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PubMed2003

Crystal structure of the ascorbate peroxidase-ascorbate complex.

Heme peroxidases catalyze the H2O2-dependent oxidation of a variety of substrates, most of which are organic. Mechanistically, these enzymes are well characterized: they share a common catalytic cycle that involves formation of a two-electron, oxidized Compound I intermediate followed by two single-electron reduction steps by substrate. The substrate specificity is more diverse--most peroxidases oxidize small organic substrates, but there are prominent exceptions--and there is a notable absence of structural information for a representative peroxidase-substrate complex. Thus, the features that control substrate specificity remain undefined. We present the structure of the complex of ascorbate peroxidase-ascorbate. The structure defines the ascorbate-binding interaction for the first time and provides new rationalization of the unusual functional features of the related cytochrome c peroxidase enzyme, which has been a benchmark for peroxidase catalysis for more than 20 years. A new mechanism for electron transfer is proposed that challenges existing views of substrate oxidation in other peroxidases.

Amino Acid Sequence↗

PubMed2004

Enzyme-catalyzed mechanism of isoniazid activation in class I and class III peroxidases.

There is an urgent need to understand the mechanism of activation of the frontline anti-tuberculosis drug isoniazid by the Mycobacterium tuberculosis catalase-peroxidase. To address this, a combination of NMR spectroscopic, biochemical, and computational methods have been used to obtain a model of the frontline anti-tuberculosis drug isoniazid bound to the active site of the class III peroxidase, horseradish peroxidase C. This information has been used in combination with the new crystal structure of the M. tuberculosis catalase-peroxidase to predict the mode of INH binding across the class I heme peroxidase family. An enzyme-catalyzed mechanism for INH activation is proposed that brings together structural, functional, and spectroscopic data from a variety of sources. Collectively, the information not only provides a molecular basis for understanding INH activation by the M. tuberculosis catalase-peroxidase but also establishes a new conceptual framework for testing hypotheses regarding the enzyme-catalyzed turnover of this compound in a number of heme peroxidases.

Antitubercular Agents↗

PubMed1958

Colorimetric analysis with N, N-dimethyl-p-phenylenediamine of the uptake of intravenously injected horseradish peroxidase by various tissues of the rat.

1. A method is described for the colorimetric determination of peroxidase with N,N-dimethyl-p-phenylenediamine. The amount of red pigment formed by peroxidase is proportional to the concentration of enzyme and to the time of incubation during the first 40 to 90 seconds. The influence of the concentration of enzyme, N,N-dimethyl-p-phenylenediamine, H(2)O(2), the time of incubation, pH, the temperature, and the possible interference by oxidizing and reducing agents of tissues has been tested. 2. The method has been used to follow the uptake of intravenously injected horseradish peroxidase by 18 different tissues of the rat over a period of 30 hours. The highest concentration of the injected tracer enzyme was found in extracts of kidney, liver, bone marrow, thymus, and spleen. Considerable amounts were taken up by pancreas, prostate, epididymis, and small intestine. Lower concentrations were found in extracts of lung, stomach, heart, and skeletal muscle, aorta, skin, and connective tissue. No uptake was observed by brain and peripheral nerve tissue. 3. Tissue homogenates containing high concentrations of the injected peroxidase, in general also showed high or average activity of acid phosphatase. 4. Six hours after intravenous administration, the liver contained 27 per cent, the kidney 12 per cent, and the spleen, 1.4 per cent of the injected dose. 5. Approximately 20 per cent of the injected peroxidase was excreted in the urine during the first 6 hours, and the concentration of peroxidase in blood serum and urine fell exponentially during this time. After 6 hours, only low concentrations were excreted in the urine but low enzyme activity was still detectable after 30 hours. Approximately 6 per cent of the injected dose was excreted in the feces from 6 to 20 hours after administration. 6. After feeding through a stomach tube, low concentrations of peroxidase were found in blood serum and urine. Considerable variations in the extent of absorption from the gastrointestinal tract were observed in individual rats.

Animals↗

PubMed1983

Evidence for thiocyanate-sensitive peroxidase activity in human saliva.

A procedure was developed for determining the relative levels of lactoperoxidase, leukocyte myeloperoxidase, and thiocyanate-sensitive peroxidase in human saliva. With this procedure, most of the peroxidase activity in whole saliva from normal (those without cancer) subjects was found to be associated with lactoperoxidase and thiocyanate-sensitive peroxidase, with only a minor contribution from leukocyte myeloperoxidase. In contrast, thiocyanate-sensitive peroxidase and leukocyte myeloperoxidase were the major peroxidase activities present in the residual salivary secretion obtainable from two xerostomic patients examined, and these enzymes were present at concentrations much higher than those normally occurring in human saliva. The occurrence of thiocyanate-sensitive peroxidase in saliva has not been previously reported and may represent either an additional peroxidase activity of saliva or a form of lactoperoxidase which is particularly sensitive to inhibition by thiocyanate.

Animals↗

PubMed1976

Peroxidase-catalyzed halogenation.

Peroxidase-catalyzed halogenation reactions have been established as being important in the biosynthesis of the hormone thyroxine and in biological defense mechanisms. Recently these reactions have been recognized as valuable tools for the study of proteins as well as their arrangement in macromolecular structures. The pathways of peroxidase catalyses can be accommodated within the framework of the classical Chance-George mechanism. This implies that the initial steps of the reaction invariably involve oxidation of peroxidases by peroxides--and that the resulting derivative, compound I, is the oxidant of the halide ions. Such reactions may result either in the formation of hypohalous acids, or in halogenation of the enzyme apoprotein, followed by transhalogenation to substrate for halogenation. Chloro- and myeloperoxidases catalyze oxidation of all halide ions, except F-; oxidation of bromide and iodide is mediated by lactoperoxidase, but horseradish peroxidase only oxidizes iodide. All of the above enzymes except horseradish will oxidize the pseudo halide thiocyanate. The origins of this differentiation remain to be defined, but they presumably reflect significant variation in oxidation potential of different peroxidase-peroxide derivatives, rather than constraints on the peroxidase-donor interactions. As pointed out above, halogenation of the amino acids tyrosine and histidine or these residues in proteins can take place on the enzyme. This makes lactoperoxidase-catalyzed iodination selective. The amino acid residues in proteins that are iodinated depend not only on reactivity of the amino acid residue but also on its geometric location. Thus lactoperoxidase-catalyzed iodination can be a useful tool in the study of protein structure and function. It is also useful in establishing the geometric position of proteins within macromolecular structures. Thyroid peroxidase catalyzes iodination of thyroglobulin and is involved in a second important step, the coupling of the iodotyrosines to form thyroxine or triiodothyronine. A proposed mechanism for this reaction suggests that the oxidation is mediated by the iodoenzyme derivative mentioned above followed by a prototropic rearrangement and scission to form the ether bound of thyronine and a serine residue on thyroglobulin.

Amino Acids↗

PubMed1982

One- and two-electron oxidations of tyrosine, monoiodotyrosine, and diiodotyrosine catalyzed by hog thyroid peroxidase.

Stopped flow experiments were carried out with purified hog thyroid peroxidase (A413 nm/A280 nm = 0.42). In the steady state of oxidations of L- and D-tyrosines, N-acetyltyrosinamide, and monoiodotyrosine, thyroid peroxidase existed in the form of Compound I, the primary catalytic intermediate of peroxidase in its reaction with H2O2. Kinetic results led us to conclude that thyroid peroxidase catalyzes two-electron oxidations of these molecules. In the steady state of oxidation of diiodotyrosine, on the other hand, the enzyme was found in the form of compound II at pH 7.4, but in the form of compound I at pH 5.5. The result implies that the mechanism of diiodotyrosine oxidation varied from a one-electron to a two-electron type as the pH decreased. The selection of mechanisms of oxidation appears to be peculiar to thyroid peroxidase; horseradish peroxidase and lactoperoxidase catalyzed only one-electron oxidations of these five donor molecules. Rate constants for rate-limiting steps in the reactions of these donor molecules with the three peroxidases were measured by overall kinetic and stopped flow kinetic methods.

Animals↗

PubMed1986

Prostaglandin H synthase and hydroperoxides: peroxidase reaction and inactivation kinetics.

The reaction kinetics of the peroxidase activity of prostaglandin H synthase have been examined with 15-hydroperoxyeicosatetraenoic acid and hydrogen peroxide as substrates and tetramethylphenylenediamine as cosubstrate. The apparent Km and Vmax values for both hydroperoxides were found to increase linearly with the cosubstrate concentration. The overall reaction kinetics could be interpreted in terms of an initial reaction of the synthase with hydroperoxide to form an intermediate equivalent to horseradish peroxidase Compound I, followed by reduction of this intermediate by cosubstrate to regenerate the resting enzyme. The rate constants estimated for the generation of synthase Compound I were 7.1 X 10(7) M-1 s-1 with the lipid hydroperoxide and 9.1 X 10(4) M-1 s-1 with hydrogen peroxide. The rate constants estimated for the rate-determining step in the regeneration of resting enzyme by cosubstrate were 9.2 X 10(6) M-1 s-1 in the case of the reaction with lipid hydroperoxide and 3.5 X 10(6) M-1 s-1 in the case of reaction with hydrogen peroxide. The intrinsic affinities of the synthase peroxidase for substrate (Ks) were estimated to be on the order of 10(-8) M for lipid hydroperoxide and 10(-5) M for hydrogen peroxide. These affinities are quite similar to the reported affinities of the synthase for these hydroperoxides as activators of the cyclooxygenase. The peroxidase activity was found to be progressively inactivated during the peroxidase reaction. The rate of inactivation of the peroxidase was increased by increases in hydroperoxide level, and decreased by increases in peroxidase cosubstrate. The inactivation of the peroxidase appeared to occur by a hydroperoxide-dependent process, originating from synthase Compound I or Compound II.

Animals↗

PubMed1998

Anaerobic stopped-flow studies of indole-3-acetic acid oxidation by dioxygen catalysed by horseradish C and anionic tobacco peroxidase at neutral pH: catalase effect.

The effect of order of reagent mixing in the absence and in the presence of catalase on the transient kinetics of indole-3-acetic acid (IAA) oxidation by dioxygen catalysed by horseradish peroxidase C and anionic tobacco peroxidase at neutral pH has been studied. The data suggest that haem-containing plant peroxidases are able to catalyse the reaction in the absence of exogenous hydroperoxide. The initiation proceeds via the formation of the ternary complex enzyme-->IAA-->oxygen responsible for IAA primary radical generation. The horseradish peroxidase-catalysed reaction is independent of catalase indicating a significant contribution of free radical processes into the overall mechanism. This is in contrast to the tobacco peroxidase-catalysed reaction where the peroxidase cycle plays an important role. The transient kinetics of IAA oxidation catalysed by tobacco peroxidase exhibits a biphasic character with the first phase affected by catalase. The first phase is therefore associated with the common peroxidase cycle while the second is ascribed to native enzyme interaction with skatole peroxy radicals yielding directly Compound II.

Journal Article↗

PubMed1998

Identification of skatolyl hydroperoxide and its role in the peroxidase-catalysed oxidation of indol-3-yl acetic acid.

Indol-3-yl acetic acid (IAA, auxin) is a plant hormone whose degradation is a key determinant of plant growth and development. The first evidence for skatolyl hydroperoxide formation during the plant peroxidase-catalysed degradation of IAA has been obtained by electrospray MS. Skatolyl hydroperoxide degrades predominantly non-enzymically to oxindol-3-yl carbinol but in part enzymically into indol-3-yl methanol via a peroxidase cycle in which IAA acts as an electron donor. Skatolyl hydroperoxide is degradable by catalase. Horseradish peroxidase isoenzyme C (HRP-C) and anionic tobacco peroxidase (TOP) exhibit differences in their mechanisms of reaction. The insensitivity of the HRP-C-catalysed reaction to catalase is ascribed to the formation of HRP-C Compound III at the initiation step and its subsequent role in radical propagation. This is in contrast with the TOP-catalysed process in which skatolyl hydroperoxide has a key role. Indol-3-yl aldehyde is produced not via the peroxidase cycle but by catalysis involving ferrous peroxidase. Because indol-3-yl aldehyde is one of the main IAA-derived products identified in planta, we conclude that ferrous peroxidases participate in IAA catalytic transformations in vivo. A general scheme for peroxidase-catalysed IAA oxidation is presented.

Carbon Monoxide↗

PubMed1994

Inhibition of Apoplastic and Symplastic Peroxidase Activity from Norway Spruce by the Photooxidant Hydroxymethyl Hydroperoxide.

Young, clonal Norway spruce trees (Picea abies L.) were exposed for 2 years at high altitudes to ambient atmospheric concentrations of photooxidants containing hydroxymethyl hydroperoxide (HMHP) as an important constituent. In spruce needles from a site with higher concentrations of organic peroxides in air, the apoplastic peroxidase activities were significantly lower than in needles exposed to lower organic peroxide concentrations. Guaiacol peroxidase activities in total needle extracts were not affected. In vitro HMHP at a concentration of 35 [mu]M inhibited apoplastic and total needle guaiacol peroxidase activities by 50% at pH 5.25. At the same pH, ascorbate-specific peroxidase activity required about 100 [mu]M HMHP for 50% inhibition. At pH 7, 1.46 mM HMHP caused a 50% reduction in guaiacol peroxidase and a 13% reduction in ascorbate peroxidase activity. The present results suggest that HMHP in ambient air may affect peroxidase activity in spruce needles. Peroxidases located in the relatively acidic aqueous phase of the cell walls appear to be more susceptible to HMHP inhibition than those present in neutral or slightly alkaline symplastic compartments of cells such as the cytosol or chloroplasts.

Journal Article↗

PubMed1998

Localized changes in peroxidase activity accompany hydrogen peroxide generation during the development of a nonhost hypersensitive reaction in lettuce

Peroxidase activity was characterized in lettuce (Lactuca sativa L.) leaf tissue. Changes in the activity and distribution of the enzyme were examined during the development of a nonhost hypersensitive reaction (HR) induced by Pseudomonas syringae (P. s.) pv phaseolicola and in response to an hrp mutant of the bacterium. Assays of activity in tissue extracts revealed pH optima of 4.5, 6.0, 5.5 to 6.0, and 6.0 to 6.5 for the substrates tetramethylbenzidine, guaiacol, caffeic acid, and chlorogenic acid, respectively. Inoculation with water or with wild-type or hrp mutant strains of P. s. pv phaseolicola caused an initial decline in total peroxidase activity; subsequent increases depended on the hydrogen donor used in the assay. Guaiacol peroxidase recovered more rapidly in tissues undergoing the HR, whereas changes in tetramethylbenzidine peroxidase were generally similar in the two interactions. In contrast, increases in chlorogenic acid peroxidase were significantly higher in tissues inoculated with the hrp mutant. During the HR, increased levels of Mn2+/2, 4-dichlorophenol-stimulated NADH and NADPH oxidase activities, characteristic of certain peroxidases, were found in intercellular fluids and closely matched the accumulation of H2O2 in the apoplast. Histochemical analysis of peroxidase distribution by electron microscopy revealed a striking, highly localized increase in activity within the endomembrane system and cell wall at the sites of bacterial attachment. However, no clear differences in peroxidase location were observed in tissue challenged by the wild-type strain or the hrp mutant. Our results highlight the significance of the subcellular control of oxidative reactions leading to the generation of reactive oxygen species, cell wall alterations, and the HR.

Journal Article↗

PubMed1969

Studies on polyphenol content, activities and isozymes of polyphenol oxidase and peroxidase during air-curing in three tobacco types.

The change in polyphenol content in the primed leaves of burley, flue-cured, and Turkish tobaccos during air-curing was related to the activities and isozymes of polyphenol oxidase and peroxidase. The quantity of chlorogenic acid was rapidly reduced during the first week of curing. The decrease in rutin content during curing was less significant, especially when the concentration of chlorogenic acid was high in leaf tissues. This result was further confirmed by in vitro assays with partially purified tobacco polyphenol oxidase.The polyphenol oxidase activity did not differ at any stage of curing in the 3 tobaccos. When the activity was measured by the oxidation of 3,4-dihydroxyphenylalanine it rose rapidly during the first day of curing and then decreased sharply so that in the fully cured leaf only 15% activity remained. The increase in activity was not observed when chlorogenic acid was used as the substrate. A similar level of peroxidase activity was found in the 3 tobaccos before curing. Peroxidase activities increased rapidly during the first 24 hr of curing, declined thereafter, and remained highest in the flue-cured tobacco, less in the Turkish line, and least in the burley at the end of curing process.By polyacrylamide gel block electrophoresis, 10 peroxidase isozyme bands, 2 cationic and 8 anionic, appeared identical in all 3 tobaccos. When catechol replaced benzidine-2 HCl as the electron donor, 1 cationic and 2 anionic peroxidase isozymes did not form. Of interest is that the same 10 peroxidase isozyme bands also exhibited polyphenol oxidase activities when treated with 3,4-dihydroxyphenylalanine or chlorogenic acid. Results suggest that in the crude tobacco leaf extract the peroxidase and polyphenol oxidase may associate as protein complexes, and peroxidase isozymes may differ in electron-donor requirements. Isozyme patterns for both oxidases at various curing intervals differed only quantitatively.

Journal Article↗

PubMed1984

Peroxidase Release Induced by Ozone in Sedum album Leaves: Involvement of Ca.

The effect of ozone was studied on the peroxidase activity from various compartments of Sedum album leaves (epidermis, intercellular fluid, residual cell material, and total cell material). The greatest increase following a 2-hour ozone exposure (0.4 microliters O(3) per liter) was observed in extracellular peroxidases. Most of the main bands of peroxidase activity separated by isoelectric focusing exhibited an increase upon exposure to ozone. Incubation experiments with isolated peeled or unpeeled leaves showed that leaves from ozone-treated plants release much more peroxidases in the medium than untreated leaves. The withdrawal of Ca(2+) ions reduced the level of extracellular peroxidase activity either in whole plants or in incubation experiments. This reduction and the activation obtained after addition of Ca(2+) resulted from a direct requirement of Ca(2+) by the enzyme and from an effect of Ca(2+) on peroxidase secretion. The ionophore A23187 promoted an increase of extracellular peroxidase activity only in untreated plants. The release of peroxidases by untreated and ozone-treated leaves is considerably lowered by metabolic inhibitors (3-(3,4-dichlorophenyl)-1,1-dimethylurea and sodium azide) and by puromycin.

Journal Article↗

PubMed1985

Peroxidase Activity in Relation to Suberization and Respiration in White Spruce (Picea glauca [Moench] Voss) Seedling Roots.

Peroxidase (EC 1.11.1.7) activity is associated with suberization during endodermal development and metacutization in roots of white spruce (Picea glauca [Moench] Voss) seedlings. Histochemical analysis indicates a relationship between suberization and peroxidase activity, but peroxidase is ubiquitous. Increased peroxidase activity results from the induction of four anodic peroxidase isozymes in addition to quantitative increases in two anodic peroxidase isozymes. Four of these polymerized eugenol. Cold temperatures induce formation of two anodic isozymes and result in suberization. The increased peroxidase activity associated with suberization is correlated to residual respiration. In an attempt to elucidate this relationship, the effect of respiratory inhibitors on respiration and peroxidase activity are compared.

Journal Article↗

PubMed1989

Hormonal regulation and distribution of peroxidase isoenzymes in the Cucurbitaceae.

Ethylene enhanced the levels of peroxidases in the roots, stems, leaves, and cotyledons of 2-week-old cucumber Cucumis sativus cv Poinsett 76 seedlings. Antibodies to the isoelectric point (pl) 9 and pl 4 isoenzymes were used in a radial immuno-diffusion assay to demonstrate that ethylene induced similar peroxidases in other cultivars of C. sativus, other species of Cucumis and other genera of Cucurbitaceae. Examination of ethylene-induced peroxidases, using isoelectric focusing gels, demonstrated the presence of a series of other peroxidases, mostly slightly acidic, whose isoelectric focusing pH was approximately 6. These pl 6 peroxidases were partially purified on a cation exchange column. Ouchterlony double diffusion gels indicated that these proteins cross-reacted with antibodies to both the pl 9 and pl 4 peroxidase. The data presented here suggest that the induction of peroxidase isoenzymes during ethylene-induced senescence is a common response in this family of plants. In addition, antibody and isoelectric focusing studies indicate that both acidic and basic peroxidase are highly conserved in members of this family.

Journal Article↗

PubMed1990

Anions activate the oxidation of indoleacetic Acid by peroxidases from tomato and other sources.

Anionic peroxidase from tomato (Lycopersicon esculentum) fruit oxidized indoleacetic acid (IAA) slowly in the presence of Mn(2+) and dichlorophenol in acetate buffers. The addition of certain anions to the reaction mixture increased the rate of oxidation. Phosphate was one of the effective anions and exerted maximal activation at 0.1 molar. The most effective activator of tomato peroxidase was nitrilotriacetate (NTA) at an optimum concentration of 60 micromolar. Only 0.17 nanomolar peroxidase was needed to oxidize 0.1 micromole IAA/5 minutes in the presence of NTA compared to 650 nanomolar peroxidase for the same rate in the absence of NTA. Other effective anions were oxalate, pyrophosphate, malate, and citrate. Each activator exhibited an optimum concentration and higher concentrations were inhibitory. Anionic peroxidase from horseradish was activated by the same anions. A cationic peroxidase from horseradish and lactoperoxidase oxidized IAA in acetate buffer although anions activated these enzymes severalfold. Microperoxidase and other hematoporphrins also catalyzed IAA oxidation in the presence of anions. It is proposed that IAA oxidation by peroxidase may be important when vacuolar contents mix with peroxidase as during plant injury.

Journal Article↗

PubMed2000

Molecular characterization and physiological role of ascorbate peroxidase from halotolerant Chlamydomonas sp. W80 strain.

A cDNA clone encoding an ascorbate peroxidase was isolated from the cDNA library from halotolerant Chlamydomonas W80 by a simple screening method based on the bacterial expression system. The cDNA clone contained an open reading frame encoding a mature protein of 282 amino acids with a calculated molecular mass of 30,031 Da, preceded by the chloroplast transit peptide consisting of 37 amino acids. In fact, ascorbate peroxidase was localized in the chloroplasts of Chlamydomonas W80 cells; the activity was detected in the stromal fraction but not in the thylakoid membrane. The deduced amino acid sequence of the cDNA showed 54 and 49% homology to chloroplastic and cytosolic ascorbate peroxidase isoenzymes of spinach leaves, respectively. The enzyme from Chlamydomonas W80 cells was purified to electrophoretic homogeneity. The molecular properties of the purified enzyme were similar to those of the other algal ascorbate peroxidases rather than those of ascorbate peroxidases from higher plants. The enzyme was relatively stable in ascorbate-depleted medium compared with the chloroplastic ascorbate peroxidase isoenzymes of higher plants. The presence of NaCl (3%) as well as of beta-d-thiogalactopyranoside was needed for the expression of Chlamydomonas W80 ascorbate peroxidase in Escherichia coli.

Amino Acid Sequence↗

PubMed2001

Ascorbate-independent electron transfer between cytochrome b561 and a 27 kDa ascorbate peroxidase of bean hypocotyls.

Cytochrome b561 (cyt b561) is a trans-membrane cytochrome probably ubiquitous in plant cells. In vitro, it is readily reduced by ascorbate or by juglonol, which in plasma membrane (PM) preparations from plant tissues is efficiently produced by a PM-associated NAD(P)H:quinone reductase activity. In bean hypocotyl PM, juglonol-reduced cyt b561 was not oxidized by hydrogen peroxide alone, but hydrogen peroxide led to complete oxidation of the cytochrome in the presence of a peroxidase found in apoplastic extracts of bean hypocotyls. This peroxidase active on cyt b561 was purified from the apoplastic extract and identified as an ascorbate peroxidase of the cytosolic type. The identification was based on several grounds, including the ascorbate peroxidase activity (albeit labile), the apparent molecular mass of the subunit of 27 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the dimeric native structure, the typical spectral properties of a heme-containing peroxidase, and an N-terminal sequence strongly conserved with cytosolic ascorbate peroxidases of plants. Cyt b561 used in the experiments was purified from bean hypocotyl PM and juglonol was enzymatically produced by recombinant NAD(P)H:quinone reductase. It is shown that NADPH, NAD(P)H:quinone reductase, juglone, cyt b561, the peroxidase interacting with cyt b561, and H2O2, in this order, constitute an artificial electron transfer chain in which cyt b561 is indirectly reduced by NADPH and indirectly oxidized by H2O2.

Amino Acid Sequence↗