Metabolic response to treatment with cold, paraquat, or 3-amino-1,2,4-triazole in leaves of winter wheat.

We treated leaves of winter wheat (Triticum aestivum L.) with cold, paraquat, or 3-amino-1,2,4-triazole and compared the responses. We assayed the activities of glucose-6-phosphate dehydrogenase, catalase, dehydroascorbate reductase and ascorbate free radical reductase and levels of hydrogen peroxide, glucose-6-phosphate, fructose-6-phosphate, ascorbate, dehydroascorbate, reduced and oxidized glutathione. With any of the three treatments, contents of cellular peroxides and hexose phosphates were raised. The content of ascorbate was lowered markedly by paraquat treatment, which produces active oxygen species, whereas such a decrease did not occur in other two treatments. When the plants were treated with 3-amino-1,2,4-triazole, which is a specific inhibitor of catalase, the content of oxidized glutathione increased severalfold. The glucose-6-phosphate dehydrogenase activity increased with all three treatments, but it decreased after glyphosate treatment, which does not stimulate the formation of peroxides. The activities of catalase and dehydroascorbate reductase were increased by the treatment of cold and paraquat, while 3-amino-1,2,4-triazole did not affect the dehydroascorbate reductase activity. The activity of ascorbate free radical reductase increased after treatment by paraquat only.     

Antioxidant defences of the apoplast

Summary The apoplast of barley and oat leaves contained superoxide dismutase (SOD), catalase, ascorbate peroxidase, dehydroascorbate reductase, monodehydroascorbate reductase, and glutathione reductase activities. The activities of these enzymes in the apoplastic extracts were greatly modified 24 h after inoculation with the biotrophic fungal pathogenBlumeria graminis. The quantum efficiency of photosystem II, which is related to photosynthetic electron transport flux, was comparable in inoculated and healthy leaves during this period. Apoplastic soluble acid invertase activity was also modified in inoculated leaves. Inoculation-dependent increases in apoplastic SOD activity were observed in all lines. Major bands of SOD activity, observed in apoplastic protein extracts by activity staining of gels following isoelectric focusing, were similar to those observed in whole leaves but two additional minor bands were found in the apoplastic fraction. The apoplastic extracts contained substantial amounts of dehydroascorbate (DHA) but little or no glutathione (GSH). Biotic stress decreased apoplastic ascorbate and DHA but increased apoplastic GSH in resistant lines. The antioxidant cycle enzymes may function to remove apoplastic H₂O₂ with ascorbate and GSH derived from the cytoplasm. DHA and oxidized glutathione may be reduced in the apoplast or returned to the cytosol for rereduction.Abbreviations AA reduced ascorbate - APX ascorbate peroxidase - DHA dehydroascorbate (oxidised ascorbate) - DHAR dehydroascorbate reductase - G6PDH glucose-6-phosphate dehydrogenase - GSH reduced glutathione - GSSG glutathione disulphide - GR glutathione reductase - MDHA monodehydroascorbate - MDHAR monodehydroascorbate reductase - SOD superoxide dismutase     

Antioxidants and Manganese Deficiency in Needles of Norway Spruce (Picea abies L.) Trees

Chlorotic and green needles from Norway spruce (Picea abies L.) trees were sampled in the Calcareous Bavarian Alps in winter. The needles were used for analysis of the mineral and pigment contents, the levels of antioxidants (ascorbate, glutathione), and the activities of protective enzymes (superoxide dismutase, catalase, ascorbate peroxidase, monodehydroascorbate radical reductase, dehydroascorbate reductase, glutathione reductase). In addition, the activities of two respiratory enzymes (glucose-6-phosphate dehydrogenase, NAD-malate dehydrogenase), which might provide the NADPH necessary for functioning of the antioxidative system, were determined. We found that chlorotic needles were severely manganese deficient (3 to 6 micrograms Mn per gram dry weight as compared with up to 190 micrograms Mn per gram dry weight in green needles) but had a similar dry weight to fresh weight ratio, had a similar protein content, and showed no evidence for enhanced lipid peroxidation as compared with green needles. In chlorotic needles, the level of total ascorbate and the activities of superoxide dismutase, monodehydroascorbate radical reductase, NAD-malate dehydrogenase, and glucose-6-phosphate dehydrogenase were significantly increased, whereas the levels of ascorbate peroxidase, dehydroascorbate reductase, glutathione reductase, and glutathione were not affected. The ratio of ascorbate to dehydroascorbate was similar in both green and chlorotic needles. These results suggest that in spruce needles monodehydroascorbate radical reductase is the key enzyme involved in maintaining ascorbate in its reduced state. The reductant necessary for this process may have been supplied at the expense of photosynthate.     

Effect of chronic ethanol treatment under partial catalase inhibition on the activity of enzymes related to peroxide metabolism in rat liver and heart

1. In order to test the hypothesis that the alcoholic cardiomyopathy under partial catalase inhibition is associated with the activation of lipid peroxidation in cardiomyocytes (Panchenko et al., Experientia 43, 580-581, 1987), the effects of ethanol and catalase inhibitor 3-amino-1,2,4-triazole (aminotriazole) on rat heart and liver content of reduced glutathione and on the activity of enzymes related to peroxide metabolism: catalase, superoxide dismutase, glutathione peroxidase, glutathione reductase, glutathione-S-transferase and glucose-6-phosphate dehydrogenase were investigated. 2. In accordance with the data obtained by Kino (J. molec, cell. Cardiol. 13, 5-12, 1981), when ethanol (36% of dietary calories) and aminotriazole were simultaneously administered an alcoholic cardiomyopathy developed while in the liver moderate fatty degeneration was revealed. 3. Chronic combined or separate administration of ethanol and aminotriazole was shown to increase glutathione concentration and glutathione-S-transferase activity in rat liver. In the groups of animals which received isocaloric carbohydrates in the diet instead of ethanol the liver glucose-6-phosphate dehydrogenase was increased. 4. Acute and chronic aminotriazole injections led to catalase inactivation and in the latter case also to inhibition of the liver superoxide dismutase and glutathione peroxidase activities. 5. Ethanol and aminotriazole treatment did not alter the glutathione level and the activity of all enzymes tested (except catalase) in rat myocardium.     

Inhibition of catalase by aminotriazole <Emphasis Type="Italic">in vivo</Emphasis> results in reduction of glucose-6-phosphate dehydrogenase activity in <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> cells

The inhibitor of catalase 3-amino-1,2,4-triazole (AMT) was used to study the physiological role of catalase in the yeast Saccharomyces cerevisiae under starvation. It was shown that AMT at the concentration of 10 mM did not affect the growth of the yeast. In vivo and in vitro the degree of catalase inhibition by AMT was concentration- and time-dependent. Peroxisomal catalase in bakers' yeast was more sensitive to AMT than the cytosolic one. In vivo inhibition of catalase by AMT in S. cerevisiae caused a simultaneous decrease in glucose-6-phosphate dehydrogenase activity and an increase in glutathione reductase activity. At the same time, the level of protein carbonyls, a marker of oxidative modification, was not affected. Possible mechanisms compensating the negative effects caused by AMT inhibition of catalase are discussed.     

Changes in apoplastic antioxidants induced by powdery mildew attack in oat genotypes with race non-specific resistance

Three oat (Avena sativa L.) lines which show differential responses to attack by the biotrophic fungal pathogen Blumeria graminis DC f. sp. avenae Marchal, which causes powdery mildew, were studied: Maldwyn shows the strongest resistance in adult plants; Selma shows greater susceptibility; while a Selma × Maldwyn hybrid, OM1387, has a similar degree of resistance to Maldwyn. Host responses to pathogen attack were complete 48 h after inoculation but largely accomplished within the first 24 h, the point when material was taken for enzyme and metabolic assays. In Maldwyn and OM1387 about 80% of attacked cells showed localized autofluorescent host-cell responses but this fell to less than 20% in Selma. A cytoplasmic marker enzyme, glucose 6-phosphate dehydrogenase, was used to determine contamination of the apoplastic extracts by cellular components. After correction for cytoplasmic contamination, up to 4% of the total foliar activities of superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase, dehydroascorbate reductase and monodehydroascorbate reductase activities were detected in the apoplast. The apoplast contained about 2% of the total foliar glutathione pool and dehydroascorbate, but not ascorbate, at values amounting to 10% of the total foliar ascorbate plus dehydroascorbate pool. Twenty-four hours after inoculation the foliar or apoplastic ascorbate pools were similar in inoculated and control leaves. Foliar catalase activity increased in both susceptible and resistant responses. Resistance correlated with increased total foliar glutathione, an increase in the ratio of reduced to oxidized glutathione and with decreased total activities of foliar ascorbate peroxidase, glutathione reductase, dehydroascorbate reductase and monodehydroascorbate reductase. Received: 17 April 1998 / Accepted: 28 August 1998     

Peroxide-Scavenging Systems during Cold Acclimation of Apple Callus in Culture

The relationship between peroxide-scavenging systems and coldacclimation was studied in apple callus in culture during acclimationunder artificial conditions. Unacclimated callus did not survivefreezing at –10?C, whereas callus acclimated at 0?C exhibitedgradually increased resistance to freezing and, after acclimationfor 20 days, it survived at temperatures as low as –15–C.During acclimation of callus, there was an immediate and abruptincrease in the activities of ascorbate peroxidase (EC 1.11.1.11 [EC] ),peroxidase (EC 1.11.1.7 [EC] ) and catalase (EC 1.11.1.6 [EC] ), which reachedmaximum values after acclimation for 10 days, at the same timeas the very beginning of the increase in cold hardiness wasobserved. An increase in the activity of glyceraldehyde-3-phosphatedehydrogenase (EC 1.2.1.12 [EC] ) occurred during the first 5 daysof cold treatment. The activities of glucose-6-phosphate dehydrogenase(EC 1.1.1.49 [EC] ), hexokinase (EC 2.7.1.1 [EC] ), glutathione reductase(EC 1.6.4.2 [EC] ), glutathione peroxidase (EC 1.11.1.9 [EC] ) and dehydro-ascorbatereductase (EC 1.8.5.1 [EC] ) increased gradually during the cold treatment.In contrast, the activity of glucosephosphate isomerase (EC5.3.1.9 [EC] ) decreased gradually during acclimation. Furthermore,during acclimation, the levels of glucose-6-phosphate, fructose-6-phosphateand glucose-1-phosphate increased slowly and steadily, and thelevels of GSH and ascorbate remained at consistently higherlevels. In addition, acclimation caused marked cytological changes.The most striking of these changes was the microvacuolationand thickening of the cell wall. These results indicate thatthe enhancement of peroxide-scavenging systems at the time ofcold acclimation proceeds in two stages: during the first stage,the enzymatic activities involved in the degradation of peroxides(i.e., the activities of ascorbate peroxidase, peroxidase andcatalase) increase; and, in the second stage, an alternativeenzymatic system develops for detoxification of peroxides, coupledwith the pentose phosphate cycle. (Received July 20, 1990; Accepted April 16, 1991)     

Pulmonary and hepatic glutathione levels, glutathione shuttle enzymes and lipid peroxidation in rats exposed intratracheally to coal fly ash

Fly ash and fly ash residue increased the formation of conjugated dienes and the levels of oxidized glutathione (GSSG) and reduced the levels of reduced glutathione (GSH) in lung and liver whereas fly ash extract administration had no effect on the formation of conjugated dienes and glutathione levels in lung and liver. Fly ash and fly ash residue reduced the activity of glutathione reductase both in lung and liver but did not alter the activity of glutathione peroxidase. Fly ash and fly ash extract significantly increased glucose-6-phosphate dehydrogenase activity in lung whereas in liver, fly ash and fly ash residue reduced the activity of glucose-6-phosphate dehydrogenase. Fly ash residue did not alter the activity of glucose-6-phosphate dehydrogenase in lung whereas fly ash extract was not effective in liver.     

Effects of Iron Excess on Nicotiana plumbaginifolia Plants (Implications to Oxidative Stress)

Fe excess is believed to generate oxidative stress. To contribute to the understanding of Fe metabolism, Fe excess was induced in Nicotiana plumbaginifolia grown in hydroponic culture upon root cutting. Toxicity symptoms leading to brown spots covering the leaf surface became visible after 6 h. Photosynthesis was greatly affected within 12 h; the photosynthetic rate was decreased by 40%. Inhibition of photosynthesis was accompanied by photoinhibition, increased reduction of photosystem II, and higher thylakoid energization. Fe excess seemed to stimulate photorespiration because catalase activity doubled. To cope with cellular damage, respiration rate increased and cytosolic glucose-6-phosphate dehydrogenase activity more than doubled. Simultaneously, the content of free hexoses was reduced. Indicative of generation of oxidative stress was doubling of ascorbate peroxidase activity within 12 h. Contents of the antioxidants ascorbate and glutathione were reduced by 30%, resulting in equivalent increases of dehydroascorbate and oxidized glutathione. Taken together, moderate changes in leaf Fe content have a dramatic effect on plant metabolism. This indicates that cellular Fe concentrations must be finely regulated to avoid cellular damage most probably because of oxidative stress induced by Fe.     

Up-regulation of the leaf mitochondrial and peroxisomal antioxidative systems in response to salt-induced oxidative stress in the wild salt-tolerant tomato species Lycopersicon pennellii

The response of the antioxidative systems of leaf cell mitochondria and peroxisomes of the cultivated tomato Lycopersicon esculentum (Lem) and its wild salt-tolerant related species Lycopersicon pennellii (Lpa) to NaCl 100 mM stress was investigated. Salt-dependent oxidative stress was evident in Lem mitochondria as indicated by their raised levels of lipid peroxidation and H2O2 content whereas their reduced ascorbate and reduced glutathione contents decreased. Concomitantly, SOD activity decreased whereas APX and GPX activities remained at control level. In contrast, the mitochondria of salt-treated Lpa did not exhibit salt-induced oxidative stress. In their case salinity induced an increase in the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and glutathione-dependent peroxidase (GPX). Lpa peroxisomes exhibited increased SOD, APX, MDHAR and catalase activity and their lipid peroxidation and H2O2 levels were not affected by the salt treatment. The activities of all these enzymes remained at control level in peroxisomes of salt-treated Lem plants. The salt-induced increase in the antioxidant enzyme activities in the Lpa plants conferred cross-tolerance towards enhanced mitochondrial and peroxisomal reactive oxygen species production imposed by salicylhydroxamic acid (SHAM) and 3-amino-1,2,4-triazole (3-AT), respectively.     

Effects of paraquat administration on longevity, oxygen consumption, lipid peroxidation, superoxide dismutase, catalase, glutathione reductase, inorganic peroxides and glutathione in the adult housefly

The effects of oxidative stress in the adult male housefly were examined by the administration of 1 mM paraquat. Houseflies exhibit NADH and NADPH-diaphorase activity. Paraquat caused a significant decrease in life span, metabolic rate and the concentration of thiobarbituric acid-reactants. Concentrations of reduced glutathione and inorganic peroxides were increased by paraquat. Paraquat stimulated the activity of catalase but did not affect activities of superoxide dismutase and glutathione reductase. The levels of oxidized glutathione and the rate of fluorescent age pigment accumulation were unaffected by paraquat. Results indicate that paraquat toxicity does not result from lipid peroxidation.     

Adaptive response of antioxidant enzymes to catalase inhibition by aminotriazole in goldfish liver and kidney

This study was undertaken to clarify the physiological role of catalase in the maintenance of pro/antioxidant balance in goldfish tissues by inhibiting the enzyme in vivo with 3-amino 1,2,4-triazole. Intraperitoneal injection of aminotriazole (0.5 mg/g wet mass) caused a decrease in liver catalase activity by 83% after 24 h that was sustained after 168 h post-injection. In kidney catalase activity was reduced by approximately 50% and 70% at the two time points, respectively. Levels of protein carbonyls were unchanged in liver but rose by 2-fold in kidney after 168 h. Levels of thiobarbituric acid-reactive substances were elevated in both tissues after 24 h but were reversed by 168 h. Glutathione peroxidase and glutathione-S-transferase activities increased in kidney after aminotriazole treatment whereas activities of glutathione peroxidase and glutathione reductase in liver decreased after 24 h but rebounded by 168 h. Liver glucose-6-phosphate dehydrogenase activity was reduced at both time points. Activities of these three enzymes in liver correlated inversely with the levels of lipid damage products (R2=0.65-0.81) suggesting that they may have been oxidatively inactivated. Glutathione-S-transferase activity also correlated inversely with catalase (R2=0.86). Hence, the response to catalase depletion involves compensatory changes in the activities of enzymes of glutathione metabolism.     

Effects of paraquat on anti-oxidant system in rats

The toxic effects of paraquat on the anti-oxidant defense system of male albino rats were evaluated, after administering either a single dose (1.5 and 7.5 mg/kg of body weight) or continuous daily doses (same as above, i.e., 1.5 mg/kg and 7.5 mg/kg of body weight) for 3 and 7 days. Glutathione levels in blood cells, liver, lung and kidney tissues decreased in a dose and time dependent manner. Glutathione reductase and glucose-6-phosphate dehydrogenase activity decreased, whereas the activity of glutathione-S-transferase, glutathione peroxidase, catalase and superoxide dismutase increased in paraquat exposure. Malondialdehyde formation also increased in a dose and time dependent manner. The alterations of anti-oxidant system particularly glutathione can be utilized as biomarkers during management of paraquat poisoning.     

Hydrogen-peroxide-scavenging systems within pea chloroplasts

The subcellular distribution of ascorbate peroxidase and glutathione reductase (EC 1.6.4.2) in pea leaves was compared with that of organelle markers. Enzyme distribution was found to be similar to that of the chloroplast enzyme NADPH-glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13). Isolated chloroplasts showed a close correlation between intactness and the percentage of enzyme activity recovered. Chloroplasts of 85% intactness were found to contain a high proportion of leaf dehydroascorbate reductase activity (EC 1.8.5.1), 10% of leaf glutathione and 30% of leaf ascorbate. These results are discussed in relation to the potential role of chloroplast antioxidant systems in plant resistance to environmental and other stress conditions.Abbreviations GSH reduced glutathione - GSSG oxidized glutathione - NADPH-GPD glyceraldehyde-3-phosphate dehydrogenase - SOD superoxide dismutase     

Ethanol-induced alterations of the antioxidant defense system in rat kidney

We report here the effects of chronic ethanol consumption on the antioxidant defense system in rat kidney. Thirty-two male Wistar rats were randomly divided in two identical groups and were treated as follows: control group (water for fluid) and the ethanol-fed group (2 g/kg body weight/24 h). The animals were sacrificed after 10 weeks, and respectively 30 weeks of ethanol consumption, and the renal tissue was isolated and analyzed. Results revealed that kidney alcohol dehydrogenase activities increased significantly after ethanol administration, but the electrophoretic pattern of alcohol dehydrogenase isoforms was unmodified. The SDS polyacrylamidegel electrophoretic study of kidney proteins has revealed the appearance of two new protein bands after long-term ethanol consumption. The kidney reduced glutathione/oxidized glutathione ratio decreased, indicating an oxidative stress response due to ethanol ingestion. The malondialdehyde contents and xanthine oxidase activities were unchanged. The antioxidant enzymatic defense system showed a different response during the two periods of ethanol administration. After 10 weeks, catalase, glutathione peroxidase, glutathione reductase, and glucose-6-phosphate dehydrogenase were activated, while superoxide dismutase, glutathione transferase, and gamma-glutamyltranspeptidase levels were stationary. After 30 weeks, superoxide dismutase and glutathione peroxidase activities were unmodified, but catalase, glutathione transferase, gamma-glutamyltranspeptidase, glutathione reductase, and glucose-6-phosphate dehydrogenase activities were significantly increased. Remarkable changes have been registered after 30 weeks of ethanol administration for glutathione reductase and glucose-6-phosphate dehydrogenase activities, including an increase by 106 and 216' of control values, respectively. These results showed specific changes in rat kidney antioxidant system and glutathione status as a consequence of long-term ethanol administration.     

Oxidant-antioxidant imbalance in the erythrocytes of sporadic amyotrophic lateral sclerosis patients correlates with the progression of disease

Free radicals are implicated in numerous disease processes including motor neuron degeneration (MND). Antioxidant defense enzymes: superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSHPx), glutathione reductase (GR) and glucose-6-phosphate dehydrogenase (G-6-PDH) in the erythrocytes are capable of detoxifying reactive oxygen species produced endogenously or exogenously. In the present study, the extent of lipid peroxidation (LPO) and antioxidant defenses were evaluated in the erythrocytes of 20 sporadic amyotrophic lateral sclerosis (ALS) patients and 20 controls. We observed that lipid peroxidation in the erythrocytes of amyotrophic lateral sclerosis patients significantly increased with respect to controls (P<0.001). On the other hand, catalase activity was found to be significantly lower (P<0.001). The activities of glucose-6-phosphate dehydrogenase, glutathione reductase and glutathione levels were also found to be significantly reduced in ALS patients compared to healthy subjects (P<0.001, P<0.01 and P<0.01, respectively). It was further observed that lipid peroxidation started to increase and catalase, glutathione reductase, glucose-6-phosphate dehydrogenase enzyme activities and glutathione levels started to decrease as amyotrophic lateral sclerosis progressed from 6 to 24 months, suggesting a correlation between these parameters and duration of amyotrophic lateral sclerosis. This study confirms the involvement of oxidative stress during the progression of amyotrophic lateral sclerosis and the need to develop specific peripheral biomarkers.     

Glucose-6-phosphate dehydrogenase activity and NADPH/NADP+ ratio in liver and pancreas are dependent on the severity of hyperglycemia in rat

Hyperglycemia is associated with metabolic disturbances affecting cell redox potential, particularly the NADPH/NADP+ ratio and reduced glutathione levels. Under oxidative stress, the NADPH supply for reduced glutathione regeneration is dependent on glucose-6-phosphate dehydrogenase. We assessed the effect of different hyperglycemic conditions on enzymatic activities involved in glutathione regeneration (glucose-6-phosphate dehydrogenase and glutathione reductase), NADP(H) and reduced glutathione concentrations in order to analyze the relative role of these enzymes in the control of glutathione restoration. Male Sprague-Dawley rats with mild, moderate and severe hyperglycemia were obtained using different regimens of streptozotocin and nicotinamide. Fifteen days after treatment, rats were killed and enzymatic activities, NADP(H) and reduced glutathione were measured in liver and pancreas. Severe hyperglycemia was associated with decreased body weight, plasma insulin, glucose-6-phosphate dehydrogenase activity, NADPH/NADP+ ratio and glutathione levels in the liver and pancreas, and enhanced NADP+ and glutathione reductase activity in the liver. Moderate hyperglycemia caused similar changes, although body weight and liver NADP+ concentration were not affected and pancreatic glutathione reductase activity decreased. Mild hyperglycemia was associated with a reduction in pancreatic glucose-6-phosphate dehydrogenase activity. Glucose-6-phosphate dehydrogenase, NADPH/NADP+ ratio and glutathione level, vary inversely in relation to blood glucose concentrations, whereas liver glutathione reductase was enhanced during severe hyperglycemia. We conclude that glucose-6-phosphate dehydrogenase and NADPH/NADP+ were highly sensitive to low levels of hyperglycemia. NADPH/NADP+ is regulated by glucose-6-phosphate dehydrogenase in the liver and pancreas, whereas levels of reduced glutathione are mainly dependent on the NADPH supply.     

Pathogen-Induced Changes in the Antioxidant Status of the Apoplast in Barley Leaves

Leaves of two barley (Hordeum vulgare L.) isolines, Alg-R, which has the dominant Mla1 allele conferring hypersensitive race-specific resistance to avirulent races of Blumeria graminis, and Alg-S, which has the recessive mla1 allele for susceptibility to attack, were inoculated with B. graminis f. sp. hordei. Total leaf and apoplastic antioxidants were measured 24 h after inoculation when maximum numbers of attacked cells showed hypersensitive death in Alg-R. Cytoplasmic contamination of the apoplastic extracts, judged by the marker enzyme glucose-6-phosphate dehydrogenase, was very low (less than 2%) even in inoculated plants. Dehydroascorbate, glutathione, superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase, monodehydroascorbate reductase, and dehydroascorbate reductase were present in the apoplast. Inoculation had no effect on the total foliar ascorbate pool size or the redox state. The glutathione content of Alg-S leaves and apoplast decreased, whereas that of Alg-R leaves and apoplast increased after pathogen attack, but the redox state was unchanged in both cases. Large increases in foliar catalase activity were observed in Alg-S but not in Alg-R leaves. Pathogen-induced increases in the apoplastic antioxidant enzyme activities were observed. We conclude that sustained oxidation does not occur and that differential strategies of antioxidant response in Alg-S and Alg-R may contribute to pathogen sensitivity.     

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).