared to standard quinone redox cycling conditions, OH production was increased 1.2- and 3.0-fold by the presence of anisaldehyde and Mn²⁺, respectively, and 3.1-fold by substituting Fe³⁺-EDTA with Fe³⁺-oxalate. A 6.3-fold increase was obtained by combining Mn²⁺ and Fe³⁺-oxalate. These increases were due to enhanced production of H₂O₂ via anisaldehyde redox cycling and O₂⁻ reduction by Mn²⁺. They were also caused by the acceleration of the DBQ redox cycle as a consequence of DBQH₂ oxidation by both Fe³⁺-oxalate and the Mn³⁺ generated during O₂⁻ reduction. Finally, induction of OH production through quinone redox cycling enabled P. eryngii to oxidize phenol and the dye reactive black 5, obtaining a high correlation between the rates of OH production and pollutant oxidation.The degradation of lignin and pollutants by white-rot fungi is an oxidative and rather nonspecific process based on the production of substrate free radicals (36). These radicals are produced by ligninolytic enzymes, including laccase and three kinds of peroxidases: lignin peroxidase, manganese peroxidase, and versatile peroxidase (VP) (23). The H₂O₂ required for peroxidase activities is provided by several oxidases, such as glyoxal oxidase and aryl-alcohol oxidase (AAO) (9, 18). This free-radical-based degradative mechanism leads to the production of a broad variety of oxidized compounds. Common lignin depolymerization products are aromatic aldehydes and acids, and quinones (34). In addition to their high extracellular oxidation potential, white-rot fungi show strong ability to reduce these lignin depolymerization products, using different intracellular and membrane-bound systems (4, 25, 39). Since reduced electron acceptors of oxidized compounds are donor substrates for the above-mentioned oxidative enzymes, the simultaneous actions of both systems lead to the establishment of redox cycles (35). Although the function of these redox cycles is not fully understood, they have been hypothesized to be related to further metabolism of lignin depolymerization products that require reduction to be converted in substrates of the ligninolytic enzymes (34). A second function attributed to these redox cycles is the production of reactive oxygen species, i.e., superoxide anion radicals (O₂⁻), H₂O₂, and hydroxyl radicals (OH), where lignin depolymerization products and fungal metabolites act as electron carriers between intracellular reducing equivalents and extracellular oxygen. This function has been studied in Pleurotus eryngii, whose ligninolytic system is composed of laccase (26), VP (24), and AAO (9). Incubation of this fungus with different aromatic aldehydes has been shown to provide extracellular H₂O₂ on a constant basis, due to the establishment of a redox cycle catalyzed by an intracellular aryl-alcohol dehydrogenase (AAD) and the extracellular AAO (7, 10). The process was termed aromatic aldehyde redox cycling, and 4-methoxybenzaldehyde (anisaldehyde) serves as the main Pleurotus metabolite acting as a cycle electron carrier (13). A second cyclic system, involving a cell-bound quinone reductase activity (QR) and laccase, was found to produce O₂⁻ and H₂O₂ during incubation of P. eryngii with different quinones (11). The process was described as the cell-bound divalent reduction of quinones (Q) by QR, followed by extracellular laccase oxidation of hydroquinones (QH₂) into semiquinones (Q⁻), which autoxidized to some extent, producing O₂⁻ (Q⁻ + O₂ ⇆ Q + O₂⁻). H₂O₂ was formed by O₂⁻ dismutation (O₂⁻ + HO₂ + H⁺ → O₂ + H₂O₂). In an accompanying paper, we describe the extension of this O₂⁻ and H₂O₂ generation mechanism to OH radical production by the addition of Fe³⁺-EDTA to incubation mixtures of several white-rot fungi with different quinones (6). Among them, those derived from 4-hydroxyphenyl, guaiacyl, and syringyl lignin units were used: 1,4-benzoquinone (BQ), 2-methoxy-1,4-benzoquinone (MBQ), and 2,6-dimethoxy-1,4-benzoquinone (DBQ), respectively. Semiquinone autoxidation under these conditions was catalyzed by Fe³⁺-EDTA instead of being a direct electron transfer to O₂. The intermediate Fe²⁺-EDTA reduced not only O₂, but also H₂O₂, leading to OH radical production by the Fenton reaction (H₂O₂ + Fe²⁺ → OH + OH⁻ + Fe³⁺).Although OH radicals are the strongest oxidants produced by white-rot fungi (2, 14), studies of their involvement in pollutant degradation are quite scarce. In this context, the objectives of this study were to (i) determine possible factors enhancing the production of OH radicals by P. eryngii via quinone redox cycling and (ii) test the validity of this inducible OH production mechanism as a strategy for pollutant degradation. Our selection of possible OH production promoters was guided by two observations (6). First, the redox cycle of benzoquinones working with washed P. eryngii mycelium is rate limited by hydroquinone oxidation, since the amounts of the ligninolytic enzymes that remained bound to the fungus under these conditions were not large. Second, H₂O₂ is the limiting reagent for OH production by the Fenton reaction.With these considerations in mind, anisaldehyde and Mn²⁺ were selected to increase H₂O₂ production. As mentioned above, anisaldehyde induces H₂O₂ production in P. eryngii via aromatic aldehyde redox cycling (7). Mn²⁺ has been shown to enhance H₂O₂ production during the oxidation of QH₂ by P. eryngii laccase by reducing the O₂⁻ produced in the semiquinone autoxidation reaction (Mn²⁺ + O₂⁻ → Mn³⁺ + H₂O₂ + 2 H⁺) (26). Mn²⁺ was also selected to increase the hydroquinone oxidation rate, since this reaction has been shown to be propagated by the Mn³⁺ generated in the latter reaction (QH₂ + Mn³⁺ → Q⁻ + Mn²⁺ + 2 H⁺). The replacement of Fe³⁺-EDTA by Fe³⁺-oxalate was also planned in order to increase the QH₂ oxidation rate above that resulting from the action of laccase. Oxalate is a common extracellular metabolite of wood-rotting fungi to which the function of chelating iron and manganese has been attributed (16, 45). The use of Fe³⁺-oxalate and nonchelated Fe³⁺, both QH₂ oxidants, has been proven to enable quinone redox cycling in fungi that do not produce ligninolytic enzymes, such as the brown-rot fungus Gloeophyllum trabeum (17, 40, 41). Finally, phenol and the azo dye reactive black 5 (RB5) were selected as model pollutants.     

Ferricytochrome c Directly Oxidizes Aminoacetone to Methylglyoxal,a Catabolite Accumulated in Carbonyl Stress

Age-related diseases are associated with increased production of reactive oxygen and carbonyl species such as methylglyoxal. Aminoacetone, a putative threonine catabolite, is reportedly known to undergo metal-catalyzed oxidation to methylglyoxal, NH₄ ⁺ ion, and H₂O₂ coupled with (i) permeabilization of rat liver mitochondria, and (ii) apoptosis of insulin-producing cells. Oxidation of aminoacetone to methylglyoxal is now shown to be accelerated by ferricytochrome c, a reaction initiated by one-electron reduction of ferricytochrome c by aminoacetone without amino acid modifications. The participation of O₂ ^•− and HO^• radical intermediates is demonstrated by the inhibitory effect of added superoxide dismutase and Electron Paramagnetic Resonance spin-trapping experiments with 5,5′-dimethyl-1-pyrroline-N-oxide. We hypothesize that two consecutive one-electron transfers from aminoacetone (E₀ values = −0.51 and −1.0 V) to ferricytochrome c (E₀ = 0.26 V) may lead to aminoacetone enoyl radical and, subsequently, imine aminoacetone, whose hydrolysis yields methylglyoxal and NH₄ ⁺ ion. In the presence of oxygen, aminoacetone enoyl and O₂ ^•− radicals propagate aminoacetone oxidation to methylglyoxal and H₂O₂. These data endorse the hypothesis that aminoacetone, putatively accumulated in diabetes, may directly reduce ferricyt c yielding methylglyoxal and free radicals, thereby triggering redox imbalance and adverse mitochondrial responses.     

Reduction of Ferric Iron in Anaerobic, Marine Sediment and Interaction with Reduction of Nitrate and Sulfate

Studies were carried out to elucidate the nature and importance of Fe³⁺ reduction in anaerobic slurries of marine surface sediment. A constant accumulation of Fe²⁺ took place immediately after the endogenous NO₃⁻ was depleted. Pasteurized controls showed no activity of Fe³⁺ reduction. Additions of 0.2 mM NO₃⁻ and NO₂⁻ to the active slurries arrested the Fe³⁺ reduction, and the process was resumed only after a depletion of the added compounds. Extended, initial aeration of the sediment did not affect the capacity for reduction of NO₃⁻ and Fe³⁺, but the treatments with NO₃⁻ increased the capacity for Fe³⁺ reduction. Addition of 20 mM MoO₄²⁻ completely inhibited the SO₄²⁻ reduction, but did not affect the reduction of Fe³⁺. The process of Fe³⁺ reduction was most likely associated with the activity of facultative anaerobic, NO₃⁻-reducing bacteria. In surface sediment, the bulk of the Fe³⁺ reduction may be microbial, and the process may be important for mineralization in situ if the availability of NO₃⁻ is low.     

Exogenous 5-Aminolevulenic Acid Promotes Seed Germination in Elymus nutans against Oxidative Damage Induced by Cold Stress

The protective effects of 5-aminolevulenic acid (ALA) on germination of Elymus nutans Griseb. seeds under cold stress were investigated. Seeds of E. nutans (Damxung, DX and Zhengdao, ZD) were pre-soaked with various concentrations (0, 0.1, 0.5, 1, 5, 10 and 25 mg l⁻¹) of ALA for 24 h before germination under cold stress (5°C). Seeds of ZD were more susceptible to cold stress than DX seeds. Both seeds treated with ALA at low concentrations (0.1–1 mg l⁻¹) had higher final germination percentage (FGP) and dry weight at 5°C than non-ALA-treated seeds, whereas exposure to higher ALA concentrations (5–25 mg l⁻¹) brought about a dose dependent decrease. The highest FGP and dry weight of germinating seeds were obtained from seeds pre-soaked with 1 mg l⁻¹ ALA. After 5 d of cold stress, pretreatment with ALA provided significant protection against cold stress in the germinating seeds, significantly enhancing seed respiration rate and ATP synthesis. ALA pre-treatment also increased reduced glutathione (GSH), ascorbic acid (AsA), total glutathione, and total ascorbate concentrations, and the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR), whereas decreased the contents of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂), and superoxide radical (O₂ ^•−) release in both germinating seeds under cold stress. In addition, application of ALA increased H⁺-ATPase activity and endogenous ALA concentration compared with cold stress alone. Results indicate that ALA considered as an endogenous plant growth regulator could effectively protect E. nutans seeds from cold-induced oxidative damage during germination without any adverse effect.     

Visualization of Vascular Ca2+ Signaling Triggered by Paracrine Derived ROS

Oxidative stress has been implicated in a number of pathologic conditions including ischemia/reperfusion damage and sepsis. The concept of oxidative stress refers to the aberrant formation of ROS (reactive oxygen species), which include O₂^•-, H₂O₂, and hydroxyl radicals. Reactive oxygen species influences a multitude of cellular processes including signal transduction, cell proliferation and cell death^1-6. ROS have the potential to damage vascular and organ cells directly, and can initiate secondary chemical reactions and genetic alterations that ultimately result in an amplification of the initial ROS-mediated tissue damage. A key component of the amplification cascade that exacerbates irreversible tissue damage is the recruitment and activation of circulating inflammatory cells. During inflammation, inflammatory cells produce cytokines such as tumor necrosis factor-α (TNFα) and IL-1 that activate endothelial cells (EC) and epithelial cells and further augment the inflammatory response⁷. Vascular endothelial dysfunction is an established feature of acute inflammation. Macrophages contribute to endothelial dysfunction during inflammation by mechanisms that remain unclear. Activation of macrophages results in the extracellular release of O₂^•- and various pro-inflammatory cytokines, which triggers pathologic signaling in adjacent cells⁸. NADPH oxidases are the major and primary source of ROS in most of the cell types. Recently, it is shown by us and others^9,10 that ROS produced by NADPH oxidases induce the mitochondrial ROS production during many pathophysiological conditions. Hence measuring the mitochondrial ROS production is equally important in addition to measuring cytosolic ROS. Macrophages produce ROS by the flavoprotein enzyme NADPH oxidase which plays a primary role in inflammation. Once activated, phagocytic NADPH oxidase produces copious amounts of O₂^•- that are important in the host defense mechanism^11,12. Although paracrine-derived O₂^•- plays an important role in the pathogenesis of vascular diseases, visualization of paracrine ROS-induced intracellular signaling including Ca²⁺ mobilization is still hypothesis. We have developed a model in which activated macrophages are used as a source of O₂^•- to transduce a signal to adjacent endothelial cells. Using this model we demonstrate that macrophage-derived O₂^•- lead to calcium signaling in adjacent endothelial cells.     

NO? Binds Human Cystathionine β-Synthase Quickly and Tightly

The hexa-coordinate heme in the H₂S-generating human enzyme cystathionine β-synthase (CBS) acts as a redox-sensitive regulator that impairs CBS activity upon binding of NO^• or CO at the reduced iron. Despite the proposed physiological relevance of this inhibitory mechanism, unlike CO, NO^• was reported to bind at the CBS heme with very low affinity (K_d = 30–281 μm). This discrepancy was herein reconciled by investigating the NO^• reactivity of recombinant human CBS by static and stopped-flow UV-visible absorption spectroscopy. We found that NO^• binds tightly to the ferrous CBS heme, with an apparent K_d ≤0.23 μm. In line with this result, at 25 °C, NO^• binds quickly to CBS (k_on ∼ 8 × 10³ m⁻¹ s⁻¹) and dissociates slowly from the enzyme (k_off ∼ 0.003 s⁻¹). The observed rate constants for NO^• binding were found to be linearly dependent on [NO^•] up to ∼ 800 μm NO^•, and >100-fold higher than those measured for CO, indicating that the reaction is not limited by the slow dissociation of Cys-52 from the heme iron, as reported for CO. For the first time the heme of human CBS is reported to bind NO^• quickly and tightly, providing a mechanistic basis for the in vivo regulation of the enzyme by NO^•. The novel findings reported here shed new light on CBS regulation by NO^• and its possible (patho)physiological relevance, enforcing the growing evidence for an interplay among the gasotransmitters NO^•, CO, and H₂S in cell signaling.     

The Selenium-independent Inherent Pro-oxidant NADPH Oxidase Activity of Mammalian Thioredoxin Reductase and Its Selenium-dependent Direct Peroxidase Activities

Mammalian thioredoxin reductase (TrxR) is an NADPH-dependent homodimer with three redox-active centers per subunit: a FAD, an N-terminal domain dithiol (Cys⁵⁹/Cys⁶⁴), and a C-terminal cysteine/selenocysteine motif (Cys⁴⁹⁷/Sec⁴⁹⁸). TrxR has multiple roles in antioxidant defense. Opposing these functions, it may also assume a pro-oxidant role under some conditions. In the absence of its main electron-accepting substrates (e.g. thioredoxin), wild-type TrxR generates superoxide (O), which was here detected and quantified by ESR spin trapping with 5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide (DEPMPO). The peroxidase activity of wild-type TrxR efficiently converted the O adduct (DEPMPO/HOO^•) to the hydroxyl radical adduct (DEPMPO/HO^•). This peroxidase activity was Sec-dependent, although multiple mutants lacking Sec could still generate O. Variants of TrxR with C59S and/or C64S mutations displayed markedly reduced inherent NADPH oxidase activity, suggesting that the Cys⁵⁹/Cys⁶⁴ dithiol is required for O generation and that O is not derived directly from the FAD. Mutations in the Cys⁵⁹/Cys⁶⁴ dithiol also blocked the peroxidase and disulfide reductase activities presumably because of an inability to reduce the Cys⁴⁹⁷/Sec⁴⁹⁸ active site. Although the bulk of the DEPMPO/HO^• signal generated by wild-type TrxR was due to its combined NADPH oxidase and Sec-dependent peroxidase activities, additional experiments showed that some free HO^• could be generated by the enzyme in an H₂O₂-dependent and Sec-independent manner. The direct NADPH oxidase and peroxidase activities of TrxR characterized here give insights into the full catalytic potential of this enzyme and may have biological consequences beyond those solely related to its reduction of thioredoxin.     

Kinin B1 Receptor Enhances the Oxidative Stress in a Rat Model of Insulin Resistance: Outcome in Hypertension,Allodynia and Metabolic Complications

Background

Kinin B₁ receptor (B₁R) is induced by the oxidative stress in models of diabetes mellitus. This study aims at determining whether B₁R activation could perpetuate the oxidative stress which leads to diabetic complications.

Methods and Findings

Young Sprague-Dawley rats were fed with 10% D-Glucose or tap water (controls) for 8–12 weeks. A selective B₁R antagonist (SSR240612) was administered acutely (3–30 mg/kg) or daily for a period of 7 days (10 mg/kg) and the impact was measured on systolic blood pressure, allodynia, protein and/or mRNA B₁R expression, aortic superoxide anion (O₂ ^•−) production and expression of superoxide dismutase (MnSOD) and catalase. SSR240612 reduced dose-dependently (3–30 mg/kg) high blood pressure in 12-week glucose-fed rats, but had no effect in controls. Eight-week glucose-fed rats exhibited insulin resistance (HOMA index), hypertension, tactile and cold allodynia and significant increases of plasma levels of glucose and insulin. This was associated with higher aortic levels of O₂ ^•−, NADPH oxidase activity, MnSOD and catalase expression. All these abnormalities including B₁R overexpression (spinal cord, aorta, liver and gastrocnemius muscle) were normalized by the prolonged treatment with SSR240612. The production of O₂ ^•− in the aorta of glucose-fed rats was also measured in the presence and absence of inhibitors (10–100 µM) of NADPH oxidase (apocynin), xanthine oxidase (allopurinol) or nitric oxide synthase (L-NAME) with and without Sar[D-Phe⁸]des-Arg⁹-BK (20 µM; B₁R agonist). Data show that the greater aortic O₂ ^•− production induced by the B₁R agonist was blocked only by apocynin.

Conclusions

Activation of kinin B₁R increased O₂ ^•− through the activation of NADPH oxidase in the vasculature. Prolonged blockade of B₁R restored cardiovascular, sensory and metabolic abnormalities by reducing oxidative stress and B₁R gene expression in this model.