Superoxide dismutase of the eye: relative functions of superoxide dismutase and catalase in protecting the ocular lens from oxidative damage.

1. Activities of superoxide dismutase (superoxide: superoxide oxidoreductase, EC 1.15.1.1) have been estimated in eye tissues. In rabbit eye, superoxide dismutase is present in corneal epithelium, corneal endothelium, lens, iris, ciliary body and retina. In lens the activity is in capsule epithelium. 2. Copper chelator diethyldithiocarbamate inhibited lens superoxide dismutase in vitro and in vivo in rabbit. 3. H2O2 caused inhibition of superoxide dismutase activity of lens extract, and this inhibition was potentiated by the catalase inhibitor 3-amino-1H-1,2,4-triazole (3-aminotriazole) or NaN3. 3-Aminotriazole or NaN3 had no effect on lens superoxide dismutase. Thus endogenous catalase of lens affords protection to the lens superoxide dismutase from inactivation by H2O2. 4. In rabbit having early cataract (vacuolar stage) induced by feeding-3-aminotriazole, there was a decrease in superoxide dismutase of lens, a fall in ascorbic acid of ocular humors and lens, and a 2--3-Fold increase in H2O2 of aqueous humor and vitreous humor. We conclude that catalase of eye affords protection to the lens from H2O2 and it also protects superoxide dismutase of lens from inactivation by H2O2. Superoxide dismutase, in turn, protects the lens from the superoxide radical, O2.-. It is likely that inhibition of these enzymes may lead to production of the highly reactive oxidant, the hydroxyl radical, under pathological conditions when H2O2 concentration in vivo exceeds physiological limits as in cataract induced by 3-aminotriazole. A scheme of reaction mechanism has been proposed to explain the relative functions of ocular catalase and superoxide dismutase. Such a mechanism may be involved in cataractogenic process in the human.     

Oxy Radicals in the Eye Tissues of Rabbits After Diquat In Vivo

It is our hypothesis that oxygen free radicals are the triggering agents in cataractogenesis. However, besides H₂O₂ there is no direct evidence of generation of oxy radicals in the eye tissues. Due to extremely short life of O^?₂, and OH. it is not possible to measure their cellular steady state levels. We found that indirect spectrophotometric techniques based on superoxide dismutase (SOD)-inhibitable cytochrome c reduction for estimation of O^?₂. salicylate hydroxylation for OH. and peroxidase catalysed reoxidation of 2,6-dichlorophenolindophenol for H₂O₂ were suitable, sensitive and reproducible for measurements of the reactive species of O₂ produced in the eye tissues by oxy radical enhancer, diquat in the rabbit eye in vivo, After a single intravitreal injection of 60,120 or 300 nmole diquat in the right eyes, there was a dose-dependent rise in O^?₂ levels, 106–265 fold in the aqueous humor, 34–87 fold in the vitreous humor, 6–19 fold in the lens, and 43–88 fold in the retina as compared to 0.16 μM. 0.21 μM, 2.47 nmole/g and 5.56 nmole/g in tissues of the normal eyes, respectively. There were similar increases of OH * in the eye tissues, and of H₂O₂ in the aqueous humor and vitreous humor after diquat injection.

We propose that endogenous reductants of the eye tissues univalently reduce diquat to its free radical which spontaneously reacts with O₂ generaiing O^?₂, in excessive amounts, further giving rise to H₂O₂ and OH triggering cataractogenesis.     

Free Radical Enhancer Xenobiotic is an Inducer of Cataract in Rabbit

Free radical enhancers, diquat, paraquat, plumbagin and juglone were used to study the oxy radical-induced damage to the rabbit lens in vitro and in vivo. Each compound caused a 6–8 fold increase in malondialdehyde (MDA) and a 30–55% decrease in reduced glutathione of the lens in vim. These peroxidative and oxidative changes were potentiated in the presence of 100% 0., abolished by N, and prevented by desferal-Mn (III) (DF-Mn) or liposomal superoxide dismutase (LSOD) indicating the involvement of O₂^?.

Diquat injected intravitreally as a single dose (300nmole in 30μl of isotonic saline) in the right eye of a 5-wk-old Dutch belted rabbit, induced early cataract after 24–72h. The lens of the contralateral control eye injected with isotonic saline had no change. In the right eye, O₂,^? and OH -productions were significantly (P < 0.01) higher; O₂-, was about 16 fold higher in the aqueous humor and vitreous humor, and 5 fold in the lens and retina, and OH. was 35 fold higher in the aqueous humor, 2 fold in vitreous humor and 5 fold in the lens and retina as compared to the respective tissues of the control eye. Enhanced lipid peroxidation in the lens was apparent from the higher levels of MDA and formation of aminophospholipid-MDA Schiff-base conjugates.

We propose that cyclic oxidation-reduction of xenobiotics coupled to the endogenous redox systems in the eye, could generate oxy radicals in excessive amounts, triggering cataractogcnesis.     

Peroxide-metabolizing systems of the crystalline lens

The ability of transparent and cataractous human, rabbit and mice lenses to metabolize hydrogen peroxide in the surrounding medium was evaluated. Using a chemiluminescence method in a system of luminol-horseradish peroxidase and a photometric technique, the temperature-dependent kinetics of H₂O₂ decomposition by lenses were measured. The ability of opaque human lenses to catalyze the decomposition of 10^?4 M H₂O₂ was significantly decreased. However, this was reserved by the addition of GSH to the incubation medium. Incubation of the mice lenses with the initial concentration H₂O₂ 10^?4 M led to partial depletion of GSH in normal and cataractous lenses. Human cataractous lenses showed decreased activities of glutathione reductase, glutathione peroxidase (catalyzing reduction of organic hydroperoxides including hydroperoxides of lipids), superoxide dismutase, but no signs of depletion in activities of catalase or glutathione peroxidase (utilizing H₂O₂). The findings indicated an impairment in peroxide metabolism of the mature cataractous lenses compared to normal lenses to be resulted from a deficiency of GSH. An oxidative stress induced by accumulation of lipid peroxidation products in the lens membranes during cataract progression could be considered as a primary cause of GSH deficiency and disturbance of the redox balance in the lens.     

Studies on the change of the respiratory system in roots in relation to the growth of plants II. Activity of iron-containing oxidases in roots of cereal crops with the aging of plants

Distribution of iron-containing oxidases in aging nodal rootsof rice and wheat was studied. Activities of cytochrome c oxidase(1.9.3.1 [EC] , cytochrome c : O₂ oxidoreductase), catalase (1.11.1.6 [EC] ,H₂O₂: H₂O₂ oxidoreductase) and peroxidase (1.11.1.7 [EC] , donor:H₂O₂ oxidoreductase) in wheat roots were comparatively higherthan were those in rice roots at corresponding stages. Cytochromec oxidase in roots remained active throughout the lives of therice and wheat crops. In rice roots, catalase seemed to playa distinct role around the panicle formation stage. Decay ofcatalase activity took place earlier than did that of peroxidaseand cytochrome c oxidase activities. In wheat roots similarenzyme activity changes were not observed. Data may suggestthat the high activity of iron containing oxidases at the panicleformation stage (I) may be chiefly due to catalase activityin rice roots. ¹Paper presented at the 14th Annual Meeting of the Society ofthe Science of Soil and Manure, Japan (1968). (Received November 21, 1968; )     

Nomenclature for catalase genes

Summary Gene product: catalase (H₂O₂:H₂O₂ oxidoreductase, EC 1.11.1.6) Mnemonic:Cat0 Gene product number:2.1.11.1.6     

Arsenite treatment induces oxidative stress,upregulates antioxidant system,and causes phytochelatin synthesis in rice seedlings

The effects of arsenite treatment on generation of reactive oxygen species, induction of oxidative stress, response of antioxidative system, and synthesis of phytochelatins were investigated in two indica rice (Oryza sativa L.) cvs. Malviya-36 and Pant-12 grown in sand cultures for a period of 5–20 days. Arsenite (As₂O₃; 25 and 50 μM) treatment resulted in increased formation of superoxide anion (O₂^.−), elevated levels of H₂O₂ and thiobarbituric acid reactive substances, showing enhanced lipid peroxidation. An enhanced level of ascorbate (AA) and glutathione (GSH) was observed irrespective of the variation in the level of dehydroascorbate (DHA) and oxidized glutathione (GSSG) which in turn influenced redox ratios AA/DHA and GSH/GSSG. With progressive arsenite treatment, synthesis of total acid soluble thiols and phytochelatins (PC) increased in the seedlings. Among antioxidative enzymes, the activities of superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6), total ascorbate peroxidase (APX, EC 1.11.1.11), chloroplastic ascorbate peroxidase, guaiacol peroxidase (EC 1.11.1.7), monodehydroascorbate reductase (EC 1.6.5.4), and glutathione reductase (EC 1.6.4.2) increased in arsenite treated seedlings, while dehyroascorbate reductase (EC 1.8.5.1) activity declined initially during 5–10 days and increased thereafter. Results suggest that arsenite treatment causes oxidative stress in rice seedlings, increases the levels of many enzymatic and non-enzymatic antioxidants, and induces synthesis of thiols and PCs, which may serve as important components in mitigating arsenite-induced oxidative damage.     

H<Subscript>2</Subscript>O<Subscript>2</Subscript> production and antioxidant responses in seeds and early seedlings of two different rice varieties exposed to aluminum

The effect of Al stress on H₂O₂ production of rice (Oryza sativa L.) seedlings and difference in responses of antioxidant enzymes between Al-tolerant variety (Azucena) and Al-sensitive rice one (IR 64) were investigated. Aluminum-induced H₂O₂ production and malondialdehyde (MDA) content were more pronounced for IR 64 than for Azucena. In the presence of 2 mM Al, addition of 10 mM imidazole (inhibitor of NADPH oxidase) and 1 mM azide (inhibitor of peroxidase) significantly decreased H₂O₂ production by 16% and 43% for Azucena, and 21% and 68% for IR 64, respectively. Under Al treatment, the Al-tolerant variety Azucena had significantly higher activities of catalase, ascorbate peroxidase, dehydroascorbate reducase, glutathione peroxidase and glutathione reductase, and higher concentrations of reduced glutathione than the Al-sensitive one IR 64. Treatment with buthionine sulfoximine, a specific inhibitor of GSH synthesis, significantly increased H₂O₂ production in both varieties in the presence and absence of Al. In contrast, the treatment with GSH significantly decreased the production of H₂O₂ induced by Al stress. Results suggest that GSH may play an important role in scavenging H₂O₂ caused by Al stress.     

Nitric oxide acts as an antioxidant and delays methyl jasmonate-induced senescence of rice leaves

In the present study, we evaluate the protective effect of nitric oxide (NO) against senescence of rice leaves promoted by methyl jasmonate (MJ). Senescence of rice leaves was determined by the decrease of protein content. MJ treatment resulted in (1) induction of leaf senescence, (2) increase in H₂O₂ and malondialdehyde (MDA) contents, (3) decrease in reduced form glutathione (GSH) and ascorbic acid (AsA) contents, and (4) increase in antioxidative enzyme activities (ascorbate peroxidase, glutathione reductase, peroxidase and catalase). All these MJ effects were reduced by free radical scavengers such as sodium benzoate and GSH. NO donors [N-tert-butyl-α-phenylnitrone (PBN), sodium nitroprusside, 3-morpholinosydonimine, and AsA+NaNO₂] were effective in reducing MJ-induced leaf senescence. PBN prevented MJ-induced increase in the contents of H₂O₂ and MDA, decrease in the contents of GSH and AsA, and increase in the activities of antioxidative enzymes. The protective effect of PBN on MJ-promoted senescence, MJ-increased H₂O₂ content and lipid peroxidation, MJ-decreased GSH and AsA, and MJ-increased antioxidative enzyme activities was reversed by 2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide, a NO-specific scavenger, suggesting that the protective effect of PBN is attributable to NO released. Reduction of MJ-induced senescence by NO in rice leaves is most likely mediated through its ability to scavenge active oxygen species including H₂O₂     

Salicylic acid increases tolerance to oxidative stress induced by hydrogen peroxide accumulation in leaves of cadmium-exposed flax (Linum usitatissimum L.)

The aim of this study is to investigate the impacts of exogenous salicylic acid (SA) pretreatments on hydrogen peroxide (H₂O₂) accumulation, protein oxidation, and H₂O₂-scavenging enzymes in leaves of Cd-treated flax seedlings. Cd-enhanced H₂O₂ levels were related to increased activities of guaiacol peroxidase (POX, EC 1.11.1.7) and ascorbate peroxidase (APX, EC 1.11.1.11), and were independent of changes in catalase (CAT, EC 1.11.1.6) and superoxide dismutase (SOD, EC 1.15.1.1) activities. In control flax seedlings, exogenous SA pretreatments inhibited the activity of CAT, resulted in an enhanced production of H₂O₂ suggesting that SA requires H₂O₂ to initiate an oxidative stress. However, although leaves of Cd-free flax seedlings pretreated with SA accumulated in vivo H₂O₂ by 1.2-fold compared with leaves of Cd-only exposed ones; the damage to growth and proteins after the exposure to Cd was significantly less, indicating that SA can regulate the Cd-induced oxidative stress. Moreover, the Cd-treated seedlings primed with SA exhibited a higher level of total antioxidant capacities and increased activities of H₂O₂-detoxifying enzymes.     

Cinnamic acid pretreatment mitigates chilling stress of cucumber leaves through altering antioxidant enzyme activity

To elucidate the physiological mechanism of chilling stress mitigated by cinnamic acid (CA) pretreatment, a cucumber variety (Cucumis sativus cv. Jinchun no. 4) was pretreated with 50 μM CA for 2 d and was then cultivated at two temperatures (15/8 and 25/18 °C) for 1 d. We investigated whether exogenous CA could protect cucumber plantlets from chilling stress (15/8 °C) and examined whether the protective effect was associated with the regulation of antioxidant enzymes and lipid peroxidation. At 2 d, exogenous CA did not influence plant growth, but induced the activities of some antioxidant enzymes, including superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), guaiacol peroxidase (GPX, EC 1.11.1.7), glutathione peroxidase (GSH-Px, EC 1.6.4.2) and ascorbate peroxidase (APX, EC 1.11.1.11) in cucumber leaves, and it also elevated the contents of reduced glutathione (GSH) and ascorbate (AsA). When CA was rinsed and the CA-pretreated seedlings were exposed to different temperatures, the antioxidant activities in leaves at 3 d had undergone additional change. Chilling increased the activities of CAT, GSH-PX, APX, GSH and AsA in leaves, but the combination of CA pretreatment and chilling enhanced the antioxidant activities even more. Moreover, chilling inhibited plant growth and increased the contents of malonaldehyde (MDA), superoxide radical (O₂⁻) and hydrogen peroxide (H₂O₂) in cucumber leaves, and the stress resulted in 87.5% of the second leaves being withered. When CA pretreatment was combined with the chilling stress, we observed alleviated growth inhibition and decreased contents of MDA, H₂O₂ and O₂⁻ in comparison to non-pretreated stressed plants, and found that the withered leaves occurred at a rate of 25.0%. We propose that CA pretreatment increases antioxidant enzyme activities in chilling-stressed leaves and decreases lipid peroxidation to some extent, enhancing the tolerance of cucumber leaves to chilling stress.     

外源H_2S对NO_3~-胁迫下番茄幼苗生理生化特性的影响

采用营养液水培方法,通过外源施加H2S供体NaHS(100μmol/L),研究了信号分子H2S对100mmol/L NO3-胁迫下番茄幼苗生理生化特性的影响。结果表明:(1)NO3-胁迫下,随着处理时间的延长,番茄幼苗的株高、根长、鲜重和干重显著降低,叶绿素(a、b)含量、净光合速率、气孔导度、蒸腾速率均显著降低,而胞间CO2浓度以及丙二醛(MDA)、H2O2含量增加,超氧化物歧化酶(SOD)、过氧化物酶(POD)、过氧化氢酶(CAT)和抗坏血酸过氧化物酶(APX)活性显著降低,抗坏血酸(AsA)和还原性谷胱甘肽(GSH)含量显著降低。(2)与NO3-胁迫处理相比,外源NaHS处理1、3、5d后,番茄幼苗的株高、根长、鲜重和干重显著增加,叶绿素(a、b)含量、净光合速率、气孔导度、蒸腾速率均显著升高,而胞间CO2浓度显著降低;MDA和H2O2含量降低,SOD、POD、CAT和APX活性显著增强,AsA和GSH含量显著增加,而且幼苗的硝酸还原酶、谷氨酰胺合成酶、谷氨酸合酶的活性显著增强;L-半胱氨酸脱巯基酶活性和内源H2S含量增加。研究认为,外源H2S可能通过提高抗氧化物酶的活性和增加抗氧化物质含量来缓解NO3-对番茄幼苗造成的伤害,从而增强其对NO3-胁迫耐性。     

Ascorbate deficient semi-dwarf asfL1 mutant of Lathyrus sativus exhibits alterations in antioxidant defense

An ascorbate-deficient semi-dwarf mutant asfL-1 was detected in 250 Gy γ-ray treated grass pea (Lathyrus sativus L.) cv. BioR-231. The mutant contained only 42 % of leaf and 20 % of root ascorbate content of mother control (MC). I investigated the possible causes of ascorbate deficiency and its effect on growth and antioxidant defense in control and 150 mM NaCl-treated seedling after 60 d growth period. Ascorbate deficiency was due to significant reduction in activities of monodehydroascorbate reductase and dehydroascorbate reductase as well as increase in ascorbate oxidase, leading to considerable decrease in redox state. Despite low ascorbate pool and decrease in ascorbate peroxidase activity, shoot and root biomass production in asfL-1 mutant were similar to MC plants, even at NaCl treatment. High accumulation of glutathione (GSH) coupled with high activities of GSH reductase, catalase, GSH peroxidase and peroxidase in both tissues of the mutant permitted efficient recycling of GSH and scavenging of H₂O₂ through well integrated catalase/peroxidase system, despite high superoxide dismutase activity under NaCl treatment. The collapse of this system led to inhibition of growth in NaCl-treated mother plants. Together, the results suggested that asfL-1 plants undertook a major reshuffle in its antioxidant defense machinery, which effectively counterbalanced the negative impact of ascorbate deficiency and remained unperturbed by NaCl treatment to maintain normal growth and biomass production.     

Glutathione peroxidase II of guinea pig liver cytosol: Relationship to glutathione S-transferases

GSH peroxidase II activity is not associated with all GSH-S-transferase (EC 2.5.1.18) proteins. In guinea pig liver GSH peroxidase II (nonseleno and specific for organic hydroperoxides) is associated almost entirely with GSH-S-transferase peak aa and a smaller peak designated aa′. Transferase a shows a slight peroxidase activity, transferase b is absent, and transferase c has no peroxidase activity. GSH peroxidase II of guinea pig liver has an isoelectric point of 8.9 and a molecular weight of 45,000. It consists of two subunits of similar size (26,000). The GSH peroxidase II and the GSH-S-transferase activities of transferase aa have not been resolved into separate proteins and presumably reside in the same protein. In rat liver GSH peroxidase II activity is present with the highest specific activity in GSH-S-transferase AA. There is no AA′. Transferase B also shows peroxidase activity. Transferases A and C show low but measurable peroxidase activity. Transferase peak E shows peroxidase activity, but it is contaminated by large amounts of GSH peroxidase I (EC 1.11.1.9), recognized by its activity on H₂O₂.     

Metabolism of hydrogen peroxide in isolated hepatocytes: Relative contributions of catalase and glutathione peroxidase in decomposition of endogenously generated H2O2

The compartmentation of hydrogen peroxide catabolism was studied in isolated hepatocytes. Hydrogen peroxide generation in the peroxisomal compartment was stimulated by addition of glycolate and in the endoplasmic reticular compartment (cytosolic compartment) by ethylmorphine. The rate of catabolism by catalase was estimated from the concentration of methanol required to decrease the steady-state concentration of catalase Compound I to the half-maximal value. The rate of catabolism by glutathione peroxidase was assessed in a semiquantitative manner by the rate of GSSG efflux. The relationship of GSSG efflux to catalase-dependent metabolism of H₂O₂ in the presence of increasing concentrations of glycolate was sigmoidal. This indicates that the function of glutathione peroxidase is small relative to that of catalase at low rates of H₂O₂ production in the peroxisomal fraction, but that the contribution of the former system increases as the peroxisomal H₂O₂ production rate is enhanced, and suggests that the accumulation of a steady-state concentration of H₂O₂ in the nanomolar range in the peroxisomes is sufficient to allow diffusion of H₂O₂ into the cytosol. Following pretreatment of animals with aminotriazole to inhibit catalase, glycolate caused GSSG release at rates nearly double those in control cells. This indicates that even incomplete inhibition of catalase in cells can result in enhanced release of H₂O₂ into the cytosol and demonstrates the relationship of GSSG release to H₂O₂ production under these conditions. An estimate of the rate of H₂O₂ diffusion to catalase during ethylmorphine metabolism was made from the steady-state level of Compound I and measured formate concentrations. This rate increased threefold as the rate of GSH loss increased from 1 to 2 nmol/10⁶ cells per min, indicating that as the rate of H₂O₂ production in the endoplasmic reticulum becomes maximally stimulated in the presence of ethylmorphine, the rate of H₂O₂ metabolism by catalase becomes larger. A comparison of ethylmorphine-stimulated rates of GSSG efflux from cells of control and aminotriazole-treated rats shows that, unlike experiments with glycolate, no difference in the rate of efflux is observed. These results support the conclusion that in hepatocytes catalase has a relatively minor role in catabolism of H₂O₂ at low rates of H₂O₂ generation in the endoplasmic reticulum, but that the catalase function increases as the rate of H₂O₂ production is enhanced.     

Protective effect of sesaminol from Sesamum indicum Linn. against oxidative damage in PC12 cells

Sesaminol is one component of sesame oil and has been widely used as the stabilizer to extend the storage period of food oil in China. In this study, we tried to investigate the antioxidant activity of sesaminol on rat pheochromocytoma (PC12) cells oxidative damaged by H₂O₂. Cell viability, LDH level and apoptosis of the PC12 cells were assayed after treatment with sesaminol for 3 h and exposure to H₂O₂. Furthermore, superoxide (SOD), catalase (CAT), glutathione peroxidase (GSH‐Px) and intracellular ROS were assayed after exposure of the PC12 cells to H₂O₂. The results showed that pre‐treatment with sesaminol prior to H₂O₂ exposure significantly elevated cell survival rate and SOD, CAT and GSH‐Px activity. Meanwhile, sesaminol declined the secreted LDH level, apoptosis rate and ROS level of H₂O₂ exposed cells. Thus, sesaminol may protect PC12 against oxidative injury. Copyright © 2012 John Wiley & Sons, Ltd.     

Antioxidative responses to different altitudes in leaves of alpine plant Polygonum viviparum in summer

Mountain environmental stresses result in increased formation of hydrogen peroxide (H₂O₂) and accumulation of malondialdehyde (MDA) in leaves of Polygonum viviparum. The activities of several antioxidative system enzymes such as superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), peroxidase (POD, EC 1.11.1.7), glutathione reductase (GR, EC 1.6.4.2), dehydroascorbate reductase (DHAR, EC 1.8.5.1) and the contents of several non-enzymatic antioxidants such as reduced form of ascorbate (ASC), dehydroascorbate (DHA), reduced glutathione (GSH), and oxidized glutathione (GSSG) were investigated in leaves of P. viviparum, which were collected from three altitudes (2,200, 3,200, and 3,900 m) of Tianshan Mountain in China. The activities of these four antioxidative enzymes were accompanied by increases of H₂O₂ levels from 2,200 to 3,200 m. However, the activities of CAT and POD were decreased, whereas the activities of SOD and GR continually increased at 3,900 m. Analyses of isoforms of SOD, CAT, POD, and GR showed that the leaves of P. viviparum exposed different altitude conditions are capable of differentially altering the intensity. Additionally, two new isoforms of SOD were detected at 3900 m. A continual increase in the ASC, ASC to DHA ratio, GSH and GSH/[GSH + GSSG] ratio, and the activity of DHAR were observed in leaves of P. viviparum with the elevation of altitude. These results suggest that the higher contents of ASC, GSH as well as an increase in reduced redox state may be essential to antioxidation processes in the leaves of P. viviparum, whereas antioxidant enzymes system is a cofactor in the processes.     

Role of peroxidase in lignification of tobacco cells : I. Oxidation of nicotinamide adenine dinucleotide and formation of hydrogen peroxide by cell wall peroxidases

The two peroxidase isoenzyme groups (G_I and G_III) localized in the cell walls of tobacco (Nicotiana tabacum L.) tissues were compared with respect to their capacity for NADH-dependent H₂O₂ formation. Peroxidases of the G_III group are slightly more active than those of the G_I group when both are assayed under optimal conditions. This difference is probably not of major regulatory importance. NADH-dependent formation of H₂O₂ required the presence of Mn²⁺ and a phenol as cofactors. The addition of H₂O₂ to the reaction mixture accelerated subsequent NADH-dependent H₂O₂ formation. In the presence of both cofactors or Mn²⁺ alone, catalase oxidized NADH. However, if the cofactors were absent or if only dichlorophenol was present, catalase inhibited NADH oxidation. No H₂O₂ accumulation occurred in the presence of catalase. Superoxide dismutase inhibited NADH oxidation quite significantly indicating the involvement of the superoxide radical in the peroxidase reaction. These results are interpreted to mean that the reactions whereby tobacco cell wall peroxidases catalyze NADH-dependent H₂O₂ formation are similar to those proposed for horseradish peroxidase (Halliwell 1978 Planta 140: 81-88).     

Antioxidative response of <Emphasis Type="Italic">Hordeum maritimum</Emphasis> L<Emphasis Type="Italic">.</Emphasis> to potassium deficiency

The objective of the present study was to determine the influence of potassium deprivation on the halophyte species Hordeum maritimum grown in hydroponics for 2 weeks. Treatments were with potassium (+K) or without potassium (−K). Growth, water status, mineral nutrition, parameters of oxidative stress [malondialdehyde (MDA), carbonyl groups (C=O), and hydrogen peroxide concentration (H₂O₂) contents], antioxidant enzyme activities [superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), guaiacol peroxidase (GPX, EC 1.11.1.7), ascorbate peroxidase (APX, EC 1.11.1.11), monodehydroascorbate peroxidase (MDHAR, EC 1.6.5.4), dehydroascorbate peroxidase (DHAR, EC 1.8.5.1), and glutathione reductase (GR, EC 1.6.4.2)], and antioxidant molecules [ascorbate (ASC), and glutathione (GSH)] were determined. Results showed that the growth of vegetative organs decreased owing to potassium deficiency with roots (−36%) more affected than shoots (−12%). Water status was only diminished in roots (reduction of 24%). Potassium deprivation decreased potassium concentration in both organs, this decrease was more pronounced in roots (−81%) than in shoots (−55%). In contrast to carbonyl groups, MDA content increased owing to potassium deprivation. Except for CAT activity that remained unaffected; SOD, GPX, APX, GR, MDHAR, and DHAR activities were significantly increased. H₂O₂ concentration was negatively correlated with the activities of enzymes and the accumulation of non-enzymatic antioxidants implicated in its detoxification. In conclusion, a cooperative process between the antioxidant systems is important for the tolerance of H. maritimum to potassium deficiency.