H2O2 induces rapid biophysical and permeability changes in the plasma membrane of Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the diffusion rate of hydrogen peroxide (H₂O₂) through the plasma membrane decreases during adaptation to H₂O₂ by means of a mechanism that is still unknown. Here, evidence is presented that during adaptation to H₂O₂ the anisotropy of the plasma membrane increases. Adaptation to H₂O₂ was studied at several times (15min up to 90min) by applying the steady-state H₂O₂ delivery model. For wild-type cells, the steady-state fluorescence anisotropy increased after 30min, or 60min, when using 2-(9-anthroyloxy) stearic acid (2-AS), or diphenylhexatriene (DPH) membrane probe, respectively. Moreover, a 40% decrease in plasma membrane permeability to H₂O₂ was observed at 15min with a concomitant two-fold increase in catalase activity. Disruption of the ergosterol pathway, by knocking out either ERG3 or ERG6, prevents the changes in anisotropy during H₂O₂ adaptation. H₂O₂ diffusion through the plasma membrane in S. cerevisiae cells is not mediated by aquaporins since the H₂O₂ permeability constant is not altered in the presence of the aquaporin inhibitor mercuric chloride. Altogether, these results indicate that the regulation of the plasma membrane permeability towards H₂O₂ is mediated by modulation of the biophysical properties of the plasma membrane.     

Cytoprotective Effects of Grape Seed Extract on Human Gingival Fibroblasts in Relation to Its Antioxidant Potential

Cytoprotective effects of short-term treatment with grape seed extract (GSE) upon human gingival fibroblasts (hGFs) were evaluated in relation to its antioxidant properties and compared with those of a water-soluble analog of vitamin E: trolox (Tx). GSE and Tx showed comparable antioxidant potential in vitro against di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium (DPPH; a stable radical), hydroxyl radical (^•OH), singlet oxygen (¹O₂), and hydrogen peroxide (H₂O₂). Pretreatment or concomitant treatment with GSE for 1 min protected hGFs from oxidative stressors, including H₂O₂, acid-electrolyzed water (AEW), and ¹O₂, and attenuated the intracellular formation of reactive oxygen species induced by H₂O₂ and AEW. Tx also reduced the H₂O₂- and AEW-induced intracellular formation of reactive oxygen species, but showed no cytoprotective effects on hGFs exposed to H₂O₂, AEW, or ¹O₂. These results suggest that the cytoprotective effects of GSE are likely exerted independently of its antioxidant potential.     

Synergistic effect of hydrogen peroxide on polyploidization during the megakaryocytic differentiation of K562 leukemia cells by PMA

The human myelogenous cell line, K562 has been extensively used as a model for the study of megakaryocytic (MK) differentiation, which could be achieved by exposure to phorbol 12-myristate 13-acetate (PMA). In this study, real-time PCR analysis revealed that the expression of catalase (cat) was significantly repressed during MK differentiation of K562 cells induced by PMA. In addition, PMA increased the intracellular reactive oxygen species (ROS) concentration, suggesting that ROS was a key factor for PMA-induced differentiation. PMA-differentiated K562 cells were exposed to hydrogen peroxide (H₂O₂) to clarify the function of ROS during MK differentiation. Interestingly, the percentage of high-ploidy (DNA content >4N) cells with H₂O₂ was 34.8±2.3% at day 9, and was 70% larger than that without H₂O₂ (21.5±0.8%). Further, H₂O₂ addition during the first 3 days of PMA-induced MK differentiation had the greatest effect on polyploidization. In an effort to elucidate the mechanisms of enhanced polyploidization by H₂O₂, the BrdU assay clearly indicated that H₂O₂ suppressed the division of 4N cells into 2N cells, followed by the increased polyploidization of K562 cells. These findings suggest that the enhancement in polyploidization mediated by H₂O₂ is due to synergistic inhibition of cytokinesis with PMA. Although H₂O₂ did not increase ploidy during the MK differentiation of primary cells, we clearly observed that cat expression was repressed in both immature and mature primary MK cells, and that treatment with the antioxidant N-acetylcysteine effectively blocked and/or delayed the polyploidization of immature MK cells. Together, these findings suggest that MK cells are more sensitive to ROS levels during earlier stages of maturation.     

High-frequency underwater plasma discharge application in antibacterial activity

Plasma discharge is a novel disinfection and effectual inactivation approach to treat microorganisms in aqueous systems. Inactivation of Gram-negative Escherichia coli (E. coli) by generating high-frequency, high-voltage, oxygen (O₂) injected and hydrogen peroxide (H₂O₂) added discharge in water was achieved. The effect of H₂O₂ dose and oxygen injection rate on electrical characteristics of discharge and E. coli disinfection has been reported. Microbial log reduction dependent on H₂O₂ addition with O₂ injection was observed. The time variation of the inactivation efficiency quantified by the log reduction of the initial E. coli population on the basis of optical density measurement was reported. The analysis of emission spectrum recorded after discharge occurrence illustrated the formation of oxidant species (OH^?, H, and O). Interestingly, the results demonstrated that O₂ injected and H₂O₂ added, underwater plasma discharge had fabulous impact on the E. coli sterilization. The oxygen injection notably reduced the voltage needed for generating breakdown in flowing water and escalated the power of discharge pulses. No impact of hydrogen peroxide addition on breakdown voltage was observed. A significant role of oxidant species in bacterial inactivation also has been identified. Furthermore the E. coli survivability in plasma treated water with oxygen injection and hydrogen peroxide addition drastically reduced to zero. The time course study also showed that the retardant effect on E. coli colony multiplication in plasma treated water was favorable, observed after long time. High-frequency underwater plasma discharge based biological applications is technically relevant and would act as baseline data for the development of novel antibacterial processing strategies.     

Cross-talks between Ca<Superscript>2+</Superscript>/CaM and H<Subscript>2</Subscript>O<Subscript>2</Subscript> in abscisic acid-induced antioxidant defense in leaves of maize plants exposed to water stress

Here we examined whether Ca²⁺/Calmodulin (CaM) is involved in abscisic acid (ABA)-induced antioxidant defense and the possible relationship between CaM and H₂O₂ in ABA signaling in leaves of maize (Zea mays L.) plants exposed to water stress. An ABA-deficient mutant vp5 and its wild type were used for the experimentation. We found that water stress enhanced significantly the contents of CaM and H₂O₂, and the activities of chloroplastic and cytosolic superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR), and the gene expressions of the CaM1, cAPX, GR1 and SOD4 in leaves of wild-type maize. However, the increases mentioned above were almost arrested in vp5 plants and in the wild-type plants pretreated with ABA biosynthesis inhibitor tungstate (T), suggesting that ABA is required for water stress-induced H₂O₂ production, the enhancement of CaM content and antioxidant defense. Besides, we showed that the up-regulation of water stress-induced antioxidant defense was almost completely blocked by pretreatment with Ca²⁺ inhibitors, CaM antagonists and reactive oxygen (ROS) manipulators. Moreover, the analysis of time course of CaM and H₂O₂ production under water stress showed that the increase in CaM content preceded that of H₂O₂. These results suggested that Ca²⁺/CaM and H₂O₂ were involved in the ABA-induced antioxidant defense under water stress, and the increases of Ca²⁺/CaM contents triggered H₂O₂ production, which inversely affected the contents of CaM. Thus, a cross-talk between Ca²⁺/CaM and H₂O₂ may play a pivotal role in the ABA signaling.     

Putative role of the malate valve enzyme NADP–malate dehydrogenase in H2O2 signalling in Arabidopsis

In photosynthetic organisms, sudden changes in light intensity perturb the photosynthetic electron flow and lead to an increased production of reactive oxygen species. At the same time, thioredoxins can sense the redox state of the chloroplast. According to our hypothesis, thioredoxins and related thiol reactive molecules downregulate the activity of H₂O₂-detoxifying enzymes, and thereby allow a transient oxidative burst that triggers the expression of H₂O₂ responsive genes. It has been shown recently that upon light stress, catalase activity was reversibly inhibited in Chlamydomonas reinhardtii in correlation with a transient increase in the level of H₂O₂. Here, it is shown that Arabidopsis thaliana mutants lacking the NADP–malate dehydrogenase have lost the reversible inactivation of catalase activity and the increase in H₂O₂ levels when exposed to high light. The mutants were slightly affected in growth and accumulated higher levels of NADPH in the chloroplast than the wild-type. We propose that the malate valve plays an essential role in the regulation of catalase activity and the accumulation of a H₂O₂ signal by transmitting the redox state of the chloroplast to other cell compartments.     

Light-Stimulated Changes in the Acidity of Suspensions of Oat Protoplasts: Dependence upon Photosynthesis

Light induced an alkalinization and stimulated a subsequent acidification of the medium surrounding oat (Avena sativa L. cv Garry) leaf protoplasts. Blue light was less effective than would be predicted from photosynthetic action spectra. Nonetheless, 3-(3,4-dichlorophenyl)-1,1-dimethylurea prevented alkalinization and reduced acidification to the dark rate for protoplast suspensions exposed to all light regimes tested.

Alkalinization increased in parallel with initial rates of O₂ evolution as the quantum flux density of white light was raised to 75 microeinsteins per square meter per second. Alkalinization was accompanied by a decrease in the CO₂ content of the medium; therefore, it was attributed to photosynthetically induced CO₂ uptake. The effect of CO₂ depletion on the acidity of the medium appeared to be mainly restricted to the first 15 minutes of exposure to light. Consequently, subsequent pH changes primarily reflected a constant net proton efflux. Acidification occurred in the dark, but rates of acidification increased in response to increased light approximately in parallel with changes in a concomitant net O₂ efflux. The results indicated that protoplasts could acidify the medium in response to nonphotosynthetic activity, but that photosynthesis mediated light stimulation of acidification.

     

Virucidal Effect of Cold Atmospheric Gaseous Plasma on Feline Calicivirus,a Surrogate for Human Norovirus

Minimal food-processing methods are not effective against foodborne viruses, such as human norovirus (NV). It is important, therefore, to explore novel nonthermal technologies for decontamination of foods eaten fresh, minimally processed and ready-to-eat foods, and food contact surfaces. We studied the in vitro virucidal activity of cold atmospheric gaseous plasma (CGP) against feline calicivirus (FCV), a surrogate of NV. Factors affecting the virucidal activity of CGP (a so-called radio frequency atmospheric pressure plasma jet) were the plasma generation power, the exposure time and distance, the plasma feed gas mixture, and the virus suspension medium. Exposure to 2.5-W argon (Ar) plasma caused a 5.55 log₁₀ unit reduction in the FCV titer within 120 s. The reduction in the virus titer increased with increasing exposure time and decreasing exposure distance. Of the four plasma gas mixtures studied (Ar, Ar plus 1% O₂, Ar plus 1% dry air, and Ar plus 0.27% water), Ar plus 1% O₂ plasma treatment had the highest virucidal effect: more than 6.0 log₁₀ units of the virus after 15 s of exposure. The lowest virus reduction was observed with Ar plus 0.27% water plasma treatment (5 log₁₀ unit reduction after 120 s). The highest reduction in titer was observed when the virus was suspended in distilled water. Changes in temperature and pH and formation of H₂O₂ were not responsible for the virucidal effect of plasma. The oxidation of viral capsid proteins by plasma-produced reactive oxygen and nitrogen species in the solution was thought to be responsible for the virucidal effect. In conclusion, CGP exhibits virucidal activity in vitro and has the potential to combat viral contamination in foods and on food preparation surfaces.     

The reactivity of human erythrocyte membrane cholesterol with a cholesterol oxidase

Cholesterol oxidase (EC 1.1.3.6, Brevibacterium sp.), which catalyzes the reaction: cholesterol + O₂ → Δ4-cholestenone + H₂O₂, has no effect on the cholesterol of intact (human) erythrocytes and of “resealed” ghosts, when it is present only outside these ghosts. The cholesterol of “leaky” ghosts, of “resealed” ghosts with enzyme trapped within, and of “inside-out” vesicles, was completely oxidized. This pattern indicates that the inner (cytoplasmic) membrane surface must be exposed to the enzyme for the reaction to occur, and that outer surface cholesterol only becomes reactive after the membrane has been degraded by the oxidation of inner surface cholesterol. The enzymatic oxidations followed monotonic first-order kinetics, and hence gave no evidence to support the two states of cholesterol in the membrane that had been postulated earlier from studies on the plasma lipoprotein extraction of cholesterol from the membrane.     

Protective and Antioxidative Effects of GM1 Ganglioside in PC12 Cells Exposed to Hydrogen Peroxide are Mediated by Trk Tyrosine Kinase

GM1 ganglioside was found to increase the survival of PC12 cells exposed to H₂O₂, its action was blocked by Trk tyrosine kinase inhibitor K-252a. Thus, the inhibition of H₂O₂ cytotoxic action by GM1 constituted 52.8 ± 4.3%, but in the presence of 1.0 μM K-252a it was only 11.7 ± 10.8%, i.e. the effect of GM1 became insignificant. Exposure to GM1 markedly reduced the increased accumulation of reactive oxygen species (ROS) and diminished the inactivation of Na⁺,K⁺-ATPase induced in PC12 cells by H₂O₂, but in the presence of K-252a GM1 did not change these metabolic parameters. The inhibitors of extracellular signal-regulated protein kinase, phosphatidyl inositol 3-kinase and protein kinase C decreased the effects of GM1. A combination of these protein kinase inhibitors reduced inhibition of H₂O₂ cytotoxic action by GM1 to the larger extent than each of the inhibitors and practically abolished the ability of GM1 to decrease H₂O₂-induced ROS accumulation. The protective and antioxidative effects of GM1 in PC12 cells exposed to H₂O₂ appear to be mediated by activation of Trk receptor tyrosine kinase and the protein kinases downstream from this enzyme.     

Aquaporin as a membrane transporter of hydrogen peroxide in plant response to stresses

Hydrogen peroxide (H₂O₂) is a reactive oxygen species that signals between cells, and H₂O₂ signaling is essential for diverse cellular processes, including stress response, defense against pathogens, and the regulation of programmed cell death in plants. Although plasma membrane intrinsic proteins (PIPs) have been known to transport H₂O₂ across cell membranes, the permeability of each family member of PIPs toward H₂O₂ has not yet been determined in most plant species. In a recent study, we showed that certain isoforms of Arabidopsis thaliana AtPIPs, including AtPIP2;2, AtPIP2;4, AtPIP2;5, and AtPIP2;7, are permeable for H₂O₂ in yeast cells. Since the expression of PIPs is differently modulated in Arabidopsis by abiotic stress or H₂O₂ treatment, it is important to investigate the integrated regulation of aquaporin expression and their physiological significance in H₂O₂ transport and plant response to diverse abiotic stresses.     

Hydrogen Sulfide Protects HUVECs against Hydrogen Peroxide Induced Mitochondrial Dysfunction and Oxidative Stress

Background

Hydrogen sulfide (H₂S) has been shown to have cytoprotective effects in models of hypertension, ischemia/reperfusion and Alzheimer''s disease. However, little is known about its effects or mechanisms of action in atherosclerosis. Therefore, in the current study we evaluated the pharmacological effects of H₂S on antioxidant defenses and mitochondria protection against hydrogen peroxide (H₂O₂) induced endothelial cells damage.

Methodology and Principal Findings

H₂S, at non-cytotoxic levels, exerts a concentration dependent protective effect in human umbilical vein endothelial cells (HUVECs) exposed to H₂O₂. Analysis of ATP synthesis, mitochondrial membrane potential (ΔΨm) and cytochrome c release from mitochondria indicated that mitochondrial function was preserved by pretreatment with H₂S. In contrast, in H₂O₂ exposed endothelial cells mitochondria appeared swollen or ruptured. In additional experiments, H₂S was also found to preserve the activities and protein expressions levels of the antioxidants enzymes, superoxide dismutase, catalase, glutathione peroxidase and glutathione-S-transferase in H₂O₂ exposed cells. ROS and lipid peroxidation, as assessed by measuring H₂DCFDA, dihydroethidium (DHE), diphenyl-l-pyrenylphosphine (DPPP) and malonaldehyde (MDA) levels, were also inhibited by H₂S treatment. Interestingly, in the current model, D, L-propargylglycine (PAG), a selective inhibitor of cystathionine γ-lyase (CSE), abolished the protective effects of H₂S donors.

Innovation

This study is the first to show that H₂S can inhibit H₂O₂ mediated mitochondrial dysfunction in human endothelial cells by preserving antioxidant defences.

Significance

H₂S may protect against atherosclerosis by preventing H₂O₂ induced injury to endothelial cells. These effects appear to be mediated via the preservation of mitochondrial function and by reducing the deleterious effects of oxidative stress.     

Hydrogen peroxide-induced apoptosis-like cell death in Entamoeba histolytica

The microaerophilic intestinal parasitic protozoan Entamoeba histolytica has been previously shown to be highly susceptible to oxidative stress induced by hydrogen peroxide. However the mechanism of cell death was not investigated. Studies presented in this paper demonstrate several morphological features in the parasite when exposed to H₂O₂ which are identical to metazoan apoptotic phenotype indicating a possible apoptosis-like cell death exhibited by E. histolytica in response to H₂O₂ treatment. Trophozoite cell shrinkage, DNA fragmentation, phosphatidyl serine externalization and increased endogenous reactive oxygen species level have been observed in the protozoan parasite when exposed to 2.0 mM H₂O₂ for different time periods. Although the parasite genome is completely devoid of any of the homologues of mammalian caspases it still codes for a huge number of cysteine proteases which may take over the apoptotic function of the caspases. But the present study indicates the existence of a cysteine protease independent programmed cell death in the parasite since E-64 the specific cysteine protease inhibitor could not rescue the cells from H₂O₂ induced apoptosis-like cell death.     

Hydrogen peroxide spraying alleviates drought stress in soybean plants

To ascertain the effect of exogenously applied hydrogen peroxide (H₂O₂) on drought stress, we examined whether the spraying of soybean leaves with H₂O₂ would alleviate the symptoms of drought stress. Pre-treatment by spraying leaves with H₂O₂ delayed foliar wilting caused by drought stress compared to leaves sprayed with distilled water (DW). Additionally, the relative water content of drought-stressed leaves pre-treated with H₂O₂ was higher than that of leaves pre-treated with DW. Therefore, we analyzed the effect of H₂O₂ spraying on photosynthetic parameters and on the biosynthesis of oligosaccharides related to water retention in leaves during drought stress. Under conditions of drought stress, the net photosynthetic rate and stomatal conductance of leaves pre-treated with H₂O₂ were higher than those of leaves pre-treated with DW. In contrast to DW spraying, H₂O₂ spraying immediately caused an increase in the mRNA levels of d-myo-inositol 3-phosphate synthase 2 (GmMIPS2) and galactinol synthase (GolS), which encode key enzymes for the biosynthesis of oligosaccharides known to help plants tolerate drought stress. In addition, the levels of myo-inositol and galactinol were higher in H₂O₂-treated leaves than in DW-treated leaves. These results indicated that H₂O₂ spraying enabled the soybean plant to avoid drought stress through the maintenance of leaf water content, and that this water retention was caused by the promotion of oligosaccharide biosynthesis rather than by rapid stomatal closure.     

Hydrogen peroxide mediated killing of bacteria

Summary Polymorphonuclear leukocytes (PMN) or neutrophils have multiple systems available for killing ingested bacteria. Nearly each of these incorporates H₂O₂ indicating the essential nature of this reactive oxygen intermediate for microbicidal activity. Following ingestion of bacteria by PMN, H₂O₂ is formed by the respiratory burst which consumes O₂ and generates H₂O₂ from O_2–. H₂O₂ is deposited intracellularly near bacteria within phagocytic vacuoles where it can react with the MPO-H₂O₂-halide system to form toxic hyperchlorous acid (HOCl) and/or possibly singlet oxygen (¹O₂). H₂O₂ can also react with O_2– and/or iron (Fe⁺⁺) from lactoferrin or bacteria to form the highly toxic hydroxyl radical (¹OH). These mechanisms appear important since deficiencies of H₂O₂ production, myeloperoxidase or lactoferrin frequently increases their owner's susceptibility to infection. In particular, examination of PMN from infection prone patients with chronic granulomatous disease (CGD) most clearly demonstrates the importance of H₂O₂ in killing of bacteria. CGD PMN lack the capacity to effectively generate H₂O₂ and subsequently have impaired ability to kill catalase positive (H₂O₂ producing) but not catalase negative (not H₂O₂ producing) bacteria. PMN also have catalase and glutathione peroxidase systems in their cytoplasms to protect themselves from the toxicity of H₂O₂. Finally, while H₂O₂ is critical for host defense, it can also be released extracellularly and thereby play a significant role in PMN mediated tissue injury.     

d-β-Hydroxybutyrate inhibited the apoptosis of PC12 cells induced by H2O2 via inhibiting oxidative stress

Oxidative stress has an important role in neurodegenerative diseases and cerebral ischemic injury. It is reported that d-β-hydroxybutyrate (DβHB), the major component of ketone bodies, is neuroprotective in recent studies. Therefore, in the present work the neuroprotective effects of DβHB on H₂O₂-induced apoptosis mediated by oxidative stress was investigated. PC12 cells were exposed to H₂O₂ with different concentrations of H₂O₂ for different times after DβHB pretreatment. MTT assay, apoptotic rates, intracellular reactive oxygen species (ROS) level, GSH content, mitochondrial membrane potential (MMP) and caspase-3 activity were determined. The results showed that DβHB inhibited the decrease of cell viability induced by H₂O₂ in PC12 cells. DβHB decreased the apoptotic rates induced by H₂O₂. The changes of intracellular ROS, GSH, MMP and caspase-3 activity due to H₂O₂ exposure were partially reversed in PC12 cells. So DβHB inhibited the apoptosis of PC12 cells induced by H₂O₂ via inhibiting oxidative stress.     

The involvement of hydrogen peroxide in plant responses to stresses

The role of reactive oxygen species, especially H₂O₂, in plant response to stresses has been the focus of much attention. Hydrogen peroxide has been postulated to play multiple functions in plant defence against pathogens. (1) H₂O₂ may possess direct microbicidal activity at the sites of pathogen invasion. (2) It is used for cell-wall reinforcing processes: lignification and oxidative cross-linking of hydroxyproline-rich proteins and other cell-wall polymers. (3) It was found to be necessary for phytoalexin synthesis. (4) H₂O₂ may trigger programmed plant cell death during the hypersensitive response that restricts the spread of infection. (5) H₂O₂ has been suggested to act as a signal in the induction of systemic acquired resistance and (6) it induces defence genes. Recently H₂O₂ has been proposed to be involved in the signal transduction pathways leading to acclimation and protection from abiotic stresses. The present review discusses new insights into the function of H₂O₂ in plant responses to biotic and abiotic stresses.     

The involvement of hydrogen peroxide in the differentiation of secondary walls in cotton fibers

H₂O₂ is a widespread molecule in many biological systems. It is created enzymatically in living cells during various oxidation reactions and by leakage of electrons from the electron transport chains. Depending on the concentration H₂O₂ can induce cell protective responses, programmed cell death, or necrosis. Here we provide evidence that H₂O₂ may function as a developmental signal in the differentiation of secondary walls in cotton (Gossypium hirsutum) fibers. Three lines of evidence support this conclusion: (a) the period of H₂O₂ generation coincided with the onset of secondary wall deposition, (b) inhibition of H₂O₂ production or scavenging the available H₂O₂ from the system prevented the wall differentiation process, and (c) exogenous addition of H₂O₂ prematurely promoted secondary wall formation in young fibers. Furthermore, we provide support for the concept that H₂O₂ generation could be mediated by the expression of the small GTPase Rac, the accumulation of which was shown previously to be strongly induced during the onset of secondary wall differentiation. In support of Rac's role in the activation of NADPH oxidase and the generation of reactive oxygen species, we transformed soybean (Glycine max) and Arabidopsis cells with mutated Rac genes. Transformation with a dominantly activated cotton Rac13 gene resulted in constitutively higher levels of H₂O₂, whereas transformation with the antisense and especially with dominant-negative Rac constructs decreased the levels of H₂O₂.     

A Mechanism of Resistance to Hydrogen Peroxide in Vibrio rumoiensis S-1

A possible mechanism of resistance to hydrogen peroxide (H₂O₂) in Vibrio rumoiensis, isolated from the H₂O₂-rich drain pool of a fish processing plant, was examined. When V. rumoiensis cells were inoculated into medium containing either 5 mM or no H₂O₂, they grew in similar manners. A spontaneous mutant strain, S-4, derived from V. rumoiensis and lacking catalase activity did not grow at all in the presence of 5 mM H₂O₂. These results suggest that catalase is inevitably involved in the resistance and survival of V. rumoiensis in the presence of H₂O₂. Catalase activity was constitutively present in V. rumoiensis cells grown in the absence of H₂O₂, and its occurrence was dependent on the age of the cells, a characteristic which is observed for the HP II-type catalase of Escherichia coli. The presence of the HP II-type catalase in V. rumoiensis cells was evidenced by partial sequencing of the gene encoding the HP II-type catalase from this organism. A notable difference between V. rumoiensis and E. coli is that catalase is accumulated at very high levels (~2% of the total soluble proteins) in V. rumoiensis, in contrast to the case for E. coli. When V. rumoiensis cells which had been exposed to 5 mM H₂O₂ were centrifuged, most intracellular proteins, including catalase, were recovered in the medium. On the other hand, when V. rumoiensis cells were grown on plates containing various concentrations of H₂O₂, individual cells had a colony-forming ability inferior to those of E. coli, Bacillus subtilis, and Vibrio parahaemolyticus. Thus, it is suggested that when V. rumoiensis cells are exposed to high concentrations of H₂O₂, most cells will immediately be broken by H₂O₂. In addition, the cells which have had little or no damage will start to grow in a medium where almost all H₂O₂ has been decomposed by the catalase released from broken cells.