Kinetic and molecular orbital studies on the rate of oxidation of monosubstituted phenols and anilines by horseradish peroxidase compound II

The second-order rate constant (k4) for the oxidation of a series of aromatic donor molecules (monosubstituted phenols and anilines) by horseradish peroxidase (HRP) compound II was examined with a stopped-flow apparatus. The electronic states of these substrates were calculated by an ab initio molecular orbital method. It was found that in both phenols and anilines log k4 values correlate well with the highest occupied molecular orbital (HOMO) energy level and the lowest unoccupied molecular orbital (LUMO) energy level, but not with the net charge or frontier electron density on atoms of these molecules. The HOMO and LUMO energy levels of phenols and anilines further showed linear relationships with Hammett's sigma values with negative slopes. Similar results were obtained in the oxidation of substrates by HRP compound I, except that the rate of reaction was much higher than in the case of HRP compound II. In addition, the rates of oxidation of phenols by compound I or II were found to be about 1000 times higher than those of anilines with similar HOMO energy levels. On the basis of these results, the mechanism of electron transfer from the substrate to the heme iron of HRP compound II is discussed.     

Enhancement of Lipid Peroxidation by Indole-3-Acetic Acid and Derivatives: Substituent Effects

The peroxidation of liposomes by a haem peroxidase and hydrogen peroxide in the presence of indole-3-acetic acid and derivatives was investigated. It was found that these compounds can accelerate the lipid peroxidation up to 65 fold and this is attributed to the formation of peroxyl radicals that may react with the lipids, possibly by hydrogen abstraction. The peroxyl radicals are formed by peroxidase-catalyzed oxidation of the enhancers to radical cations which undergo cleavage of the carbon-carbon bond on the side-chain to yield CO2 and carbon-centred radicals that rapidly add oxygen. In competition with decarboxylation, the radical cations deprotonate reversibly from the Nl position. Rates of decarboxylation,pK_a values and rate of reaction with the peroxidase compound I indicate consistent substituent effects which, however, can not be quantitatively related to the usual Hammett or Brown parameters. Assuming that the rate of decarboxylation of the radical cations taken is a measure of the electron density of the molecule (or radical), it is found that the efficiency of these compounds as enhancers of lipid peroxidation increases with increasing electron density, suggesting that, at least in the model system, the oxidation of the substrates is the limiting step in causing lipid peroxidation.     

Hydroxyurea and p-aminophenol are the suicide inhibitors of ascorbate peroxidase

Guaiacol peroxidase from spinach catalyzes the oxidation of p-aminophenol to produce the aminophenoxy radical as the primary product which is converted further into a stable oxidation product with an absorption peak at 470 nm. The p-aminophenol radicals oxidize ascorbate (AsA) to produce monodehydroascorbate radicals. Kinetic analysis indicates that p-aminophenol radicals also oxidize monodehydroascorbate to dehydroascorbate. Incubation of AsA peroxidase from tea leaves and hydrogen peroxide with p-aminophenol, p-cresol, hydroxyurea, or hydroxylamine results in the inactivation of the enzyme. No inactivation of the enzyme was found upon incubation of the enzyme with these compounds either in the absence of hydrogen peroxide or with the stable oxidized products of these compounds. The enzyme was protected from inactivation by the inclusion of AsA in the incubation mixture. The radicals of p-aminophenol and hydroxyurea were produced by AsA peroxidase as detected by their ESR signals. These signals disappeared upon the addition of AsA, and the signal characteristic of monodehydroascorbate was found. Thus, AsA peroxidase is inactivated by the radicals of p-aminophenol, p-cresol, hydroxyurea, and hydroxylamine which are produced by the peroxidase reaction, and it is protected from inactivation by AsA via the scavenging of the radicals. Thus, these compounds are the suicide inhibitors for AsA peroxidase. Isozyme II of AsA peroxidase, which is localized in chloroplasts, is more sensitive to these compounds than isozyme I. In contrast to AsA peroxidase, guaiacol peroxidase was not affected by these various compounds, even though each was oxidized by it and the corresponding radicals were produced.     

Enhancement of the peroxidase-mediated oxidation of butylated hydroxytoluene to a quinone methide by phenolic and amine compounds

We have recently demonstrated that butylated hydroxyanisole (BHA) markedly stimulates the peroxidase-dependent oxidation of butylated hydroxytoluene (BHT) to the potentially toxic BHT-quinone methide. Using both horseradish peroxidase and prostaglandin H synthase we now report the ability of a wide variety of compounds to stimulate peroxidase-dependent activation of BHT. These compounds include several phenolic compounds commonly present in pharmacologic preparations or occurring naturally in foods. The ability of a given compound to stimulate BHT oxidation was found to depend on the type of radical it forms upon peroxidase oxidation. Compounds which have been shown to form phenoxy radicals or nitrogen-centered cation radicals were observed to enhance BHT oxidation. Conversely, compounds which are known to form peroxy radicals or semiquinone radicals either inhibited or had no effect on BHT oxidation. Compounds which enhanced BHT oxidation (monitored by covalent binding of [14C]BHT to protein) were also observed to stimulate the formation of BHT-quinone methide and stilbenequinone. This suggested a common mechanism of interaction of these compounds with BHT. The stimulation of BHT covalent binding by BHA was also seen in various human and animal tissues using either arachidonic acid or hydrogen peroxide as substrate. The possible toxicologic implications of the enhancement of peroxidase-catalyzed BHT oxidation to BHT-quinone methide are discussed.     

Oxidation of 4-tert-butylcatechol and dopamine by hydrogen peroxide catalysed by horseradish peroxidase.

The catalytic cycle of horseradish peroxidase (HRP; donor:hydrogen peroxide oxidoreductase; EC 1.11.1.7) is initiated by a rapid oxidation of it by hydrogen peroxide to give an enzyme intermediate, compound I, which reverts to the resting state via two successive single electron transfer reactions from reducing substrate molecules, the first yielding a second enzyme intermediate, compound II. To investigate the mechanism of action of horseradish peroxidase on catechol substrates we have studied the oxidation of both 4-tert-butylcatechol and dopamine catalysed by this enzyme. The different polarity of the side chains of both o-diphenol substrates could help in the understanding of the nature of the rate-limiting step in the oxidation of these substrates by the enzyme. The procedure used is based on the experimental data to the corresponding steady-state equations and permitted evaluation of the more significant individual rate constants involved in the corresponding reaction mechanism. The values obtained for the rate constants for each of the two substrates allow us to conclude that the reaction of horseradish peroxidase compound II with o-diphenols can be visualised as a two-step mechanism in which the first step corresponds to the formation of an enzyme-substrate complex, and the second to the electron transfer from the substrate to the iron atom. The size and hydrophobicity of the substrates control their access to the hydrophobic binding site of horseradish peroxidase, but electron density in the hydroxyl group of C-4 is the most important feature for the electron transfer step.     

Enhancement of Lipid Peroxidation by Indole-3-Acetic Acid and Derivatives: Substituent Effects

The peroxidation of liposomes by a haem peroxidase and hydrogen peroxide in the presence of indole-3-acetic acid and derivatives was investigated. It was found that these compounds can accelerate the lipid peroxidation up to 65 fold and this is attributed to the formation of peroxyl radicals that may react with the lipids, possibly by hydrogen abstraction. The peroxyl radicals are formed by peroxidase-catalyzed oxidation of the enhancers to radical cations which undergo cleavage of the carbon-carbon bond on the side-chain to yield CO2 and carbon-centred radicals that rapidly add oxygen. In competition with decarboxylation, the radical cations deprotonate reversibly from the Nl position. Rates of decarboxylation,pK_a values and rate of reaction with the peroxidase compound I indicate consistent substituent effects which, however, can not be quantitatively related to the usual Hammett or Brown parameters. Assuming that the rate of decarboxylation of the radical cations taken is a measure of the electron density of the molecule (or radical), it is found that the efficiency of these compounds as enhancers of lipid peroxidation increases with increasing electron density, suggesting that, at least in the model system, the oxidation of the substrates is the limiting step in causing lipid peroxidation.     

Plasmodium falciparum: discovery of peroxidase active organelles

Staining with 3,3' diaminobenzidine tetrahydrochloride (DAB) is a common method used for the detection of peroxidases. Using this histochemical staining method in conjunction with transmission electron microscopy, we observed oxidation of DAB that was localized to a discrete set of organelles displaying morphological similarity to small (75-90 nm diameter) versions of higher eukaryotic microbodies or peroxisomes. These single membrane bounded organelles were characterized by an asymmetrical matrix capable of oxidizing DAB to an electron dense inclusion. Oxidation of DAB was further found to be dependent upon hydrogen peroxide (H2O2) as a substrate. Given a lack of peroxisomal import proteins and enzymes, it is unlikely that these represent conventional peroxisomes. Rather, they likely represent specialized organelles containing endogenous peroxidase or pseudo-peroxidase activity.     

Fluorene Oxidation In Vivo by Phanerochaete chrysosporium and In Vitro during Manganese Peroxidase-Dependent Lipid Peroxidation

The oxidation of fluorene, a polycyclic hydrocarbon which is not a substrate for fungal lignin peroxidase, was studied in liquid cultures of Phanerochaete chrysosporium and in vitro with P. chrysosporium extracellular enzymes. Intact fungal cultures metabolized fluorene to 9-hydroxyfluorene via 9-fluorenone. Some conversion to more-polar products was also observed. Oxidation of fluorene to 9-fluorenone was also obtained in vitro in a system that contained manganese(II), unsaturated fatty acid, and either crude P. chrysosporium peroxidases or purified recombinant manganese peroxidase. The oxidation of fluorene in vitro was inhibited by the free-radical scavenger butylated hydroxytoluene but not by the lignin peroxidase inhibitor NaVO(inf3). Manganese(III)-malonic acid complexes could not oxidize fluorene. These results indicate that fluorene oxidation in vitro was a consequence of lipid peroxidation mediated by P. chrysosporium manganese peroxidase. The rates of fluorene and diphenylmethane disappearance in vitro were significantly faster than those of true polycyclic aromatic hydrocarbons or fluoranthenes, whose rates of disappearance were ionization potential dependent. This result indicates that the initial oxidation of fluorene proceeds by mechanisms other than electron abstraction and that benzylic hydrogen abstraction is probably the route for oxidation.     

para-Substituted N-nitroso-N-oxybenzenamine ammonium salts: a new class of redox-sensitive nitric oxide releasing compounds

N-Nitroso-N-oxybenzenamine ammonium salts with -OMe, -Me, -H, -F, -Cl, -CF3, and -SO2Me substituents at the para position of the phenyl ring constitute a new class of-redox sensitive nitric oxide (NO) releasing compounds. These compounds yield nitric oxide and the corresponding nitrosobenzene derivatives by a spontaneous dissociation mechanism after undergoing a one electron oxidation. Oxidation of these compounds can be achieved through chemical, electrochemical and enzymatic methods. It was observed electrochemically that the amount of NO generated was dependent on the substituent effect and the applied oxidation potential. Electron-withdrawing substituents increase the oxidation potential of the compound. A linear correlation was observed when the peak potentials for the oxidation were graphed versus the Hammett substituent constant. Density functional theory calculations were also performed on this series of compounds. The theoretical oxidation energies of the corresponding anions show a strong linear correlation with the experimental potentials. Furthermore, enzymatic oxidation using horseradish peroxidase showed a similar substituent effect. These results indicate that substitution at the para position of the phenyl ring has a profound effect on the stability, oxidation potential and enzymatic kinetic properties of the compounds. Thus para-substituted N-nitroso-N-oxybenzenamine salts comprise a new class of redox-sensitive nitric oxide releasing agents.     

Kinetic and molecular orbital studies on the rate of oxidation of monosubstituted phenols and anilines by lactoperoxidase compound II in comparison with the case of horseradish peroxidase

The second-order rate constant (k4) for the oxidation of monosubstituted phenols and anilines by lactoperoxidase compound II was examined by Chance's method [B. Chance, Arch. Biochem. Biophys. 71 (1957), 130–136]. When the electronic states of these substrates were calculated by an ab initio molecular orbital method, it was found that the log k4 value correlates well with the highest occupied molecular orbital (HOMO) energy level but not with the net charge or frontier electron density. These results are essentially similar to those reported previously in the case of horseradish peroxidase [J. Sakurada, R. Sekiguchi, K. Sato, and T. Hosoya, Biochemistry 29 (1990), 4093–4098], showing some dissimilar features which are considered to reflect the structural difference between the two enzymes.Abbreviations HOMO highest occupied molecular orbital - HRP horseradish peroxidase - LPO lactoperoxidase (EC 1.11.1.7) - LUMO lowest unoccupied molecular orbital     

Formation of iminoxyl and nitroxide free radicals from nitrosonaphthols: an electron spin resonance study

The possible metabolic activation of nitrosonaphthols, suspected carcinogens, was investigated by electron spin resonance (ESR) spectroscopy. Free radicals were found to be the primary metabolites formed during both the reduction and oxidation of these compounds. Whereas the one-electron oxidation of nitrosonaphthols is enzymatic and catalyzed by the peroxidase prototype, horseradish peroxidase, their one-electron reduction by reducing cofactors such as NADH or NADPH was not enhanced by rat liver microsomal enzymes. The ESR spectra of the radicals found during the oxidation of nitrosonaphthols were analyzed and characterized as iminoxyl free radicals. The reduction pathway leads to nitroxide free radicals with unusually low nitrogen hyperfine constants.     

Arylamine oxidations by chloroperoxidase

Chloroperoxidase was found to catalyze the peroxide oxidation of most of the anilines investigated to the corresponding nitroso compounds. Michaelis-Menten constants and maximal velocities were determined for each substrate at the optimal hydrogen peroxide concentration. The major factor controlling the rate of oxidation of the aniline substrates was found to be substituent size, with highly bulky substituents resulting in a marked decrease in the rate of oxidation. Turnover numbers for oxidizable substrates ranged from 67 200 min^?1 for aniline down to 6800 min^?1 for 4-isopropylaniline.     

Lignin Peroxidase Oxidation of Aromatic Compounds in Systems Containing Organic Solvents

Lignin peroxidase from Phanerochaete chrysosporium was used to study the oxidation of aromatic compounds, including polycyclic aromatic hydrocarbons and heterocyclic compounds, that are models of moieties of asphaltene molecules. The oxidations were done in systems containing water-miscible organic solvents, including methanol, isopropanol, N, N-dimethylformamide, acetonitrile, and tetrahydrofuran. Of the 20 aromatic compounds tested, 9 were oxidized by lignin peroxidase in the presence of hydrogen peroxide. These included anthracene, 1-, 2-, and 9-methylanthracenes, acenaphthene, fluoranthene, pyrene, carbazole, and dibenzothiophene. Of the compounds studied, lignin peroxidase was able to oxidize those with ionization potentials of <8 eV (measured by electron impact). The reaction products contain hydroxyl and keto groups. In one case, carbon-carbon bond cleavage, yielding anthraquinone from 9-methylanthracene, was detected. Kinetic constants and stability characteristics of lignin peroxidase were determined by using pyrene as the substrate in systems containing different amounts of organic solvent. Benzyl alkylation of lignin peroxidase improved its activity in a system containing water-miscible organic solvent but did not increase its resistance to inactivation at high solvent concentrations.     

Computer calculation-based quantitative structure-activity relationships for the oxidation of phenol derivatives horseradish peroxidase compound II

 The second-order rate constants for the oxidation of a series of phenol derivatives by horseradish peroxidase compound II were compared to computer-calculated chemical parameters characteristic for this reaction step. The phenol derivatives studied were phenol, 4-chlorophenol, 3-hydroxyphenol, 3-methylphenol, 4-methylphenol, 4-hydroxybenzoate, 4-methoxyphenol and 4-hydroxybenzaldehyde. Assuming a reaction of the phenolic substrates in their non-dissociated, uncharged forms, clear correlations (r = 0.977 and r = 0.905) were obtained between the natural logarithm of the second-order rate constants (ln k _app and ln k ₂ respectively) for their oxidation by compound II and their calculated ionisation potential, i.e. minus the energy of their highest occupied molecular orbital [E(HOMO)]. In addition to this first approach in which the quantitative structure-activity relationship (QSAR) was based on a calculated frontier orbital parameter of the substrate, in a second and third approach the relative heat of formation (ΔΔHF) calculated for the process of one-electron abstraction and H^• abstraction from the phenol derivatives was used as a parameter. Plots of the natural logarithms of the second-order rate constants (k _app and k ₂) for the reaction and the calculated ΔΔHF values for the process of one-electron abstraction also provide clear QSARs with correlation coefficients of –0.968 and –0.926 respectively. Plots of the natural logarithms of the second-order rate constants (k _app and k ₂) for the reaction and the calculated ΔΔHF values for the process of H^• abstraction provide QSARs with correlation coefficients of –0.989 and –0.922 respectively. Since both mechanisms considered, i.e. initial electron abstraction versus initial H^• abstraction, provided clear QSARs, the results could not be used to discriminate between these two possible mechanisms for phenol oxidation by horseradish peroxidase compound II. The computer calculation-based QSARs thus obtained for the oxidation of the various phenol derivatives by compound II from horseradish peroxidase indicate the validity of the approaches investigated, i.e. both the frontier orbital approach and the approach in which the process is described by calculated relative heats of formation. The results also indicate that outcomes from computer calculations on relatively unrelated phenol derivatives can be reliably compared to one another. Furthermore, as the actual oxidation of peroxidase substrates by compound II is known to be the rate-limiting step in the overall catalysis by horseradish peroxidase, the QSARs of the present study may have implications for the differences in the overall rate of substrate oxidation of the phenol derivatives by horseradish peroxidase. Received: 29 March 1996 / Accepted: 17 July 1996     

Dihydrotetramethylrosamine: a long wavelength, fluorogenic peroxidase substrate evaluated in vitro and in a model phagocyte

Dihydrotetramethylrosamine, a fluorogenic substrate for peroxidase, and its fluorescent oxidation product, tetramethylrosamine chloride, were evaluated. The substrate is colorless and nonfluorescent while the oxidized dye absorbs at 550 nm and emits at 574 nm in both methanol and water. In vitro assays demonstrated that the substrate was oxidized to the fluorophore by horseradish peroxidase in the presence of hydrogen peroxide. In vivo uptake and oxidation of the substrate by Amoeba proteus was characterized by the initial appearance of fluorescent phagocytic vacuoles with subsequent localization in vesicular organelles the size and shape of protozoan mitochondria. Similar staining patterns occurred in cells incubated with substrate, oxidized rosamine or rhodamine 123, a known mitochondrial stain.     

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.     

A sensitive photometric assay for monoamine oxidase.

A new spectrophotometric assay for the determination of monoamine oxidase activity is described. This simple and sensitive method is based on a coupled indicator reaction measuring the monoamine oxidase-dependent production of hydrogen peroxide. In this reaction the hydrogen peroxide-dependent oxidation of leuco-2′,7′-dichlorofluorescein to 2′,7′-dichlorofluorescein catalyzed by horseradish peroxidase is followed at 502 nm. Using benzylamine and seven biogenic amines as substrates, linear relationships between 2′,7′-dichlorofluorescein formation rate and monoamine oxidase concentration were found. The assay is especially suitable for determining substrate specificities for physiological amines as well as for inhibitor studies with pargyline or the monoamine oxidase A- and B-specific inhibitors clorgyline and deprenyl.     

Effect of functional groups on antioxidant properties of substituted selenoethers

Selenoethers attached to functional groups through propyl chain viz., bis(3-carboxypropyl)selenide (SeBA), bis(3-hydroxypropyl)selenide (SePOH) and bis(3-aminopropyl)selenide dihydrochloride (SePAm), have been examined for their ability to inhibit peroxyl radical mediated DNA damage, peroxyl radical scavenging ability and glutathione peroxidase (GPx) like activity. The DNA damage was monitored by gel electrophoresis, bimolecular rate constants for scavenging of model peroxyl radical were determined by pulse radiolysis and the GPx activity was followed by their ability to reduce hydrogen peroxide in the presence of glutathione utilizing NADPH decay and HPLC analysis. Among these compounds, SeBA showed maximum DNA protecting activity and it was also the most efficient in scavenging peroxyl radicals with the highest GPx mimicking activity. Quantum chemical calculations confirmed that SeBA with the highest energy level of HOMO (highest occupied molecular orbital) is the easiest to undergo oxidation and therefore exhibits better radical scavenging, GPx mimicking and DNA protecting activity than SePOH or SePAm.     

Oxidation of polycyclic aromatic hydrocarbons and dibenzo[p]-dioxins by Phanerochaete chrysosporium ligninase

The lignin peroxidase (ligninase) of Phanerochaete chrysosporium catalyzes the oxidation of a variety of lignin-related compounds. Here we report that this enzyme also catalyzes the oxidation of certain aromatic pollutants and compounds related to them, including polycyclic aromatic hydrocarbons with ionization potentials less than or equal to approximately 7.55 eV. This result demonstrates that the H2O2-oxidized states of lignin peroxidase are more oxidizing than the analogous states of classical peroxidases. Experiments with pyrene as the substrate showed that pyrene-1,6-dione and pyrene-1,8-dione are the major oxidation products (84% of total as determined by high performance liquid chromatography), and gas chromatography/mass spectrometry analysis of ligninase-catalyzed pyrene oxidations done in the presence of H2(18)O showed that the quinone oxygens come from water. We found that whole cultures of P. chrysosporium also transiently oxidize pyrene to these quinones. Experiments with dibenzo[p]dioxin and 2-chlorodibenzo[p]dioxin showed that they are also substrates for ligninase. The immediate product of dibenzo[p]dioxin oxidation is the dibenzo[p]dioxin cation radical, which was observed in enzymatic reactions by its electron spin resonance and visible absorption spectra. The cation radical mechanism of ligninase thus applies not only to lignin, but also to other environmentally significant aromatics.     

A model of peroxidase activity with inhibition by hydrogen peroxide

The rate of color formation in an activity assay consisting of phenol and hydrogen peroxide as substrates and 4-aminoantipyrine as chromogen is significantly influenced by hydrogen peroxide concentration due to its inhibitory effect on catalytic activity. A steady-state kinetic model describing the dependence of peroxidase activity on hydrogen peroxide concentration is presented. The model was tested for its application to soybean peroxidase (SBP) and horseradish peroxidase (HRP) reactions based on experimental data which were measured using simple spectrophotometric techniques. The model successfully describes the dependence of enzyme activity for SBP and HRP over a wide range of hydrogen peroxide concentrations. Model parameters may be used to compare the rate of substrate utilization for different peroxidases as well as their susceptibility to compound III formation. The model indicates that SBP tends to form more compound III and is catalytically slower than HRP during the oxidation of phenol.