Horseradish peroxidase catalyzed hydroxylations: mechanistic studies

Generation of Antifungal Stilbenes Using the Enzymatic Secretome of Botrytis cinerea

The protein secretome of Botrytis cinerea was used to perform the biotransformation of resveratrol, pterostilbene, and a mixture of both. Metabolite profiling by UHPLC-HRMS revealed the presence of compounds with unusual molecular formula, suggesting the existence of new products. To isolate these products, the reactions were scaled-up, and 21 analogues were isolated and fully characterized by NMR and HRESIMS analyses. The reaction with pterostilbene afforded five new compounds, while the reaction with a mixture of pterostilbene and resveratrol afforded seven unusual stilbene dimers. The antifungal properties of these compounds were evaluated using in vitro bioassays against Plasmopara viticola. The cytological effects of the isolated antifungal compounds on the ultrastructure of P. viticola were also evaluated.

Horseradish peroxidase (HRP) as a tool in green chemistry

The horseradish peroxidase (HRP) potential in organic synthesis.

A mechanistic study of the formation of hydroxyl radicals induced by horseradish peroxidase with NADH

During the oxidation of NADH by horseradish peroxidase (HRP-Fe(3+)), superoxide (O(-)(2)) is produced, and HRP-Fe(3+) is converted to compound III. Superoxide dismutase inhibited both the generation of O(-)(2) and the formation of compound III. In contrast, catalase inhibited only the generation of O(-)(2). Under anaerobic conditions, the formation of compound III did not occur in the presence of NADH, thus indicating that compound III is produced via formation of a ternary complex consisting of HRP-Fe(3+), NADH and oxygen. The generation of hydroxyl radicals was dependent upon O(-)(2) and H(2)O(2) produced by HRP-Fe(3+)-NADH. The reaction of compound III with H(2)O(2) caused the formation of compound II without generation of hydroxyl radicals. Only HRP-Fe(3+)-NADH (but not K(+)O(-)(2) and xanthine oxidase-hypoxanthine) was able to induce the conversion of metmyoglobin to oxymyoglobin, thus suggesting the participation of a ternary complex made up of HRP-Fe(2+…)O(2)(…)NAD(.) (but not free O(-)(2) or H(2)O(2)) in the conversion of metmyoglobin to oxymyoglobin. It appears that a cyclic pathway is formed between HRP-Fe(3+), compound III and compound II in the presence of NADH under aerobic conditions, and a ternary complex plays the central roles in the generation of O(-)(2) and hydroxyl radicals.

A heme peroxidase with a functional role as an L-tyrosine hydroxylase in the biosynthesis of anthramycin

We report the first characterization and classification of Orf13 (S. refuineus) as a heme-dependent peroxidase catalyzing the ortho-hydroxylation of L-tyrosine to L-DOPA. The putative tyrosine hydroxylase coded by orf13 of the anthramycin biosynthesis gene cluster has been expressed and purified. Heme b has been identified as the required cofactor for catalysis, and maximal L-tyrosine conversion to L-DOPA is observed in the presence of hydrogen peroxide. Preincubation of L-tyrosine with Orf13 prior to the addition of hydrogen peroxide is required for L-DOPA production. However, the enzyme becomes inactivated by hydrogen peroxide during catalysis. Steady-state kinetic analysis of L-tyrosine hydroxylation revealed similar catalytic efficiency for both L-tyrosine and hydrogen peroxide. Spectroscopic data from a reduced-CO(g) UV-vis spectrum of Orf13 and electron paramagnetic resonance of ferric heme Orf13 are consistent with heme peroxidases that have a histidyl-ligated heme iron. Contrary to the classical heme peroxidase oxidation reaction with hydrogen peroxide that produces coupled aromatic products such as o,o'-dityrosine, Orf13 is novel in its ability to catalyze aromatic amino acid hydroxylation with hydrogen peroxide, in the substrate addition order and for its substrate specificity for L-tyrosine. Peroxygenase activity of Orf13 for the ortho-hydroxylation of L-tyrosine to L-DOPA by a molecular oxygen dependent pathway in the presence of dihydroxyfumaric acid is also observed. This reaction behavior is consistent with peroxygenase activity reported with horseradish peroxidase for the hydroxylation of phenol. Overall, the putative function of Orf13 as a tyrosine hydroxylase has been confirmed and establishes the first bacterial class of tyrosine hydroxylases.

Structural Diversity of Peroxidase-Catalyzed Oxidation Products ofo-Methoxyphenols

[reaction: see text] The biocatalytic oxidation of o-methoxyphenolic compounds led to a variety of oligophenols (dimers to pentamers) and some of their oxidation products. The reaction was carried out in an aqueous medium at room temperature with hydrogen peroxide as the terminal oxidant in a facile and green route to potentially bioactive compounds. Detailed structural information on the products of peroxidase-catalyzed oxidation of o-methoxyphenols is presented for the first time.

Suicide inactivation of peroxidases and the challenge of engineering more robust enzymes

As the number of industrial applications for proteins continues to expand, the exploitation of protein engineering becomes critical. It is predicted that protein engineering can generate enzymes with new catalytic properties and create desirable, high-value, products at lower production costs. Peroxidases are ubiquitous enzymes that catalyze a variety of oxygen-transfer reactions and are thus potentially useful for industrial and biomedical applications. However, peroxidases are unstable and are readily inactivated by their substrate, hydrogen peroxide. Researchers rely on the powerful tools of molecular biology to improve the stability of these enzymes, either by protecting residues sensitive to oxidation or by devising more efficient intramolecular pathways for free-radical allocation. Here, we discuss the catalytic cycle of peroxidases and the mechanism of the suicide inactivation process to establish a broad knowledge base for future rational protein engineering.

Oxidation of hydroquinones by the versatile ligninolytic peroxidase from Pleurotus eryngii. H2O2 generation and the influence of Mn2+

Formation of H2O2 during the oxidation of three lignin-derived hydroquinones by the ligninolytic versatile peroxidase (VP), produced by the white-rot fungus Pleurotus eryngii, was investigated. VP can oxidize a wide variety of phenols, including hydroquinones, either directly in a manner similar to horseradish peroxidase (HRP), or indirectly through Mn3+ formed from Mn2+ oxidation, in a manner similar to manganese peroxidase (MnP). From several possible buffers (all pH 5), tartrate buffer was selected to study the oxidation of hydroquinones as it did not support the Mn2+-mediated activity of VP in the absence of exogenous H2O2 (unlike glyoxylate and oxalate buffers). In the absence of Mn2+, efficient hydroquinone oxidation by VP was dependent on exogenous H2O2. Under these conditions, semiquinone radicals produced by VP autoxidized to a certain extent producing superoxide anion radical (O2*-) that spontaneously dismutated to H2O2 and O2. The use of this peroxide by VP produced quinone in an amount greater than equimolar to the initial H2O2 (a quinone/H2O2 molar ratio of 1 was only observed under anaerobic conditions). In the presence of Mn2+, exogenous H2O2 was not required for complete oxidation of hydroquinone by VP. Reaction blanks lacking VP revealed H2O2 production due to a slow conversion of hydroquinone into semiquinone radicals (probably via autooxidation catalysed by trace amounts of free metal ions), followed by O2*- production through semiquinone autooxidation and O2*- reduction by Mn2+. This peroxide was used by VP to oxidize hydroquinone that was mainly carried out through Mn2+ oxidation. By comparing the activity of VP to that of MnP and HRP, it was found that the ability of VP and MnP to oxidize Mn2+ greatly increased hydroquinone oxidation efficiency.

Microperoxidase 8 catalyzed nitration of phenol by nitrogen dioxide radicals

Microperoxidase 8 (MP8) is a heme octapeptide obtained by hydrolytic digestion of horse heart cytochrome c. At pH below 9, the heme iron is axially coordinated to the imidazole side chain of His18 and to a water molecule. Replacement of this weak ligand by H2O2 allows the formation of high-valent iron-oxo species which are responsible for both peroxidase-like and cytochrome P450-like activities of MP8. This paper shows that MP8 is able to catalyze the nitration of phenol by nitrite. The reaction requires H2O2 and is inhibited by ligands having a high affinity for the iron, catalase and radical scavengers. This suggests that the nitrating species could be NO2* radicals formed by the oxidation of nitrite by high-valent iron-oxo species. This new activity of MP8 opens a new access to nitro-aromatic compounds under mild conditions and validates the use of this minienzyme to mimick heme peroxidases, especially in the reactions of NO-derived species with biomolecules under oxidative stress conditions.

Soybean peroxidase as an effective bromination catalyst*

Soybean peroxidase (SBP), an acidic peroxidase isolated from the seed coat, has been shown to be an effective catalyst for the oxidation of a variety of organic compounds. In the present study, we demonstrate that SBP can catalyze halogenation reactions. In the presence of H(2)O(2), SBP catalyzed the oxidation of bromide and iodide but not chloride. Veratryl alcohol (3,4-dimethoxybenzyl alcohol) served as a useful substrate for SBP-catalyzed halogenations yielding the 6-bromo derivative. Halogenation of veratryl alcohol was optimal at pHs below 2.5 with rates of 2.4 µm/min, achieving complete conversions of 150-µm veratryl alcohol in 24 h. The enzyme showed essentially no brominating activity at pHs above 5.5. SBP-catalyzed bromination of veratryl alcohol proceeded with a maximum reaction velocity, (V(max))(apparent), of 5.8 x 10(-1) µm/min, a K(m) of 78 µm and a catalytic efficiency (k(cat)/K(m) of 1.37 x 10(5) M/min at pH 4.0, assuming all of the enzyme's active sites participate in the reaction. SBP also catalyzed the bromination of several other organic substrates including pyrazole to produce a single product 1-bromopyrazole, indole to yield both 5-bromoindole and 5-hydroxyindole, and the decarboxylative bromination of 3,4 dimethoxy-trans-cinnamic acid to trans-2-bromo-1-(3,4 dimethoxyphenyl)ethylene. A catalytic mechanism for SBP-catalyzed bromination has been proposed based on experimental results in this and related studies.

Hydroxyl-radical production in physiological reactions. A novel function of peroxidase

Peroxidases catalyze the dehydrogenation by hydrogen peroxide (H2O2) of various phenolic and endiolic substrates in a peroxidatic reaction cycle. In addition, these enzymes exhibit an oxidase activity mediating the reduction of O2 to superoxide (O2.-) and H2O2 by substrates such as NADH or dihydroxyfumarate. Here we show that horseradish peroxidase can also catalyze a third type of reaction that results in the production of hydroxyl radicals (.OH) from H2O2 in the presence of O2.-. We provide evidence that to mediate this reaction, the ferric form of horseradish peroxidase must be converted by O2.- into the perferryl form (Compound III), in which the haem iron can assume the ferrous state. It is concluded that the ferric/perferryl peroxidase couple constitutes an effective biochemical catalyst for the production of .OH from O2.- and H2O2 (iron-catalyzed Haber-Weiss reaction). This reaction can be measured either by the hydroxylation of benzoate or the degradation of deoxyribose. O2.- and H2O2 can be produced by the oxidase reaction of horseradish peroxidase in the presence of NADH. The .OH-producing activity of horseradish peroxidase can be inhibited by inactivators of haem iron or by various O2.- and .OH scavengers. On an equimolar Fe basis, horseradish peroxidase is 1-2 orders of magnitude more active than Fe-EDTA, an inorganic catalyst of the Haber-Weiss reaction. Particularly high .OH-producing activity was found in the alkaline horseradish peroxidase isoforms and in a ligninase-type fungal peroxidase, whereas lactoperoxidase and soybean peroxidase were less active, and myeloperoxidase was inactive. Operating in the .OH-producing mode, peroxidases may be responsible for numerous destructive and toxic effects of activated oxygen reported previously.

Biotransformations with peroxidases

Enzymes are chiral catalysts and are able to produce optically active molecules from prochiral or racemic substrates by catalytic asymmetric induction. One of the major challenges in organic synthesis is the development of environmentally acceptable chemical processes for the preparation of enantiomerically pure compounds, which are of increasing importance as pharmaceuticals and agrochemicals. Enzymes meet this challenge! For example, a variety of peroxidases effectively catalyze numerous selective oxidations of electron-rich substrates, which include the hydroxylation of arenes, the oxyfunctionalizations of phenols and aromatic amines, the epoxidation and halogenation of olefins, the oxygenation of heteroatoms and the enantioselective reduction of racemic hydroperoxides. In this review, we summarize the important advances achieved in the last few years on peroxidase-catalyzed transformations, with major emphasis on preparative applications.

Chloroperoxidase-catalyzed enantioselective oxidation of methyl phenyl sulfide with dihydroxyfumaric acid/oxygen or ascorbic acid/oxygen as oxidants

The chloroperoxidase catalyzed oxidation of methyl phenyl sulfide to (R)-methyl phenyl sulfoxide was investigated, both in batch and membrane reactors, using as oxidant H2O2, or O2 in the presence of either dihydroxyfumaric acid or ascorbic acid. The effects of pH and nature and concentration of the oxidants on the selectivity, stability, and productivity of the enzyme were evaluated. The highest selectivity was displayed by ascorbic acid/O2, even though the activity of chloroperoxidase with this system was lower than that obtained with the others. When the reaction was carried out in a membrane reactor, it was possible to reuse the enzyme for several conversion cycles. The results obtained with ascorbic acid/O2 and dihydroxyfumaric acid/O2 as oxidants do not seem to be compatible with either a mechanism involving hydroxyl radicals as the active species or with the hypothesis that oxidation occurs through the initial formation of H2O2. Copyright 1999 John Wiley & Sons, Inc.

Microperoxidase/H2O2-catalyzed aromatic hydroxylation proceeds by a cytochrome-P-450-type oxygen-transfer reaction mechanism

The mechanism of aromatic hydroxylation of aniline and phenol derivatives in a H2O2-driven microperoxidase-8(MP8)-catalyzed reaction was investigated. It was shown that the reaction was not inhibited by the addition of scavengers of superoxide anion or hydroxyl radicals, which demonstrates that the reaction mechanism differs from that of the aromatic hydroxylation catalyzed by a horseradish peroxidase/ dihydroxyfumarate system. Additional experiments with 18O-labelled H2 18O2 demonstrated that the oxygen incorporated into aniline to give 4-aminophenol originates from H2O2. Furthermore, it was found that the addition of ascorbic acid efficiently blocks all peroxidase-type reactions that can be catalyzed by the MP8/H2O2 system, but does not inhibit the aromatic hydroxylation of aniline and phenol derivatives. Together, these observations exclude reaction mechanisms for the aromatic hydroxylation that proceed through peroxidase-type mechanisms in which the oxygen incorporated into the substrate originates from O2 or H2O. The mechanism instead seems to proceed by an initial attack of the high-valent iron-oxo intermediate of MP8 on the pi-electrons of the aromatic ring of the substrate leading to product formation by a cytochrome-P-450-type of sigma-O-addition or oxygen-rebound mechanism. This implies that MP8, which has a histidyl and not a cysteinate fifth axial ligand, is able to react by a cytochrome-P-450-like oxygen-transfer reaction mechanism.

Involvement of peroxidase in chorion hardening in Aedes aegypti

Peroxidase activity is detectable in Aedes aegypti ovaries, containing developing eggs, at 24 h following blood feeding, and peak peroxidase activity is reached at 36-48 h after the blood-meal. Peroxidase is associated with the chorion layer in mature eggs and the majority of the enzyme is released from the chorion layer by treating the isolated chorion fraction with SDS/urea. Analysis of the SDS/urea solubilized chorion proteins using SDS-PAGE with tropolone/H2O2 or dopa staining verified the presence of both peroxidase and phenol oxidase in the released chorion proteins. The molecular weight of chorion peroxidase is about 61,000 Da as determined by SDS-PAGE analysis. Incubation of the solubilized chorion proteins with tyrosine and H2O2 produces dityrosine, and hyrolysis of hardened egg chorion results in the detection of dityrosine and trityrosine in the chorion hydrolysate. Data suggest that chorion peroxidase is involved in the hardening of the mosquito egg chorion by catalyzing the formation of ditryrosine through tyrosine residues on structural proteins. The overall hardening of the A. aegypti egg chorion includes both peroxidase-mediated chorion protein crosslinking through dityrosine formation and phenol oxidase-catalyzed chorion melanization.

Autocatalytic generation of dopa in the engineered protein R2 F208Y from Escherichia coli ribonucleotide reductase and crystal structure of the dopa-208 protein

The mutant form Phe-208-->Tyr of the R2 protein of Escherichia coli ribonucleotide reductase contains an intrinsic ferric-Dopa cofactor with characteristic absorption bands at 460 and ca. 700 nm [Ormö, M., de Maré, F., Regnström, K., Aberg, A., Sahlin, M., Ling, J., Loehr, T. M., Sanders-Loehr, J., & Sjöberg, B. M. (1992) J. Biol. Chem. 267, 8711-8714]. The three-dimensional structure of the mutant protein, solved to 2.5-A resolution, shows that the Dopa is localized to residue 208 and that it is a bidentate ligand of Fe1 of the binuclear iron center of protein R2. Nascent apoR2 F208Y, lacking metal ions, can be purified from overproducing cells grown in iron-depleted medium. ApoR2 F208Y is rapidly and quantitatively converted to the Dopa-208 form in vitro by addition of ferrous iron in the presence of oxygen. Other metal ions (Cu2+, Mn2+, Co2+) known to bind to the metal site of wild-type apoR2 do not generate a Dopa in apoR2 F208Y. The autocatalytic generation of Dopa does not require the presence of a tyrosine residue at position 122, the tyrosine which in a wild-type R2 protein acquires the catalytically essential tyrosyl radical. It is proposed that generation of Dopa initially follows the suggested reaction mechanism for tyrosyl radical generation in the wild-type protein and involves a ferryl intermediate, which in the case of the mutant R2 protein oxygenates Tyr 208. This autocatalytic metal-mediated reaction in the engineered R2 F208Y protein may serve as a model for formation of covalently bound quinones in other proteins.

Thianthrene 5-oxide as a probe of the electrophilicity of hemoprotein oxidizing species

Thianthrene 5-oxide (T-5-O), which is oxidized to the 5,10- and 5,5-dioxides, respectively, by electrophilic and nucleophilic agents, has been used to determine the electronic properties of hemoprotein oxidizing species. Cytochrome P450 oxidizes T-5-O to the 5,10- rather than the 5,5-dioxide but oxidizes the 5,5-dioxide rapidly and the 5,10-dioxide slowly to the 5,5,10-trioxide. Chloroperoxidase oxidizes T-5-O to the 5,10-dioxide but very poorly oxidizes it further to the 5,5,10-trioxide. It does, however, readily oxidize the 5,5-dioxide to the trioxide. The oxidizing species of cytochrome P450 and chloroperoxidase are thus comparably electrophilic, but the former is more powerful. T-5-O is not detectably oxidized by horseradish peroxidase/H2O2 but is oxidized exclusively to the 5,5-dioxide when the peroxide is replaced by dihydroxyfumaric acid (DHFA). The oxygen incorporated into the 5,5-dioxide in this reaction derives from molecular oxygen. This is consistent with the involvement of a DHFA-derived co-oxidizing species. Oxidation of T-5-O by human hemoglobin and H2O2 yields the 5,5- and 5,10-dioxides and the 5,5,10-trioxide. The oxygen in these products derives primarily (greater than 80%) from H2O2. Hemoglobin and H2O2 thus form both a P450-like electrophilic oxidant (5,10-dioxide) and a peroxide-derived nucleophilic oxidant (5,5-dioxide). A large difference in the cis:trans ratios of the 5,10-dioxides produced from T-5-O by cytochrome P450 (1.3:1) and chloroperoxidase (2.5:1) vs hemoglobin (0.1:1) suggests that the hemoglobin active site severely constrains the geometry of the electrophilic oxidation.(ABSTRACT TRUNCATED AT 250 WORDS)

Hydroxylation of phenol to hydroquinone catalyzed by a human myeloperoxidase-superoxide complex: possible implications in benzene-induced myelotoxicity

Benzene, a known human myelotoxin and leukemogen is metabolized by liver cytochrome P-450 monooxygenase to phenol. Further hydroxylation of phenol by cytochrome P-450 monooxygenase results in the formation of mainly hydroquinone, which accumulates in the bone marrow. Bone marrow contains high levels of myeloperoxidase. Here we report that phenol hydroxylation to hydroquinone is also catalyzed by human myeloperoxidase in the presence of a superoxide anion radical generating system, hypoxanthine and xanthine oxidase. No hydroquinone formation was detected in the absence of myeloperoxidase. At low concentrations superoxide dismutase stimulated, but at high concentrations inhibited, the conversion of phenol to hydroquinone. The inhibitory effect at high superoxide dismutase concentrations indicates that the active hydroxylating species of myeloperoxidase is not derived from its interaction with hydrogen peroxide. Furthermore, catalase a hydrogen peroxide scavenger, was found to have no significant effect on hydroxylation of phenol to hydroquinone, supporting the lack of hydrogen peroxide involvement. Mannitol (a hydroxyl radical scavenger) was found to have no inhibitory effect, but histidine (a singlet oxygen scavenger) inhibited hydroquinone formation. Based on these results we postulate that a myeloperoxidase-superoxide complex spontaneously rearranges to generate singlet oxygen and that this singlet oxygen is responsible for phenol hydroxylation to hydroquinone. These results also suggest that myeloperoxidase dependent hydroquinone formation could play a role in the production and accumulation of hydroquinone in bone marrow, the target organ of benzene-induced myelotoxicity.

Free radical generation by redox cycling of estrogens

Natural and synthetic estrogens elicit normal hormonal responses in concentrations in a clearly defined yet low range. At elevated doses, metabolic reactions of the phenolic moiety, while harmless at low levels, may become the predominant biochemical activity and may exert deleterious effects. These metabolic pathways, such as i) oxidation of estrogens to catechol estrogens and further to their respective quinones, and ii) free radical generation by redox cycling between catechol estrogens or diethylstilbestrol and their quinones, are investigated for their influence in physiological or pathophysiological processes. In this review, the in vitro capacity of various enzymes to oxidize estrogen hydroquinones to quinones or to reduce corresponding quinones to hydroquinones is evaluated. The in vivo activities of enzymes supporting redox cycling of estrogens and free radical generation is correlated with induction of kidney tumors in Syrian hamsters. Concomitant changes in activities in quinone reductase and other detoxifying enzymes in kidneys of hamsters treated with estrogen support a role of free radicals in the induction of tumors by estrogen. Free radical damage to protein and possibly to DNA in kidneys of estrogen-treated hamsters may be used as markers of free radical action in vivo.

Kinetics of the reaction of compound II of horseradish peroxidase with hydrogen peroxide to form compound III

The kinetics of the reaction of H2O2 with compound II of horseradish peroxidase were studied as a function of pH at 25 degrees C and constant ionic strength of 0.11 M. The reaction of H2O2 with compound II involves the transient formation of ferric peroxidase and superoxide anion as the first step followed by the reaction of the intermediate species with H2O2 to form compound III. Both reactions are also observed with peracetic acid as substrate, though the amplitude of the first step was too small for the rate to be measured. Observation of the first reaction was not possible below pH 8.5 under the conditions of this investigation. It tends to occur faster at lower pH so an increasing fraction is lost in the dead time of the stopped-flow apparatus. The rate constants for the second reaction, leading to compound III formation, are small at all pH values, with a maximum of 20 M-1 s-1 at pH 7.0. Groups on the enzyme intermediate species with pKa values of 4.2 and 9.1 appear to be involved in this reaction. Compound III formation is accompanied by oxidation of aromatic amino acid groups on the protein. The compound III formed from horseradish peroxidase compound II and hydrogen peroxide has bands with molar absorption coefficients in excellent agreement with those obtained by flash photolysis of aerated carbonmonoxyperoxidase [Wittenberg, J. B., Noble, R. W., Wittenberg, B. A., Antonini, E., Brunori, M. and Wyman, J. (1967) J. Biol. Chem. 242, 626-634]. Attempts to use m-chloroperbenzoic acid as oxidant resulted in the destruction of compound II.

Peroxidase catalysed formation of prostaglandins from arachidonic acid

Horseradish peroxidase and bovine lactoperoxidase (EC 1.11.1.7), when incubated aerobically with arachidonate, gave rise to the formation of substances identified by bioassay as prostaglandin F2 alpha (PGF2 alpha)- and prostaglandin E2 (PGE2)-like compounds. Boiling of enzymes, which suppressed their capacity to peroxidize guaiacol, also destroyed their capacity to convert arachidonate into PG-like compounds. The rates of formation of PG-like compounds rapidly declined with time, approaching zero after 10 and 20 min for PGE2 alpha- and PGE2-like compounds, respectively. Addition of more enzyme further promoted the reaction. Horseradish and lacto-peroxidases showed optimum pH values of 9.0 and 10.0, respectively. Both enzymes exhibited apparent Km values of about 5 x 10(-5) M for arachidonate. Some reducing agents such as ascorbic acid, NADH and adrenaline dose-dependently inhibited this reaction. The haem poison, phenylhydrazine, also inhibited, with an IC50 of 1 x 10(-7) M. Indomethacin inhibited only the formation of PGE2-like compounds with an IC50 of about 3 x 10(-6) M. As compared to a standard commercial preparation of horseradish peroxidase, the purified horseradish basic and acidic isoenzymes exhibited a higher activity, towards arachidonate whereas other haemoproteins, possessing peroxidase activity, were less active. TLC and GC-MS analyses performed on the reaction products led to the identification of PGF2 alpha, PGE2 and PG6K1 alpha and other unidentified arachidonate derivatives. At 25 degrees, pH 9.5, horseradish peroxidase, acting on saturating concentration of arachidonate, catalysed the formation of 60 mumol/min/mmole enzyme of PGE2 + PGF2 alpha. This appears to be the first report of the synthesis of prostaglandins catalysed by peroxidases.

Role of oxygen during horseradish peroxidase turnover and inactivation

Horseradish peroxidase catalyzed oxidation of phenol has been reinvestigated to determine the requirements of facile enzyme autoinactivation. Turnover of this peroxidase was monitored spectrophotometrically at 400 nm and found dependent on the concentration of phenol and hydrogen peroxide. The inactivation of the peroxidase required both substrates, phenol and H2O2, but surprisingly was also potentiated by molecular oxygen. Exclusion of diffusible superoxide or hydroxyl radicals had slight effect on product formation or loss of catalytic activity. A mechanism is proposed to explain the unanticipated role of oxygen during enzyme inactivation.

The role of hydroxyl radicals in irreversible inactivation of lactoperoxidase by excess H2O2. A spin-trapping/ESR and absorption spectroscopy study

H2O2 is catalytically metabolized by ferric lactoperoxidase (LPO)----compound (cpd) I----cpd II----ferric LPO cycles. An excess of the substrate, however, is degraded by a ferric LPO----cpd I----cpd II----cpd III----ferrous LPO----ferric LPO cycle. This latter pathway leads to the partial or total irreversible inactivation of the enzyme depending on the excess of H2O2 (H. Jenzer, W. Jones, and H. Kohler (1986) J. Biol. Chem. 261, 15550-15556). Spin-trapping/ESR data indicate that in the course of the reaction superoxide (HO2./O2-) and hydroxyl radicals (OH.) are formed. Since many substances known to scavenge radicals, such as a spin trap (e.g., 5,5-dimethyl-1-pyrroline-N-oxide) desferrioxamine, albumin, or mannitol, do not prevent enzyme inactivation, we conclude that OH. generation is a site-specific reaction at or near the active center of LPO where bulky scavenger molecules may not be able to penetrate. We suggest the formation of OH. by a Fenton-like reaction between H2O2 and the intermediate ferrous state of the enzyme, which substitutes for Fe2+ in the Fenton reaction. OH. is a powerful oxidant which in turn may attack rapidly the nearest partner available, either H2O2 to produce HO2. and H2O, or the prosthetic group to give rise to oxidative cleavage of the porphyrin ring structure of the heme moiety of LPO and thus to the liberation of iron.

Cooxidation of styrene by horseradish peroxidase and phenols: a biochemical model for protein-mediated cooxidation

Styrene is oxidized to styrene oxide and benzaldehyde when incubated with horseradish peroxidase, H2O2, and 4-methylphenol. Styrene oxide is not formed in the absence of any of these reaction components or of molecular oxygen. The coupling products 2-(4-methylphenoxy)-1-phenylethane, 2-(4-methylphenoxy)-1-phenylethan-1-ol, and 2-(4-methylphenoxy)-2-phenylethan-1-ol are not formed, but the ortho-linked dimer of 4-methylphenol is a major product. The epoxide oxygen is labeled in the presence of 18O2 but not H218O2. Styrene oxide formation is not inhibited by mannitol or superoxide dismutase. The stereochemistry of trans-[1-2H]styrene is partially scrambled in the epoxide product. EPR signals attributable to the 2,4-dihydroxy-5-methylphenoxy radical, a product of the oxidation of 4-methylcatechol, are observed if Zn2+ is added to stabilize the radical. This radical is only detected in the presence of styrene. The results imply that styrene is epoxidized by the hydroperoxy radical generated by addition of molecular oxygen to the 4-methylphenoxy radical. The epoxidation mimics the chemistry proposed to occur in the protein-mediated cooxidation of styrene by hemoglobin and myoglobin.

Enzyme-catalyzed detoxication reactions: mechanisms and stereochemistry

Enzyme catalyzed detoxication reactions are one of the primary defenses organisms have against chemical insult. This article reviews current chemical approaches to understanding the cooperative role of enzymes in the metabolism of foreign compounds. Emphasis is placed on chemical and stereochemical studies which help elucidate the mechanism of action and active-site topologies of the detoxication enzymes. The stereoselectivity of the cytochromes P-450 and flavin containing monooxygenases as well as the role of hemoglobin and lipid peroxidation in the primary metabolism of xenobiotics is discussed. Current knowledge of the mechanism and stereoselectivity of epoxide hydrolase is also presented. Three enzymes involved in secondary metabolism of xenobiotics, UDP-glucuronosyltransferase, sulfotransferase and glutathione S-transferase are discussed with particular emphasis on active site topology and cooperative participation with the enzymes of primary metabolism.