Production of reactive oxygen species by photosystem II

Photosysthetic cleavage of water molecules to molecular oxygen is a crucial process for all aerobic life on the Earth. Light-driven oxidation of water occurs in photosystem II (PSII) — a pigment-protein complex embedded in the thylakoid membrane of plants, algae and cyanobacteria. Electron transport across the thylakoid membrane terminated by NADPH and ATP formation is inadvertently coupled with the formation of reactive oxygen species (ROS). Reactive oxygen species are mainly produced by photosystem I; however, under certain circumstances, PSII contributes to the overall formation of ROS in the thylakoid membrane. Under limitation of electron transport reaction between both photosystems, photoreduction of molecular oxygen by the reducing side of PSII generates a superoxide anion radical, its dismutation to hydrogen peroxide and the subsequent formation of a hydroxyl radical terminates the overall process of ROS formation on the PSII electron acceptor side. On the PSII electron donor side, partial or complete inhibition of enzymatic activity of the water-splitting manganese complex is coupled with incomplete oxidation of water to hydrogen peroxide. The review points out the mechanistic aspects in the production of ROS on both the electron acceptor and electron donor side of PSII.     

Magnificant role of intracellular reactive oxygen species production and its scavenging encompasses downstream processes

Environmental stresses are often associated with production of certain deleterious chemical entities called reactive oxygen species (ROS), which include hydrogen peroxide (H₂O₂), superoxide radical (O₂^?), hydroxyl radical (OH^?). In plants, ROS are formed by the inevitable leakage of electrons onto O₂ from the electron transport activities of chloroplasts, mitochondria, peroxisomes, vacuole and plasma membranes or as a byproduct of various metabolic pathways. Plants have their own antioxidant defense mechanisms to encounter ROS that is of enzymic and non-enzymic nature. Coordinated activities of these antioxidants regulate ROS detoxification and reduces oxidative load in plants. Though ROS are always regarded to impart negative impact on plants, some reports consider them to be important in regulating key cellular functions; however, such reports in plant are limited. On the other hand, specific ROS function as signaling molecules and activate signal transduction processes in response to various stresses is a matter of investigation.     

Dark production of reactive oxygen species in photosystem II membrane particles at elevated temperature: EPR spin-trapping study

In our study, EPR spin-trapping technique was employed to study dark production of two reactive oxygen species, hydroxyl radicals (OH) and singlet oxygen (¹O₂), in spinach photosystem II (PSII) membrane particles exposed to elevated temperature (47 °C). Production of OH, evaluated as EMPO-OH adduct EPR signal, was suppressed by the enzymatic removal of hydrogen peroxide and by the addition of iron chelator desferal, whereas externally added hydrogen peroxide enhanced OH production. These observations reveal that OH is presumably produced by metal-mediated reduction of hydrogen peroxide in a Fenton-type reaction. Increase in pH above physiological values significantly stimulated the formation of OH, whereas the presence of chloride and calcium ions had the opposite effect. Based on our results it is proposed that the formation of OH is linked to the thermal disassembly of water-splitting manganese complex on PSII donor side. Singlet oxygen production, followed as the formation of nitroxyl radical TEMPO, was not affected by OH scavengers. This finding indicates that the production of these two species was independent and that the production of ¹O₂ is not closely linked to PSII donor side.     

Dark production of reactive oxygen species in photosystem II membrane particles at elevated temperature: EPR spin-trapping study

In our study, EPR spin-trapping technique was employed to study dark production of two reactive oxygen species, hydroxyl radicals (OH.) and singlet oxygen ((1)O2), in spinach photosystem II (PSII) membrane particles exposed to elevated temperature (47 degrees C). Production of OH., evaluated as EMPO-OH adduct EPR signal, was suppressed by the enzymatic removal of hydrogen peroxide and by the addition of iron chelator desferal, whereas externally added hydrogen peroxide enhanced OH. production. These observations reveal that OH. is presumably produced by metal-mediated reduction of hydrogen peroxide in a Fenton-type reaction. Increase in pH above physiological values significantly stimulated the formation of OH., whereas the presence of chloride and calcium ions had the opposite effect. Based on our results it is proposed that the formation of OH. is linked to the thermal disassembly of water-splitting manganese complex on PSII donor side. Singlet oxygen production, followed as the formation of nitroxyl radical TEMPO, was not affected by OH. scavengers. This finding indicates that the production of these two species was independent and that the production of (1)O2 is not closely linked to PSII donor side.     

Quality control of photosystem II: reactive oxygen species are responsible for the damage to photosystem II under moderate heat stress

Moderate heat stress (40 degrees C for 30 min) on spinach thylakoid membranes induced cleavage of the reaction center-binding D1 protein of photosystem II, aggregation of the D1 protein with the neighboring polypeptides D2 and CP43, and release of three extrinsic proteins, PsbO, -P, and -Q. These heat-induced events were suppressed under anaerobic conditions or by the addition of sodium ascorbate, a general scavenger of reactive oxygen species. In accordance with this, singlet oxygen and hydroxyl radicals were detected in spinach photosystem II membranes incubated at 40 degrees C for 30 min with electron paramagnetic resonance spin-trapping spectroscopy. The moderate heat stress also induced significant lipid peroxidation under aerobic conditions. We suggest that the reactive oxygen species are generated by heat-induced inactivation of a water-oxidizing manganese complex and through lipid peroxidation. Although occurring in the dark, the damages caused by the moderate heat stress to photosystem II are quite similar to those induced by excessive illumination where reactive oxygen species are involved.     

The production and scavenging of reactive oxygen species in the plastoquinone pool of chloroplast thylakoid membranes

Reactive oxygen species (ROS) resulting from oxygen reduction, superoxide anion radical O and hydrogen peroxide H ₂O₂ are very significant in the cell metabolism of aerobic organisms. They can be destructive and lead to apoptosis and they can also serve as signal molecules. In the light, chloroplasts are known to be one of the main sources of ROS in plants. However, the components involved in oxygen reduction and the detailed chemical mechanism are not yet well established. The present review describes the experimental data and theoretical considerations that implicate the plastoquinone pool (PQ‐pool) in this process. The evidence indicates that the PQ‐pool has a dual role: (1) the reduction of O₂ by plastosemiquinone to superoxide and (2) the reduction of superoxide by plastohydroquinone to hydrogen peroxide. The second role represents not only the scavenging of superoxide, but also the generation of hydrogen peroxide as an important signaling molecule. The regulatory and protective functions of the PQ‐pool are discussed in the context of these reactions.     

Mitochondrial production of reactive oxygen species

Numerous biochemical studies are aimed at elucidating the sources and mechanisms of formation of reactive oxygen species (ROS) because they are involved in cellular, organ-, and tissue-specific physiology. Mitochondria along with other cellular organelles of eukaryotes contribute significantly to ROS formation and utilization. This review is a critical account of the mitochondrial ROS production and methods for their registration. The physiological and pathophysiological significance of the mitochondrially produced ROS are discussed.     

Production of reactive oxygen species in chloride- and calcium-depleted photosystem II and their involvement in photoinhibition

Mixed photosystem II (PSII) samples consisting of Cl(-)-depleted and active, or Ca(2+)-depleted and active PSII enriched membrane fragments, respectively, were investigated with respect to their susceptibility to light. In the presence of Cl(-)-depleted PSII, active centers were damaged more severely, most likely caused by a higher amount of reactive oxygen species formed in the nonfunctional centers. Cl(-) depletion led to an increased H(2)O(2) production, which seemed to be responsible for the stimulation of PSII activity loss. To distinguish between direct H(2)O(2) formation by partial water oxidation and indirect H(2)O(2) formation by oxygen reduction involving the prior formation of O(2)(-?), the production of reactive oxygen species was followed by spin trapping EPR spectroscopy. All samples investigated, i.e. PSII with a functional water splitting complex, Ca(2+)- and Cl(-)-depleted PSII, produced upon illumination O(2)(-?) and OH(?) radicals on the acceptor side, while Cl(-)-depleted PSII produced additionally OH(?) radicals originating from H(2)O(2) formed on the donor side of PSII.     

Molecular mechanisms of cellular injury produced by neurotoxic amino acids that generate reactive oxygen species

Summary There is now strong experimental evidence that the basic precursors for the synthesis of catechol(amine) and indolamine neurotransmitters, tyrosine and tryptophan can act as generators of ROS (reactive oxygen species): peroxides, superoxide and peroxyradicals. The consequences of free radicals formation from precursors during oxidative degradation process, their possible participation in electron transfer/addition reactions and chain processes involving cell antioxidant defense system were presented and discussed. Although the generation of neurotoxic ROS by tyrosine and tryptophan is accepted to occur in the presented model systems, doubts can exist as to the situationin vivo, which may be completely different and remain to be explored. The relevance of the present findings with regard to a variety of neurological diseases cannot be ignored.     

Singlet oxygen production in herbicide-treated photosystem II

Photo-generated reactive oxygen species in herbicide-treated photosystem II were investigated by spin-trapping. While the production of .OH and O2-* was herbicide-independent, 1O2 with a phenolic was twice that with a urea herbicide. This correlates with the reported influence of these herbicides on the redox properties of the semiquinone QA-* and fits with the hypothesis that 1O2 is produced by charge recombination reactions that are stimulated by herbicide binding and modulated by the nature of the herbicide. When phenolic herbicides are bound, charge recombination at the level of P+*Pheo-* is thermodynamically favoured forming a chlorophyll triplet and hence 1O2. With urea herbicides this pathway is less favourable.     

叶绿体光合代谢中活性氧的产生与清除

有氧代谢不可避免产生活性氧(ROS),叶绿体的PSI和PSII反应中心均是ROS产生的主要位点。叶绿体产生的ROS主要有超氧阴离子(O2-)、过氧化氢(H2O2)、羟自由基(.OH)和单线氧(1O2),其中在PSI产生的O2-将进一步产生H2O2和.OH,而1O2产生在PSII。正常生理代谢条件下,叶绿体内抗氧化系统和光能吸收利用的调节保持活性氧产生和消灭的平衡,不会影响植物的正常生理功能。     

The production of reactive oxygen species by dietary flavonols

Flavonols are a group of naturally occurring compounds which are widely distributed in nature where they are found glycosylated primarily in vegetables and fruits. A number of studies have found both anti- and prooxidant effects for many of these compounds. The most widely studied because of their ubiquitous nature have been quercetin, a B-dihydroxylated and myricetin, a B-trihydroxylated flavonol. Some of their prooxidant properties have been attributed to the fact that they can undergo autooxidation when dissolved in aqueous buffer. Studying a number of factors affecting autooxidation, we found the rate of autooxidation for both quercetin and myricetin to be highly pH dependent with no autooxidation detected for quercetin at physiologic pH. Both the addition of iron for the two flavonols and the addition of iron followed by SOD for quercetin at physiologic pH. Both the addistantially. Neither kaempferol, a monohydroxylated flavonol nor rutin, a glycosylated quercetin showed any ability to autooxidize. The results with rutin differ from what we expected based on the B-ring structural similarity to quercetin. The autooxidation of quercetin and myricetin was further studied by electron spin resonance spectroscopy (ESR). Whereas quercetin produced a characteristic DMPO-OH radical, it was not detected below a pH of 9. However, the addition of iron allowed the signal to be detected at a pH as low as 8.0. On the other hand, myricetin autooxidation yielded a semiquinone signal which upon the addition of iron, converted to a DMPO-OH signal detected at a pH of 7.5. In a microsome-NADPH system, quercetin produced an increase in oxygen utilization and with ESR, an ethanol-derived radical signal which could be completely suppressed by catalase indicating the dependence of the signal on hydrogen peroxide. These studies demonstrate that the extracellular production of active oxygen species by dietary flavonols is not likely to occur in vivo but the potential for intracellular redox cycling may have toxicologic significance.     

Mitochondrial reactive oxygen species and inflammation: Molecular mechanisms,diseases and promising therapies

Over the last few decades, many different groups have been engaged in studies of new roles for mitochondria, particularly the coupling of alterations in the redox pathway with the inflammatory responses involved in different diseases, including Alzheimer’s disease, Parkinson's disease, atherosclerosis, cerebral cavernous malformations, cystic fibrosis and cancer. Mitochondrial dysfunction is important in these pathological conditions, suggesting a pivotal role for mitochondria in the coordination of pro-inflammatory signaling from the cytosol and signaling from other subcellular organelles. In this regard, mitochondrial reactive oxygen species are emerging as perfect liaisons that can trigger the assembly and successive activation of large caspase-1- activating complexes known as inflammasomes. This review offers a glimpse into the mechanisms by which inflammasomes are activated by mitochondrial mechanisms, including reactive oxygen species production and mitochondrial Ca²⁺ uptake, and the roles they can play in several inflammatory pathologies.     

In vitro reactive oxygen species production by histatins and copper(I,II)

The ability of the histidine-rich peptides, histatin-5 (Hst-5) and histatin-8 (Hst-8), to support the generation of reactive oxygen species during the Cu-catalyzed oxidation of ascorbate and cysteine has been evaluated. High levels of hydrogen peroxide (70–580 mol/mol Cu/h) are produced by aqueous solutions containing Cu(II), Hst-8 or Hst-5, and a reductant, either ascorbate or cysteine, as determined by the postreaction Amplex Red assay. When the reactions are conducted in the presence of superoxide dismutase, the total hydrogen peroxide produced is decreased, more so in the presence of the peptides (up to 50%), suggesting the intermediacy of superoxide in these reactions. On the other hand, the presence of sodium azide or sodium formate, traps for hydroxyl radicals, has no appreciable effect on the total hydrogen peroxide production for the Cu–Hst systems. EPR spin-trapping studies using 5-(2,2-dimethyl-1,3-propoxy cyclophosphoryl)-5-methyl-1-pyrroline N-oxide (CYPMPO) in the cysteine–Cu(II) reactions reveal the formation of the CYPMPO–hydroperoxyl and CYPMPO–hydroxyl radical adducts in the presence of Hst-8, whereas only the latter was observed with Cu alone.     

Protein synthesis is the primary target of reactive oxygen species in the photoinhibition of photosystem II

Photoinhibition of photosystem II (PSII) occurs when the rate of photodamage to PSII exceeds the rate of the repair of photodamaged PSII. Recent examination of photoinhibition by separate determinations of photodamage and repair has revealed that the rate of photodamage to PSII is directly proportional to the intensity of incident light and that the repair of PSII is particularly sensitive to the inactivation by reactive oxygen species (ROS). The ROS-induced inactivation of repair is attributable to the suppression of the synthesis de novo of proteins, such as the D1 protein, that are required for the repair of PSII at the level of translational elongation. Furthermore, molecular analysis has revealed that the ROS-induced suppression of protein synthesis is associated with the specific inactivation of elongation factor G via the formation of an intramolecular disulfide bond. Impairment of various mechanisms that protect PSII against photoinhibition, including photorespiration, thermal dissipation of excitation energy, and the cyclic transport of electrons, decreases the rate of repair of PSII via the suppression of protein synthesis. In this review, we present a newly established model of the mechanism and the physiological significance of repair in the regulation of the photoinhibition of PSII.     

A new paradigm for the action of reactive oxygen species in the photoinhibition of photosystem II

Inhibition of the activity of photosystem II (PSII) under strong light is referred to as photoinhibition. This phenomenon is due to the imbalance between the rate of photodamage to PSII and the rate of the repair of damaged PSII. Photodamage is initiated by the direct effects of light on the oxygen-evolving complex and, thus, photodamage to PSII is unavoidable. Studies of the effects of oxidative stress on photodamage and subsequent repair have revealed that reactive oxygen species (ROS) act primarily by inhibiting the repair of photodamaged PSII and not by damaging PSII directly. Thus, strong light has two distinct effects on PSII; it damages PSII directly and it inhibits the repair of PSII via production of ROS. Investigations of the ROS-induced inhibition of repair have demonstrated that ROS suppress the synthesis de novo of proteins and, in particular, of the D1 protein, that are required for the repair of PSII. Moreover, a primary target for inhibition by ROS appears to be the elongation step of translation. Inhibition of the repair of PSII by ROS is accelerated by the deceleration of the Calvin cycle that occurs when the availability of CO₂ is limited. In this review, we present a new paradigm for the action of ROS in photoinhibition.     

A new paradigm for the action of reactive oxygen species in the photoinhibition of photosystem II

Inhibition of the activity of photosystem II (PSII) under strong light is referred to as photoinhibition. This phenomenon is due to the imbalance between the rate of photodamage to PSII and the rate of the repair of damaged PSII. Photodamage is initiated by the direct effects of light on the oxygen-evolving complex and, thus, photodamage to PSII is unavoidable. Studies of the effects of oxidative stress on photodamage and subsequent repair have revealed that reactive oxygen species (ROS) act primarily by inhibiting the repair of photodamaged PSII and not by damaging PSII directly. Thus, strong light has two distinct effects on PSII; it damages PSII directly and it inhibits the repair of PSII via production of ROS. Investigations of the ROS-induced inhibition of repair have demonstrated that ROS suppress the synthesis de novo of proteins and, in particular, of the D1 protein, that are required for the repair of PSII. Moreover, a primary target for inhibition by ROS appears to be the elongation step of translation. Inhibition of the repair of PSII by ROS is accelerated by the deceleration of the Calvin cycle that occurs when the availability of CO(2) is limited. In this review, we present a new paradigm for the action of ROS in photoinhibition.     

Photoinduced production of reactive oxygen species by retinal derivatives and conjugates

Formation of reactive oxygen species (ROS) during all-trans-retinal (ATR) illumination was studied by EPR. The quantity of ROS was estimated from the EPR signal of a spin adduct of DEPMPO (5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide). The EPR signal was found to be a superposition of the adducts: 80% superoxide anion radical, 15% hydroxyl radical, and 5% unknown radical. Albumin at an equimolar concentration decreased the relative quantum yield of ROS five times, and a 4-fold albumin excess decreased ROS production 30 times. It is supposed that ATR and A2E in the photoreceptor membrane are the sources of photodamage induced by ROS. Since ATR in the cell is known to be transported by interphotoreceptor retinal-binding proteins of the albumin family, its binding to protein is supposed to play an important protective role, preventing ROS production in the photoreceptor cell.