Plant peroxisomes,reactive oxygen metabolism and nitric oxide

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. Like mitochondria and chloroplasts, 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 O2*- radicals. Besides catalase, several antioxidative systems have been demonstrated in plant peroxisomes, including different superoxide dismutases, the four enzymes of the ascorbate-glutathione cycle plus ascorbate and glutathione, and three NADP-dependent dehydrogenases. A CuZn-SOD and two Mn-SODs have been purified and characterized from different types of plant peroxisomes. The presence of the enzyme nitric oxide synthase (NOS) and its reaction product, nitric oxide (NO*), has been recently demonstrated in plant peroxisomes. Different experimental evidence has suggested that peroxisomes have a ROS-mediated cellular function in leaf senescence and in stress situations induced by xenobiotics and heavy metals. Peroxisomes could also have a role in plant cells as a source of signal molecules like NO*, O2*- radicals, H2O2, and possibly S-nitrosoglutathione (GSNO). It seems reasonable to think that a signal molecule-producing function similar to that postulated for plant peroxisomes could also be performed by human, animal and yeast peroxisomes, where research on oxy radicals, antioxidants and nitric oxide is less advanced than in plant peroxisomes.     

Evidence for the Presence of the Ascorbate-Glutathione Cycle in Mitochondria and Peroxisomes of Pea Leaves

The presence of the enzymes of the ascorbate-glutathione cycle was investigated in mitochondria and peroxisomes purified from pea (Pisum sativum L.) leaves. All four enzymes, ascorbate peroxidase (APX; EC 1.11.1.11), monodehydroascorbate reductase (EC 1.6.5.4), dehydroascorbate reductase (EC 1.8.5.1), and glutathione reductase (EC 1.6.4.2), were present in mitochondria and peroxisomes, as well as in the antioxidants ascorbate and glutathione. The activity of the ascorbate-glutathione cycle enzymes was higher in mitochondria than in peroxisomes, except for APX, which was more active in peroxisomes than in mitochondria. Intact mitochondria and peroxisomes had no latent APX activity, and this remained in the membrane fraction after solubilization assays with 0.2 M KCl. Monodehydroascorbate reductase was highly latent in intact mitochondria and peroxisomes and was membrane-bound, suggesting that the electron acceptor and donor sites of this redox protein are not on the external side of the mitochondrial and peroxisomal membranes. Dehydroascorbate reductase was found mainly in the soluble peroxisomal and mitochondrial fractions. Glutathione reductase had a high latency in mitochondria and peroxisomes and was present in the soluble fractions of both organelles. In intact peroxisomes and mitochondria, the presence of reduced ascorbate and glutathione and the oxidized forms of ascorbate and glutathione were demonstrated by high-performance liquid chromatography analysis. The ascorbate-glutathione cycle of mitochondria and peroxisomes could represent an important antioxidant protection system against H2O2 generated in both plant organelles.     

Activated oxygen‐mediated metabolic functions of leaf peroxisomes

Peroxisomes are subcellular organelles with an essentially oxidative type of metabolism. The presence in these organelles of superoxide dismutases and the generation of superoxide radicals (O₂^??) was first demonstrated in plant tissues and in recent years different experimental evidence has suggested the existence of cellular functions related to activated oxygen species. Some of these functions are analyzed in this work. In purified intact peroxisomes from pea (Pisum sativum L.) leaves, xanthine oxidase and urate oxidase were found to be present. The occurrence and the level of the metabolites xanthine, hypoxanthine, uric acid, and allantoin were studied in extracts of pea leaf peroxisomes by HPLC. Xanthine, uric acid, and allantoin were detected in peroxisomes. These results suggest a cellular role for leaf peroxisomes in the catabolism of purines. In peroxisomal membranes, 3 polypeptides (PMPs) with molecular masses of 18, 29 and 32 kDa, respectively, have been shown to generate superoxide radicals. These PMPs were purified from pea leaf peroxisomal membranes and characterized. While the 18- and 32-kDa PMPs use NADH as electron donor for O₂^?? production, the 29-kDa PMP was clearly dependent on NADPH. Very recently, the occurrence in pea leaf peroxisomes of all the enzymes of the ascorbate-glutathione cycle has been demonstrated. NADPH is required for the glutathione reductase activity of the cycle and this implies the reduction of NADP⁺ to NADPH. This recycling function could be carried out by the NADP-dependent glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), and isocitrate dehydrogenase (ICDH). These 3 dehydrogenases have been demonstrated to be present in the matrix of pea leaf peroxisomes. The catabolism of purines, the superoxide-generating PMPs, the ascorbate-glutathione cycle, and the dehydrogenase-mediated recycling of NADPH, are activated oxygen roles of leaf peroxisomes that add to other functions previously known for peroxisomes from eukaryotic cells.     

Antioxidative enzymes from chloroplasts, mitochondria, and peroxisomes during leaf senescence of nodulated pea plants

In this work the influence of the nodulation of pea (Pisum sativum L.) plants on the oxidative metabolism of different leaf organelles from young and senescent plants was studied. Chloroplasts, mitochondria, and peroxisomes were purified from leaves of nitrate-fed and Rhizobium leguminosarum-nodulated pea plants at two developmental stages (young and senescent plants). In these cell organelles, the activity of the ascorbate-glutathione cycle enzymes ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR), and the ascorbate and glutathione contents were determined. In addition, the total superoxide dismutase (SOD) activity, the pattern of mitochondrial and peroxisomal NADPH-generating dehydrogenases, some of the peroxisomal photorespiratory enzymes, the glyoxylate cycle and oxidative metabolism enzymes were also analysed in these organelles. Results obtained on the metabolism of cell organelles indicate that nodulation with Rhizobium accelerates senescence in pea leaves. A considerable decrease of the ascorbate content of chloroplasts, mitochondria, and peroxisomes was found, and in these conditions a metabolic conversion of leaf peroxisomes into glyoxysomes, characteristic of leaf senescence, took place.     

Proteome of plant peroxisomes: new perspectives on the role of these organelles in cell biology

Peroxisomes are cell organelles bounded by a single membrane with a basically oxidative metabolism. Peroxisomes house catalase and H₂O₂‐producing flavin‐oxidases as the main protein constituents. However, since their discovery in early fifties, a number of new enzymes and metabolic pathways have been reported to be also confined to these organelles. Thus, the presence of exo‐ and endo‐peptidases, superoxide dismutases, the enzymes of the plant ascorbate‐glutathione cycle plus ascorbate and glutathione, several NADP‐dehydrogenases, and also L‐arginine‐dependent nitric oxide synthase activity has evidenced the relevant role of these organelles in cell physiology. In recent years, the study of new functions of peroxisomes has become a field of intensive research in cell biology, and these organelles have been proposed to be a source of important signal molecules for different transduction pathways. In plants, peroxisomes participate in seed germination, leaf senescence, fruit maturation, response to abiotic and biotic stress, photomorphogenesis, biosynthesis of the plant hormones jasmonic acid and auxin, and in cell signaling by reactive oxygen and nitrogen species (ROS and RNS, respectively). In order to decipher the nature and specific role of the peroxisomal proteins in these processes, several approaches including in vivo and in vitro import assays and generation of mutants have been used. In the last decade, the development of genomics and the report of the first plant genomes provided plant biologists a powerful tool to assign to peroxisomes those proteins which harbored any of the two peroxisomal targeting signals (PTS, either PTS1 or PTS2) described so far. Unfortunately, those molecular approaches could not give any response to those proteins previously localized in plant peroxisomes by classical biochemical and cell biology methods that did not contain any PTS. However, more recently, proteomic studies of highly purified organelles have provided evidence of the presence in peroxisomes of new proteins not previously reported. Thus, the contribution of proteomic approaches to the biology of peroxisomes is essential, not only for elucidation of the mechanisms involved in the import of the PTS1‐ and PTS2‐independent proteins, but also to the understanding of the role of these organelles in the cell physiology of plant growth and development.     

Fungal pathogen-induced changes in the antioxidant systems of leaf peroxisomes from infected tomato plants

Peroxisomes, being one of the main organelles where reactive oxygen species (ROS) are both generated and detoxified, have been suggested to be instrumental in redox-mediated plant cell defence against oxidative stress. We studied the involvement of tomato (Lycopersicon esculentum Mill.) leaf peroxisomes in defence response to oxidative stress generated upon Botrytis cinerea Pers. infection. The peroxisomal antioxidant potential expressed as superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6) and glutathione peroxidase (GSH-Px, EC 1.11.1.19) as well as the ascorbate-glutathione (AA-GSH) cycle activities was monitored. The initial infection-induced increase in SOD, CAT and GSH-Px indicating antioxidant defence activation was followed by a progressive inhibition concomitant with disease symptom development. Likewise, the activities of AA-GSH cycle enzymes: ascorbate peroxidase (APX, EC 1.11.1.11), monodehydroascorbate reductase (MDHAR, EC 1.6.5.4), dehydroascorbate reductase (DHAR, EC 1.8.5.1) and glutathione reductase (GR, EC 1.6.4.2) as well as ascorbate and glutathione concentrations and redox ratios were significantly decreased. However, the rate and timing of these events differed. Our results indicate that B. cinerea triggers significant changes in the peroxisomal antioxidant system leading to a collapse of the protective mechanism at advanced stage of infection. These changes appear to be partly the effect of pathogen-promoted leaf senescence.     

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.     

Role of the Ascorbate-Glutathione Cycle of Mitochondria and Peroxisomes in the Senescence of Pea Leaves

We investigated the relationship between H₂O₂ metabolism and the senescence process using soluble fractions, mitochondria, and peroxisomes from senescent pea (Pisum sativum L.) leaves. After 11 d of senescence the activities of Mn-superoxide dismutase, dehydroascorbate reductase (DHAR), and glutathione reductase (GR) present in the matrix, and ascorbate peroxidase (APX) and monodehydroascorbate reductase (MDHAR) activities localized in the mitochondrial membrane, were all substantially decreased in mitochondria. The mitochondrial ascorbate and dehydroascorbate pools were reduced, whereas the oxidized glutathione levels were maintained. In senescent leaves the H₂O₂ content in isolated mitochondria and the NADH- and succinate-dependent production of superoxide (O₂^·−) radicals by submitochondrial particles increased significantly. However, in peroxisomes from senescent leaves both membrane-bound APX and MDHAR activities were reduced. In the matrix the DHAR activity was enhanced and the GR activity remained unchanged. As a result of senescence, the reduced and the oxidized glutathione pools were considerably increased in peroxisomes. A large increase in the glutathione pool and DHAR activity were also found in soluble fractions of senescent pea leaves, together with a decrease in GR, APX, and MDHAR activities. The differential response to senescence of the mitochondrial and peroxisomal ascorbate-glutathione cycle suggests that mitochondria could be affected by oxidative damage earlier than peroxisomes, which may participate in the cellular oxidative mechanism of leaf senescence longer than mitochondria.     

Peroxisomes as a source of reactive oxygen species and nitric oxide signal molecules in plant cells

The important role of plant peroxisomes in a variety of metabolic reactions such as photorespiration, fatty acid beta-oxidation, the glyoxylate cycle and generation-degradation of hydrogen peroxide is well known. In recent years, the presence of a novel group of enzymes, mainly involved in the metabolism of oxygen free-radicals, has been shown in peroxisomes. In addition to hydrogen peroxide, peroxisomes can generate superoxide-radicals and nitric oxide, which are known cellular messengers with a variety of physiological roles in intra- and inter-cellular communication. Nitric oxide and hydrogen peroxide can permeate the peroxisomal membrane and superoxide radicals can be produced on the cytosolic side of the membrane. The signal molecule-generating capacity of peroxisomes can have important implications for cellular metabolism in plants, particularly under biotic and abiotic stress.     

Activities of SOD and the ascorbate-glutathione cycle enzymes in subcellular compartments in leaves and roots of the cultivated tomato and its wild salt-tolerant relative Lycopersicon pennellii

The activities of the ascorbate-glutathione cycle enzymes ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) and SOD were studied in cell organelles of the cultivated tomato Lycopersicon esculentum (M82) and its wild salt-tolerant related species Lycopersicon pennellii (Lpa). All four enzymes of the ascorbate-glutathione cycle were present in chloroplasts/plastids, mitochondria and peroxisomes of leaf and root cells of both tomato species. In all leaf and root organelles of both species, the activity of MDHAR was similar to, or higher than, that of APX, while the activity of DHAR was one order of magnitude lower than that of MDHAR. Based on these results, it is suggested that in the organelles of both tomato species, ascorbate is regenerated mainly by MDHAR. In both tomato species, GR activity, and to a lesser extent DHAR activity, was found to reside in the soluble fraction of all leaf and root cell organelles, while APX and MDHAR activities were distributed between the membrane and soluble fractions. A higher SOD to APX activity ratio in all Lpa organelles was the major difference between the two tomato species. It is possible that this higher ratio contributes to the inherently better protection of Lpa from salt stress, as was previously reported.     

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.     

Subcellular Localization of Oxygen Defense Enzymes in Soybean (Glycine max [L.] Merr.) Root Nodules

Soybean (Glycine max [L.] Merr.) root nodules contain the enzymes of the ascorbate-glutathione pathway to minimize oxidative damage. In the present study, fractionation and immunocytochemistry were used to determine the subcellular location of the enzymes of this pathway. All four enzymes (ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase) were present in the soluble fraction from nodule plant cells and in isolated mitochondria. No activity was detected in peroxisomes. Bacteroids contained glutathione reductase but not the other enzymes of this pathway. Immunogold localization indicated that ascorbate peroxidase was present in the cytosol of infected and uninfected cells but not in the peribacteroid space. Results of immunogold and immunofluorescence studies indicated that monodehydroascorbate reductase was located primarily in the cell wall, suggesting that ascorbate regeneration in the cytoplasm may proceed primarily through the action of dehydroascorbate reductase. The possible roles of monodehydroascorbate reductase in cell wall metabolism are discussed.     

Peroxisomes as a cellular source of reactive nitrogen species signal molecules

Peroxisomes are single membrane-bounded subcellular organelles with an essentially oxidative type of metabolism and are probably the major sites of intracellular H₂O₂ production. These organelles also generate superoxide radicals () and besides catalase they have a complex battery of antioxidative enzymes. In recent years the existence of l-arginine-dependent nitric oxide synthase (NOS) activity and the generation of the reactive nitrogen species (RNS) nitric oxide (NO) have been demonstrated in plant peroxisomes. The inter-cellular and intracellular NO carrier S-nitrosoglutathione (GSNO) can be generated inside peroxisomes and the presence of this RNS has been demonstrated in peroxisomes from several plant species. This review analyzes the available evidence concerning the properties of the NOS activity and the generation of the RNS messengers NO and GSNO in peroxisomes in the context of the cellular function of these organelles as a source of RNS signaling molecules. The important physiological functions displayed by NO and other RNS in intra- and inter-cellular communication in different organisms indicate that more attention should be payed to the RNS signaling function of peroxisomes in human, animal and fungal cells, where it is very likely that similar mechanisms to those found in plant peroxisomes are also operative.     

Role of peroxisomes in the oxidative injury induced by 2,4-dichlorophenoxyacetic acid in leaves of pea plants

The role of peroxisomes in the oxidative injury induced by the auxin herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) in leaves of pea (Pisum sativum L.) plants was studied. Applications of (2,4-D) on leaves or to root substrate increased the superoxide radical production in leaf peroxisomes. Foliar application also increased H₂O₂ contents in leaf peroxisomes. Reactive oxygen species (ROS) overproduction was accompanied by oxidative stress, as shown by the changes in lipid peroxidation, protein carbonyls, total and protein thiols, and by the up-regulation of the activities of superoxide dismutase, ascorbate peroxidase, glutathione reductase, catalase, glucose 6-phosphate dehydrogenase and NADP⁺-dependent isocitrate dehydrogenase. Foliar or root 2,4-D applications also induced senescence symptoms in pea leaf peroxisomes, as shown by the decrease of protein content and glycolate oxidase and hydroxypyruvate reductase activities, and by the increase of endopeptidase, xanthine oxidase, isocitrate lyase and acyl-CoA oxidase activities as well as of 3-ketoacyl-CoA thiolase and thiol-protease protein contents. 2,4-D did not induce proliferation of pea leaf peroxisomes but induced senescence-like morphological changes in these organelles. Results suggest that peroxisomes might contribute to 2,4-D toxicity in pea leaves by overproducing cell-damaging ROS and by participating actively in 2,4-D-induced leaf senescence.     

Peroxisomes from pepper fruits (Capsicum annuum L.): purification, characterisation and antioxidant activity

Pepper is a vegetable of importance in human nutrition. Currently, one of the most interesting properties of natural products is their antioxidant content. In this work, the purification and characterisation of peroxisomes from fruits of a higher plant was carried out, and their antioxidative enzymatic and non-enzymatic content was investigated. Green and red pepper fruits (Capsicum annuum L., type Lamuyo) were used in this study. The analysis by electron microscopy showed that peroxisomes from both types of fruits contained crystalline cores which varied in shape and size, and the presence of chloroplasts and chromoplasts in green and red pepper fruits, respectively, was confirmed.

Peroxisomes were purified by differential and sucrose density-gradient centrifugations. In the peroxisomal fractions, the activity of the photorespiration, β-oxidation and glyoxylate cycle enzymes, and the ROS-related enzymes catalase, superoxide dismutase, xanthine oxidase, glutathione reductase and NADP⁺-dehydrogenases, was determined. Most enzymes studied had higher specific activity and protein content in green than in red fruits. By native PAGE and western blot analysis, the localisation of a Mn-SOD in fruit peroxisomes was demonstrated. The ascorbate and glutathione levels were also determined in crude extracts and in peroxisomes purified from both green and red peppers. The total ascorbate content (200-220 mg per 100 g FW) was similar in crude extracts from the two types of fruits, but higher in peroxisomes from red peppers. The glutathione concentration was 2-fold greater in green pepper crude extracts than in red fruits, whereas peroxisomes from both tissues showed similar values. The presence in pepper peroxisomes of different antioxidative enzymes and their corresponding metabolites implies that these organelles might be an important pool of antioxidants in fruit cells, where these enzymes could also act as modulators of signal molecules (O₂^˙-, H₂O₂) during fruit maturation.     

The effect of Botrytis cinerea infection on the antioxidant profile of mitochondria from tomato leaves

Infection of tomato leaves with the necrotrophic fungus Botrytis cinerea resulted in substantial changes in enzymatic and non-enzymatic components of the ascorbate-glutathione cycle as well as in superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), glutathione transferase (GST), and l-galactono-gamma-lactone dehydrogenase (GLDH) activities. In the initial phase of the 5 d experiment CuZn SOD was the most rapidly induced isoform (up to 209% of control), whereas later on its activity increase was not concomitant with the constant total SOD enhancement. Starting from the second day B. cinerea infection diminished the mitochondrial antioxidant capacity by decreasing activities of ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) as well as declining ascorbate and glutathione contents. This was accompanied by dehydroascorbate (DHA) and oxidized glutathione (GSSG) accumulation that resulted in ascorbate and glutathione redox ratios decreases. The strongest redox ratio decline of 29% for ascorbate and of 34% for glutathione was found on the 3rd and 2nd days, respectively. Glutathione reductase (GR) induction (185% of control 2 d after inoculation) was insufficient to overcome the decreased antioxidant potential of glutathione. Changes in the ascorbate pool size were closely related to the activity of l-galactono-gamma-lactone dehydrogenase (GLDH). The activities of two glutathione-dependent enzymes: GSH-Px and GST were increased from day 1 to day 4. These results demonstrated that in B. cinerea-tomato interaction mitochondria could be one of the main targets for infection-induced oxidative stress.     

Dehydrogenases of the pentose phosphate pathway in rat liver peroxisomes

Subcellular distribution of pentose-phosphate cycle enzymes in rat liver was investigated, using differential and isopycnic centrifugation. The activities of the NADP+-dependent dehydrogenases of the pentose-phosphate pathway (glucose-6-phosphate dehydrogenase and phosphogluconate dehydrogenase) were detected in the purified peroxisomal fraction as well as in the cytosol. Both dehydrogenases were localized in the peroxisomal matrix. Chronic administration of the hypolipidemic drug clofibrate (ethyl-alpha-p-chlorophenoxyisobutyrate) caused a 1.5-2.5-fold increase in the amount of glucose-6-phosphate and phosphogluconate dehydrogenases in the purified peroxisomes. Clofibrate decreased the phosphogluconate dehydrogenase, but did not alter glucose-6-phosphate dehydrogenase activity in the cytosolic fraction. The results obtained indicate that the enzymes of the non-oxidative segment of the pentose cycle (transketolase, transaldolase, triosephosphate isomerase and glucose-phosphate isomerase) are present only in a soluble form in the cytosol, but not in the peroxisomes or other particles, and that ionogenic interaction of the enzymes with the mitochondrial and other membranes takes place during homogenization of the tissue in 0.25 M sucrose. Similar to catalase, glucose-6-phosphate dehydrogenase and phosphogluconate dehydrogenase are present in the intact peroxisomes in a latent form. The enzymes have Km values for their substrates in the millimolar range (0.2 mM for glucose-6-phosphate and 0.10-0.12 mM for 6-phosphogluconate). NADP+, but not NAD+, serves as a coenzyme for both enzymes. Glucose-6-phosphate dehydrogenase was inhibited by palmitoyl-CoA, and to a lesser extent by NADPH. Peroxisomal glucose-6-phosphate and phosphogluconate dehydrogenases have molecular mass of 280 kDa and 96 kDa, respectively. The putative functional role of pentose-phosphate cycle dehydrogenases in rat liver peroxisomes is discussed.     

The oxidant/antioxidant network: role of melatonin

Melatonin is now known to be a multifaceted free radical scavenger and antioxidant. It detoxifies a variety of free radicals and reactive oxygen intermediates including the hydroxyl radical, peroxynitrite anion, singlet oxygen and nitric oxide. Additionally, it reportedly stimulates several antioxidative enzymes including glutathione peroxidase, glutathione reductase, glucose-6-phosphate dehydrogenase and superoxide dismutase; conversely, it inhibits a prooxidative enzyme, nitric oxide synthase. Melatonin also crosses all morphophysiological barriers, e.g., the blood-brain barrier, placenta, and distributes throughout the cell; these features increase the efficacy of melatonin as an antioxidant. Melatonin has been shown to markedly protect both membrane lipids and nuclear DNA from oxidative damage. In every experimental model in which melatonin has been tested, it has been found to resist macromolecular damage and the associated dysfunction associated with free radicals.     

Activation of peroxisomal acyl-CoA oxidase and lipid peroxidation in the rat myocardium as affected by prolonged administration of ethanol

The influence of chronic alcoholic intoxication on the activity of peroxisomal acyl-CoA oxidase and antioxidative defensive enzymes (catalase, glutathione reductase, glutathione-S-transferase, superoxide dismutase, glucose-6-phosphate dehydrogenase) was studied in the rat myocardium. The parameters of lipid peroxidation in cardiomyocytes (the level of spontaneous chemiluminescence, accumulation of thiobarbituric acid-reactive material), as well as reduced glutathione content were also examined. The data obtained suggest that ethanol-induced activation of lipid peroxidation in the myocardium may be due to the elevation of hydrogen peroxide-generating activity of peroxisomes.