Inhibition Of Catalase Activity Research Articles

Response of soil catalase activity to chromium contamination

The impact of chromium(III) and (VI) forms on soil catalase activity was presented. The Orthic Podzol, Haplic Phaeozem and Mollic Gleysol from different depths were used in the experiment. The soil samples were amended with solution of Cr(III) using CrCl3, and with Cr(VI) using K2Cr2O7 in the concentration range from 0 to 20 mg/kg, whereas the samples without the addition of chromium served as control. Catalase activity was assayed by one of the commonly used spectrophotometric methods. As it was demonstrated in the experiment, both Cr(III) and Cr(VI) have an ability to reduce soil catalase activity. A chromium dosage of 20 mg/kg caused the inhibition of catalase activity and the corresponding contamination levels ranged from 75% to 92% for Cr(III) and 68% to 76% for Cr(VI), with relation to the control. Catalase activity reached maximum in the soil material from surface layers (0-25 cm), typically characterized by the highest content of organic matter creating favorable conditions for microorganisms.

Cadmium Induced Changes in Metabolic Function of Mitochondrial Isolated from Potato Tissue (Solanum tuberosum L.)

Problem statement: Cadmium is highly toxic at low concentrations, but the mechanism of its toxicity is still not understood particularly at the cellular and subcellular level. Approach: In this study we examined the effects of cadmium on the oxidophosphorylation properties of mitochondria isolated from potatoes. Results: Cadmium strongly disturbed the respiratory metabolism of mitochondria isolated especially in the transfer of electrons by cyanide pathway. Meanwhile, cadmium altered the composition of lipid fatty acids polar while inhibiting catalase activity, a key enzyme in the detoxification (antioxidant) process. In addition, cadmium caused an increase in mitochondrial volume associated with strong inhibition of ATPase activity, which could be explained by a transport of the potassium ion stimulation at the origin of the massive influx of H+ by antiport through the K+/H+ leading to a decoupling (cut) of mitochondrial oxidative phosphorylation. The swelling of mitochondria was accompanied by the rupture of the mitochondrial outer membrane and thus the release of Cytochrome C, which appears to be the initial phase of apoptosis. Conclusion: Following this study, it appeared that cadmium generates in potato the isolated mitochondria a concentration-dependent oxidative stress.

Modulation of reactive oxygen species by antioxidants in chronic myeloid leukemia cells enhances imatinib sensitivity through survivin downregulation

Survivin, a member of the inhibitor of apoptosis protein family and a target for new drugs, is modulated by reactive oxygen species in several types of neoplasms including leukemias. The aim of this study is to find mechanisms to enhance sensitivity to imatinib in imatinib-responsive cells. In this study, we demonstrated through fluorescein isothiocyanate-labeled annexin V for apoptotic cells detection and western blotting that by inhibiting catalase activity, imatinib apoptosis induction was significantly enhanced (P<0.05) through diminishing survivin expression in K562 cells. These findings might be of clinical relevance and might help improve the chemotherapeutic use of imatinib mesylate.

Combustion products of 1,3-butadiene inhibit catalase activity and induce expression of oxidative DNA damage repair enzymes in human bronchial epithelial cells

1,3-Butadiene, an important petrochemical, is commonly burned off when excess amounts need to be destroyed. This combustion process produces butadiene soot (BDS), which is composed of a complex mixture of polycyclic aromatic hydrocarbons in particulates ranging in size from <1 microm to 1 mm. An organic extract of BDS is both cytotoxic and genotoxic to normal human bronchial epithelial (NHBE) cells. Based on the oxidizing potential of BDS, we hypothesized that an organic extract of this particulate matter would (1) cause enzyme inactivation due to protein amino acid oxidation and (2) induce oxidative DNA damage in NHBE cells. Thus, our aims were to determine the effect of butadiene soot ethanol extract (BSEE) on both enzyme activity and the expression of proteins involved in the repair of oxidative DNA damage. Catalase was found to be sensitive to BDS as catalase activity was potently diminished in the presence of BSEE. Using Western analysis, both the alpha isoform of human 8-oxoguanine DNA glycosylase (alpha-hOGG1) and human apurinic/apyrimidinic endonuclease (APE-1) were shown to be significantly overexpressed as compared to untreated controls after exposure of NHBE cells to BSEE. Our results indicate that BSEE is capable of effectively inactivating the antioxidant enzyme catalase, presumably via oxidation of protein amino acids. The presence of oxidized biomolecules may partially explain the extranuclear fluorescence that is detected when NHBE cells are treated with an organic extract of BDS. Overexpression of both alpha-hOGG1 and APE-1 proteins following treatment of NHBE cells with BSEE suggests that this mixture causes oxidative DNA damage.

Hydrogen peroxide mediates the radiation-induced mutator phenotype in mammalian cells

Chronic oxidative stress has been associated with genomic instability following exposure to ionizing radiation. However, results showing direct causal linkages between specific ROS (reactive oxygen species) and the ionizing radiation-induced mutator phenotype are lacking. The present study demonstrates that ionizing radiation-induced genomically unstable cells (characterized by chromosomal instability and an increase in mutation and gene amplification frequencies) show a 3-fold increase in steady-state levels of hydrogen peroxide, but not superoxide. Furthermore, stable clones isolated from parallel studies showed significant increases in catalase and GPx (glutathione peroxidase) activity. Treatment of unstable cells with PEG-CAT (polyethylene glycol-conjugated catalase) reduced the mutation frequency and mutation rate in a dose-dependent fashion. In addition, inhibiting catalase activity in the stable clones using AT (3-aminotriazole) increased mutation frequency and rate. These results clearly demonstrate the causal relationship between chronic oxidative stress mediated by hydrogen peroxide and the mutator phenotype that persists for many generations following exposure of mammalian cells to ionizing radiation.

Regulation of functional and regressing stages of corpus luteum development in mice. Role of reactive oxygen species

The endocrine and immune systems modulate ovarian function. The aim of the present work was to compare the status of various modulating factors in two well-defined stages of corpus luteum (CL) development (the functional stage and the regressing stage) by means of a gonadotropin-synchronised mouse model. At the regressing stage of CL development, we found that ovarian tissue showed increased prostaglandin (PG) F(2alpha) and diminished PGE levels concomitantly with enhanced protein abundance of ovarian cyclooxygenase 2, the inducible isoform of the limiting enzyme of PG synthesis. We also found both enhanced lipid peroxidation and enhanced total superoxide dismutase activity, as well as inhibited catalase activity and inhibited total hydroxyl radical scavenger capacity, when compared with ovaries at the functional stage. In addition, at the regressing stage we observed an increased percentage of CD8+ (cytotoxic/suppressor) T-cells and a decreased percentage of CD4+ (helper) T-cells from ovarian-draining lymph nodes. Also, the serum interleukin (IL)-2, IL-4 and IL-10 were diminished as compared with the functional stage. We conclude that a pro-oxidant status together with a pro-inflammatory response is responsible for the loss of luteal function.

Aluminium induced oxidative stress and DNA damage in root cells of Allium cepa L.

Aluminium (Al) was evaluated for induction of oxidative stress and DNA damage employing the growing roots of Allium cepa L. as the assay system. Intact roots of A. cepa were treated with different concentrations, 0, 1, 10, 50, 100, or 200 μM of aluminium chloride, at pH 4.5 for 4 h (or 2 h for comet assay) at room temperature, 25±1 °C. Following treatment the parameters investigated in root tissue were Al-uptake, cell death, extra cellular generation of reactive oxygen intermediates (ROI), viz. O 2 − , H 2O 2 and OH, lipid peroxidation, protein oxidation, activities of antioxidant enzymes namely catalase (CAT), superoxide dismutase (SOD), guaiacol peroxidase (GPX), ascorbate peroxidase (APX); and DNA damage, assessed by comet assay. The findings indicated that Al triggered generation of extra-cellular ROI following a dose-response. Through application of specific enzyme inhibitors it was demonstrated that extra-cellular generation of ROI was primarily due to the activity of cell wall bound NADH-PX. Generation of ROI in root tissue as well as cell death was better correlated to the levels of root Al-uptake rather than to the concentrations of Al in ambient experimental solutions. Induction of lipid peroxidation and protein oxidation by Al were statistically significant. Whereas Al inhibited CAT activity, enhanced SOD, GPX and APX activities significantly; that followed dose-response. Comet assay provided evidence that Al induced DNA damage in a range of concentrations 50–200 μM, which was comparable to that induced by ethylmethane sulfonate (EMS), an alkylating mutagen served as the positive control. The findings provided evidence that Al comparable to biotic stress induced oxidative burst at the cell surface through up- or down-regulation of some of the key enzymes of oxidative metabolism ultimately resulting in oxidative stress leading to DNA damage and cell death in root cells of A. cepa.

Inhibition of catalase activity as an early response of Arabidopsis thaliana cultured cells to the phytotoxin fusicoccin

In Arabidopsis thaliana cells, fusicoccin (FC) treatment induced an early and marked increase in the extracellular H(2)O(2) level. It also increased the huge hypo-osmotic stress-induced oxidative wave and, in addition, prevented the H(2)O(2) peak drop. These effects were apparently not linked to changes in either cytoplasmic pH or cytoplasmic free calcium concentration, since they occurred independently of the activity state of the plasma membrane (PM) H(+)-ATPase and neither influx nor efflux of (45)Ca(2+) was modified by FC. In the presence of diphenylene iodonium (DPI), inhibiting the PM NADPH oxidase presumably responsible for reactive oxygen species (ROS) production, no apoplastic H(2)O(2) development was detected either with or without FC. However, no increase in DPI-sensitive ferricyanide reduction, but rather a gradual decrease, occurred with FC. These results suggested that the H(2)O(2) increase observed with FC was not due to a overproduction of ROS but, more probably, to a reduced capability of FC-treated cells to degrade the H(2)O(2) formed. This view, at first supported by the finding that FC-treated cells failed to break down exogenously supplied H(2)O(2), was clearly confirmed by a series of measurements on exogenous catalase activity, tested in cell-free media of FC-treated samples. This assay, in fact, allowed ascertainment and partial characterization of an as yet unidentified factor increasingly accumulating in the incubation medium of FC-treated cells, behaving as a non-competitive catalase inhibitor and able to reduce markedly the cell's capability for H(2)O(2) scavenging.

Selenium deficiency alters epithelial cell morphology and responses to influenza

It is unknown whether nutritional deficiencies affect the morphology and function of structural cells, such as epithelial cells, and modify the susceptibility to viral infections. We developed an in vitro system of differentiated human bronchial epithelial cells (BEC) grown either under selenium-adequate (Se+) or selenium-deficient (Se–) conditions, to determine whether selenium deficiency impairs host defense responses at the level of the epithelium. Se– BECs had normal SOD activity, but decreased activity of the selenium-dependent enzyme GPX1. Interestingly, catalase activity was also decreased in Se– BECs. Both Se– and Se+ BECs differentiated into a mucociliary epithelium; however, Se– BEC demonstrated increased mucus production and increased Muc5AC mRNA levels. This effect was also seen in Se+ BEC treated with 3-aminotriazole, an inhibitor of catalase activity, suggesting an association between catalase activity and mucus production. Both Se– and Se+ were infected with influenza A/Bangkok/1/79 and examined 24 h postinfection. Influenza-induced IL-6 production was greater while influenza-induced IP-10 production was lower in Se– BECs. In addition, influenza-induced apoptosis was greater in Se– BEC as compared to the Se+ BECs. These data demonstrate that selenium deficiency has a significant impact on the morphology and influenza-induced host defense responses in human airway epithelial cells.

Inhibiting catalase activity sensitizes 36B10 rat glioma cells to oxidative stress

Gliomas are extremely resistant to anticancer therapies resulting in poor patient survival, due, in part, to altered expression of antioxidant enzymes. The primary antioxidant enzyme, catalase, is elevated constitutively in gliomas compared to normal astrocytes. We hypothesized that downregulating catalase in glioma cells would sensitize these cells to oxidative stress. To test this hypothesis, we implemented two approaches. The first, a pharmacological approach, used 3-amino-1,2,4-triazole, an irreversible inhibitor that reduced catalase enzymatic activity by 75%. Pharmacological inhibition of catalase was not associated with a reduction in rat 36B10 glioma cell viability until the cells were challenged with additional oxidative stress, i.e., ionizing radiation or hydrogen peroxide (H 2O 2). In the second molecular approach, we generated 36B10 glioma cells stably expressing catalase shRNA; a stable cell line displayed a 75% reduction in catalase immunoreactive protein and enzymatic activity. This was accompanied by an increase in intracellular reactive oxygen species and extracellular H 2O 2. These cells exhibited increased sensitivity to radiation and H 2O 2, which was rescued by the antioxidant, N-acetylcysteine. These results support the hypothesis that catalase is a major participant in the defense of 36B10 glioma cells against oxidative stress mediated by anticancer agents capable of increasing steady-state levels of H 2O 2.

Enhancement of Imatinib-Induced Apoptosis through Catalase Activity Modulation in Imatinib-Resistant Chronic Myelogenous Leukemia Blastic Phase Cells.

Therapeutic selectivity and drug resistance are two major issues in cancer chemotherapy. Strategies to improve therapeutic selectivity largely rely on our understanding of the biological difference between cancer and normal cells, and on the availability of therapeutic agents that target biological events critical for cancer cells but not essential for normal cells. Imatinib is a selective inhibitor of the BCR-ABL tyrosine kinase activity in chronic myelogenous leukemia (CML). Resistance to imatinib is a major problem in CML patients especially in blast phase (BP). Recently, it was also described that reactive oxygen species (ROS) is another mechanism through which imatinib induces apoptosis. To evaluate the possible role of catalase (a well known ROS enzyme scavenger) in imatinib resistant cells, we studied through apoptosis/necrosis analysis, K562 cells in presence or absence of 3-amino-1,2,4-triazole (AT), an inhibitor of catalase activity. Cell apoptosis index was tested in cells labeled with Annexin V-FITC/PI and analysed by flow cytometry. In order to examine the efficacy of imatinib apoptosis induction we evaluated firstly the apoptosis index in K562 cells after 24 hours of incubation with imatinib at the less cytotoxic concentrations of 0.5, 1.0 and 5.0 μM. Induced imatinib early apoptosis, was significantly higher, (25%, p < 0.05) than control in all concentrations tested. After that, we conducted the same assay in K562 cells previously treated with aminotriazole (AT-K562) and, to address the question of whether the resistance to imatinib could also be counteracted by glutathione modulation, another ROS scavenger, K562 cells previously treated with buthionine S,R-sulphoximine (BSO) a specific inhibitor of g-glutamylcysteine synthetase the rate limiting enzyme in glutathione synthesis, were also analysed. Untreated K562 cells and those previously treated with AT or BSO were incubated with imatinib (0.5, 1.0 and 5.0 μM) and hydrogen peroxide (H2O2) 1.0 μM, a well known apoptosis inductor inhibited by catalase. Only AT-K562 cells had their apoptosis significantly enhanced when compared with non treated cells (p < 0.05). Indeed, we showed that inhibition of catalase activity in K562 cells overcame imatinib resistance in 35% of these cells in the final concentration of 0.5 μM which is equivalent to the half mean plasma concentration used in clinical practice. The present findings suggest that combination of catalase inhibitors with imatinib in lower dosage represent an emerging therapeutic concept for preventing or overcoming resistance formation in CML. Therefore, further studies with drugs clinically available that inhibit catalase activity should be explored.

Bioaccumulation of paraquat by Lumbriculus variegatus in the presence of dissolved natural organic matter and impact on energy costs, biotransformation and antioxidative enzymes

Dissolved organic matter from natural sources (DNOM) is omnipresent in aquatic ecosystems. Besides affecting bioavailability of substances including xenobiotics, it directly influences physico-chemistry of the habitat and there is increasing evidence for it is interaction with organisms. We investigated direct and interacting effects of DNOM from three sources, Lake Valkea-Kotinen, Svartberget Brook, and Lake Fuchskuhle with the herbicide paraquat on the oligochaete worm Lumbriculus variegatus. Bioavailability of paraquat to L. variegates as well as activities of antioxidative enzymes catalase (CAT) and peroxidase (POD) and biotransformation enzyme soluble glutathione S-transferase (sGST) were assessed without and in the presence of DNOM. Furthermore, metabolic heat dissipation due to the exposure was quantified. Uptake of paraquat into the worms was concentration dependently reduced by DNOM, and with differences concerning the DNOM sources. sGST and CAT responded with increased activities to DNOM (5 and 25 mg C l −1) and paraquat (5.0, 50, and 500 μg l −1) separately. Paraquat at 5.0 μg l −1 and DNOM in combination caused increased activities of sGST, especially at 5 mg C l −1, but inhibition of CAT activities. The latter probably occurred due to saturation of the enzyme. Changes in enzyme activities were independent from the source of DNOM. Increasing DNOM concentrations raised metabolic heat dissipation in L. variegatus with maximum at 3 h of exposure. In the combined treatments, metabolic heat dissipation changed more due to the source of DNOM than due to the bioavailability of paraquat.

Induction of H 2O 2-metabolizing enzymes and total protein synthesis by antagonistic yeast and salicylic acid in harvested sweet cherry fruit

The immersion of sweet cherry fruit in Pichia membranefaciens at a concentration of 5 × 10 7 cells ml −1 or in salicyclic acid (SA) at 0.5 mM for 10 min reduced the incidence of decay and lesion size caused by Penicillium expansum. Without pathogen inoculation, peroxidase (POD) activity was enhanced in yeast-treated fruit, but activities of catalase (CAT) and superoxide dismutase (SOD) showed a decrease in the same fruit. SA-treatment significantly inhibited CAT activity, but stimulated SOD and POD activities. After inoculation with P. expansum, CAT activity decreased and SOD activity increased in both yeast- and SA-treated fruit. No obvious difference was found in POD activity between treatments and water control. Treatments with yeast and SA changed the expression of POD isozymes. In addition, yeast and SA treatment increased total protein content of sweet cherry and up-regulated 33 and 47 kDa protein bands shown by SDS-PAGE. These results indicated that yeast- and SA-treatments induced synthesis of anti-oxidant enzymes and specific proteins, which may play a role in the resistance against postharvest blue mold.

Acute depletion of reduced glutathione causes extensive carbonylation of rat brain proteins

This study was aimed at establishing whether oxidative stress induced by acute depletion of brain glutathione (GSH) is sufficient to generate protein carbonyls (PCOs). To this end, rat brain slices were incubated separately with the GSH depletors 1,3-bis[2-chloroethyl]-1-nitrosourea (BCNU) and diethyl maleate (DEM), and protein carbonylation was assessed on Western blots after derivatization with dinitrophenyl hydrazine. Incubation with 1 mM BCNU or 10 mM DEM for 2 hr decreased GSH levels by > 70%. Under these conditions the carbonylation of several proteins (40-120 kDa) increased by 2-3 fold. Isolation of carbonylated proteins showed that augmented PCOs represents a rise in the amount of oxidized protein. The iron chelator deferoxamine, the superoxide scavenger rutin and the H2O2 quencher dimethylthiourea all prevented DEM-induced protein carbonylation and lipid peroxidation (TBARS), indicating that the underlying mechanism involves the iron-catalyzed generation of hydroxyl radicals from H(2)O(2) (Fenton reaction). Inhibition of catalase activity with sodium azide and aminotriazole, and glutathione peroxidase activity with mercaptosuccinic acid did not increase PCOs or TBARS, suggesting that increased production of reactive oxygen species (ROS) rather than compromised cellular antioxidant defenses is the cause for the accumulation of H2O2 after GSH depletion. PCO formation was not affected by the xanthine oxidase inhibitor oxypurinol but it was reduced by SKF-525A and carbonyl cyanide 3-chlorophenylhydrazone, indicating that the microsomal monooxygenase system and the mitochondrial electron transport system are the major sources of ROS. Consistent with these findings, subcellular fractionation studies showed that mitochondria and synaptosomes are the major PCO-containing organelles. These results were also supported by the anatomic distribution of PCOs in brain. Our observations may be important in the context of multiple sclerosis where decreased GSH, mitochondrial dysfunction, excessive production of ROS, and increased protein carbonylation have all been reported.

Salicylic acid and H2O2 function by independent pathways in the induction of freezing tolerance in potato

The effects of salicylic acid (SA) and hydrogen peroxide (H2O2) on freezing tolerance were studied in two potato (Solanum tuberosum) cultivars (Alpha and Atlantic) that differ in cold sensitivity, Alpha being more tolerant to freezing than Atlantic. Lowest freezing survival rates were observed in 4‐week‐old plants. Freezing treatments consisting of exposure to 6° C for 4 h in the dark were applied 24 h after plants had been transferred from in vitro culture to soil. Catalase activity and H2O2 were estimated at the following harvest points: stage (a) 4‐week‐old in vitro plants treated with either 0.1 mM SA or 5 mM H2O2; stage (b) as in (a) but 24 h following transfer to soil prior to freezing treatment; stage (c) as in (b) but measured 15 days after a 4‐h freezing treatment. The results show that (1) SA induced freezing tolerance in both cultivars; (2) SA inhibited ascorbate peroxidase activities in both cultivars at all harvest points but inhibited catalase activities in only at stage (a); (3) SA induced H2O2 accumulation only in Atlantic at stage (a); (4) H2O2 enhanced shoot catalase activities in Atlantic at stages (a) and (b) whereas this treatment had no effect on shoot catalase activities in Alpha; (5) H2O2 treatment induced freezing tolerance in Atlantic, even though shoot catalase activities were lower than those of the controls following exposure to freezing temperatures. We conclude that SA does not always lead to H2O2 accumulation even though catalase and ascorbate peroxidase activities are decreased as a result of the treatment. Moreover, H2O2 accumulation is not always associated with the induction of freezing tolerance, for example at stage (a) where SA‐induced tolerance in Alpha was not accompanied by H2O2 accumulation. H2O2 was able to induce freezing tolerance only in Atlantic, even though H2O2 accumulated in both cultivars following this treatment.

Protective mechanism in UV-B treated Crotalaria juncea L. seedlings

There is concern that some anthropogenic atmospheric pollutants may result in a global reduction of stratospheric ozone. This would undoubtedly increase the level of ultraviolet radiation reaching the surface of the earth, which might have important biological consequences. The antioxidant defence system of a plant consists of a variety of antioxidant molecules and enzymes. The role of antioxidant enzyme activities in &lt;I&gt;Crotalaria juncea&lt;/I&gt; under control without UV-B treatment and ultraviolet-B supplemental radiation (UV-B) was investigated. UV-B treatment for 6 h for 4 days resulted in severe inhibition in catalase activity. On the other hand, the activities of peroxidase, polyphenol oxidase, superoxide dismutase and phenylalanine ammonialyase increased after the UV-B treatment when compared to control seedlings. These increases could be an adaptive mechanism to minimise the effects of UV-B radiation.

Susceptibility to oxidative stress of mussels ( Mytilus galloprovincialis) in the Venice Lagoon (Italy)

The aim of this study was to evaluate the susceptibility to pollutant mediated oxidative stress of the Mediterranean mussel Mytilus galloprovincialis in the Venice lagoon (Italy). In June 2003, mussels from a farm were transplanted to eight sites in the lagoon for five weeks. Oxidative stress responses were measured by: (i) total oxyradical scavenging capacity (TOSC) assay, for an overall evaluation of the oxidative stress response capability; (ii) catalase (CAT), as a key enzyme involved in the antioxidant defence system; (iii) malondialdehyde (MDA), as an indicator of lipid peroxidation, to evaluate an oxidative damage; (iv) metallothioneins (MTs), as they play a role in the antioxidant defence. The TOSC analysis revealed a reduced capability to eliminate: (i) peroxyl radical in mussels transplanted at Palude della Rosa, Valle Millecampi and Chioggia; (ii) hydroxyl radical at Campalto and Valle Millecampi; (iii) peroxynitrite at Valle Millecampi. Inhibition in CAT activity, observed in all the monitored sites, confirms the presence of an oxidative pressure in transplanted mussels. In addition, Pearson correlation analysis was performed in order to observe possible links between the various parameters. The PCA was a powerful tool to discriminate impacted sites, suggesting that the mussels transplanted throughout the Venice lagoon were subjected to different levels of oxidative pressure. Furthermore, it provided an easy and useful tool to summarize the obtained results.

Mushroom Tyrosinase: Catalase Activity, Inhibition, and Suicide Inactivation

Mushroom tyrosinase exhibits catalase activity with hydrogen peroxide (H(2)O(2)) as substrate. In the absence of a one-electron donor substrate, H(2)O(2) is able to act as both oxidizing and reducing substrate. The kinetic parameters V(max) and K(m) that characterize the reaction were determined from the initial rates of oxygen gas production (V(0)(O)()2) under anaerobic conditions. The reaction can start from either of the two enzyme species present under anaerobic conditions: met-tyrosinase (E(m)) and deoxy-tyrosinase (E(d)). Thus, a molecule of H(2)O(2) can reduce E(m) to E(d) via the formation of oxy-tyrosinase (E(ox)) (E(m) + H(2) <==> O(2) right harpoon over left harpoon E(ox)), E(ox) releases oxygen into the medium and is transformed into E(d), which upon binding another molecule of H(2)O(2) is oxidized to E(m). The effect of pH and the action of inhibitors have also been studied. Catalase activity is favored by increased pH, with an optimum at pH = 6.4. Inhibitors that are analogues of o-diphenol, binding to the active site coppers diaxially, do not inhibit catalase activity but do reduce diphenolase activity. However, chloride, which binds in the equatorial orientation to the protonated enzyme (E(m)H), inhibits both catalase and diphenolase activities. Suicide inactivation of the enzyme by H(2)O(2) has been demonstrated. A kinetic mechanism that is supported by the experimental results is presented and discussed.

LESION SIMULATING DISEASE 1 is required for acclimation to conditions that promote excess excitation energy.

The lsd1 mutant of Arabidopsis fails to limit the boundaries of hypersensitive cell death response during avirulent pathogen infection and initiates unchecked lesions in long day photoperiod giving rise to the runaway cell death (rcd) phenotype. We link here the initiation and propagation of rcd to the activity of photosystem II, stomatal conductance and ultimately to photorespiratory H(2)O(2). A cross of lsd1 with the chlorophyll a/b binding harvesting-organelle specific (designated cao) mutant, which has a reduced photosystem II antenna, led to reduced lesion formation in the lsd1/cao double mutant. This lsd1 mutant also had reduced stomatal conductance and catalase activity in short-day permissive conditions and induced H(2)O(2) accumulation followed by rcd when stomatal gas exchange was further impeded. All of these traits depended on the defense regulators EDS1 and PAD4. Furthermore, nonphotorespiratory conditions retarded propagation of lesions in lsd1. These data suggest that lsd1 failed to acclimate to light conditions that promote excess excitation energy (EEE) and that LSD1 function was required for optimal catalase activity. Through this regulation LSD1 can influence the effectiveness of photorespiration in dissipating EEE and consequently may be a key determinant of acclimatory processes. Salicylic acid, which induces stomatal closure, inhibits catalase activity and triggers the rcd phenotype in lsd1, also impaired acclimation of wild-type plants to conditions that promote EEE. We propose that the roles of LSD1 in light acclimation and in restricting pathogen-induced cell death are functionally linked.

Antioxidant Enzymes and Apoptosis

The role of antioxidant enzymes can be interpreted in terms of fine tuning of the concentration of reactive oxygen species which are required in the redox regulation of the cell cycle and of programmed cell death. This review summarizes findings from papers published in the last few years which deal with the relation between apoptosis and the two antioxidant enzymes, manganous superoxide dismutase (MnSOD) and catalase. With respect to MnSOD, the literature is much in favor of an inhibitory action in apoptosis. Increased MnSOD activity has been shown to prevent cell death via the receptor‐mediated apoptotic pathway as well as cell death via the mitochondrial pathway. The literature on the influence of catalase activity on apoptosis is less consistent. Evidence for both an antiapoptotic and a proapoptotic role of catalase can be found. From the results reviewed here, two schemes for the involvement of MnSOD and catalase in the regulation of apoptosis can be extracted: 1) Both MnSOD and catalase inhibit apoptosis by removing superoxide anion radicals or H2O2, respectively, because these reactive oxygen species are mediators required for the apoptotic program or inhibit a survival pathway. 2) An increase in H2O2 by downregulation or inhibition of catalase activity and/or downregulation of MnSOD activity inhibits apoptosis while a decrease in H2O2 by upregulation of catalase activity and/or upregulation of MnSOD activity supports apoptosis, possibly because of a supportive role of H2O2 in a survival pathway. The data reported so far do not allow for an explanation why some cell models appear to fit the first scheme while the second scheme appears to correctly describe other cell models. The present state of the literature reveals that antioxidant enzymes play a more intricate role in cell physiology than previously assumed.