Oxygen activation at the plasma membrane: relation between superoxide and hydroxyl radical production by isolated membranes

Production of reactive oxygen species (hydroxyl radicals, superoxide radicals and hydrogen peroxide) was studied using EPR spin-trapping techniques and specific dyes in isolated plasma membranes from the growing and the non-growing zones of hypocotyls and roots of etiolated soybean seedlings as well as coleoptiles and roots of etiolated maize seedlings. NAD(P)H mediated the production of superoxide in all plasma membrane samples. Hydroxyl radicals were only produced by the membranes of the hypocotyl growing zone when a Fenton catalyst (FeEDTA) was present. By contrast, in membranes from other parts of the seedlings a low rate of spontaneous hydroxyl radical formation was observed due to the presence of small amounts of tightly bound peroxidase. It is concluded that apoplastic hydroxyl radical generation depends fully, or for the most part, on peroxidase localized in the cell wall. In soybean plasma membranes from the growing zone of the hypocotyl pharmacological tests showed that the superoxide production could potentially be attributed to the action of at least two enzymes, an NADPH oxidase and, in the presence of menadione, a quinone reductase.     

Plasma membrane-generated reactive oxygen intermediates and their role in cell growth of plants

Reactive oxygen species (ROS) produced as intermediates in the reduction of O2 to H2O (superoxide radical, hydrogen peroxide, hydroxyl radical), are generally regarded as harmful products of oxygenic metabolism causing cell damage in plants, animals and microorganisms. However, oxygen radical chemistry can also play useful roles if it takes place outside of the protoplast. In plants, the production of these ROS initiated by the plasma membrane NAD(P)H oxidase can be used for controlled polymer breakdown leading to wall loosening during extension growth. Backbone cleavage of cell wall polysaccharides can be accomplished by hydroxyl radicals produced from hydrogen peroxide and superoxide in a reaction catalyzed by cell wall peroxidase. Growing plant organs such as coleoptiles or roots of maize seedlings produce these ROS specifically in the apoplast of actively growing tissues, e.g. in the epidermis of the coleoptile and the growing zone of the root. Auxin promotes the release of hydroxyl radicals when inducing elongation growth. Experimental generation of hydroxyl radicals in the wall causes an increase in wall extensibility in vitro and replaces auxin in inducing growth. Auxin-induced growth can be inhibited by scavengers of ROS or inhibitors interfering with the formation of these molecules in the cell wall. These results provide the experimental background for a novel hypothesis on the mechanism of plant cell growth in which the generation of hydroxyl radicals, initiated by the plasma membrane NAD(P)H oxidase, plays a central role.     

DNA damage during glycation of lysine by methylglyoxal: assessment of vitamins in preventing damage

Summary. Amino acids react with methylglyoxal to form advanced glycation end products. This reaction is known to produce free radicals. In this study, cleavage to plasmid DNA was induced by the glycation of lysine with methylglyoxal in the presence of iron(III). This system was found to produce superoxide as well as hydroxyl radicals. The abilities of various vitamins to prevent damage to plasmid DNA were evaluated. Pyridoxal-5-phosphate showed maximum protection, while pyridoxamine showed no protection. The protective abilities could be directly correlated to inhibition of production of hydroxyl and superoxide radicals. Pyridoxal-5-phosphate exhibited low radical scavenging ability as evaluated by its TEAC, but showed maximum protection probably by interfering in free radical production. Pyridoxamine did not inhibit free radical production. Thiamine and thiamine pyrophosphate, both showed protective effects albeit to different extents. Tetrahydrofolic acid showed better antioxidant activity than folic acid but was found to damage DNA by itself probably by superoxide generation.     

Extracellular superoxide production by Enterococcus faecalis requires demethylmenaquinone and is attenuated by functional terminal quinol oxidases

The intestinal commensal bacterium, Enterococcus faecalis, is unusual among prokaryotic organisms in its ability to produce substantial extracellular superoxide. Transposon mutagenesis, allelic replacement, and electron spin resonance (ESR)-spin trapping showed that superoxide production and generation of derivative hydroxyl radical were dependent on membrane-associated demethylmenaquinone. Extracellular superoxide was generated through univalent reduction of oxygen by reduced demethylmenaquinone. Moreover, extracellular superoxide production was inhibited by exogenous haematin, an essential cofactor for cytochrome bd, and by fumarate, a substrate for fumarate reductase. As integral membrane quinol oxidases, cytochrome bd and fumarate reductase redox cycle demethylmenaquinone, and are necessary for aerobic and anaerobic respiration respectively. A rat model of intestinal colonization demonstrated that conditions exist in the mammalian intestinal tract that permit a mode of respiration for E. faecalis that results in the formation of hydroxyl radical. These results identify and characterize the mechanism by which E. faecalis generates extracellular free radicals.     

Free radicals in iron-containing systems

All oxidative damage in biological systems arises ultimately from molecular oxygen. Molecular oxygen can scavenge carbon-centered free radicals to form organic peroxyl radicals and hence organic hydroperoxides. Molecular oxygen can also be reduced in two one-electron steps to hydrogen peroxide in which case superoxide anion is an intermediate; or it can be reduced enzymatically so that no superoxide is released. Organic hydroperoxides or hydrogen peroxide can diffuse through membranes whereas hydroxyl radicals or superoxide anion cannot. Chain reactions, initiated by chelated iron and peroxides, can cause tremendous damage. Chain carriers are chelated ferrous ion; hydroxyl radical .OH, or alkoxyl radical .OR, and superoxide anion O2-. or organic peroxyl radical RO2.. Of these free radicals .OH and RO2. appear to be most harmful. All of the biological molecules containing iron are potential donors of iron as a chain initiator and propagator. An attacking role for superoxide dismutase is proposed in the phagocytic process in which it may serve as an intermediate enzyme between NADPH oxidase and myeloperoxidase. The sequence of reactants is O2----O2-.----H2O2----HOCl.     

Superoxide and peroxyl radical generation from the reduction of polyunsaturated fatty acid hydroperoxides by soybean lipoxygenase.

Soybean lipoxygenase is shown to catalyze the breakdown of polyunsaturated fatty acid hydroperoxides to produce superoxide radical anion as detected by spin trapping with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). In addition to the DMPO/superoxide radical adduct, the adducts of peroxyl, acyl, carbon-centered, and hydroxyl radicals were identified in incubations containing linoleic acid and lipoxygenase. These DMPO radical adducts were observed just prior to the system becoming anaerobic. Only a carbon-centered radical adduct was observed under anaerobic conditions. The superoxide radical production required the presence of fatty acid substrates, fatty acid hydroperoxides, active lipoxygenase, and molecular oxygen. Superoxide radical production was inhibited when nordihydroguaiaretic acid, butylated hydroxytoluene, or butylated hydroxyanisole was added to the incubation mixtures. We propose that polyunsaturated fatty acid hydroperoxides are reduced to form alkoxyl radicals and that after an intramolecular rearrangement, the resulting hydroxyalkyl radical reacts with oxygen, forming a peroxyl radical which subsequently eliminates superoxide radical anion.     

Hydrogen peroxide causes the fatal injury to human fibroblasts exposed to oxygen radicals

Oxygen radicals are suspected as being a cause of the cellular damage that occurs at sites of inflammation. The phagocytic cells that accumulate in areas of inflammation produce superoxide, hydrogen peroxide, hydroxyl radical, and probably singlet oxygen in the extracellular fluid. The mechanism by which these oxygen molecules kill cells is unknown. To determine which of the oxygen species is responsible for the cellular killing, we exposed human fibroblasts in culture to oxygen radicals generated by the enzymatic action of xanthine oxidase upon acetaldehyde. Using the amount of chromium-51 released from labeled fibroblasts as an index of cellular death, we found that cells were protected only by interventions that reduce hydrogen peroxide concentration. Agents that inactivate superoxide, hydroxyl radical, and singlet oxygen were ineffective in limiting oxygen radical-induced cellular death.     

The catalytic effect of the carcinogen "4-nitroquinoline-N-oxide" on the oxidation of vitamin C.

The carcinogen 4-nitroquinoline-N-oxide was found to mediate the reaction between ascorbate and oxygen. The oxidation of ascorbate was initiated by the production of the nitro radical anion which reacted with oxygen to produce the oxygen superoxide radical anion, peroxide and hydroxyl radical. The production of partially reduced oxygen intermediates resulted in additional reactions with ascorbate. The consumption of oxygen could be either completely blocked by reacting the nitro radical with ferricytochrome c or partially blocked by the combined effects of superoxide dismutase and catalase. The consumption of oxygen could be enhanced by reducing the hydroxyl radicals with dimethylsulfoxide.     

Hydroxyl radical-mediated, cytochrome P-450-dependent metabolic activation of benzene in microsomes and reconstituted enzyme systems from rabbit liver

The mechanism of benzene oxygenation in liver microsomes and in reconstituted enzyme systems from rabbit liver was investigated. It was found that the NADPH-dependent transformation of benzene to water-soluble metabolites and to phenol catalyzed by cytochrome P-450 LM2 in membrane vesicles was inhibited by catalase, horseradish peroxidase, superoxide dismutase, and hydroxyl radical scavengers such as mannitol, dimethyl sulfoxide, and catechol, indicating the participation of hydrogen peroxide, superoxide anions, and hydroxyl radicals in the process. The cytochrome P-450 LM2-dependent, hydroxyl radical-mediated destruction of deoxyribose was inhibited concomitantly to the benzene oxidation. Also the microsomal benzene metabolism, which did not exhibit Michaelis-Menten kinetics, was effectively inhibited by six different hydroxyl radical scavengers. Biphenyl was formed in the reconstituted system, indicating the cytochrome P-450-dependent production of a hydroxycyclohexadienyl radical as a consequence of interactions between hydroxyl radicals and benzene. The formation of benzene metabolites covalently bound to protein was efficiently inhibited by radical scavengers but not by epoxide hydrolase. The results indicate that the microsomal cytochrome P-450-dependent oxidation of benzene is mediated by hydroxyl radicals formed in a modified Haber-Weiss reaction between hydrogen peroxide and superoxide anions and suggest that any cellular superoxide-generating system may be sufficient for the metabolic activation of benzene and structurally related compounds.     

The production of free radicals during the autoxidation of cysteine and their effect on isolated rat hepatocytes

Autoxidizing cysteine has been shown to produce thiyl and hydroxyl radicals. Hydrogen peroxide increased the yield of both radicals which was inhibited by catalase but stimulated by copper / zinc superoxide dismutase. This effect is due to increased bydrogen peroxide production by copper / zinc superoxide dismutase as a result of superoxide dismutation. The production of superoxide radicals could not be detected probably because of its low reactivity, however, measurement of oxygen uptake and reduction of ferricytochrome c by autoxidizing cysteine clearly implicate the involvement of super oxide radicals. The production of hydroxyl radicals is postulated to proceed through a Fenton reaction, however, this may not necessarily be metal ion controlled. Autoxidizing cysteine disrupts the integrity of heptocytes causing release of glutathione, adenosine triphosphate and lactate dehydrogenase indicating that it is of little use as a therapeutic agent.     

Activation of the superoxide-generating NADPH oxidase of intestinal lymphocytes produces highly reactive free radicals from sulfite.

The objective of this study was to investigate the ability of immune cells of the small intestine to produce highly reactive free radicals from the food additive sulfites. These free radicals were characterized with a spin-trapping technique using the spin traps 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO). In the presence of glucose, purified lymphocytes from intestinal Peyer's patches (PP) and mesenteric lymph nodes (MLN) were stimulated with phorbol 12-myristate 13-acetate (PMA) to produce superoxide and hydroxyl DEPMPO radical adducts. The formation of these adducts was inhibited by superoxide dismutase or diphenyleneiodonium chloride, indicating that these cells produced superoxide radical during reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activation. With the treatment of sodium sulfite, PMA-stimulated PP lymphocytes produced a DEPMPO-sulfite radical adduct and an unknown radical adduct. When DEPMPO was replaced with DMPO, DMPO-sulfite and hydroxyl radical adducts were detected. The latter adduct resulted from DMPO oxidation by sulfate radical, which was capable of oxidizing formate or ethanol. Oxygen consumption rates were further increased after the addition of sulfite to PMA-stimulated lymphocytes, suggesting the presence of sulfiteperoxyl radical. Taken together, oxidants generated by stimulated lymphocytes oxidized sulfite to sulfite radical, which subsequently formed sulfiteperoxyl and sulfate radicals. The latter two radicals are highly reactive, contributing to increased oxidative stress, which may lead to sulfite toxicity, altered functions in intestinal lymphocytes, or both.     

巨噬细胞产生NO.和O_2~-自由基的分子机理

建立了用顺磁共振（ＥＳＲ）和化学发光技术测定巨噬细胞产生ＮＯ和氧自由基的方法．捕捉到了巨噬细胞受佛波酯刺激产生的ＮＯ．和Ｏ－２自由基．测定了在不同浓度Ｌ－精氨酸存在时佛波酯刺激后巨噬细胞产生的ＮＯ自由基．研究了巨噬细胞产生的ＮＯ和氧自由基的分子机理．结果表明巨噬细胞不仅产生氧自由基而且产生ＮＯ自由基．ＮＡＤＰＨ氧化酶产生氧自由基的部位位于巨噬细胞膜的外侧．ＮＯ合成酶活化产生ＮＯ自由基比ＮＡＤＰＨ氧化酶活化产生氧自由基晚几分钟．     

Insulin-induced peroxynitrite production in human platelet-rich plasma

Recent data support the possible role of nitric oxide (NO*) in the development of insulin signalling. The aim of this study was to examine the effect of insulin on NO* production by platelets. The chemiluminescence of platelet-rich plasma prepared from the blood of healthy volunteers was measured in the presence of luminol. Indirect detection of NO* by luminol is possible in the form of peroxynitrite produced in the reaction of NO* with a superoxide free radical. Luminol oxidation induced by hydroxyl free radical and lipid peroxidation was prevented by 150 micromol/l of desferrioxamine mesylate. Insulin, in the range of 0.084-840 nmol/l, induced a concentration-dependent increase in chemiluminescence, which was inhibited both by the competitive antagonist of the NO* synthase enzyme. N(omega)-nitro-L-arginine methyl ester (at concentrations of 2.0-4.0 mmol/l, P<0.001), and by the elimination of superoxide free radicals using superoxide dismutase (72-144 IU/ml, P<0.001). In conclusion, we assume that the insulin-induced increase in chemiluminescence of platelet-rich plasma was due to increased production of NO* and superoxide free radicals forming peroxynitrite. The data are consistent with production of peroxynitrite from human platelets under insulin stimulation.     

Target cell-derived superoxide anions cause efficiency and selectivity of intercellular induction of apoptosis

Transformed fibroblasts are specifically eliminated by their nontransformed neighbors through intercellular induction of apoptosis. This process depends on the number of nontransformed effector cells and on the local density of transformed target cells. Intercellular signalling is inhibited by SOD (a scavenger of superoxide anions), taurine (a scavenger of HOCl), 4-aminobenzoyl hydrazide (a mechanism-based inhibitor of peroxidase), DMSO (a hydroxyl radical scavenger), and two inhibitors of NO synthase. Therefore, selective apoptosis induction seems to be based on superoxide anion production by transformed cells, their spontaneous dismutation to hydrogen peroxide, and HOCl generation by a novel effector cell-derived peroxidase. HOCl then interacts with target cell–derived superoxide anions to yield hydroxyl radicals. Due to the short diffusion pathway of superoxide anions, hydroxyl radical generation is confined to the intimate vicinity of transformed cells. In parallel, NO derived from effector cells interacts with superoxide anions of target cells to yield the apoptosis inducer peroxynitrite. Reconstitution experiments using transformed or nontransformed cells in conjunction with myeloperoxidase, HOCl, or an NO donor demonstrated that superoxide anions generated extracellularly by transformed cells participate in intercellular signalling and at the same time determine transformed cells as selective targets for intercellular induction of apoptosis.     

Protein-mediated hydroxyl radical generation--the primary event in NADH oxidation and oxygen reduction by the granule rich fraction of human resting leukocytes.

The granule rich-fraction isolated from human resting polymorphonuclear leukocytes is capable of CN-insensitive NADH oxidation and O₂-uptake, accompanied by production of superoxide anion, hydroxyl radicals and H₂O₂. We showed that H₂O₂ initiates and maintains NADH oxidation and O₂-uptake but is also necessary for the formation of superoxide anion and hydroxyl radicals. It acts as a primary substrate for CN-insensitive protein-mediated formation of hydroxyl radicals, which in turn produce superoxide anions, probably through univalent oxidation of NADH as an intermediary.     

Redox cycling of potential antitumor aziridinyl quinones

The formation of reactive oxygen intermediates (ROI) during redox cycling of newly synthesized potential antitumor 2,5-bis (1-aziridinyl)-1,4-benzoquinone (BABQ) derivatives has been studied by assaying the production of ROI (superoxide, hydroxyl radical, and hydrogen peroxide) by xanthine oxidase in the presence of BABQ derivatives. At low concentrations (< 10 microM) some BABQ derivatives turned out to inhibit the production of superoxide and hydroxyl radicals by xanthine oxidase, while the effect on the xanthine-oxidase-induced production of hydrogen peroxide was much less pronounced. Induction of DNA strand breaks by reactive oxygen species generated by xanthine oxidase was also inhibited by BABQ derivatives. The DNA damage was comparable to the amount of hydroxyl radicals produced. The inhibiting effect on hydroxyl radical production can be explained as a consequence of the lowered level of superoxide, which disrupts the Haber-Weiss reaction sequence. The inhibitory effect of BABQ derivatives on superoxide formation correlated with their one-electron reduction potentials: BABQ derivatives with a high reduction potential scavenge superoxide anion radicals produced by xanthine oxidase, leading to reduced BABQ species and production of hydrogen peroxide from reoxidation of reduced BABQ. This study, using a unique series of BABQ derivatives with an extended range of reduction potentials, demonstrates that the formation of superoxide and hydroxyl radicals by bioreductively activated antitumor quinones can in principle be uncoupled from alkylating activity.     

Remote delivery of hydroxyl radicals via secondary chemistry of a nonthermal plasma effluent

Electron paramagnetic resonance spectroscopy is used to observe hydroxyl radicals produced by an atmospheric pressure nonthermal plasma device at distances greater than 1 m from the discharge. The plasma device is an indirect treatment setup with closed loop airflow and hydrogen peroxide additives that is effective in deactivating bacteria on time scales of seconds. The generation of the detected hydroxyl radicals is shown to occur in secondary chemical processes near the point of delivery of the plasma treated air stream. The production of hydroxyl radicals is correlated with humidity of the air stream and ability to lyse bacterial membranes. The overall mechanisms of bacteria inactivation are found to be a combinatorial effect of effluent species. The results indicate the feasibility of selective plasma induced free radical delivery for biomedical applications even in the case of short‐lived species like the hydroxyl radical. Biotechnol. Bioeng. 2013; 110: 1936–1944. © 2013 Wiley Periodicals, Inc.     

Hydroxyl radical generation by red tide algae.

The unicellular marine phytoplankton Chattonella marina is known to have toxic effects against various living marine organisms, especially fishes. However, details of the mechanism of the toxicity of this plankton remain obscure. Here we demonstrate the generation of superoxide and hydroxyl radicals from a red tide unicellular organism, C. marina, by using ESR spectroscopy with the spin traps 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and N-t-butyl-alpha-phenylnitrone (PBN), and by using the luminol-enhanced chemiluminescence response. The spin-trapping assay revealed productions of spin adduct of superoxide anion (O2-) (DMPO-OOH) and that of hydroxyl radical (.OH) (DMPO-OH) in the algal suspension, which was not observed in the ultrasonic-ruptured suspension. The addition of superoxide dismutase (500 U/ml) almost completely inhibited the formation of both DMPO-OOH and DMPO-OH, and carbon-centered radicals were generated with the disappearance of DMPO-OH after addition of 5% dimethyl sulfoxide (Me2SO) and 5% ethanol. Furthermore, the generation of methyl and methoxyl radicals, which are thought to be produced by the reaction of hydroxyl radical and Me2SO under aerobic condition, was identified using spin trapping with a combination of PBN and Me2SO. Luminol-enhanced chemiluminescence assay also supported the above observations. These results clearly indicate that C. marina generates and releases the superoxide radical followed by the production of hydroxyl radical to the surrounding environment. The velocity of superoxide generation by C. marina was about 100 times faster than that by mammalian phagocytes per cell basis. The generation of oxygen radical is suggested to be a pathogenic principle in the toxication of red tide to susceptible aquaculture fishes and may be directly correlated with the coastal pollution by red tide.     

Oxyradicals under UV-b stress and their quenching by antioxidants

Formation of oxyradicals under UV-B stress was investigated using cucumber cotyledons. UV-B radiation induced production of free radicals which were analyzed by ESR spectroscopy. Evidence was obtained for the formation of superoxide and hydroxyl radicals in the tissues by comparing PBN-adducts formed with radicals obtained by chemical autooxidation of KO2 and Fenton's reaction. Addition of superoxide dismutase (SOD) to the reaction mixture partially reduced the intensity of signals confirming the production of superoxide radical as well as hydroxyl radicals. These radicals were quenched in vitro by the natural antioxidants alpha-tocopherol, ascorbic acid and benzoquinone. Changes in the level of antioxidants were also monitored under UV-B stress. The endogenous level of ascorbic acid was enhanced and alpha-tocopherol level was reduced in the tissue after exposure to UV-B radiation. The present report happens to be the first direct evidence obtained for the formation of superoxide and hydroxyl radicals in plant tissues exposed to UV-B radiation.     

Effect of zinc on superoxide-dependent hydroxyl radical production in vitro

Trace elements play an important role in oxygen metabolism and therefore in the formation of free radicals. Whereas iron and copper are usually the main enhancers of free radical formation, other trace elements, such as zinc and selenium, protect against the harmful effects of these radicals. To investigate the different protective mechanisms of zinc on radical formation, we examined the effects of added zinc and copper on superoxide dismutase activity. We also studied the effects of copper and iron on xanthine oxidase activity and on the Haber-Weiss cycle (iron, superoxide, and hydrogen peroxide), which generates hydroxyl radicals in vitro. The hypoxanthine/xanthine oxidase radical generating system contained a variety of different physiological ligands for binding the iron. This study confirmed the inhibitory effect of copper on xanthine oxidase activity. Moreover, it demonstrated that zinc inhibited hydroxyl radical formation when this formation was catalyzed by a citrate-iron complex in the hypoxanthine/xanthine oxidase reaction. Finally, human blood plasma inhibited citrate-iron-dependent hydroxyl radical formation under the same conditions. Although trace elements seemed responsible for this antioxidant activity of plasma, it is likely that zinc played no role as a plasma antioxidant. Indeed, calcium appeared to be responsible for most of this effect under our experimental conditions.