Immunocytochemical evidence for a peroxisomal localization of manganese superoxide dismutase in leaf protoplasts from a higher plant

Old dogs, new tricks: New insights into the iron/manganese superoxide dismutase family

Superoxide dismutases (SODs) are ancient enzymes of widespread importance present in all domains of life. Many insights have been gained into these important enzymes over the 50 years since their initial description, but recent studies in the context of microbial pathogenesis have resulted in findings that challenge long established dogmas. The repertoire of SODs that bacterial pathogens encode is diverse both in number and in metal dependencies, including copper, copper and zinc, manganese, iron, and cambialistic enzymes. Other bacteria also possess nickel dependent SODs. Compartmentalization of SODs only partially explains their diversity. The need for pathogens to maintain SOD activity across distinct hostile environments encountered during infection, including those limited for essential metals, is also a driver of repertoire diversity. SOD research using pathogenic microbes has also revealed the apparent biochemical ease with which metal specificity can change within the most common family of SODs. Collectively, these studies are revealing the dynamic nature of SOD evolution, both that of individual SOD enzymes that can change their metal specificity to adapt to fluctuating cellular metal availability, and of a cell's repertoire of SOD isozymes that can be differentially expressed to adapt to fluctuating environmental metal availability in a niche.

Metal tolerance protein MTP6 is involved in Mn and Co distribution in poplar

With the booming demand of the electric vehicle industry, the concentration of manganese (Mn) and cobalt (Co) flowing into land ecosystems has also increased significantly. While these transition metals can promote the growth and development of plants, they may become toxic under high concentrations. It is thus important to understand how Mn and Co are distributed in plants to develop novel germplasms for the remediation of these heavy metals in contaminated soils. Here, an MTP gene that encodes the CDF (cation diffusion facilitator) protein in Populus trichocarpa, PtrMTP6, was screened as the key gene involved in the distribution of both Mn and Co in poplar. The PtrMTP6-GFP fusion protein was co-localized with the mRFP-VSR2, showing that PtrMTP6 proteins are present at the pre-vacuolar compartment (PVC). Yeast mutant complementation assays further identified that PtrMTP6 serves as a Mn and Co transporter, reducing yeast cell toxicity after exposure to excessive Mn or Co. Histochemical analyses showed that PtrMTP6 was mainly expressed in phloem, suggesting that PtrMTP6 probably involved in the Mn and Co transport via phloem in plants. Under excess Co, PtrMTP6 overexpressing poplar lines were more severely damaged than the control due to higher Co accumulations in young tissue. PtrMTP6 overexpressing lines showed little change in their tolerance to excess Mn, although young tissues also accumulated more Mn. PtrMTP6 play important roles in Mn and Co distribution in poplar and further research on its regulation will be important to increase bioremediation in Mn and Co polluted ecosystems.

Nitric Oxide (NO) Scaffolds the Peroxisomal Protein-Protein Interaction Network in Higher Plants

The peroxisome is a single-membrane subcellular compartment present in almost all eukaryotic cells from simple protists and fungi to complex organisms such as higher plants and animals. Historically, the name of the peroxisome came from a subcellular structure that contained high levels of hydrogen peroxide (H2O2) and the antioxidant enzyme catalase, which indicated that this organelle had basically an oxidative metabolism. During the last 20 years, it has been shown that plant peroxisomes also contain nitric oxide (NO), a radical molecule than leads to a family of derived molecules designated as reactive nitrogen species (RNS). These reactive species can mediate post-translational modifications (PTMs) of proteins, such as S-nitrosation and tyrosine nitration, thus affecting their function. This review aims to provide a comprehensive overview of how NO could affect peroxisomal metabolism and its internal protein-protein interactions (PPIs). Remarkably, many of the identified NO-target proteins in plant peroxisomes are involved in the metabolism of reactive oxygen species (ROS), either in its generation or its scavenging. Therefore, it is proposed that NO is a molecule with signaling properties with the capacity to modulate the peroxisomal protein-protein network and consequently the peroxisomal functions, especially under adverse environmental conditions.

Effects of lead ions on germination, initial growth, and physiological characteristics of Lolium perenne L. species and its bioaccumulation potential

The present study was conducted to investigate the responses of Lolium perenne L. species to lead ions. To do this, the effects of lead ions at five levels: control (blank), 250, 500, 750, and 1000 mg/kg or mg/L (depending on germination in the soil or petri dish) on the germination, initial growth, and physiological characteristics of Lolium perenne were investigated. The results showed that the difference between various lead concentrations was statistically significant at 1% confidence level in all of the germination, vegetative, and physiological characteristics. In addition, the results of translocation and stress factors indicated that there was a significant difference between the control treatment and the concentrations of 250, 500, 750, and 1000 mg/L of lead ions. Results show that the mean value of stress, which was 0.3196 in the control value, reached 0.4154 at the concentrations 1000 mg/L. Different levels of lead ions had significant effect on the estimated characteristics including germination percentage, seed vigor, germination index, chlorophyll a, chlorophyll b, carotenoids, root, and shoot. The average germination percentage in the control was 46.66%, which decreased by 5% at the highest lead concentration. In addition, the average of seed vigor, which was 34.06 in the control conditions, decreased to 0.72 at the highest lead concentration. Also, the chlorophyll a dropped from 0.5261 mg/g in the control conditions to 0.3149 mg/g. On the other hand, increase in lead ion concentration affected the physiological characteristics of Lolium perenne species. Results suggest that Lolium perenne is capable of accumulating lead and is well tolerant to lead in soil. Therefore, it is concluded that it can be used for sowing on lands which are polluted to this heavy metal (up to the concentration of 1000 mg/kg).

The Biochemical Properties of Manganese in Plants

Manganese (Mn) is an essential micronutrient with many functional roles in plant metabolism. Manganese acts as an activator and co-factor of hundreds of metalloenzymes in plants. Because of its ability to readily change oxidation state in biological systems, Mn plays and important role in a broad range of enzyme-catalyzed reactions, including redox reactions, phosphorylation, decarboxylation, and hydrolysis. Manganese(II) is the prevalent oxidation state of Mn in plants and exhibits fast ligand exchange kinetics, which means that Mn can often be substituted by other metal ions, such as Mg(II), which has similar ion characteristics and requirements to the ligand environment of the metal binding sites. Knowledge of the molecular mechanisms catalyzed by Mn and regulation of Mn insertion into the active site of Mn-dependent enzymes, in the presence of other metals, is gradually evolving. This review presents an overview of the chemistry and biochemistry of Mn in plants, including an updated list of known Mn-dependent enzymes, together with enzymes where Mn has been shown to exchange with other metal ions. Furthermore, the current knowledge of the structure and functional role of the three most well characterized Mn-containing metalloenzymes in plants; the oxygen evolving complex of photosystem II, Mn superoxide dismutase, and oxalate oxidase is summarized.

Impact of Nitric Oxide (NO) on the ROS Metabolism of Peroxisomes

Nitric oxide (NO) is a gaseous free radical endogenously generated in plant cells. Peroxisomes are cell organelles characterized by an active metabolism of reactive oxygen species (ROS) and are also one of the main cellular sites of NO production in higher plants. In this mini-review, an updated and comprehensive overview is presented of the evidence available demonstrating that plant peroxisomes have the capacity to generate NO, and how this molecule and its derived products, peroxynitrite (ONOO⁻) and S-nitrosoglutathione (GSNO), can modulate the ROS metabolism of peroxisomes, mainly throughout protein posttranslational modifications (PTMs), including S-nitrosation and tyrosine nitration. Several peroxisomal antioxidant enzymes, such as catalase (CAT), copper-zinc superoxide dismutase (CuZnSOD), and monodehydroascorbate reductase (MDAR), have been demonstrated to be targets of NO-mediated PTMs. Accordingly, plant peroxisomes can be considered as a good example of the interconnection existing between ROS and reactive nitrogen species (RNS), where NO exerts a regulatory function of ROS metabolism acting upstream of H₂O₂.

Identification of novel superoxide dismutase isoenzymes in the olive (Olea europaea L.) pollen

BACKGROUND

Among antioxidant enzymes, the superoxide dismutase (SOD) family is a major actor in catalysing the disproportionation of superoxide. Apart from its role as antioxidant, these enzymes have a role in cell signalling, and Cu,Zn-SOD proteins are also major pollen allergens. In order to deepen our understanding of the SOD isoenzymes present in olive pollen and to analyse the molecular variability of the pollen Cu,Zn-SOD family, we carried out biochemical, transcriptomic and localization studies of pollen grains from different olive cultivars and other allergenic species.

RESULTS

Olive pollen showed a high rate of total SOD activity in all cultivars assayed, which did not correlate with pollen viability. Mass spectrometry analysis together with activity assays and Western blotting experiments enabled us to identify new forms of Cu,Zn-SOD enzyme (including chloroplastidic and peroxisomal forms) as well as differentially expressed Mn-, Fe- and Cu,Zn-SOD isoenzymes among the pollen of different olive cultivars and allergenic species. Ultrastructural localization of Cu,Zn-SOD revealed its plastidial localization in the pollen grain. We also identified the occurrence of a shorter form of one of the cytosolic Cu,Zn-SOD enzymes, likely as the result of alternative splicing. This shorter enzyme showed lower SOD activity as compared to the full length form.

CONCLUSIONS

The presence of multiple SOD isoenzymes in the olive pollen could be related to the need of finely tuning the ROS metabolism during the transition from its quiescent condition at maturity to a highly metabolically active state at germination.

The activity of superoxide dismutases (SODs) at the early stages of wheat deetiolation

Unbound tetrapyrroles, i.e. protochlorophyllide (Pchlide), chlorophyllide and chlorophylls, bring the risk of reactive oxygen species (ROS) being generated in the initial stages of angiosperm deetiolation due to inefficient usage of the excitation energy for photosynthetic photochemistry. We analyzed the activity of superoxide dismutases (SODs) in etiolated wheat (Triticum aestivum) leaves and at the beginning of their deetiolation. Mn-SOD and three isoforms of Cu/Zn-SODs were identified both in etiolated and greening leaves of T. aestivum. Two Cu/Zn-SODs, denoted as II and III, were found in plastids. The activity of plastidic Cu/Zn-SOD isoforms as well as that of Mn-SOD correlated with cell aging along a monocot leaf, being the highest at leaf tips. Moreover, a high Pchlide content at leaf tips was observed. No correlation between SOD activity and the accumulation of photoactive Pchlide, i.e. Pchlide bound into ternary Pchlide:Pchlide oxidoreductase:NADPH complexes was found. Cu/Zn-SOD I showed the highest activity at the leaf base. A flash of light induced photoreduction of the photoactive Pchlide to chlorophyllide as well as an increase in all the SODs activity which occurred in a minute time-scale. In the case of seedlings that were deetiolated under continuous light of moderate intensity (100 μmol photons m-2 s-1), only some fluctuations in plastidic Cu/Zn-SODs and Mn-SOD within the first four hours of greening were noticed. The activity of SODs is discussed with respect to the assembly of tetrapyrroles within pigment-protein complexes, monitored by fluorescence spectroscopy at 77 K.

Trace metal metabolism in plants

Many trace metals are essential micronutrients, but also potent toxins. Due to natural and anthropogenic causes, vastly different trace metal concentrations occur in various habitats, ranging from deficient to toxic levels. Therefore, one focus of plant research is on the response to trace metals in terms of uptake, transport, sequestration, speciation, physiological use, deficiency, toxicity, and detoxification. In this review, we cover most of these aspects for the essential micronutrients copper, iron, manganese, molybdenum, nickel, and zinc to provide a broader overview than found in other recent reviews, to cross-link aspects of knowledge in this very active research field that are often seen in a separated way. For example, individual processes of metal usage, deficiency, or toxicity often were not mechanistically interconnected. Therefore, this review also aims to stimulate the communication of researchers following different approaches, such as gene expression analysis, biochemistry, or biophysics of metalloproteins. Furthermore, we highlight recent insights, emphasizing data obtained under physiologically and environmentally relevant conditions.

Proteome of Plant Peroxisomes

Plant peroxisomes are required for a number of fundamental physiological processes, such as primary and secondary metabolism, development and stress response. Indexing the dynamic peroxisome proteome is prerequisite to fully understanding the importance of these organelles. Mass Spectrometry (MS)-based proteome analysis has allowed the identification of novel peroxisomal proteins and pathways in a relatively high-throughput fashion and significantly expanded the list of proteins and biochemical reactions in plant peroxisomes. In this chapter, we summarize the experimental proteomic studies performed in plants, compile a list of ~200 confirmed Arabidopsis peroxisomal proteins, and discuss the diverse plant peroxisome functions with an emphasis on the role of Arabidopsis MS-based proteomics in discovering new peroxisome functions. Many plant peroxisome proteins and biochemical pathways are specific to plants, substantiating the complexity, plasticity and uniqueness of plant peroxisomes. Mapping the full plant peroxisome proteome will provide a knowledge base for the improvement of crop production, quality and stress tolerance.

Plant peroxisomes: A nitro-oxidative cocktail

Although peroxisomes are very simple organelles, research on different species has provided us with an understanding of their importance in terms of cell viability. In addition to the significant role played by plant peroxisomes in the metabolism of reactive oxygen species (ROS), data gathered over the last two decades show that these organelles are an endogenous source of nitric oxide (NO) and related molecules called reactive nitrogen species (RNS). Molecules such as NO and H2O2 act as retrograde signals among the different cellular compartments, thus facilitating integral cellular adaptation to physiological and environmental changes. However, under nitro-oxidative conditions, part of this network can be overloaded, possibly leading to cellular damage and even cell death. This review aims to update our knowledge of the ROS/RNS metabolism, whose important role in plant peroxisomes is still underestimated. However, this pioneering approach, in which key elements such as β-oxidation, superoxide dismutase (SOD) and NO have been mainly described in relation to plant peroxisomes, could also be used to explore peroxisomes from other organisms.

Responses of transgenic Arabidopsis plants and recombinant yeast cells expressing a novel durum wheat manganese superoxide dismutase TdMnSOD to various abiotic stresses

In plant cells, the manganese superoxide dismutase (Mn-SOD) plays an elusive role in the response to oxidative stress. In this study, we describe the isolation and functional characterization of a novel Mn-SOD from durum wheat (Triticum turgidum L. subsp. Durum), named TdMnSOD. Molecular phylogeny analysis showed that the durum TdMnSOD exhibited high amino acids sequence identity with other Mn-SOD plants. The three-dimensional structure showed that TdMnSOD forms a homotetramer and each subunit is composed of a predominantly α-helical N-terminal domain and a mixed α/β C-terminal domain. TdMnSOD gene expression analysis showed that this gene was induced by various abiotic stresses in durum wheat. The expression of TdMnSOD enhances tolerance of the transformed yeast cells to salt, osmotic, cold and H2O2-induced oxidative stresses. Moreover, the analysis of TdMnSOD transgenic Arabidopsis plants subjected to different environmental stresses revealed low H2O2 and high proline levels as compared to the wild-type plants. Compared with the non-transformed plants, an increase in the total SOD and two other antioxidant enzyme activities including catalase (CAT) and peroxidases (POD) was observed in the three transgenic lines subjected to abiotic stress. Taken together, these data provide evidence for the involvement of durum wheat TdMnSOD in tolerance to multiple abiotic stresses in crop plants.

ROS Generation in Peroxisomes and its Role in Cell Signaling

In plant cells, as in most eukaryotic organisms, peroxisomes are probably the major sites of intracellular H₂O₂ production, as a result of their essentially oxidative type of metabolism. In recent years, it has become increasingly clear that peroxisomes carry out essential functions in eukaryotic cells. The generation of the important messenger molecule hydrogen peroxide (H₂O₂) by animal and plant peroxisomes and the presence of catalase in these organelles has been known for many years, but the generation of superoxide radicals (O₂^·- ) and the occurrence of the metalloenzyme superoxide dismutase was reported for the first time in peroxisomes from plant origin. Further research showed the presence in plant peroxisomes of a complex battery of antioxidant systems apart from catalase. The evidence available of reactive oxygen species (ROS) production in peroxisomes is presented, and the different antioxidant systems characterized in these organelles and their possible functions are described. Peroxisomes appear to have a ROS-mediated role in abiotic stress situations induced by the heavy metal cadmium (Cd) and the xenobiotic 2,4-D, and also in the oxidative reactions of leaf senescence. The toxicity of Cd and 2,4-D has an effect on the ROS metabolism and speed of movement (dynamics) of peroxisomes. The regulation of ROS production in peroxisomes can take place by post-translational modifications of those proteins involved in their production and/or scavenging. In recent years, different studies have been carried out on the proteome of ROS metabolism in peroxisomes. Diverse evidence obtained indicates that peroxisomes are an important cellular source of different signaling molecules, including ROS, involved in distinct processes of high physiological importance, and might play an important role in the maintenance of cellular redox homeostasis.

Modulation of superoxide dismutase (SOD) isozymes by organ development and high long-term salinity in the halophyte Cakile maritima

Superoxide dismutase (SOD) activity catalyzes the disproportionation of superoxide radicals into hydrogen peroxide and oxygen. This enzyme is considered to be a first line of defense for controlling the production of reactive oxygen species (ROS). In this study, the number and type of SOD isozymes were identified in the principal organs (roots, stems, leaves, flowers, and seeds) of Cakile maritima. We also analyzed the way in which the activity of these SOD isozymes is modulated during development and under high long-term salinity (400 mM NaCl) stress conditions. The data indicate that this plant contains a total of ten SOD isozymes: two Mn-SODs, one Fe-SOD, and seven CuZn-SODs, with the Fe-SOD being the most prominent isozyme in the different organs analyzed. Moreover, the modulation of SOD isozymes, particularly CuZn-SODs, was only detected during development and under severe salinity stress conditions. These data suggest that, in C. maritima, the occurrence of these CuZn-SODs in roots and leaves plays an adaptive role since this CuZn-SOD isozyme might replace the diminished Fe-SOD activity under salinity stress to overcome this adverse environmental condition.

Coordinated Changes in Antioxidative Enzymes Protect the Photosynthetic Machinery from Salinity Induced Oxidative Damage and Confer Salt Tolerance in an Extreme Halophyte Salvadora persica L

Salinity-induced modulations in growth, photosynthetic pigments, relative water content (RWC), lipid peroxidation, photosynthesis, photosystem II efficiency, and changes in activity of various antioxidative enzymes were studied in the halophyte Salvadora persica treated with various levels of salinity (0, 250, 500, 750, and 1000 mM NaCl) to obtain an insight into the salt tolerance ability of this halophyte. Both fresh and dry biomass as well as leaf area (LA) declined at all levels of salinity whereas salinity caused an increase in leaf succulence. A gradual increase was observed in the Na(+) content of leaf with increasing salt concentration up to 750 mM NaCl, but at higher salt concentration (1000 mM NaCl), the Na(+) content surprisingly dropped down to the level of 250 mM NaCl. The chlorophyll and carotenoid contents of the leaf remained unaffected by salinity. The photosynthetic rate (PN), stomatal conductance (gs), the transpiration rate (E), quantum yield of PSII (ΦPSII), photochemical quenching (qP), and electron transport rate remained unchanged at low salinity (250 to 500 mM NaCl) whereas, significant reduction in these parameters were observed at high salinity (750 to 1000 mM NaCl). The RWC% and water use efficiency (WUE) of leaf remained unaffected by salinity. The salinity had no effect on maximum quantum efficiency of PS II (Fv/Fm) which indicates that PS II is not perturbed by salinity-induced oxidative damage. Analysis of the isoforms of antioxidative enzymes revealed that the leaves of S. persica have two isoforms each of Mn-SOD and Fe-SOD and one isoform of Cu-Zn SOD, three isoforms of POX, two isoforms of APX and one isoform of CAT. There was differential responses in activity and expression of different isoforms of various antioxidative enzymes. The malondialdehyde (MDA) content (a product of lipid peroxidation) of leaf remained unchanged in S. persica treated with various levels of salinity. Our results suggest that the absence of pigment degradation, the reduction of water loss, and the maintenance of WUE and protection of PSII from salinity-induced oxidative damage by the coordinated changes in antioxidative enzymes are important factors responsible for salt tolerance of S. persica.

Characterization of antioxidant enzymes and peroxisomes of olive (Olea europaea L.) fruits

The presence of peroxisomes in olive (Olea europaea L.) fruits and different antioxidant enzymes occurring in this plant tissue is reported for the first time. Ultrastructural analysis showed that olive cells were characterized by the presence of large vacuoles and lipid drops. Plastids, mitochondria and peroxisomes were placed near the cell wall, showing some type of association with it. Olive fruit peroxisomes were purified by sucrose density-gradient centrifugation, and catalase, glutathione reductase and ascorbate peroxidase were found in peroxisomes. In olive fruit tissue the presence of a battery of antioxidant enzymes was demonstrated, including catalase, four superoxide dismutase isozymes (mainly an Fe-SOD plus 2 Cu,Zn-SOD and a Mn-SOD), all the enzymes of the ascorbate-glutathione cycle, reduced and oxidized glutathione, ascorbate, and four NADPH-recycling dehydrogenases. The knowledge of the full composition of antioxidants (enzymatic and non-enzymatic) in olive fruits is crucial to be able to understand the processes regulating the antioxidant composition of olive oil.

Spent metal working fluids produced alterations on photosynthetic parameters and cell-ultrastructure of leaves and roots of maize plants

In this work we assess the capacity of maize (Zea mays) plants to phytoremediate spent metal working fluids (MWFs) and its effects on photosynthesis and ultrastructure of mesophyll and root cells. A corn-esparto fibre system patented by us has been used to phytoremediate MWFs in hydroponic culture. Furthermore, a plant growth promoting rhizobacteria (PGPR) has been used to improve the process. The results show that this system is capable of significantly reducing the chemical oxygen demand, under local legislation limits. However, plant systems are really damaged, mainly its photosynthetic system, as shown by the photosynthetical parameters. Nevertheless, strain inoculated improves these parameters, especially Hill reaction. The ultrastructure of photosynthetic apparatus was also affected. Chloroplast number decreased and becomes degraded in the mesophyll of MWFs treated plants. In some cases even plasmolysis of chloroplast membrane was detected. Early senescence symptoms were detected in root ultrastructural study. Severe cellular damage was observed in the parenchymal root cells of plants grown with MWFs, while vascular bundles cell remained unchanged. It seems that the inoculation minimises the damage originated by the MWFs pollutants, appearing as less degenerative organelles and higher chloroplast number than in non-inoculated ones.

Role of peroxisomes as a source of reactive oxygen species (ROS) signaling molecules

Peroxisomes are very dynamic and metabolically active organelles and are a very important source of reactive oxygen species (ROS), H2O2, O2 (.-) and · OH, which are mainly produced in different metabolic pathways, including fatty acid β-oxidation, photorespiration, nucleic acid and polyamine catabolism, ureide metabolism, etc. ROS were originally associated to oxygen toxicity; however, these reactive species also play a central role in the signaling network regulating essential processes in the cell. Peroxisomes have the capacity to rapidly produce and scavenge H2O2 and O2 (.-) which allows to regulate dynamic changes in ROS levels. This fact and the plasticity of these organelles, which allows adjusting their metabolism depending on different developmental and environmental cues, makes these organelles play a central role in cellular signal transduction. The use of catalase and glycolate oxidase loss-of-function mutants has allowed to study the consequences of changes in the levels of endogenous H2O2 in peroxisomes and has improved our knowledge of the transcriptomic profile of genes regulated by peroxisomal ROS. It is now known that peroxisomal ROS participate in more complex signaling networks involving calcium, hormones, and redox homeostasis which finally determine the response of plants to their environment.

Differences in the activity and concentration of elements of the antioxidant system in different layers of Brassica pekinensis head

Differences in the activity of superoxide dismutase, catalase (CAT) and ascorbate peroxidase (APX) as well as in the concentration of ascorbate, tocopherol and hydrogen peroxide (H₂O₂) were found in leaves from different layers of the Chinese cabbage (Brassica pekinensis (Lour.) Rupr.) head. The youngest chlorophyll-deficient leaves from the most inner layers of the cabbage head were characterized by a high concentration of ascorbate, high activity of iron superoxide dismutase (FeSOD), cooper/zinc superoxide dismutase (Cu/ZnSOD) and a low content of H₂O₂. On the other hand, activity of CAT, manganese superoxide dismutase (MnSOD) and APX and tocopherol content were highest in chlorophyll-rich leaves from outer parts. The results of this work are interesting from the human nutrition standpoint, as the measured antioxidants have beneficial effects on human health. They can also be utilized to improve storage conditions due to an unequivocal function of antioxidant molecules in maintaining postharvest quality of vegetables.

Functional analysis of superoxide dismutases (SODs) in sunflower under biotic and abiotic stress conditions. Identification of two new genes of mitochondrial Mn-SOD

Superoxide dismutases (SODs) are a family of metalloenzymes that catalyse the disproportionation of superoxide radicals into hydrogen peroxide and oxygen. In sunflower (Helianthus annuus L.) seedlings, two new Mn-SOD isozymes, designated as I and II, were identified. However, no evidence for a Fe-SOD was found. Both Mn-SOD I and Mn-SOD II have a cleaved sequence of 14 residues that target the mitochondrion with a probability of 81% and 95%, respectively. The gene expression of these new mitochondrial Mn-SODs as well as the previously reported cytosolic and chloroplastic CuZnSODs was analyzed by real-time quantitative reverse transcription-PCR. This was done in the main organs (roots, hypocotyls, and cotyledons) of sunflower seedlings and also under biotic (infection by the pathogen Plasmopara halstedii) and abiotic stress conditions, including high and low temperature and mechanical wounding. Both CuZn-SODs had a gene expression of 1000-fold higher than that of mitochondrial Mn-SODs. And the expression of the Mn-SOD I was approximately 12-fold higher than that of Mn-SOD II. The Mn-SOD I showed a significant modulation in response to the assayed biotic and abiotic stresses even when it had no apparent oxidative stress, such as low temperature. Thus, it is proposed that the mitochondrial Mn-SOD I gene could act as an early sensor of adverse conditions to prevent potential oxidative damage.

Peroxisomes and oxidative stress

The discovery of the colocalization of catalase with H2O2-generating oxidases in peroxisomes was the first indication of their involvement in the metabolism of oxygen metabolites. In past decades it has been revealed that peroxisomes participate not only in the generation of reactive oxygen species (ROS) with grave consequences for cell fate such as malignant degeneration but also in cell rescue from the damaging effects of such radicals. In this review the role of peroxisomes in a variety of physiological and pathological processes involving ROS mainly in animal cells is presented. At the outset the enzymes generating and scavenging H2O2 and other oxygen metabolites are reviewed. The exposure of cultured cells to UV light and different oxidizing agents induces peroxisome proliferation with formation of tubular peroxisomes and apparent upregulation of PEX genes. Significant reduction of peroxisomal volume density and several of their enzymes is observed in inflammatory processes such as infections, ischemia-reperfusion injury and hepatic allograft rejection. The latter response is related to the suppressive effects of TNFalpha on peroxisomal function and on PPARalpha. Their massive proliferation induced by a variety of xenobiotics and the subsequent tumor formation in rodents is evidently due to an imbalance in the formation and scavenging of ROS, and is mediated by PPARalpha. In PEX5-/- mice with the absence of functional peroxisomes severe abnormalities of mitochondria in different organs are observed which resemble closely those in respiratory chain disorders associated with oxidative stress. Interestingly, no evidence of oxidative damage to proteins or lipids, nor of increased peroxide production has been found in that mouse model. In this respect the role of PPARalpha, which is highly activated in those mice, in prevention of oxidative stress deserves further investigation.

Plant peroxisomes as a source of signalling molecules

Peroxisomes are pleiomorphic, metabolically plastic organelles. Their essentially oxidative function led to the adoption of the name 'peroxisome'. The dynamic and diverse nature of peroxisome metabolism has led to the realisation that peroxisomes are an important source of signalling molecules that can function to integrate cellular activity and multicellular development. In plants defence against predators and a hostile environment is of necessity a metabolic and developmental response--a plant has no place to hide. Mutant screens are implicating peroxisomes in disease resistance and signalling in response to light. Characterisation of mutants disrupted in peroxisomal beta-oxidation has led to a growing appreciation of the importance of this pathway in the production of jasmonic acid, conversion of indole butyric acid to indole acetic acid and possibly in the production of other signalling molecules. Likewise the role of peroxisomes in the production and detoxification of reactive oxygen, and possibly reactive nitrogen species and changes in redox status, suggests considerable scope for peroxisomes to contribute to perception and response to a wide range of biotic and abiotic stresses. Whereas the peroxisome is the sole site of beta-oxidation in plants, the production and detoxification of ROS in many cell compartments makes the specific contribution of the peroxisome much more difficult to establish. However progress in identifying peroxisome specific isoforms of enzymes associated with ROS metabolism should allow a more definitive assessment of these contributions in the future.

The expression of different superoxide dismutase forms is cell-type dependent in olive (Olea europaea L.) leaves

Superoxide dismutase (SOD) is a key antioxidant enzyme present in prokaryotic and eukaryotic cells as a first line of defense against the accumulation of superoxide radicals. In olive leaves, the SOD enzymatic system was characterized and was found to be comprised of three isozymes, an Mn-SOD, an Fe-SOD and a CuZn-SOD. Transcript expression analysis of whole leaves showed that the three isozymes represented 82, 17 and 0.8% of the total SOD expressed, respectively. Using the combination of laser capture microdissection (LCM) and real-time quantitative reverse transcription-PCR (RT-PCR), the expression of these SOD isozymes was studied in different cell types of olive leaves, including spongy mesophyll, palisade mesophyll, xylem and phloem. In spongy mesophyll cells, the isozyme proportion was similar to that in whole leaves, but in the other cells the proportion of expressed SOD isozymes was different. In palisade mesophyll cells, Fe-SOD was the most abundant, followed by Mn-SOD and CuZn-SOD, but in phloem cells Mn-SOD was the most prominent isozyme, and Fe-SOD was present in trace amounts. In xylem cells, only the Mn-SOD was detected. On the other hand, the highest accumulation of superoxide radicals was localized in vascular tissue which was the tissue with the lowest level of SOD transcripts. These data show that in olive leaves, each SOD isozyme has a different gene expression depending on the cell type of the leaf.

Ascorbate peroxidase, a scavenger of hydrogen peroxide in glyoxysomal membranes

Efficient destruction of hydrogen peroxide (H(2)O(2)) in peroxisomes requires the action of an anti-oxidant defense system, which consists of low molecular weight anti-oxidant compounds, such as ascorbic acid, along with protective enzymes, such as catalase and ascorbate peroxidase (APX). We investigated the contribution of the ascorbate enzyme system to the consumptions of H(2)O(2) and NADH within glyoxysomes of germinating castor beans (Ricinus communis). We solubilized the glyoxysomal membrane APX (gmAPX) using octyl-glucoside and purified its activity by gel filtration. The activity was associated with a 34kDa protein, as determined by SDS-gel electrophoresis and Western blotting. The enzymatic properties of gmAPX were studied and this enzyme was found to utilize ascorbic acid as its most effective natural electron donor but it would also use pyrogallol and guaiacol at a smaller extent. Cyanide and azide drastically inhibited gmAPX, as well as certain thiol-modifying reagents and some metal chelators. The inhibition by cyanide and azide of the enzyme combined with its absorption spectra confirmed that it is a hemoprotein. The apparent K(m) value of the enzyme for ascorbic acid was 300 microM while the K(m) for H(2)O(2) was 60 microM. APX in the glyoxysomal membrane can work in cooperation with monodehydroascorbate reductase to oxidize NADH, regenerate ascorbate, detoxify H(2)O(2), and protect the integrity of glyoxysomal proteins and membranes.

Defense potentials to NaCl in a mangrove, Bruguiera parviflora: differential changes of isoforms of some antioxidative enzymes

In order to assess the role of the antioxidative defense system against salt treatment, the activities of some antioxidative enzymes and levels of antioxidants were monitored in a true mangrove, Bruguiera parviflora, subjected to varying levels of NaCl under hydroponic culture. In the leaves of B. parviflora, salt treatment preferentially enhanced the content of H2O2 as well as the activity of ascorbate peroxidase (APX), guaiacol peroxidase (GPX), glutathione reductase (GR), and superoxide dismutase (SOD), whereas it induced the decrease of total ascorbate and glutathione (GSH+GSSG) content as well as catalase (CAT) activity. Analysis of isoforms of antioxidative enzymes by native PAGE and activity staining revealed that leaves of B. parviflora had one isoform each of Mn-SOD and Cu/Zn-SOD and three isoforms of Fe-SOD. Expression of Mn-SOD and Fe-SOD-2 was preferentially elevated by NaCl. Similarly, out of the six isoforms of GPX, the GPX-1, 2, 3 and 6 were enhanced by salt treatment but the levels of GPX-4 and -5 changed minimally as compared to those of a control. Activity staining gel revealed only one prominent isoform of APX and two isoforms of GR (GR-1 and GR-2), all of these isoforms increased upon salt exposure. Four CAT-isoforms were identified, among which the prominent CAT-2 isoform level was maximally reduced, suggesting differential down regulation of CAT isoforms by NaCl. The concentrations of malondialdehyde (MDA), a product of lipid peroxidation, remained unchanged in leaves of the plant treated with different concentrations of NaCl. This suggests that plants are protected against activated oxygen species by the elevated levels of certain antioxidative enzymes, thus avoiding lipid peroxidation during salt exposure. The differential changes in the levels of the isoforms due to NaCl treatment may be useful as markers for recognizing salt tolerance in mangroves.

Mitochondrial and peroxisomal manganese superoxide dismutase: differential expression during leaf senescence

In pea (Pisum sativum L.) leaves, manganese superoxide dismutase (Mn-SOD) is mainly localized in mitochondria as well as in peroxisomes. In this work, the effect of leaf senescence on the peroxisomal and mitochondrial Mn-SOD was studied in detached leaves from pea plants which were incubated in the dark at 25 degrees C for 3-11 d. Northern blots hybridized with a cDNA encoding mitochondrial Mn-SOD from pea leaves and a mitochondrial Mn-SOD transit peptide-specific probe showed increased Mn-SOD transcript levels during leaf senescence, due in part to in creased mitochondrial Mn-SOD mRNA. Recombinantly-expressed mitochondrial Mn-SOD was used to raise polyclonal antibodies which cross-reacted with Mn-SOD in peroxisomes purified from pea leaves. Western blot assays of crude extracts with the antibodies to pea mitochondrial Mn-SOD showed that the levels of total Mn-SOD protein gradually increased with leaf senescence. By native PAGE, the total Mn-SOD activity of pea leaves increased with senescence. EM immunocytochemistry was used to distinguish mitochondrial and peroxisomal Mn-SOD in senescent leaves. Increased Mn-SOD labelling in mitochondria was intensified, whereas the immunogold labelling of peroxisomes did not change with senescence. Overall, these results show that mitochondrial and peroxisomal Mn-SOD expression is regulated differently. The expression of mitochondrial Mn-SOD is induced during the senescence of pea leaves, whereas peroxisomal Mn-SOD could be post-translationally activated. Previously described results showing decreased mitochondrial Mn-SOD activity and increased peroxisomal Mn-SOD activity may be reflective of post-translational events regulating enzymatic activity during leaf senescence.

Reactive oxygen species, antioxidant systems and nitric oxide in peroxisomes

Peroxisomes are subcellular organelles with an essentially oxidative type of metabolism. Like chloroplasts and mitochondria, plant peroxisomes also produce superoxide radicals (O2*(-)) and there are, at least, two sites of superoxide generation: one in the organelle matrix, the generating system being xanthine oxidase, and another site in the peroxisomal membranes dependent on NAD(P)H. In peroxisomal membranes, three integral polypeptides (PMPs) with molecular masses of 18, 29 and 32 kDa have been shown to generate radicals O2*(-). Besides catalase, several antioxidative systems have been demonstrated in plant peroxisomes, including different superoxide dismutases, the ascorbate-glutathione cycle, and three NADP-dependent dehydrogenases. A CuZn-SOD and two Mn-SODs have been purified and characterized from different types of peroxisomes. The four enzymes of the ascorbate-glutathione cycle (ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase) as well as the antioxidants glutathione and ascorbate have been found in plant peroxisomes. The recycling of NADPH from NADP(+) can be carried out in peroxisomes by three dehydrogenases: glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and isocitrate dehydrogenase. In the last decade, different experimental evidence has suggested the existence of cellular functions for peroxisomes related to reactive oxygen species (ROS), but the recent demonstration of the presence of nitric oxide synthase (NOS) in plant peroxisomes implies that these organelles could also have a function in plant cells as a source of signal molecules like nitric oxide (NO*), superoxide radicals, hydrogen peroxide, and possibly S-nitrosoglutathione (GSNO).

Molecular cloning, characterization and regulation by cadmium of a superoxide dismutase from the ectomycorrhizal fungus Paxillus involutus

The gene encoding a superoxide dismutase (PiSOD) was cloned by suppressive subtractive hybridization from cDNA library of the ectomycorrhizal fungus, Paxillus involutus, grown under cadmium-stress conditions. The encoded protein was presumed to be localized in the peroxisomes because it contained a C-terminal peroxisomal localization peptide (SKL) and lacked an N-terminal mitochondrial transit peptide. Complementation of an Escherichia coli SOD null strain that is unable to grow in the presence of paraquat or cadmium indicated that cloned Pisod encoded a functional superoxide dismutase. Sensitivity of PiSOD activity to H2O2 but not KCN, and sequence homologies to other SODs strongly suggest that it is a manganese-containing superoxide dismutase. Monitoring PiSOD transcript, immunoreactive polypeptide and superoxide dismutase activity following cadmium stress suggests that the principal level of control is post-translational. This is, to our knowledge, the first insight in the characterization of molecular events that take place in an ectomycorrhizal fungus during exposure to heavy metals.

Cell biology of peroxisomes and their characteristics in aquatic organisms

The general characteristics of peroxisomes in different organisms, including aquatic organisms such as fish, crustaceans, and mollusks, are reviewed, with special emphasis on different aspects of the organelle biogenesis and mechanistic aspects of peroxisome proliferation. Peroxisome proliferation and peroxisomal enzyme inductions elicited by xenobiotics or physiological conditions have become useful tools to study the mechanisms of peroxisome biogenesis. During peroxisome proliferation, the induction of peroxisomal proteins is heterogeneous, enzymes that show increased activity being involved in different aspects of lipid homeostasis. The process of peroxisome biogenesis is coordinately triggered by a whole array of structurally dissimilar compounds known as peroxisome proliferators, and investigating the effect of some of these compounds that commonly appear as pollutants in the environment on the peroxisomes of aquatic animals inhabiting marine and estuarine habitats seems interesting. It is also important to determine whether peroxisome proliferation in these animals is a phenomenon that might occur under normal physiological or season-related conditions and plays a metabolic or functional role. This would help set the basis for understanding the process of peroxisome biogenesis in aquatic animals.

Peroxisomal NADP-Dependent Isocitrate Dehydrogenase. Characterization and Activity Regulation during Natural Senescence

The peroxisomal localization and characterization of NADP-dependent isocitrate dehydrogenase (perICDH) in young and senescent pea (Pisum sativum) leaves was studied by subcellular fractionation, kinetic analysis, immunoblotting, and immunoelectron microscopy. The subunit molecular mass for perICDH determined by immunoblotting was 46 kD. By isoelectric focusing (IEF) of the peroxisomal matrix fraction, the NADP-ICDH activity was resolved into four isoforms, perICDH-1 to perICDH-4, with isoelectric points (pIs) of 6.0, 5.6, 5.4, and 5.2, respectively. The kinetic properties of the NADP-ICDH in peroxisomes from young and senescent pea leaves were analyzed. The maximum initial velocity was the same in peroxisomes from young and senescent leaves, while the Michaelis constant value in senescent leaf peroxisomes was 11-fold lower than in young leaf peroxisomes. The protein levels of NADP-ICDH in peroxisomes were not altered during senescence. The kinetic behavior of this enzyme suggests a possible fine control of enzymatic activity by modulation of its Michaelis constant during the natural senescence of pea leaves. After embedding, electron microscopy immunogold labeling of NADP-ICDH confirmed that this enzyme was localized in the peroxisomal matrix. Peroxisomal NADP-ICDH represents an alternative dehydrogenase in these cell organelles and may be the main system for the reduction of NADP to NADPH for its re-utilization in the peroxisomal metabolism.

Protein folding coupled to disulphide bond formation

Protein folding that is coupled to disulphide bond formation has many experimental advantages. In particular, the kinetic roles and importance of all the disulphide intermediates can be determined, usually unambiguously. This contrasts with other types of protein folding, where the roles of any intermediates detected are usually not established. Nevertheless, there is considerable confusion in the literature about even the best-characterized disulphide folding pathways. This article attempts to set the record straight.

Superoxide radical generation in peroxisomal membranes: evidence for the participation of the 18 kDa integral membrane polypeptide

Peroxisomes were isolated from pea (Pisum sativum L.) leaves and the peroxisomal membranes were purified by treatment with Na2CO3. The production of superoxide radicals (O2) induced by NADH was investigated in peroxisomal membranes from intact organelles incubated with proteases (pronase E and proteinase K). Under isoosmotic conditions, in the presence of pronase E, the production of O2-. radicals was inhibited by 80%. SDS-PAGE of peroxisomal membranes after protease treatment demonstrated a decrease in the 18-kDa PMP. This suggests that this polypeptide has a small fragment exposed to the cytosolic side of the peroxisomal membrane which is essential for O2-. production. The 18-kDa PMP was purified by preparative SDS-PAGE and in the reconstituted protein the NADH-driven production of O2-. radicals was investigated. The isolated polypeptide showed a high generation rate of superoxide (about 300 nmol O2-. x mg-1 protein x min-1) which was completely inhibited by 50 mM pyridine. The 18-kDa PMP was recognized by a polyclonal antibody against Cyt b5 from human erythrocytes. The presence of b-type cytochrome in peroxisomal membranes was demonstrated by difference spectroscopy. Results obtained show that in the NADH-dependent O2-. radical generating system of peroxisomal membranes, the 18-kDa integral membrane polypeptide, which appears to be Cyt b5, is clearly involved in superoxide radical production.

Immunocytochemical localization of copper,zinc superoxide dismutase in peroxisomes from watermelon (Citrullus vulgaris Schrad.) cotyledons

In previous works using cell fractionation methods we demonstrated the presence of a Cu,Zn-containing superoxide dismutase in peroxisomes from watermelon cotyledons. In this work, this intracellular localization was evaluated by using western blot and EM immunocytochemical analysis with a polyclonal antibody against peroxisomal Cu,Zn-SOD II from watermelon cotyledons. In crude extracts from 6-day old cotyledons, analysis by western blot showed two cross-reactivity bands which belonged to the isozymes Cu,Zn-SOD I and Cu,Zn-SOD II. In peroxisomes purified by sucrose density-gradient centrifugation only one cross-reactivity band was found in the peroxisomal matrix which corresponded to the isozyme Cu,Zn-SOD II. When SOD activity was assayed in purified peroxisomes two isozymes were detected, Cu,Zn-SOD II in the matrix, and a Mn-SOD in the membrane fraction which was removed by sodium carbonate washing. EM immunocytochemistry of Cu,Zn-SOD on sections of 6-day old cotyledons, showed that gold label was mainly localized over plastids and also in peroxisomes and the cytosol, whereas mitochondria did not label for Cu,Zn-SOD.

Purification and properties of cytosolic copper, zinc superoxide dismutase from watermelon (Citrullus vulgaris Schrad.) cotyledons

Cytosolic copperzinc-superoxide dismutase (CuZn-SOD I; EC 1.15.1.1) was purified to homogeneity from watermelon (Citrullus vulgaris Schrad.) cotyledons. The stepwise purification procedure consisted of acetone precipitation, batch anion-exchange chromatography, anion-exchange Fast Protein Liquid Chromatography, gel-filtration column chromatography, and affinity chromatography on concanavalin A-Sepharose. CuZn-SOD I was purified 310-fold with a yield of 12.6 micrograms enzyme per gram cotyledons, and had a specific activity of 3,450 units per milligram protein. The relative molecular mass for cytosolic CuZn-SOD was 34000, and it was composed by two equal subunits of 16.3 kDa. CuZn-SOD I did not contain neutral carbohydrates in its molecule, and its ultraviolet and visible absorption spectra showed two absorption maxima at 254 nm and 580 nm. Metal analysis showed that the enzyme contained 1 gram-atom Cu and 1 gram-atom Zn per mole dimer. Cytosolic CuZn-SOD was recognized by the antibody against peroxisomal CuZn-SOD from watermelon cotyledons, and its enzymatic activity was inhibited by this antibody. By IEF (pH 4.2-4.9), using a new method for vertical slab gels set up in our laboratory, purified cytosolic CuZn-SOD was resolved into two equal isoforms with isoelectric point of 4.63 and 4.66.

Reactions of bovine liver catalase with superoxide radicals and hydrogen peroxide

The oxidized intermediates generated upon exposure of bovine liver catalase to hydrogen peroxide (H2O2) and superoxide radical (O2-) fluxes were examined with UV-visible spectrophotometry. H2O2 and O2- were generated by means of glucose/glucose oxidase and xanthine/xanthine oxidase systems. Serial overlay of absorption spectra in the Soret (350-450 nm) and visible (450-700 nm) regions showed that three oxidized intermediates, namely Compounds I, II and III, can be observed upon exposure of catalase to enzymatically generated H2O2 and O2-. Compound I is formed during the reaction of native enzyme with H2O2 and disappears in two ways: (i) via the catalytic reaction with H2O2 to restore native catalase and (ii) via the reaction with O2- to form Compound II. At low H2O2 concentrations (< 4.8 x 10(-9) M H2O2), Compound II reverts towards the native state mainly in a direct one-step reaction, whereas at higher H2O2 concentrations the pathway of Compound II back to the native enzyme involves Compound III. Formation of the latter from Compound II and H2O2 is irreversible and the rate constant of this reaction is 6.1 +/- 0.2 x 10(4) M-1 s-1. The formation of Compound III through the direct reaction of O2- with native enzyme has also been observed. Depending on the experimental conditions, the inactivation of catalase by O2- can be due to accumulation of Compound II ("slow" inhibition) or to the formation of Compound III ("rapid" inhibition) part of which leads to a dead end product. Formation of Compound III and of this dead end product are responsible for the irreversible inactivation in presence of an excess of H2O2.

Metabolism of oxygen radicals in peroxisomes and cellular implications

Peroxisomes are subcellular respiratory organelles which contain catalase and H2O2-producing flavin oxidases as basic enzymatic constituents. These organelles have an essentially oxidative type of metabolism and have the potential to carry out different important metabolic pathways. In recent years the presence of different types of superoxide dismutase (SOD) have been demonstrated in peroxisomes from several plant species, and more recently the occurrence of SOD has been extended to peroxisomes from human and transformed yeast cells. A copper,zinc-containing SOD from plant peroxisomes has been purified and partially characterized. The production of hydroxyl and superoxide radicals has been studied in peroxisomes. There are two sites of O2- production in peroxisomes: (1) in the matrix, the generating system being xanthine oxidase; and (2) in peroxisomal membranes, dependent on reduced nicotinamide adenine dinucleotide (NADH), and the electron transport components of the peroxisomal membrane are possibly responsible. The generation of oxygen radicals in peroxisomes could have important effects on cellular metabolism. Diverse cellular implications of oxyradical metabolism in peroxisomes are discussed in relation to phenomena such as cell injury, peroxisomal genetic diseases, peroxisome proliferation and oxidative stress, metal and salt stress, catabolism of nucleic acids, senescence, and plant pathogenic processes.

Peroxisome proliferation and oxidative stress mediated by activated oxygen species in plant peroxisomes

The existence of a relationship between clofibrate-induced peroxisome proliferation and oxidative stress mediated by activated oxygen species was studied in intact peroxisomes purified from Pisum sativum L. plants. Incubation of leaves with 1 mM clofibrate produced a remarkable increase in the peroxisomal activity of acyl-CoA oxidase and, to a lesser extent, of xanthine oxidase, whereas there was a nearly complete loss of catalase activity and a decrease in Mn-superoxide dismutase. Ultrastructural studies of intact leaves showed that clofibrate induced a five- and twofold proliferation of the peroxisomal and mitochondrial populations, respectively, in comparison with those in control leaves. Prolonged incubation with clofibrate produced considerable alterations in the ultrastructure of cells. In peroxisomal membranes, the NADH-induced generation of O2- radicals, as well as the lipid peroxidation of membranes, increased as a result of treatment of plants with clofibrate. In intact peroxisomes treated with this hypolipidemic drug, the H2O2 concentration was higher than in peroxisomes from control plants. These results demonstrate that clofibrate stimulates the production of activated oxygen species (O2- and H2O2) inside peroxisomes, as well as the lipid peroxidation of peroxisomal membranes. This effect is concomitant with a decrease of catalase and Mn-SOD activities, the main peroxisomal enzymatic defenses against H2O2 and O2-, and indicates that in the toxicity of clofibrate, at the level of peroxisomes, an oxidative stress mechanism mediated by activated oxygen species is involved.

A new cellular function for peroxisomes related to oxygen free radicals?

Although in cell biology peroxisomes are still 'young' organelles, it is becoming increasingly clear that they are involved in important cellular functions. Recent results have indicated the presence of the metalloenzyme superoxide dismutase in peroxisomes and the production of superoxide free radicals (O2-) in these oxidative organelles. These findings, together with other experimental evidence, point towards the existence of new roles for peroxisomes in cellular active oxygen metabolism, something that has a potential impact in multiple areas of cell biology, particularly in biochemistry and biomedicine.

Phylogenetic distribution of superoxide dismutase supports an endosymbiotic origin for chloroplasts and mitochondria

Three isozymes of superoxide dismutase (SOD) have been identified and characterized. The iron and manganese isozymes (Fe-SOD and Mn-SOD, respectively) show extensive primary sequence and structural homology, suggesting a common evolutionary ancestor. In contrast, the copper/zinc isozyme (CuZn-SOD) shows no homology with Fe-SOD or Mn-SOD, suggesting an independent origin for this enzyme. The three isozymes are unequally distributed throughout the biological kingdoms and are located in different subcellular compartments. Obligate anaerobes and aerobic diazotrophs contain Fe-SOD exclusively. Facultative aerobes contain either Fe-SOD or Mn-SOD or both. Fe-SOD is found in the cytosol of cyanobacteria while the thylakoid membranes of these organisms contain a tightly bound Mn-SOD. Similarly, most eukaryotic algae contain Fe-SOD in the chloroplast stroma and Mn-SOD bound to the thylakoids. Most higher plants contain a cytosol-specific and a chloroplast-specific CuZn-SOD, and possibly a thylakoid-bound Mn-SOD as well. Plants also contain Mn-SOD in their mitochondria. Likewise, animals and fungi contain a cytosolic CuZn-SOD and a mitochondrial Mn-SOD. The Mn-SOD found in the mitochondria of eukaryotes shows strong homology to the prokaryotic form of the enzyme. Taken together, the phylogenetic distribution and subcellular localization of the SOD isozymes provide strong support for the hypothesis that the chloroplasts and mitochondria of eukaryotic cells arose from prokaryotic endosymbionts.

Aspects of the structure, function, and applications of superoxide dismutase

The current status of superoxide dismutase (SOD) is that it is an enzyme with diverse ramifications. This review attempts an understanding of SOD as a structural, functional, and biological entity. Accordingly, the review is in three parts. The first part discusses SOD in terms of protein structure, proceeding from primary to secondary and three-dimensional structure for the three forms of SOD: copper/zinc SOD, manganese SOD, and iron SOD. This is the order of structural knowledge of the enzyme. Iron SOD is an enzyme of prokaryotes and some higher plants. Manganese SOD is an enzyme of prokaryotes and eukaryotes. Copper/zinc SOD is an enzyme of eukaryotes and certain prokaryotes. The evolutionary relationships of the three forms of SOD, the status of the copper/zinc SOD gene in prokaryotes, and the cloning and sequencing of SOD genes are discussed. The second part of the review deals with the catalytic mechanism of SOD in the three forms of the enzyme. Structural and mechanistic conclusions from various spectroscopic studies are critically considered. A detailed picture is given of the active site of copper/zinc SOD. The third part is a review of SOD in the general context of oxygen toxicity. After consideration of the question of superoxide toxicity and superoxide pathology, several areas in which SOD has been investigated or used as a tool in a biochemical, pharmacological, or clinical context are discussed, including population genetics; trisomy 21; development and senescence; the nutritional copper, zinc, and manganese status; hemolysis and anemia; oxygen toxicity in the lung and nervous system; inflammation, autoimmune disease and chromosome breakage, ischemia and degenerative changes; radiation damage; and malignancy. A comprehensive picture is given of measurements of SOD activity in disease states, and the question of superoxide-related disease is considered at several points.

Isolation and characterization of an iron-containing superoxide dismutase from tomato leaves, Lycopersicon esculentum

A cyanide-insensitive superoxide dismutase was purified from tomato leaves (Lycopersicon esculentum, Mill., var. Venture) to apparent homogeneity. The enzyme had twofold higher specific activity (about 4000 standard units) than ferric superoxide dismutases purified from Brassica campestris [Salin, M. L. and Bridges, S. M. (1980) Arch. Biochem. Biophys. 201, 369-374] and Nuphar luteum [Salin, M.L. and Bridges, S. M. (1982) Plant Physiol. 69, 161-165]. The protein had a relative molecular mass of about 42000 and was composed of two equal subunits noncovalently joined. It was negatively charged (pI = 4.6) and contained about 1.45 mol Fe/mol dimer and negligible amounts of Mn, Cu and Zn. Absorption spectrum and sensitivity to NaN3, H2O2 and temperature are also reminiscent of other ferric superoxide dismutases. Comparison of amino acid composition indicated, however, a closer relationship to the Mn-containing enzymes rather than to other Fe-containing superoxide dismutases. Two possible ways of Fe-containing superoxide dismutase acquisition by vascular plants were suggested.