Removal Of H2O2 Research Articles

Thylakoid-bound ascorbate peroxidase mutant exhibits impaired electron transport and photosynthetic activity.

In chloroplasts, stromal and thylakoid-bound ascorbate peroxidases (tAPX) play a major role in the removal of H(2)O(2) produced during photosynthesis. Here, we report that hexaploid wheat (Triticum aestivum) expresses three homeologous tAPX genes (TaAPX-6A, TaAPX-6B, and TaAPX-6D) mapping on group-6 chromosomes. The tAPX activity of a mutant line lacking TaAPX-6B was 40% lower than that of the wild type. When grown at high-light intensity photosystem II electron transfer, photosynthetic activity and biomass accumulation were significantly reduced in this mutant, suggesting that tAPX activity is essential for photosynthesis. Despite the reduced tAPX activity, mutant plants did not exhibit oxidative damage probably due to the reduced photochemical activity. This might be the result of a compensating mechanism to prevent oxidative damage having as a consequence a decrease in growth of the tAPX mutant plants.

Increased H(2)O(2) counteracts the vasodilator and natriuretic effects of superoxide dismutation by tempol in renal medulla.

A membrane-permeable SOD mimetic, 4-hydroxytetramethyl-piperidine-1-oxyl (tempol), has been used as an antioxidant to prevent hypertension. We recently found that this SOD mimetic could not prevent development of hypertension induced by inhibition of renal medullary SOD with diethyldithiocarbamic acid. The present study tested a hypothesis that increased H2O2 counteracts the effects of tempol on renal medullary blood flow (MBF) and Na+ excretion (UNaV), thereby restraining the antihypertensive effect of this SOD mimetic. By in vivo microdialysis and Amplex red H2O2 microassay, it was found that interstitial H2O2 levels in the renal cortex and medulla in anesthetized rats averaged 55.91 +/- 3.66 and 102.18 +/- 5.16 nM, respectively. Renal medullary interstitial infusion of tempol (30 micromol x min-1x kg-1) significantly increased medullary H2O2 levels by 46%, and coinfusion of catalase (10 mg x min-1x kg-1) completely abolished this increase. Functionally, removal of H2O2 by catalase enhanced the tempol-induced increase in MBF, urine flow, and UNaV by 28, 41, and 30%, respectively. Direct delivery of H2O2 by renal medullary interstitial infusion (7.5-30 nmol x min-1x kg-1) significantly decreased renal MBF, urine flow, and UNaV, and catalase reversed the effects of H2O2. We conclude that tempol produces a renal medullary vasodilator effect and results in diuresis and natriuresis. However, this SOD mimetic increases the formation of H2O2, which constricts medullary vessels and, thereby, counteracts its vasodilator actions. This counteracting effect of H2O2 may limit the use of tempol as an antihypertensive agent under exaggerated oxidative stress in the kidney.

H2O2, but not menadione, provokes a decrease in the ATP and an increase in the inosine levels in Saccharomyces cerevisiae. An experimental and theoretical approach.

When Saccharomyces cerevisiae cells, grown in galactose, glucose or mannose, were treated with 1.5 mm hydrogen peroxide (H2O2) for 30 min, an important decrease in the ATP, and a less extensive decrease in the GTP, CTP, UTP and ADP-ribose levels was estimated. Concomitantly a net increase in the inosine levels was observed. Treatment with 83 mm menadione promoted the appearance of a compound similar to adenosine but no appreciable changes in the nucleotide content of yeast cells, grown either in glucose or galactose. Changes in the specific activities of the enzymes involved in the pathway from ATP to inosine, in yeast extracts from (un)treated cells, could not explain the effect of H2O2 on the levels of ATP and inosine. Application of a mathematical model of differential equations previously developed in this laboratory pointed to a potential inhibition of glycolysis as the main reason for that effect. This theoretical consideration was reinforced both by the lack of an appreciable effect of 1.5 mm (or even higher concentrations) H2O2 on yeast grown in the presence of ethanol or glycerol, and by the observed inhibition of the synthesis of ethanol promoted by H2O2. Normal values for the adenylic charge, ATP and inosine levels were reached at 5, 30 and 120 min, respectively, after removal of H2O2 from the culture medium. The strong decrease in the ATP level upon H2O2 treatment is an important factor to be considered for understanding the response of yeast, and probably other cell types, to oxidative stress.

Mutational analysis of Peroxiredoxin IV: exclusion of a positional candidate for multinodular goitre

Multinodular goitre (MNG) is a common disorder characterised by an enlargement of the thyroid, occurring as a compensatory response to hormonogenesis impairment. The incidence of MNG is dependent on sex (female:male ratio 5:1) and several reports have documented a genetic basis for the disease. Last year we mapped a MNG locus to chromosome Xp22 in a region containing the peroxiredoxin IV (Prx-IV) gene. Since Prx-IV is involved in the removal of H2O2 in thyroid cells, we hypothesize that mutations in Prx-IV gene are involved in pathogenesis of MNG. Four individuals (2 affected, 2 unrelated unaffected) were sequenced using automated methods. All individuals were originated from the original three-generation Italian family described in previous studies. A Southern blot analysis using a Prx-IV full-length cDNA as a probe was performed in order to exclude genomic rearrangements and/or intronic mutations. In addition a RT-PCR of PRX-IV was performed in order to investigate expression alterations. No causative mutations were found. Two adjacent nucleotide substitutions were detected within introns 1 and 4. These changes were also detected in unaffected individuals, suggesting that they were innocuous polymorphisms. No gross genomic rearrangements and/or restriction fragment alterations were observed on Southern analysis. Finally, using RT-PCR from tissue-specific RNA, no differences of PRX-IV expression-levels were detected between affected and unaffected samples. Based on sequence and genomic analysis, Prx-IV is very unlikely to be the MNG2 gene.

H2O2 plays different roles in determining penetration failure in three diverse plant-fungal interactions.

Fungal plant pathogens that attempt to penetrate and feed on living cells frequently trigger a localized plant defence response that results in fungal penetration failure. In the current study we demonstrate that breakdown products of the cell wall released by the localized application of hemicellulase elicit localized responses including, sequentially, extracellular H2O2 generation; accumulation of phenolic compounds; and cross-linking of proteins in the cell wall. In a detailed time-course study of three plant-fungus interactions that result in a high frequency of penetration failure, only one plant-fungus combination displayed a similar profile of responses to that induced by localized cell-wall degradation. The additional generation of extracellular O2- in one interaction, and the absence of phenolic compounds in the cell wall in another, demonstrate that plant responses to the penetration process may be influenced by activities of the penetrating fungus. Significantly, H2O2 generation was the only response detected in all three plant-fungal combinations at the correct time and place to account for penetration failure, and in all three combinations the enzymatic removal of H2O2 resulted in increased penetration success. Pharmacological studies suggest that in two of the three interactions, H2O2 generation required cytoskeletal involvement but was independent of transcription or translation, although inhibition of the latter processes increased fungal penetration. In at least one of these two interactions, the data suggest that H2O2 generation and new gene expression act within the same penetration-inhibiting pathway, possibly through the involvement of phenolic materials. However, enzymatic removal of H2O2 from the third interaction almost completely eliminated penetration failure, while interference with cytoplasmic processes had no effect, suggesting that H2O2 generation in this system did not require protoplast involvement and, alone, was necessary and sufficient to account for fungal penetration failure.

Reciprocal regulation of endothelial substrate adhesion and barrier function.

To examine how cell-substrate adhesion is regulated during barrier changes produced by exposure to inflammatory mediators. Lung microvascular endothelial monolayers were treated with test agents +/- blockers, and barrier was measured by transendothelial resistance; cell-substrate adhesion was assessed by surface area conservation after trypsin treatment of monolayers. Protein phosphorylation and distribution were assayed by immunoblotting and fluorescent microscopy, respectively. H2O2, histamine, bradykinin, and thrombin, decreased endothelial barrier function, and enhanced adhesion to the substratum. H2O2 enhanced cell adhesion to the substrate in a concentration (0-1 mM)- and time (0-60 minutes)-dependent fashion. This effect of H2O2 reversed within 120 minutes of removal of H2O2 and was blocked by the mean arterial pressure (MAP) kinase inhibitor, PD98059 and by chelating cytoplasmic Ca2+ but not PKC or PKG inhibition. H2O2 also stimulated tyrosine phosphorylation of several proteins and increased the association of the focal adhesive proteins paxillin, talin, and vinculin with the cytoskeleton and may promote localization of these proteins to junctions. Our data indicate that inflammatory mediators reduce cell-cell contact, contributing to reduced solute barrier and simultaneously enhanced substrate binding, which may be reciprocal events in barrier regulation in vitro and in vivo.

Cadmium-induced changes in antioxidative systems, hydrogen peroxide content, and differentiation in Scots pine roots.

To investigate whether Cd induces common plant defense pathways or unspecific necrosis, the temporal sequence of physiological reactions, including hydrogen peroxide (H(2)O(2)) production, changes in ascorbate-glutathione-related antioxidant systems, secondary metabolism (peroxidases, phenolics, and lignification), and developmental changes, was characterized in roots of hydroponically grown Scots pine (Pinus sylvestris) seedlings. Cd (50 microM, 6 h) initially increased superoxide dismutase, inhibited the systems involved in H(2)O(2) removal (glutathione/glutathione reductase, catalase [CAT], and ascorbate peroxidase [APX]), and caused H(2)O(2) accumulation. Elongation of the roots was completely inhibited within 12 h. After 24 h, glutathione reductase activities recovered to control levels; APX and CAT were stimulated by factors of 5.5 and 1.5. Cell death was increased. After 48 h, nonspecific peroxidases and lignification were increased, and APX and CAT activities were decreased. Histochemical analysis showed that soluble phenolics accumulated in the cytosol of Cd-treated roots but lignification was confined to newly formed protoxylem elements, which were found in the region of the root tip that normally constitutes the elongation zone. Roots exposed to 5 microM Cd showed less pronounced responses and only a small decrease in the elongation rate. These results suggest that in cells challenged by Cd at concentrations exceeding the detoxification capacity, H(2)O(2) accumulated because of an imbalance of redox systems. This, in turn, may have triggered the developmental program leading to xylogenesis. In conclusion, Cd did not cause necrotic injury in root tips but appeared to expedite differentiation, thus leading to accelerated aging.

Hydrogen Peroxide Activates Na+-Dependent Ca2+ Influx in Coronary Endothelial Cells

The purpose of the present study was to examine the effect of short duration H2O2 exposure on coronary artery endothelial cell [Ca2+]i regulation. Freshly dispersed cells from porcine coronary artery were exposed to H2O2 (300 μmol/L) for 3 min while monitoring [Ca2+]i using fura-2 microfluorometry. H2O2 increased [Ca2+]i from 0.86 ± 0.03 to 2.19 ± 0.41 ratio units at 3 min of H2O2 (P < 0.05). Intracellular Ca2+ remained elevated 3 min following removal of H2O2, yet H2O2 had no effect on the subsequent [Ca2+]i response to bradykinin (0.1 μmol/L). The H2O2-induced [Ca2+]i increase was completely abolished either by removal of extracellular Ca2+ or lowering extracellular Na+. Cells exposed to the Na+ ionophore, monensin, showed an increase in [Ca2+]i with a time course similar to that seen with H2O2. Furthermore, H2O2-induced Ca2+ influx was not attenuated by either Ni2+ (300 μmol/L) or econazole (10 μmol/L), excluding Ca2+ influx via the agonist-sensitive pathway. Thus, in coronary arterial endothelial cells, H2O2 increases Ca2+ influx in an extracellular Na+-dependent manner via an agonist-insensitive pathway.

Role of glutathione in adaptation and signalling during chilling and cold acclimation in plants.

Glutathione is an important component of the ascorbate-glutathione cycle, which is involved in the regulation of hydrogen peroxide (H2O2) concentrations in plants. During chilling and cold acclimation, i.e. exposure to temperatures between 0 and 15 degrees C, H2O2 may accumulate. Excess electrons from the photosynthetic and respiratory electron transport chains can be used for the reduction of oxygen, thus producing superoxide radicals (O2.-); these are subsequently transformed to H2O2 via superoxide dismutase (SOD; EC 1.15.1.1). During the removal of excess H2O2, reduced glutathione (GSH) is converted to its oxidised form (GSSG), and GSH is regenerated by the activity of NADPH-dependent glutathione reductase (GR; EC 1.6.4.2). At low non-freezing temperatures, high GSH content and GR activity were detected in several plant species, indicating a possible contribution to chilling tolerance and cold acclimation. Changes in H2O2 concentration and GSH/GSSG ratio alter the redox state of the cells and may activate special defence mechanisms through a redox signalling chain. The finding that several defence genes have antioxidant responsive elements or GSSG binding sites in their regulatory regions supports the idea that redox signalling is involved in regulating gene expression in response to low temperature.

Physiological constraints on bromoform (CHBr3) production by Ulva lactuca (Chlorophyta)

Physiological factors affecting bromoform (CHBr3) production by Ulva lactuca were investigated using metabolic inhibitors and presumed substrates of bromoperoxidase (BrPO). The metabolic inhibitors were used at a verified physiologically active concentration. Bromoform production was nearly tripled in the light (376 ± 92 pg cm−2 h−1) compared to the dark (114 ± 70 pg cm−2 h−1), was inhibited in the light in the presence of the photosynthetic inhibitor DCMU, and was inhibited in the dark in the presence of mitochondrial respiratory inhibitor rotenone. Removal of H2O2 from seawater (treatment with catalase) decreased CHBr3 production in the light and dark. Addition of H2O2 to incubations at either 1.0 mM or 100 µM significantly decreased CHBr3 production in the light and inhibited photosynthesis. In the dark, CHBr3 production was decreased and respiration inhibited in the presence of 1 mM H2O2; CHBr3 production was enhanced and respiration was not affected in the presence of 100 µM H2O2. Removal of dissolved organic matter (DOM) from seawater decreased CHBr3 production, as did the addition of alternative BrPO substrates. These results suggest the presence of an extracellular and intracellular BrPO that protects the alga from both internally produced and externally present H2O2. The results show that H2O2 produced as a result of photosynthetic and respiratory electron transport, presumably by superoxide dismutase, is available to BrPO for bromination, and that the bromination of an unidentified metabolite (presumably Βߚketo acids) and a component of DOM leads to the production of volatile polybromomethanes.

Redox control of oxygen sensing in the rabbit ductus arteriosus.

How the ductus arteriosus (DA) closes at birth remains unclear. Inhibition of O2-sensitive K+ channels may initiate the closure but the sensor mechanism is unknown. We hypothesized that changes in endogenous H2O2 could act as this sensor. Using chemiluminescence measurements with luminol (50 [mu]M) or lucigenin (5 [mu]M) we showed significantly higher levels of reactive O2 species in normoxic, compared to hypoxic DA. This increase in chemiluminescence was completely reversed by catalase (1200 U ml-1). Prolonged normoxia caused a significant decrease in K+ current density and depolarization of membrane potential in single fetal DA smooth muscle cells. Removal of endogenous H2O2 with intracellular catalase (200 U ml-1) increased normoxic whole-cell K+ currents (IK) and hyperpolarized membrane potential while intracellular H2O2 (100 nM) and extracellular t-butyl H2O2 (100 [mu]M) decreased IK and depolarized membrane potential. More rapid metabolism of O2- with superoxide dismutase (100 U ml-1) had no significant effect on normoxic K+ currents. N-Mercaptopropionylglycine (NMPG), duroquinone and dithiothreitol all dilated normoxic-constricted DA rings, while the oxidizing agent 5,5'-dithiobis-(2-nitrobenzoic acid) constricted hypoxia-dilated rings. NMPG also increased IK. We conclude that increased H2O2 levels, associated with a cytosolic redox shift at birth, signal K+ channel inhibition and DA constriction.

Hydrogen peroxide reversibly inhibits epidermal growth factor (EGF) receptor internalization and coincident ubiquitination of the EGF receptor and Eps15.

Recently, we demonstrated that hydrogen peroxide (H2O2) inhibits the internalization of the epidermal growth factor (EGF) receptor and the EGF-induced mono-ubiquitination of EGF receptor pathway substrate clone #15 (Eps15) in fibroblasts. In addition, it was suggested that EGF receptor internalization might be inhibited by H2O2 by inhibition of ubiquitination of proteins involved in endocytosis. Here, we show that H2O2 also inhibits the poly-ubiquitination of the EGF receptor in fibroblasts. Furthermore, recovery of the cells resulted in re-establishment of ubiquitination of both the EGF receptor and Eps15 and coincided with restoration of internalization of those receptors that had bound EGF in the presence of H2O2. In addition, EGF receptor internalization was inhibited by the sulphydryl reagent N-ethylmaleimide (NEM), indicating that intact SH groups might be required for receptor-mediated endocytosis. Furthermore, H2O2 rapidly induced an increase in the cellular ratio of GSSG:GSH (oxidized glutathione:reduced glutathione) and removal of H2O2 resulted in a fast restoration of the ratio of GSSG:GSH. Therefore, these results suggest a relation between the inhibition of internalization ubiquitination and an increase in GSSG:GSH ratio, which strengthens the hypothesis that H2O2 inhibits EGF receptor internalization by an inhibition of ubiquitination of proteins involved in EGF receptor-mediated endocytosis.

Overexpression of Catalase in the Mitochondrial or Cytosolic Compartment Increases Sensitivity of HepG2 Cells to Tumor Necrosis Factor-α-induced Apoptosis

The sensitivity of HepG2 cells overexpressing catalase in either the cytosolic or mitochondrial compartment to tumor necrosis factor-alpha (TNF-alpha) and cycloheximide was studied. Cells overexpressing catalase in the cytosol (C33 cells) and especially in mitochondria (mC5 cells) were more sensitive to TNF-alpha-induced apoptosis than were control cells (Hp cells). The activities of caspase-3 and -8 were increased by TNF-alpha, with the highest activities found in mC5 cells. Sodium azide, an inhibitor of catalase, reduced the increased sensitivity of mC5 and C33 cells to TNF-alpha to the level of toxicity found with control Hp cells. Azide also decreased the elevated caspase-3 activity of mC5 cells. A pan-caspase inhibitor prevented the TNF-alpha-induced apoptosis and toxicity produced by catalase overexpression. Addition of H(2)O(2) prevented TNF-alpha-induced apoptosis and caspase activation, an effect prevented by simultaneous addition of catalase. TNF-alpha plus cycloheximide increased ATP levels, with higher levels in C33 and mC5 cells compared with Hp cells. TNF-alpha did not produce apoptosis in mC5 cells maintained in a low energy state. TNF-alpha signaling was not altered by the overexpression of catalase, as activation of nuclear factor kappaB and AP-1 by TNF-alpha was similar in the three cell lines. These results suggest that catalase, overexpressed in the cytosolic or especially the mitochondrial compartment, potentiates TNF-alpha-induced apoptosis and activation of caspases by removal of H(2)O(2).

Amphotericin B both inhibits and enhances T-cell proliferation: Inhibitory effect is mediated through H2O2 production via cyclooxygenase pathway by macrophages

Amphotericin B (AmB) has been shown to have both immunosuppressive and -enhancing effects, making its precise nature of action enigmatic. In the present study, we found that AmB inhibited concanavalin A (Con A)-induced T cell proliferation if added within first 30 min of stimulation, after which inhibition began to diminish rapidly. However, AmB did not inhibit T-cell proliferation induced by a combination of PMA and ionomycin. AmB inhibition of Con A-induced proliferation was completely overcome by cyclooxygenase inhibitor ibuprofen ([α-methyl-4-(isobutyl)phenylacetic acid]) and H2O2 scavenger catalase. In fact, in the presence of ibuprofen and catalase, AmB enhanced, instead of suppressing, Con A-induced proliferation in a dose-dependent way. The effect of catalase was limited to the removal of extracellular H2O2 only, as the enzyme did not enter the cells. AmB stimulated H2O2 production by macrophages, but not by a lymphocyte population, which was inhibited by ibuprofen. Our T-cell preparation contained about 3% macrophages, and AmB inhibition of proliferation was further pronounced by increasing the macrophage number by as little as 1%. Finally, AmB inhibition of Con A-induced T-cell proliferation was completely overcome by 2-mercaptoethanol. On the basis of these results, we suggest that AmB stimulates H2O2 production by macrophages through the activation of the cyclooxygenase pathway of arachidonate metabolism. H2O2 then inhibits Con A-induced T-cell proliferation by interfering with an early step of the T-cell receptor signaling pathway through the oxidative modification of some signaling proteins. Our results also show that AmB enhances T-cell proliferation, which can be seen only after blocking its inhibitory effect. J. Cell. Biochem. 77:361–371, 2000. © 2000 Wiley-Liss, Inc.

Removal of hydrogen peroxide by a 1-cysteine peroxiredoxin enzyme of the filarial parasite Dirofilaria immitis

Prior studies have shown that filarial nematodes can effectively metabolize hydrogen peroxide in excess of that generated by activated host cells. However, the mechanisms of H2O2 removal by the filarial parasites are unclear. Herein we report the results of studies carried out on the biochemical activity and on immunolocalization of a recombinant peroxiredoxin (Prx) enzyme from the dog filarial parasite Dirofilaria immitis. A full-length cDNA encoding a 1-Cys Prx enzyme from the dog heartworm D. immitis was expressed in Escherichia coli as a recombinant polyhistidine fusion protein (rDiPrx-1). rDiPrx-1 was capable of reducing H2O2 in the presence of dithiothreitol. The apparent kinetic constants determined for DiPrx-1 using H2O2 as a substrate were a Michaelis constant (Km) of 16.28 mM and a maximal velocity (Vmax) of 16 micromol/min(-1). Consistent with the enzyme activity, D. immitis adult worms could detoxify exogenously added H2O2 in vitro. Antibodies to rDiPrx-1 identified a 27-kDa native antigen in parasite extracts and larval and adult excretory-secretory products. The antibodies were used to localize the native antigen to the lateral hypodermal chords of both male and female worms by immunohistochemistry. In addition, labeling was seen in the afibrillar muscle cells in male worms and in some areas of the uterine wall in female worms. Thus, DiPrx-1 is the first parasite Prx to be shown to detoxify exogenously added H2O2 in an in vitro system.

Application of sample pre-oxidation of arsenite in human urine prior to speciation via on-line photo-oxidation with membrane hydride generation and ICP-MS detection

A pre-oxidation procedure which converts arsenite [As(III)] into arsenate [As(v)] was investigated in urinary arsenic speciation prior to on-line photo-oxidation hydride generation with ICP-MS detection. This sample pre-oxidation method eliminates As(III) and As(v) preservation concerns and simplifies the chromatographic separation. Four oxidants, Cl2, MnO2, H2O2 and I3-, were investigated. Chlorine (ClO-aq) and MnO2 selectively converted As(III) into As(v) in pure water samples, but the conversion was inefficient in the complex urine matrix. Oxidation of As(III) by H2O2 was least affected by the urine matrix, but the removal of excess H2O2 at pH 10 proved difficult. The most appropriate oxidant for the selective conversion of As(III) into As(v) with minimal interference from the urine matrix is I3- at pH 7. Unlike H2O2, excess oxidant can be easily removed by the addition of S2O3(2-). The I3-(-)S2O3(2-) treatment on a fortified sample of reconstituted NIST SRM 2670 freeze dried urine indicated that arsenobetaine (AsB), dimethlyarsinic acid (DMA), monomethylarsonic acid (MMA) and As(v) were not chemically degraded with recoveries ranging from 95 to 102% for all arsenicals. Sample clean-up involved pH adjustment prior to C18 filtration in order to achieve efficient As(III) conversion and quantitative recoveries of AsB and DMA. The concentrations determined in NIST SRM 2670 freeze dried urine were AsB 17.2 +/- 0.5, DMA 56 +/- 4 and MMA 10.3 +/- 0.3 with a combined total of 83 +/- 5 micrograms L-1 (+/- 2 sigma).

Interactions of nitric oxide and oxygen in cytotoxicity: Proliferation and antioxidant enzyme activities of endothelial cells in culture

Nitric oxide (NO) shows cytotoxicity, and its reaction products with reactive oxygen species, such as peroxynitrite, are potentially more toxic. To examine the role of O2 in the NO toxicity, we have examined the proliferation of cultured human umbilical vein endothelial cells in the presence or absence of NO donor, ((Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)-amino]diazen-1-ium-1,2-diolate) (DETA-NONOate) (100–500 μM), under normoxia (air), hypoxia (< 0.04% O2) or hyperoxia (88–94% O2). It was found that the dose dependency on NONOate was little affected by the ambient O2 concentration, showing no apparent synergism between the two treatments. We have also examined the effects of exogenous NO under normoxia and hyperoxia on the cellular activities of antioxidant enzymes involved in the H2O2 elimination, since many of them are known to be inhibited by NO or peroxynitrite in vitro. Under normoxia DETA-NONOate (500 μM) caused 25% decrease in catalase activity and 30% increases in glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase activities in 24 h. Under hyperoxia NO caused about 25% decreases in activities of catalase, glutathione reductase and glucose-6-phosphate dehydrogenase. The H2O2 removal rate by NO-treated cells was computed on the mathematical model for the enzyme system. It was concluded that the cellular antioxidant function is little affected by NO under normoxia but that it is partially impaired when the cells are exposed to NO under hyperoxia.

Evidence for the production of hydrogen peroxide in rainwater by lightning during thunderstorms

Rain samples were collected sequentially from individual events at a site in Miami, Florida, USA, from April 1995 to October 1996, and analyzed for H2O2, major anions, pH, temperature, and rainfall amounts. The measurements showed that in the absence of lightning, the concentration of H2O2, like that of sulfate and other conservative constituents, either remained fairly constant or decreased as a function of time during the storms depending on whether rainout or washout process was the dominant pathway for the removal of atmospheric H2O2. However, during the course of several thunderstorms, H2O2 concentration increased significantly with time, whereas the concentration of sulfate and other conservative constituents remained fairly constant or decreased as a function of time. These observations indicate that substantial amounts of H2O2 in rainwater were produced by lightning activities during thunderstorms. Possible mechanisms are proposed here.

The glutathione system of peroxide detoxification is less efficient in neurons than in astroglial cells.

The ability of neurons to detoxify exogenously applied peroxides was analyzed using neuron-rich primary cultures derived from embryonic rat brain. Incubation of neurons with H2O2 at an initial concentration of 100 microM (300 nmol/3 ml) led to a decrease in the concentration of the peroxide, which depended strongly on the seeding density of the neurons. When 3 x 10(6) viable cells were seeded per dish, the half-time for the clearance by neurons of H2O2 from the incubation buffer was 15.1 min. Immediately after application of 100 microM H2O2 to neurons, glutathione was quickly oxidized. After incubation for 2.5 min, GSSG accounted for 48% of the total glutathione. Subsequent removal of H2O2 caused an almost complete regeneration of the original ratio of GSH to GSSG within 2.5 min. Compared with confluent astroglial cultures, neuron-rich cultures cleared H2O2 more slowly from the incubation buffer. However, if the differences in protein content were taken into consideration, the ability of the cells to dispose of H2O2 was identical in the two culture types. The clearance rate by neurons for H2O2 was strongly reduced in the presence of the catalase inhibitor 3-aminotriazol, a situation contrasting with that in astroglial cultures. This indicates that for the rapid clearance of H2O2 by neurons, both glutathione peroxidase and catalase are essential and that the glutathione system cannot functionally compensate for the loss of the catalase reaction. In addition, the protein-normalized ability of neuronal cultures to detoxify exogenous cumene hydroperoxide, an alkyl hydroperoxide that is reduced exclusively via the glutathione system, was lower than that of astroglial cells by a factor of 3. These results demonstrate that the glutathione system of peroxide detoxification in neurons is less efficient than that of astroglial cells.

Distribution of hydrogen peroxide and methylhydroperoxide over the Pacific and South Atlantic Oceans

The gas phase hydrogen peroxide and methylhydroperoxide concentrations were measured in the troposphere over the tropical Pacific Ocean as a component of NASA's Global Tropospheric Experiment/Pacific Exploratory Mission‐Tropics A field campaign. Flights on two aircraft covered the Pacific from 70°S to 60°N and 110°E to 80°W and South Atlantic from 40°S to 15°N and 45°W to 70°E, and extending from 76 to 13,000 m altitude. H2O2 and CH3OOH have the highest concentrations at a given altitude at the equator and decrease with increasing latitude in both the northern and southern hemispheres. Above 4 km the gradient is substantially reduced for both H2O2 and CH3OOH with latitude, and at altitudes in excess of 8 km there is no latitudinal dependence. H2O2 and CH3OOH exhibit maximum mixing ratios between 1 and 2 km at all latitudes. The mean mixing ratio of H2O2 at the equator was 1600±600 parts per trillion by volume (pptv) decreasing to 500±250 pptv at latitudes greater than 55° north and south between 1 and 2 km altitude. CH3OOH at the equator was 1400±250 pptv, decreasing to 330±200 pptv at high latitudes at altitudes between 1 and 2 km. The concentration of peroxides at high latitudes in the northern hemisphere was generally a factor of 2 higher than at corresponding latitudes in the southern hemisphere. The ratio of H2O2 to CH3OOH was between 1 and 2 from 45°S to 35°N at altitudes below 4 km. Between 5° to 15°N the ratio is less than 1, due to preferential removal of H2O2 in the Intertropical Convergence Zone.