Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “PEROXIDASES”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 955 records · Page 53Linked to original sources
PubMed2006

Potential applications of immobilized bitter gourd (Momordica charantia) peroxidase in the removal of phenols from polluted water.

The potential applications of immobilized bitter gourd peroxidase in the treatment of model wastewater contaminated with phenols have been investigated. The synthetic water was treated with soluble and immobilized enzyme preparations under various experimental conditions. Maximum removal of phenols was found in the buffers of pH values 5.0-6.0 and at 40 degrees C in the presence of 0.75 mM H(2)O(2). Fourteen different phenols were independently treated with soluble and immobilized bitter gourd peroxidase in the buffer of pH 5.6 at 37 degrees C. Chlorinated phenols and native phenol were significantly removed while other substituted phenols were marginally removed by the treatment. Phloroglucinol and pyrogallol were recalcitrant to the action of bitter gourd peroxidase. Immobilized bitter gourd peroxidase preparation was capable of removing remarkably high percentage of phenols from the phenolic mixtures. Significantly higher level of total organic carbon was removed from the model wastewater containing individual phenol or complex mixture of phenols by immobilized bitter gourd peroxidase as compared to the soluble enzyme. 2,4-dichlorophenol and a phenolic mixture were also treated in a stirred batch reactor with fixed quantity of enzyme for longer duration. The soluble bitter gourd peroxidase ceased to function after 3h while the immobilized enzyme was active even after 6h of incubation with phenolic solutions.

Carbon↗
PubMed2006

Treatment of phenolic wastewater by horseradish peroxidase immobilized by bioaffinity layering.

Horseradish peroxidase was immobilized by bioaffinity layering and used for the treatment of wastewater containing p-chlorophenol. For this purpose, lectin Concanavalin A was bound to Sephadex beads. The glycoenzyme peroxidase was layered upon this Con A layer. Subsequently, alternate layers of the enzyme and Con A were applied. The most efficient design consisted of three layers of Con A and peroxidase each. This immobilized enzyme preparation retained 80% of the activity of the free peroxidase used for immobilization. PEG at the concentration of 0.1 mg ml(-1) was found to prevent enzyme inactivation by the products, although it increased the process time. Thus 60 U ml(-1) of enzyme completely converted the p-chlorophenol (into products) in 4 min in the absence of PEG. On the other hand, only 0.05 U ml(-1) of enzyme was required for this purpose in the presence of PEG but the process required 60 min. Peroxidase converts phenol molecules into free radicals. These free radicals then polymerize and get precipitated. As a further means of minimizing exposure of the enzyme to free radicals and enhancing the reusability, it was decided to remove the enzyme from reaction medium after 10 min. With this strategy, the bioaffinity layered peroxidase preparation could be reused five times without any loss of activity.

Chlorophenols↗
PubMed2005

Heme peroxidase clothing and inhibition with polyphenolic substances revealed by molecular modeling.

Molecular modeling techniques were applied to study oligomeric derivatives of phenols, which are produced during peroxidase-catalyzed oxidation. The interaction of substrates and oligomers with Arthromyces ramosus peroxidase (ARP) was analyzed by docking and molecular dynamics methods. The most possible interaction site of oligomers is the active center of the peroxidase. The affinity of oligomers increases with increasing length of oligomers. However, the complexed oligomers produce non-productive complexes with the peroxidase. Molecular dynamics studies showed that oligomer-peroxidase complexes are stable. It seems likely that strong and stable, but non-productive docking of the oligomers determinates peroxidase inhibition during the reaction by preventing the access of regular substrates to the active center of the enzyme.

Binding Sites↗
PubMed2006

Effect of Ilex extracts and isolated compounds on peroxidase secretion of rat submandibulary glands.

Free radicals are involved in diverse disorders such as tumoral, central nervous system alterations, immunological and inflammatory pathologies. Peroxidase is an oral enzyme involved in the defense of the oral cavity. Ilex species such as Ilex paraguariensis St. Hil. and the commercial product made with it "Yerba Mate" are used traditionally as antirheumatics and for the treatment of gastrointestinal diseases among others and also as a beverage with nutritional and stimulant properties. The presence of polyphenolic derivatives and flavonoids in the aqueous extract has been determined by HPLC analysis. In this study, the activity of aqueous extracts of I. paraguariensis and "Yerba Mate" on peroxidase secretion in female rat submandibular glands was investigated. The contribution to this pharmacological activity by some major hydrocynnamic acid derivatives present in the crude extracts, such as chlorogenic acid and caffeic acid and the most abundant methylxanthine, caffeine, was also evaluated. Spectrophotometrical determination of peroxidase activity showed that both extracts produced a significant increase in both secreted peroxidase and total peroxidase activity, though "Yerba Mate" showed a higher activity (EC(50) "Yerba Mate": 148+/-10 microg/ml; EC(50)I. paraguariensis: 841+/-20 microg/ml). The HPLC/DAD analysis of the crude extracts was performed and chlorogenic acid, caffeic acid and caffeine were identified and quantified. The results (expressed as W/W percentage of dried material) were as follows: I. paraguariensis: chlorogenic acid: 2.80+/-0.30, caffeic acid: 0.023+/-0.004, caffeine: 1.06+/-0.06; "Yerba Mate": chlorogenic acid: 1.98+/-0.37; caffeic acid: 0.020+/-0.003, caffeine: 0.70+/-0.06. Caffeine and chlorogenic acid were proved to play an important role in the induction of peroxidase secretion induced by the extracts.

Animals↗
PubMed2005

Molecular cloning and tissue-specific transcriptional regulation of the first peroxidase family member, Udp1, in stinging nettle (Urtica dioica).

A full-length cDNA clone, designated Udp1, was isolated from Urtica dioica (stinging nettle), using a polymerase chain reaction based strategy. The putative Udp1 protein is characterized by a cleavable N-terminal signal sequence, likely responsible for the rough endoplasmic reticulum entry and a 310 amino acids mature protein, containing all the important residues, which are evolutionary conserved among different members of the plant peroxidase family. A unique structural feature of the Udp1 peroxidase is defined into the short carboxyl-terminal extension, which could be associated with the vacuolar targeting process. Udp1 peroxidase is differentially regulated at the transcriptional level and is specifically expressed in the roots. Interestingly, wounding and ultraviolet radiation stress cause an ectopic induction of the Udp1 gene expression in the aerial parts of the plant. A genomic DNA fragment encoding the Udp1 peroxidase was also cloned and fully sequenced, revealing a structural organization of three exons and two introns. The phylogenetic relationships of the Udp1 protein to the Arabidopsis thaliana peroxidase family members were also examined and, in combination with the homology modelling approach, dictated the presence of distinct structural elements, which could be specifically involved in the determination of substrate recognition and subcellular localization of the Udp1 peroxidase.

Amino Acid Sequence↗
PubMed2005

Effect of cytochrome c peroxidase on corneal epithelial healing process after photorefractive keratectomy.

PURPOSE: To evaluate the role of commercially prepared cytochrome c peroxidase eyedrops in corneal epithelial healing after photorefractive keratectomy (PRK). SETTING: Department of Pathophysiological Optics, Faculty of Medicine, University of Bologna, Bologna, Italy. METHODS: Seventy-two eyes of 36 patients affected by low to moderate refractive error (myopia and myopic astigmatism) had uneventful bilateral photorefractive keratectomy (PRK). In each patient, 1 eye (32 eyes) received standard postoperative therapy plus cytochrome c peroxidase eyedrops (3 times a day for 1 week or until corneal reepithelialization was completed, corresponding to 15 000). The fellow eye served as the control and received standard postoperative therapy plus placebo. Patients were monitored daily starting the day after surgery for 7 days to evaluate the corneal reepithelialization rate using a video slitlamp camera with a cobalt blue light. Mean diameter of corneal wounds was measured. Videotaped images were recorded and analyzed by computer planimetry. RESULTS: All the eyes treated with cytochrome c peroxidase eyedrops healed completely before day 5 postsurgery, with a mean reepithelialization time of 91 hours +/- 14 (SD); the mean reepithelialization time was 154 +/- 9 in eyes receiving placebo (P<.05); the mean reepithelialization rate was 0.066 +/- 0.007 mm/hour in the cytochrome c peroxidase group and 0.039 +/- 0.006 mm/hour in the control group (P<.05). There were no statistically significant differences between groups in corneal haze presentation during follow-up (P =.70), perhaps because the time period was too brief (7 days). However, corneal clarity, on slitlamp biomicroscopy in the study group was greater than in the control group. No side effects or toxic effects were documented. CONCLUSIONS: These data suggest that cytochrome c peroxidase significantly accelerates epithelial healing after PRK. Further clinical study should be performed to prove the results obtained in this pilot study and the long-term efficacy of cytochrome c peroxidase to prevent corneal haze.

Adult↗
PubMed2006

Identification of Trp106 as the tryptophanyl radical intermediate in Synechocystis PCC6803 catalase-peroxidase by multifrequency Electron Paramagnetic Resonance spectroscopy.

The reactive intermediates formed in the catalase-peroxidase from Synechocystis PCC6803 upon reaction with peroxyacetic acid, and in the absence of peroxidase substrates, are the oxoferryl-porphyrin radical and two subsequent protein-based radicals that we have previously assigned to a tyrosyl (Tyr()) and tryptophanyl (Trp()) radicals by using multifrequency Electron Paramagnetic Resonance (EPR) spectroscopy combined with deuterium labeling and site-directed mutagenesis. In this work, we have further investigated the Trp() in order to identify the site for the tryptophanyl radical formation, among the 26 Trp residues of the enzyme and to possibly understand the protein constraints that determine the selective formation of this radical. Based on our previous findings about the absence of the Trp() intermediate in four of the Synechocystis catalase-peroxidase variants on the heme distal side (W122F, W106A, H123Q, and R119A) we constructed new variants on Trp122 and Trp106 positions. Trp122 is very close to the iron on the heme distal side while Trp106 belongs to a short stretch (11 amino acid residues on the enzyme surface) that is highly conserved in catalase-peroxidases. We have used EPR spectroscopy to characterize the changes on the heme microenvironment induced by these mutations as well as the chemical nature of the radicals formed in each variant. Our findings identify Trp106 as the tryptophanyl radical site in Synechocystis catalase-peroxidase. The W122H and W106Y variants were specially designed to mimic the hydrogen-bond interactions of the naturally occurring Trp residues. These variants clearly demonstrated the important role of the extensive hydrogen-bonding network of the heme distal side, in the formation of the tryptophanyl radical. Moreover, the fact that W106Y is the only Synechocystis catalase-peroxidase variant of the distal heme side that recovers a catalase activity comparable to the WT enzyme, strongly indicates that the integrity of the extensive hydrogen-bonding network is also essential for the catalatic activity of the enzyme.

Bacterial Proteins↗
PubMed2001

The molecular peculiarities of catalase-peroxidases.

In developing ideas of how protein structure modifies haem reactivity, the activity of Class I of the plant peroxidase superfamily (including cytochrome c peroxidase, ascorbate peroxidase and catalase-peroxidases (KatGs)) is an exciting field of research. Despite striking sequence homologies, there are dramatic differences in catalytic activity and substrate specificity with KatGs being the only member with substantial catalase activity. Based on multiple sequence alignment performed for Class I peroxidases, we present a hypothesis for the pronounced catalase activity of KatGs. In their catalytic domains KatGs are shown to possess three large insertions, two of them are typical for KatGs showing highly conserved sequence patterns. Besides an extra C-terminal copy of the ancestral hydroperoxidase gene resulting from gene duplication, these two large loops are likely to control the orientation of both the haem group and of essential residues in the active site. They seem to modulate the access of substrates to the prosthetic group at the distal side as well as the flexibility and character of the bond between the proximal histidine and the ferric iron. The hypothesis presented opens new possibilities in the rational engineering of peroxidases.

Amino Acid Sequence↗
PubMed2001

Reversed-phase and size-exclusion chromatography as useful tools in the resolution of peroxidase-mediated (+)-catechin oxidation products.

The peroxidase-catalysed oxidation of plant phenolics involves one-electron oxidation reactions, and yields unstable mono-radical species, which couple to generate heterogeneous product mixtures of different degrees of polymerisation. One such phenolic susceptible to oxidation by peroxidase is (+)-catechin. Low-pressure chromatography on Sephadex LH-20, using methanol as mobile phase, resolves the main peroxidase-mediated (+)-catechin oxidation products into a dimeric compound (dehydrodicatechin A) and an oligomeric fraction with a polymerisation degree equal or greater than 5. These pure fractions were used to develop rapid high-performance liquid chromatographic methods, both reversed-phase and size-exclusion chromatography for the direct analysis of the peroxidase-mediated (+)-catechin oxidation products. The joint use of both chromatographic systems permitted the qualitative and quantitative identification of the peroxidase-mediated (+)-catechin oxidation products, and can thus be considered as a useful tool for analysing the complex mixtures of natural bioactive plant products synthesized in reactions catalyzed by plant peroxidases.

Catechin↗
PubMed2001

Photosynthetic pigments and peroxidase activity as indicators of heavy metal stress in the Grey mangrove, Avicennia marina (Forsk.) Vierh.

Mangroves have been observed to possess a tolerance to high levels of heavy metals, yet accumulated metals may induce subcellular biochemical changes, which can impact on processes at the organism level. Six month-old seedlings of the grey mangrove, Avicennia marina (Forsk.) Vierh, were exposed to a range of Cu (0-800 micrograms/g), Pb (0-800 micrograms/g) and Zn (0-1000 micrograms/g) concentrations in sediments under laboratory conditions, to determine leaf tissue metal accumulation patterns, effects on photosynthetic pigments (chlorophyll a, chlorophyll b and carotenoids), and the activity of the antioxidant enzyme peroxidase. Limited Cu uptake to leaves was observed at low sediment Cu levels, with saturation and visible toxicity to Cu at sediment levels greater than 400 micrograms/g. Leaf Pb concentrations remained low over a range of Pb sediment concentrations, up to 400 micrograms/g Pb, above which it appeared that unrestricted transport of Pb occurred, although no visible signs of Pb toxicity were observed. Zn was accumulated linearly with sediment zinc concentration, and visible toxicity occurring at the highest concentration, 1000 micrograms/g Zn. Significant increases in peroxidase activity and decreases in photopigments were found with Cu and Zn at concentrations lower than those inducing visible toxicity. Significant increases in peroxidase activity only, were found when plants were exposed to Pb. Positive linear relationships between peroxidase activity and leaf tissue metal concentrations were found for all metals. Significant linear decreases in photosynthetic pigments with increasing leaf tissue metal concentrations were observed with Cu and Zn only. Photosynthetic pigments and peroxidase activity may be applicable as sensitive biological indicators of Cu and Zn stress, and peroxidase activity for Pb stress in A. marina.

Carotenoids↗
PubMed2000

A 37-kDa peroxidase secreted from liverworts in response to chemical stress.

A peroxidase was purified from the culture medium of a suspension culture of Marchantia polymorpha (liverwort) after treatment with bornyl acetate, which acts as a chemical stress agent to the cells. The peroxidase was characterised as a glycoprotein of molecular mass 37-kDa having a pl of about 10 and an optimal pH of 6.5. The peroxidase was thermally stable at 50 degrees C for up to 60 min. The partial amino acid sequence of the peroxidase was determined and found to be dissimilar to the amino acid sequences of other higher plant peroxidases. The oxidative polymerization of lunularin by this peroxidase was examined and the formation of a dimer, a trimer and a tetramer was demonstrated by negative ion Fast Atom Bombardment (FAB)-mass spectroscopy of the reaction products.

Bibenzyls↗
PubMed2002

Ascorbic acid and flavonoid-peroxidase reaction as a detoxifying system of H(2)O(2) in grapevine leaves.

Biosynthesis of both ascorbic acid (AsA) and peroxidase activity were induced by light in cv. Sultana grapevine leaves. Induced peroxidase activity mainly involved basic isoenzymes of pI 9.8 and 9.6 and catalyzed the oxidation of flavonoids like quercetin and kaempferol and derivatives of hydroxycinnamic acids such as ferulic and p-coumaric acids, but not AsA. However, the peroxidase-dependent oxidation of ferulic acid and quercetin was temporarily suppressed by AsA as long as it remained in the reaction medium. Kinetics and spectroscopic results indicated that AsA was oxidized to dehydroascorbic acid only in the presence of phenols or flavonoids, and did not interfere with the catalytic activity of the peroxidase. Ascorbate peroxidase isoenzymes (APx), whose activities are widely considered central for detoxification of H(2)O(2) in most plant cells, were not detected in grape leaves extracts. The significance of light stimulus on peroxidase activity and leaf AsA content is discussed in terms of a flavonoid-redox cycle proposed as an alternative system to detoxify H(2)O(2) in grapevine leaves.

Ascorbic Acid↗
PubMed2001

Kinetic studies on veratryl alcohol transformation by horseradish peroxidase.

A number of peroxidases, such as lignin peroxidase and manganese peroxidase have proved to be useful for industrial applications. Some studies on the effects of temperature and pH stability have been carried out. It is known that veratryl alcohol increases their stability in the range 28-50 degrees C and is oxidized, leading to veratryl aldehyde formation. Similar results with horseradish peroxidase (HRP) in the presence of cofactors were found, but the oxidation of veratryl alcohol in the absence of cofactors was extremely labile at acid pH and inactivated in a few minutes. Considering the growing industrial application of HRP, knowledge of its stability and denaturation kinetics is required. In this study, horseradish peroxidase pool (HRP-VI) and its isoenzymes HRP-VIII (acid) and HRP-IX (basic) have been shown to catalyze the oxidation of veratryl alcohol to veratryl aldehyde in the presence of hydrogen peroxide at pH 5.8 in the 35-45 degrees C range and in the absence of any cofactors. Heat and pH denaturation experiments in the presence and absence of veratryl alcohol incubation were conducted with HRP-VI and HRP-IX isoenzymes. HRP-IX was the most active isoenzyme acting on veratryl alcohol but HRP-VI was the most stable for the temperature range tested. At 35 degrees C the HRP pool presented decay constant (Kd) values of 5.5 x 10(-2) h(-1) and 1.4 10(-2) h(-1) in the absence and presence of veratryl alcohol, respectively, with an effective ratio of 3.9. These results present a new feature of peroxidases that opens one more interesting application of HRP to industrial processes.

Benzyl Alcohols↗
PubMed2003

Purification and characterization of a new cationic peroxidase from fresh flowers of Cynara scolymus L.

A basic heme peroxidase isoenzyme (AKPC) has been purified to homogeneity from artichoke flowers (Cynara scolymus L.). The enzyme was shown to be a monomeric glycoprotein, M(r)=42300+/-1000, (mean+/-S.D.) with an isoelectric point >9. The native enzyme exhibits a typical peroxidase ultraviolet-visible spectrum with a Soret peak at 404 nm (epsilon=137,000+/-3000 M(-1) cm(-1)) and a Reinheitzahl (Rz) value (A(404nm)/A(280nm)) of 3.8+/-0.2. The ultraviolet-visible absorption spectra of compounds I, II and III were typical of class III plant peroxidases but unlike horseradish peroxidase isoenzyme C, compound I was unstable. Resonance Raman and UV-Vis spectra of the ferric form show that between pH 5.0 and 7.0 the protein is mainly 6 coordinate high spin with a water molecule as the sixth ligand. The substrate-specificity of AKPC is characteristic of class III (guaiacol-type) peroxidases with chlorogenic and caffeic acids, that are abundant in artichoke flowers, as particularly good substrates at pH 4.5. Ferric AKPC reacts with hydrogen peroxide to yield compound I with a second-order rate constant (k(+1)) of 7.4 x 10(5) M(-1) s(-1) which is significantly slower than that reported for most other class III peroxidases. The reaction of ferric and ferrous AKPC with nitric oxide showed a potential use of this enzyme for quantitative spectrophotometric determination of NO and as a component of novel NO sensitive electrodes.

Amino Acid Sequence↗
PubMed1998

Rhythms of glutathione peroxidase and glutathione reductase in brain of chick and their inhibition by light.

Melatonin was recently shown to be a component of the antioxidative defense system of organisms due to its free radical scavenging and antioxidant activities. Pharmacologically, melatonin stimulates the activity of the peroxide detoxifying enzyme glutathione peroxidase in rat brain and in several tissues of chicks. In this report, we studied the endogenous rhythm of two antioxidant enzymes, glutathione peroxidase and glutathione reductase, in five regions (hippocampus, hypothalamus, striatum, cortex and cerebellum) of chick brain and correlated them with physiological blood melatonin concentrations. Glutathione peroxidase exhibited a marked 24 h rhythm with peak activity in each brain region which had acrophases about 8 h after lights off and about 4 h after the serum melatonin peak was detected. Glutathione reductase activity exhibited similar robust rhythms with the peaks occurring roughly 2 h after those of glutathione peroxidase. We suggest that neural glutathione peroxidase increases due to the rise of nocturnal melatonin levels while glutathione reductase activity rises slightly later possibly due to an increase of its substrate, oxidized glutathione. The exposure of chicks to constant light for 6 days eliminated the melatonin rhythm as well as the peaks in both glutathione peroxidase and glutathione reductase activities. These findings suggest that the melatonin rhythm may be related to the nighttime increases in the enzyme activities, although other explanations cannot be excluded.

Animals↗
PubMed1997

Molecular cloning and characterization of the epididymis-specific glutathione peroxidase-like protein secreted in the porcine epididymal fluid.

The epididymis-specific glutathione peroxidase was purified from the porcine cauda epididymal fluid in order to analyze its enzymatic activity and roles in the epididymis. The purified protein was found to consist of four identical 23 kDa subunits. The complementary DNA encoding the 23 kDa subunit was cloned from the cDNA library of the porcine proximal caput epididymis, only where the 23 kDa subunit is expressed. Although the selenocysteine codon (TGA) is contained in the cDNA of the other cytosolic type of glutathione peroxidases, it is replaced by cysteine codon (TGT) in the 23 kDa subunit cDNA, similarly to the results previously obtained for cDNAs encoding the epididymis-specific form of the secreted glutathione peroxidases of mouse, rat and monkey. By the direct analysis of the selenium, the purified protein was proved to contain no selenium atom in the molecule. The activities of the purified epididymis-specific glutathione peroxidase toward hydrogen peroxide or organic hydroperoxides were by far lower than the activity of cytosolic selenium-dependent glutathione peroxidase (less than 0.1%). In addition, the concentration of glutathione in the porcine epididymal fluids was about 20 microM, which is much lower than the optimal concentration for the glutathione peroxidase activity of the purified protein. These results strongly suggest that this protein is enzymatically quiescent at least in the porcine epididymal fluid. An immunocytochemical study showed that this protein was found to bind to the acrosomal region of the epididymal sperm and to disappear during the acrosome reaction. Furthermore, this protein significantly retarded the acrosome reaction induced in vitro. The possibilities have been discussed that it protects sperm from the premature acrosome reaction and maintains sperm fertilizing ability in the epididymis.

Amino Acid Sequence↗
PubMed2001

Aedes aegypti peroxidase gene characterization and developmental expression.

The functions of insect peroxidases include detoxification, stabilization of extracellular matrices, and possible involvement in insect immunity. The current study describes the isolation of a peroxidase gene, AePox, and its cDNA from the mosquito, Aedes aegypti. AePox codes for a protein that is homologous to various heme-peroxidases from vertebrates and invertebrates, with highest identity to Drosophila melanogaster peroxidase (62%). Sequence comparison identified several functionally and structurally conserved domains in the mosquito peroxidase, including a heme environment, a calcium binding site, and five possible disulfide bridges. These results imply that AePOX may likely have a similar structure and catalytic mechanism as those described for the mammalian myeloperoxidase superfamily. Expression studies demonstrate that AePox is transcribed in mosquito larvae and pupae, but not in adults, in ovaries, or in early embryos. However, AePOX protein is present in all mosquito stages and possibly has a maturation process that is similar to that of human myeloperoxidase. Unlike most human peroxidases, the AePox gene contains a TATA box and an ecdysone response element (EcRE).

Aedes↗
PubMed2000

Catalase-peroxidase (Mycobacterium tuberculosis KatG) catalysis and isoniazid activation.

Resonance Raman spectra of native, overexpressed M. tuberculosis catalase-peroxidase (KatG), the enzyme responsible for activation of the antituberculosis antibiotic isoniazid (isonicotinic acid hydrazide), have confirmed that the heme iron in the resting (ferric) enzyme is high-spin five-coordinate. Difference Raman spectra did not reveal a change in coordination number upon binding of isoniazid to KatG. Stopped-flow spectrophotometric studies of the reaction of KatG with stoichiometric equivalents or small excesses of hydrogen peroxide revealed only the optical spectrum of the ferric enzyme with no hypervalent iron intermediates detected. Large excesses of hydrogen peroxide generated oxyferrous KatG, which was unstable and rapidly decayed to the ferric enzyme. Formation of a pseudo-stable intermediate sharing optical characteristics with the porphyrin pi-cation radical-ferryl iron species (Compound I) of horseradish peroxidase was observed upon reaction of KatG with excess 3-chloroperoxybenzoic acid, peroxyacetic acid, or tert-butylhydroperoxide (apparent second-order rate constants of 3.1 x 10(4), 1.2 x 10(4), and 25 M(-1) s(-1), respectively). Identification of the intermediate as KatG Compound I was confirmed using low-temperature electron paramagnetic resonance spectroscopy. Isoniazid, as well as ascorbate and potassium ferrocyanide, reduced KatG Compound I to the ferric enzyme without detectable formation of Compound II in stopped-flow measurements. This result differed from the reaction of horseradish peroxidase Compound I with isoniazid, during which Compound II was stably generated. These results demonstrate important mechanistic differences between a bacterial catalase-peroxidase and the homologous plant peroxidases and yeast cytochrome c peroxidase, in its reactions with peroxides as well as substrates.

Bacterial Proteins↗