Mechanism of nitrite-stimulated catalysis by lactoperoxidase

Structural evidence of the conversion of nitric oxide (NO) to nitrite ion (NO2-) by lactoperoxidase (LPO): Structure of the complex of LPO with NO2- at 1.89 Å resolution

Lactoperoxidase (LPO) is a heme containing mammalian enzyme which uses hydrogen peroxide (H2O2) to catalyze the conversion of substrates into oxidized products. LPO is found in body fluids and tissues such as milk, saliva, tears, mucosa and other body secretions. The previous structural studies have shown that LPO converts substrates, thiocyanate (SCN-) and iodide (I-) ions into oxidized products, hypothiocyanite (OSCN-) and hypoiodite (IO-) ions respectively. We report here a new structure of the complex of LPO with an oxidized product, nitrite (NO2-). This product was generated from NO using the two step reaction of LPO by adding hydrogen peroxide (H2O2) in the solution of LPO in 0.1 M phosphate buffer at pH 6.8 as the first step. In the second step, NO gas was added to the above mixture. This was crystallized using 20% (w/v) PEG-3350 and 0.2 M ammonium iodide at pH 6.8. The structure determination showed the presence of NO2- ion in the distal heme cavity of the substrate binding site of LPO. The structure also showed that the propionate group which is linked to pyrrole ring D of the heme moiety was disordered. Similarly, the side chain of Asp108, which is covalently linked to heme moiety, was also split into two components. As a result of these changes, the conformation of the side chain of Arg255 was altered allowing it to form new interactions with the disordered carboxylic group of propionate moiety. These structural changes are indicative of an intermediate state in the catalytic reaction pathway of LPO.

Reaction of human peroxidasin 1 compound I and compound II with one-electron donors

Human peroxidasin 1 (hsPxd01) is a homotrimeric multidomain heme peroxidase embedded in the extracellular matrix. It catalyses the two-electron oxidation of bromide by hydrogen peroxide to hypobromous acid which mediates the formation of essential sulfilimine cross-links between methionine and hydroxylysine residues in collagen IV. This confers critical structural reinforcement to the extracellular matrix. This study presents for the first time transient kinetic measurements of the reactivity of hsPxd01 compound I and compound II with the endogenous one-electron donors nitrite, ascorbate, urate, tyrosine and serotonin using the sequential stopped-flow technique. At pH 7.4 and 25 °C compound I of hsPxd01 is reduced to compound II with apparent second-order rate constants ranging from (1.9 ± 0.1) × 104 M-1 s-1 (urate) to (4.8 ± 0.1) × 105 M-1 s-1 (serotonin). Reduction of compound II to the ferric state occurs with apparent second-order rate constants ranging from (4.3 ± 0.2) × 102 M-1 s-1 (tyrosine) to (7.7 ± 0.1) × 103 M-1 s-1 (serotonin). The relatively fast rates of compound I reduction suggest that these reactions may take place in vivo and modulate bromide oxidation due to formation of compound II. Urate is shown to inhibit the bromination activity of hsPxd01, whereas nitrite stimulates the formation of hypobromous acid. The results are discussed with respect to known kinetic data of homologous mammalian peroxidases and to the physiological role of human peroxidasin 1.

Probing nitrite coordination in horseradish peroxidase by resonance Raman spectroscopy: Detection of two binding sites

Nitrite is a powerful oxidant that affects the activity of peroxidases towards various substrates and leads to heme macrocycle modifications in members of the peroxidase family, such as the horseradish peroxidase (HRP). We have applied resonance Raman spectroscopy to investigate the structural properties of the species formed in the reaction of NO₂^- with the ferric form of HRP. Our data demonstrate that the heme nitrovinyl group is partially formed at near neutral pH, without coordination of NO₂^- to the heme Fe. Nitrite coordinates to the heme Fe at acidic pH in the nitro binding mode, characterized by the detection of the ν(Fe-NO₂) at 563cm^-1, δ(FeNO₂) at 822cm^-1 and ν_sym(NO₂) at 1272cm^-1. The sensitivity of the vibrations of the heme Fe-nitro complex to H/D exchange indicates H-bonding interaction of the heme-bound ligand with the distal environment that determines the NO₂^- binding mode. A model describing the different modes of NO₂^- binding in HRP is presented.

Uric acid and thiocyanate as competing substrates of lactoperoxidase

The physiological function of urate is poorly understood. It may act as a danger signal, an antioxidant, or a substrate for heme peroxidases. Whether it reacts sufficiently rapidly with lactoperoxidase (LPO) to act as a physiological substrate remains unknown. LPO is a mammalian peroxidase that plays a key role in the innate immune defense by oxidizing thiocyanate to the bactericidal and fungicidal agent hypothiocyanite. We now demonstrate that urate is a good substrate for bovine LPO. Urate was oxidized by LPO to produce the electrophilic intermediates dehydrourate and 5-hydroxyisourate, which decayed to allantoin. In the presence of superoxide, high yields of hydroperoxides were formed by LPO and urate. Using stopped-flow spectroscopy, we determined rate constants for the reaction of urate with compound I (k1 = 1.1 × 10(7) M(-1) s(-1)) and compound II (k2 = 8.5 × 10(3) M(-1) s(-1)). During urate oxidation, LPO was diverted from its peroxidase cycle because hydrogen peroxide reacted with compound II to give compound III. At physiologically relevant concentrations, urate competed effectively with thiocyanate, the main substrate of LPO for oxidation, and inhibited production of hypothiocyanite. Similarly, hypothiocyanite-dependent killing of Pseudomonas aeruginosa was inhibited by urate. Allantoin was present in human saliva and associated with the concentration of LPO. When hydrogen peroxide was added to saliva, oxidation of urate was dependent on its concentration and peroxidase activity. Our findings establish urate as a likely physiological substrate for LPO that will influence host defense and give rise to reactive electrophilic metabolites.

Green tea and vitamin E enhance exercise-induced benefits in body composition, glucose homeostasis, and antioxidant status in elderly men and women

OBJECTIVE

To investigate the effects of green tea plus vitamin E in addition to exercise on body composition and metabolic and antioxidant parameters in healthy elderly individuals.

DESIGN

Interventional randomized controlled prospective trial.

METHODS

For 12 weeks, 22 elderly men and women (age: 71.1 ± 1.2 years; body mass index: 28.3 ± 0.5 kg/m(2) [mean ± SE]) undertook 30 minutes of moderately intense walking 6 d/wk. They were randomly assigned to ingest either green tea plus vitamin E (GTVE; 3 cups and 400 IU, respectively; n = 11) or placebo (n = 11). Data on anthropometrics, fasting insulin and glucose levels, physical fitness, dietary intake, safety parameters, and biomarkers of oxidation status were recorded and analyzed at the start and end of the study.

RESULTS

Though dietary intake was unchanged, improved exercise capacity was followed by a significant reduction in body weight and fasting insulin levels in all participants. Additional consumption of GTVE resulted in a twofold increase in serum vitamin E (from 20.4 to 40.6 μmol/L, p < 0.001) and a decrease of men's and women's waist circumferences (from 100.8 and 95.7 to 96.9 and 85.0 cm, p < 0.05 and p < 0.01, respectively) and fasting glucose levels (from 5.30 to 4.98 mmol/L, p < 0.01). Plasma protein carbonyls dropped (from 0.93 to 0.77 nmol/mg protein, p < 0.05), whereas erythrocyte catalase activities increased (from 26.7 to 29.7 U/g hemoglobin, p < 0.05) in the GTVE group only. Oral peroxidase activities were increased in both groups.

CONCLUSIONS

A daily dose of GTVE in healthy elderly men and women may improve exercise-induced benefits in body composition and glucose tolerance and may also lower oxidative burden.

Inactivation of the potent Pseudomonas aeruginosa cytotoxin pyocyanin by airway peroxidases and nitrite

Pyocyanin (1-hydroxy-N-methylphenazine, PCN) is a cytotoxic pigment and virulence factor secreted by the human bacterial pathogen, Pseudomonas aeruginosa. Here, we report that exposure of PCN to airway peroxidases, hydrogen peroxide (H(2)O(2)), and NaNO(2) generates unique mononitrated PCN metabolites (N-PCN) as revealed by HPLC/mass spectrometry analyses. N-PCN, in contrast to PCN, was devoid of antibiotic activity and failed to kill Escherichia coli and Staphylococcus aureus. Furthermore, in contrast to PCN, intratracheal instillation of N-PCN into murine lungs failed to induce a significant inflammatory response. Surprisingly, at a pH of ∼7, N-PCN was more reactive than PCN with respect to NADH oxidation but resulted in a similar magnitude of superoxide production as detected by electron paramagnetic resonance and spin trapping experiments. When incubated with Escherichia coli or lung A549 cells, PCN and N-PCN both led to superoxide formation, but lesser amounts were detected with N-PCN. Our results demonstrate that PCN that has been nitrated by peroxidase/H(2)O(2)/NO(2)(-) systems possesses less cytotoxic/proinflammatory activity than native PCN. Yield of N-PCN was decreased by the presence of the competing physiological peroxidase substrates (thiocyonate) SCN(-) (myeloperoxidase, MPO, and lactoperoxidase, LPO) and Cl(-) (MPO), which with Cl(-) yielded chlorinated PCNs. These reaction products also showed decreased proinflammatory ability when instilled into the lungs of mice. These observations add important insights into the complexity of the pathogenesis of lung injury associated with Pseudomonas aeruginosa infections and provide additional rationale for exploring the efficacy of NO(2)(-) in the therapy of chronic Pseudomonas aeruginosa airway infection in cystic fibrosis.

Airway peroxidases catalyze nitration of the {beta}2-agonist salbutamol and decrease its pharmacological activity

β(2)-agonists are the most effective bronchodilators for the rapid relief of asthma symptoms, but for unclear reasons, their effectiveness may be decreased during severe exacerbations. Because peroxidase activity and nitrogen oxides are increased in the asthmatic airway, we examined whether salbutamol, a clinically important β(2)-agonist, is subject to potentially inactivating nitration. When salbutamol was exposed to myeloperoxidase, eosinophil peroxidase or lactoperoxidase in the presence of hydrogen peroxide (H(2)O(2)) and nitrite (NO(2)(-)), both absorption spectroscopy and mass spectrometry indicated formation of a new metabolite with features expected for the nitrated drug. The new metabolites showed an absorption maximum at 410 nm and pK(a) of 6.6 of the phenolic hydroxyl group. In addition to nitrosalbutamol (m/z 285.14), a salbutamol-derived nitrophenol, formed by elimination of the formaldehyde group, was detected (m/z 255.13) by mass spectrometry. It is noteworthy that the latter metabolite was detected in exhaled breath condensates of asthma patients receiving salbutamol but not in unexposed control subjects, indicating the potential for β(2)-agonist nitration to occur in the inflamed airway in vivo. Salbutamol nitration was inhibited in vitro by ascorbate, thiocyanate, and the pharmacological agents methimazole and dapsone. The efficacy of inhibition depended on the nitrating system, with the lactoperoxidase/H(2)O(2)/NO(2)(-) being the most affected. Functionally, nitrated salbutamol showed decreased affinity for β(2)-adrenergic receptors and impaired cAMP synthesis in airway smooth muscle cells compared with the native drug. These results suggest that under inflammatory conditions associated with asthma, phenolic β(2)-agonists may be subject to peroxidase-catalyzed nitration that could potentially diminish their therapeutic efficacy.

Inorganic chemistry of defensive peroxidases in the human oral cavity

The innate host response system is comprised of various mechanisms for orchestrating host response to microbial infection of the oral cavity. The heterogeneity of the oral cavity and the associated microenvironments that are produced give rise to different chemistries that affect the innate defense system. One focus of this review is on how these spatial differences influence the two major defensive peroxidases of the oral cavity, salivary peroxidase (SPO) and myeloperoxidase (MPO). With hydrogen peroxide (H(2)O(2)) as an oxidant, the defensive peroxidases use inorganic ions to produce antimicrobials that are generally more effective than H(2)O(2) itself. The concentrations of the inorganic substrates are different in saliva vs. gingival crevicular fluid (GCF). Thus, in the supragingival regime, SPO and MPO work in unison for the exclusive production of hypothiocyanite (OSCN(-), a reactive inorganic species), which constantly bathes nascent plaques. In contrast, MPO is introduced to the GCF during inflammatory response, and in that environment it is capable of producing hypochlorite (OCl(-)), a chemically more powerful oxidant that is implicated in host tissue damage. A second focus of this review is on inter-person variation that may contribute to different peroxidase function. Many of these differences are attributed to dietary or smoking practices that alter the concentrations of relevant inorganic species in the oral cavity (e.g.: fluoride, F(-); cyanide, CN(-); cyanate, OCN(-); thiocyanate, SCN(-); and nitrate, NO(3)(-)). Because of the complexity of the host and microflora biology and the associated chemistry, it is difficult to establish the significance of the human peroxidase systems during the pathogenesis of oral diseases. The problem is particularly complex with respect to the gingival sulcus and periodontal pockets (where the very different defensive stratagems of GCF and saliva co-mingle). Despite this complexity, intriguing in vitro and in vivo studies are reviewed here that reveal the interplay between peroxidase function and associated inorganic chemistry.

Mammalian heme peroxidases: from molecular mechanisms to health implications

A marked increase in interest has occurred over the last few years in the role that mammalian heme peroxidase enzymes, primarily myeloperoxidase, eosinophil peroxidase, and lactoperoxidase, may play in both disease prevention and human pathologies. This increased interest has been sparked by developments in our understanding of polymorphisms that control the levels of these enzymes, a greater understanding of the basic chemistry and biochemistry of the oxidants formed by these species, the development of specific biomarkers that can be used in vivo to detect damage induced by these oxidants, the detection of active forms of these peroxidases at most, if not all, sites of inflammation, and a correlation between the levels of these enzymes and a number of major human pathologies. This article reviews recent developments in our understanding of the enzymology, chemistry, biochemistry and biologic roles of mammalian peroxidases and the oxidants that they generate, the potential role of these oxidants in human disease, and the use of the levels of these enzymes in disease prognosis.

Chlorogenic acid in coffee can prevent the formation of dinitrogen trioxide by scavenging nitrogen dioxide generated in the human oral cavity

Coffee contains antioxidants like chlorogenic acid and its isomers. In this report, effects of coffee on the nitrite-induced N2O3 formation were studied using whole saliva and bacterial fraction prepared from the saliva. The formation of N2O3 was measured by fluorescence increase due to the transformation of 4,5-diaminofluorescein to triazolfluorescein. Coffee inhibited the nitrite-induced fluorescence increase, and 50% inhibition was observed at several microg of coffee/mL in bacterial fraction of saliva as well as whole saliva. During the inhibition of the fluorescence increase, concentration of chlorogenic acid and its isomers decreased. It is discussed that the reduction of NO2 by chlorogenic acid and its isomers contributed to the coffee-dependent inhibition of the fluorescence increase as N2O3 is formed from NO and NO2. When coffee was added to whole saliva, chlorogenic acid and its isomers bound to cells in the saliva. The rate of the fluorescence increase in bacterial fraction, which was prepared at defined periods after the ingestion of coffee, was increased to the rate before the ingestion of coffee with a half-time of about 1 h. This result suggests that chlorogenic acid and its isomers remained in the oral cavity for a few hours after ingestion of coffee. The significance of coffee drinking and rinsing of the mouth with coffee for the health of the oral cavity is proposed.

Doxorubicin inhibits oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) by a lactoperoxidase/H(2)O(2) system by reacting with ABTS-derived radical

The effect of doxorubicin on oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) by lactoperoxidase and hydrogen peroxide has been investigated. It was found that: (1) oxidation of ABTS to its radical cation (ABTS*(+)) is inhibited by doxorubicin as evidenced by its induction of a lag period, duration of which depends on doxorubicin concentration; (2) the inhibition is due to doxorubicin hydroquinone reducing the ABTS*(+) radical (stoichiometry 1: 1.8); (3) concomitant with the ABTS*(+) reduction is oxidation of doxorubicin; only when the doxorubicin concentration decreases to a near zero level, net oxidation of ABTS could be detected; (4) oxidation of doxorubicin leads to its degradation to 3-methoxysalicylic acid and 3-methoxyphthalic acid; (5) the efficacy of doxorubicin to quench ABTS*(+) is similar to the efficacy of p-hydroquinone, glutathione and Trolox C. These observations support the assertion that under certain conditions doxorubicin can function as an antioxidant. They also suggest that interaction of doxorubicin with oxidants may lead to its oxidative degradation.

Radical energies and the regiochemistry of addition to heme groups. Methylperoxy and nitrite radical additions to the heme of horseradish peroxidase

The heme of hemoproteins, as exemplified by horseradish peroxidase (HRP), can undergo additions at the meso carbons and/or vinyl groups of the electrophilic or radical species generated in the catalytic oxidation of halides, pseudohalides, carboxylic acids, aryl and alkyl hydrazines, and other substrates. The determinants of the regiospecificity of these reactions, however, are unclear. We report here modification of the heme of HRP by autocatalytically generated, low-energy NO2* and CH3OO* radicals. The NO2* radical adds regioselectively to the 4- over the 2-vinyl group but does not add to the meso positions. Reaction of HRP with tert-BuOOH does not lead to heme modification; however, reaction with the F152M mutant, in which the heme vinyls are more sterically accessible, results in conversion of the heme 2-vinyl into a 1-hydroxy-2-(methylperoxy)ethyl group [-CH(OH)CH2OOCH3]. [18O]-labeling studies indicate that the hydroxyl group in this adduct derives from water and the methylperoxide oxygens from O2. Under anaerobic conditions, methyl radicals formed by fragmentation of the autocatalytically generated tert-BuO* radical add to both the delta-meso carbon and the 2-vinyl group. The regiochemistry of these and the other known additions to the heme indicate that only high-energy radicals (e.g., CH3*) add to the meso carbon. Less energetic radicals, including NO2* and CH3OO*, add to heme vinyl groups if they are small enough but do not add to the meso carbons. Electrophilic species such as HOBr, HOCl, and HOSCN add to vinyl groups but do not react with the meso carbons. This meso- versus vinyl-reactivity paradigm, which appears to be general for autocatalytic additions to heme prosthetic groups, suggests that meso hydroxylation of the heme by heme oxygenase occurs by a controlled radical reaction rather than by electrophilic addition.

Thiocyanate Cannot Inhibit the Formation of Reactive Nitrogen Species in the Human Oral Cavity in the Presence of High Concentrations of Nitrite: Detection of Reactive Nitrogen Species with 4,5-Diaminofluorescein

In the human oral cavity, nitrite is reduced to nitric oxide (NO) by certain bacteria. 4,5-Diaminofluorescein (DAF-2) was transformed to a fluorescent component triazolfluorescein (DAF-2T) in a bacterial fraction of saliva in the presence of nitrite. No detectable consumption of DAF-2 and formation of DAF-2T were observed in bacterial fraction in the absence of nitrite. The nitrite-dependent transformation of DAF-2 to DAF-2T was inhibited by catalase, SCN(-), and CN(-) suggesting the participation of peroxidases in saliva in the transformation. The formation of DAF-2T, which was observed by the addition of an NO generating reagent (+/-)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (NOR 3) to bacterial fraction, was also inhibited by catalase, SCN(-), and CN(-). The degree of the inhibition by SCN(-) decreased as the concentration of nitrite or NOR 3 was increased. Superoxide dismutase (SOD) enhanced nitrite- and NOR 3-induced fluorescence increase in the bacterial fraction, and the degree of the enhancement decreased as the concentrations of nitrite and NOR 3 were increased. In whole saliva filtrate, the inhibitory effects of SCN(-) on the fluorescence increase decreased as the concentration of nitrite was increased, but the enhancement by SOD was not significantly affected by the increase in the concentration of nitrite. As salivary bacteria produce O(2)(-), H(2)O(2), and NO and as peroxidase/H(2)O(2)/nitrite systems in saliva produce NO(2), the effects of SCN(-) are discussed taking SCN(-)-dependent inhibition of NO(2) formation by peroxidases in saliva into consideration and the effects of SOD are discussed taking O(2)(-)-dependent consumption of NO into consideration. It is concluded that when the rate of the formation of NO is high, SCN(-) is not effective enough to inhibit the formation of N(2)O(3) in the oral cavity.

Immunochemical detection of nitric oxide and nitrogen dioxide trapping of the tyrosyl radical and the resulting nitrotyrosine in sperm whale myoglobin

We demonstrate herein that nitric oxide (*NO) and nitrogen dioxide (*NO2) both react with the tyrosyl radical formed in sperm whale myoglobin (swMb) by reaction with hydrogen peroxide. The tyrosyl radical was detected by Western blotting using a novel anti-5,5-dimethyl-1-pyrroline N-oxide (DMPO) polyclonal antiserum that specifically recognizes protein radical-derived DMPO nitrone adducts. In the presence of DMPO, hydrogen peroxide reacts with swMb to form the DMPO tyrosyl radical as is known from both electron spin resonance and immuno-spin trapping investigations. Both *NO and NO2- significantly suppressed DMPO-Mb formation under the physiological oxygen tension of 30 mm Hg. If this inhibition of DMPO trapping of the tyrosyl radical is due, at least in part, to the reaction of the tyrosyl radical with *NO and *NO2, then nitrotyrosine should be formed. In line with this expectation, swMb treated with low concentrations of *NO or NO2- formed nitrotyrosine when hydrogen peroxide was added under 30 mm Hg oxygen tension as detected by Western blotting. The amount of nitrotyrosine generated with *NO was higher than with NO2-, implying that there are two different peroxynitrite-independent nitrotyrosine formation mechanisms and that *NO is not just a source of *NO2.

Standard reduction potentials of all couples of the peroxidase cycle of lactoperoxidase

Lactoperoxidase (LPO) is found in mucosal surfaces and exocrine secretions including milk, tears and saliva and has physiological significance in antimicrobial defense. Its predominant physiological role is to convert hydrogen peroxide and thiocyanate in hypothiocyanite. In this study, the standard reduction potentials of all redox couples involved in the halogenation and peroxidase cycle of LPO have been determined by multi-mixing stopped-flow spectroscopy. The standard reduction potentials of the redox couples compound I/native LPO, compound I/compound II of LPO, and compound II/native LPO are (1.09 +/- 0.01) V, (1.14 +/- 0.02) V, and (1.04 +/- 0.02) V, respectively, at pH 7 and 25 degrees C. Thus, for the first time, a full description of these important thermodynamic parameters of lactoperoxidase has been performed, allowing a better understanding in the substantial differences in the oxidation of two- and one-electron donors by LPO and other members of the mammalian heme peroxidase superfamily.

Inactivation of Anthracyclines by Cellular Peroxidase

The anticancer anthracyclines, doxorubicin and daunorubicin, are highly cytotoxic to both cancer and normal cells. In this work, we have investigated the capacity of cellular myeloperoxidase to inactivate these agents. We show that incubation of human leukemia HL-60 cells with the anthracyclines in the presence of hydrogen peroxide and nitrite causes irreversible oxidation of the drugs, suggesting an extensive modification of their chromophores. Methimazole, 4-aminobenzoic acid hydrazide, or azide inhibits the reaction, suggesting that it is mediated by the cellular myeloperoxidase, an enzyme naturally present in large amounts in HL-60 cells. In contrast to the intact drugs, the oxidatively transformed anthracyclines were substantially less cytotoxic for HL-60 (assayed by apoptosis) and PC3 prostate cancer cells and H9c2 rat cardiac myoblasts in vitro (assayed by clonogenic survival), indicating that the oxidative metabolism of these agents leads to their inactivation. Using tandem mass spectrometry, we identified two specific metabolic products of the anthracycline degradation, 3-methoxyphthalic acid and 3-methoxysalicylic acid. These two metabolic products were obtained as authentic compounds and were nontoxic to HL-60 leukemic cells and cardiac myocytes. These findings may have important implications for the cellular pharmacology of anthracyclines and for clinical oncology.

Peroxidase catalyzed nitration of tryptophan derivatives

The enzymatic nitration of tryptophan derivatives by oxidation of nitrite has been studied using lactoperoxidase and horseradish peroxidase, and compared with the chemical nitration produced by nitrogen dioxide and peroxynitrite. HPLC, mass spectra and NMR analysis of the mixture of products clearly show that nitration occurs at position 4-, 6-, 7-, and N1 of the indole ring, and nitrosation at position N1. Kinetic studies performed on peroxidase/NO2-/H2O2 systems showed substrate saturation behavior with all the tryptophan derivatives employed. The rate dependence on nitrite concentration was found to be linear with horseradish peroxidase while it exhibited saturation behavior with lactoperoxidase. The composition of the product mixture depends on the nitrating agent. While the production of 4-nitro, 6-nitro, 7-nitro and N1-nitro derivatives follows a similar trend, indicating that they are formed according to a similar mechanism, the ratio between the N1-nitroso derivative and other derivatives depends markedly on the nitrite concentration when tryptophan modification is performed by the peroxidase/H2O2/nitrite systems. Analysis of the data indicates that at low nitrite concentration the enzymatic reaction occurs through the classical peroxidase cycle. At high nitrite concentration the reaction proceeds through a different intermediate that we assume to be a protein bound peroxynitrite species.

Mechanistic insight into the peroxidase catalyzed nitration of tyrosine derivatives by nitrite and hydrogen peroxide

Peroxidases perform the nitration of tyrosine and tyrosyl residues in proteins, in the presence of nitrite and hydrogen peroxide. The nitrating species is still unknown but it is usually assumed to be nitrogen dioxide. In the present investigation, the nitration of phenolic compounds derived from tyrosine by lactoperoxidase and horseradish peroxidase was studied, with the aim of elucidating the mechanism of the reaction. The results indicate that nitrogen dioxide cannot be the only nitrating species and suggest the presence of two simultaneously operative pathways, one proceeding through enzyme-generated nitrogen dioxide and another through a more reactive species, assumed to be complexed peroxynitrite, which is generated by reaction of hydrogen peroxide with the enzyme-nitrite complex. The importance of the two pathways depends on peroxide and nitrite concentrations. With lactoperoxidase, nitration through the highly reactive intermediate is preferred except at very low nitrite concentration, while with horseradish peroxidase, the nitrogen dioxide driven mechanism is preferred except at very high nitrite concentration. The preferred mechanism for the two enzymes is that operative in the physiological nitrite concentration range.

Protein Radical Formation during Lactoperoxidase-mediated Oxidation of the Suicide Substrate Glutathione

A novel anti-5,5-dimethyl-1-pyrroline N-oxide (DMPO) polyclonal antiserum that specifically recognizes protein radical-derived DMPO nitrone adducts has been developed. In this study, we employed this new approach, which combines the specificity of spin trapping and the sensitivity of antigen-antibody interactions, to investigate protein radical formation from lactoperoxidase (LPO). When LPO reacted with GSH in the presence of DMPO, we detected an LPO radical-derived DMPO nitrone adduct using enzyme-linked immunosorbent assay and Western blotting. The formation of this nitrone adduct depended on the concentrations of GSH, LPO, and DMPO as well as pH values, and GSH could not be replaced by H(2)O(2). The level of this nitrone adduct was decreased significantly by azide, catalase, ascorbate, iodide, thiocyanate, phenol, or nitrite. However, its formation was unaffected by chemical modification of free cysteine, tyrosine, and tryptophan residues on LPO. ESR spectra showed that a glutathiyl radical was formed from the LPO/GSH/DMPO system, but no protein radical adduct could be detected by ESR. Its formation was decreased by azide, catalase, ascorbate, iodide, or thiocyanate, whereas phenol or nitrite increased it. GSH caused marked changes in the spectrum of compound II of LPO, indicating that GSH binds to the heme of compound II, whereas phenol or nitrite prevented these changes and reduced compound II back to the native enzyme. GSH also dose-dependently inhibited the peroxidase activity of LPO as determined by measuring 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) oxidation. Taken together, these results demonstrate that the GSH-dependent LPO radical formation is mediated by the glutathiyl radical, possibly via the reaction of the glutathiyl radical with the heme of compound II to form a heme-centered radical trapped by DMPO.

Nitrotyrosine, dityrosine, and nitrotryptophan formation from metmyoglobin, hydrogen peroxide, and nitrite

The biological relevance of tyrosine nitration is a subject of much interest, because extensive evidence supports formation of 3-nitrotyrosine in vivo under a variety of different pathological conditions. Several reagents are likely to be responsible for nitration in vivo, among others peroxynitrite and nitrite in the presence of H(2)O(2)/peroxidases. In this work we show that also metmyoglobin and methemoglobin can nitrate free tyrosine in the presence of nitrite and H(2)O(2). The results of these studies are simulated rather well by using a scheme that comprehends all the possible reactions that can take place in the system. Thus, a good understanding of the factors that determine the yields is achieved. Finally, we demonstrate that the system metMb/H(2)O(2)/NO(2)(-) can also lead to the nitration of tryptophan and produces, in particular, 6-, 4-, and 5-nitrotryptophan.

Human salivary peroxidase-catalyzed oxidation of nitrite and nitration of salivary components 4-hydroxyphenylacetic acid and proteins

Human saliva contains high activities of peroxidase and high concentrations of nitrite (about 0.2 mM in average). If H2O2 is provided by bacteria and leukocytes in the oral cavity, peroxidase-dependent formation of reactive nitrogen species, which can nitrate phenolics like 4-hydroxyphenylacetic acid (HPA) and tyrosine residues in salivary proteins, is possible. H2O2-dependent oxidation of nitrite and H2O2-dependent nitration of HPA were observed in dialyzed saliva and by partially purified salivary peroxidase (SPX). The nitration was inhibited by a physiological electron donor to salivary peroxidase, SCN-. When concentrations of H2O2 and nitrite were increased, nitration of HPA was also observed in control (non-dialyzed) saliva. In addition, H2O2-dependent nitration of tyrosine residues in salivary proteins was observed in dialyzed saliva as an increase in absorbance around 420 nm at pH 7.2. Kinetic studies of the increase in absorbance indicated that sulfhydryl groups in salivary proteins as well as glutathione, ascorbate, urate and SCN- could inhibit the nitration. Since the nitration of proteins can lead to impairment of their functions, it is discussed how the oral cavity is protected from the damages caused by reactive nitrogen species under normal conditions and also discussed that reactive nitrogen species generated by the H2O2/nitrite/peroxidase system can participate in the host defence mechanism in the oral cavity.

Oxidation of mitoxantrone by lactoperoxidase

The lactoperoxidase (LPO) catalysed oxidation of mitoxantrone, an anthraquinone type anti-cancer drug, was studied spectrophotometrically under turnover and single turnover conditions with a stopped flow apparatus. With Compound I and Compound II, mitoxantrone formed binding complexes that were deactivated with increasing substrate concentration. The productive second-order rate constants for reduction were 3.6 x 10(6) and 2.2 x 10(4) M(-1) s(-1) for Compound I and Compound II, respectively. Under turnover conditions, Compound II was the steady-state intermediate, but with increasing H2O2, Compound II reacted with H2O2 to form the catalytically inactive intermediate Compound III. Nitrite prevented formation of Compound III by reducing Compound II to the native state. It also modulated the pathway of mitoxantrone oxidation by increasing the level of oxidised metabolites such as MH2(2+) and the novel metabolite MH. The biological implication of drug activation by LPO with nitrite is discussed.

Kinetics of the reactions of nitrogen monoxide and nitrite with ferryl hemoglobin

Hemoglobin released in the circulation from ruptured red blood cells can be oxidized by hydrogen peroxide or peroxynitrite to generate the highly oxidizing iron(IV)oxo species HbFe(IV)z=O. Nitrogen monoxide, produced in large amounts by activated inducible nitric oxide synthase, can have indirect cytotoxic effects, mainly through the generation of peroxynitrite from its very fast reaction with superoxide. In the present work we have determined the rate constant for the reaction of HbFe(IV)z=O with NO(*), 2.4 x 10(7) M(-1)s(-1) at pH 7.0 and 20 degrees C. The reaction proceeds via the intermediate HbFe(III)ONO, which then dissociates to metHb and nitrite. As these products are not oxidizing and because of its large rate, the reaction of HbFe(IV)z=O with NO(*) may be important to remove the high valent form of hemoglobin, which has been proposed to be at least in part responsible for oxidative lesions. In addition, we have determined that the rate constant for the reaction of HbFe(IV)z=O with nitrite is significantly lower (7.5 x 10(2) M(-1)s(-1) at pH 7.0 and 20 degrees C), but increases with decreasing pH (1.8 x 10(3) M(-1)s(-1) at pH 6.4 and 20 degrees C). Thus, under acidic conditions as found in ischemic tissues, this reaction may also have a physiological relevance.

Peroxidase- and nitrite-dependent metabolism of the anthracycline anticancer agents daunorubicin and doxorubicin

Oxidation of the anticancer anthracyclines doxorubicin (DXR) and daunorubicin (DNR) by lactoperoxidase(LPO)/H(2)O(2) and horseradish peroxidase(HRP)/H(2)O(2) systems in the presence and absence of nitrite (NO(2)(-)) has been investigated using spectrophotometric and EPR techniques. We report that LPO/H(2)O(2)/NO(2)(-) causes rapid and irreversible loss of anthracyclines' absorption bands, suggesting oxidative degradation of their chromophores. Both the initial rate and the extent of oxidation are dependent on both NO(2)(-) concentration and pH. The initial rate decreases when the pH is changed from 7 to 5, and the reaction virtually stops at pH 5. Oxidation of a model hydroquinone compound, 2,5-di-tert-butylhydroquinone, by LPO/H(2)O(2) is also dependent on NO(2)(-); however, in contrast to DNR and DXR, this oxidation is most efficient at pH 5, indicating that LPO/H(2)O(2)/NO(2)(-) is capable of efficiently oxidizing simple hydroquinones even in the neutral form. Oxidation of anthracyclines by HRP/H(2)O(2)/NO(2)(-) is substantially less efficient relative to that by LPO/H(2)O(2)/NO(2)(-) at either pH 5 or pH 7, most likely due to the lower rate of NO(2)(-) metabolism by HRP/H(2)O(2). EPR measurements show that interaction of anthracyclines and 2,5-di-tert-butylhydroquinone with LPO/H(2)O(2)/NO(2)(-) generates the corresponding semiquinone radicals presumably via one-electron oxidation of their hydroquinone moieties. The possible role of the (*)NO(2) radical, a putative LPO metabolite of NO(2)(-), in oxidation of these compounds is discussed. Because in vivo the anthracyclines may co-localize with peroxidases, H(2)O(2), and NO(2)(-) in tissues, their oxidation via the proposed mechanism is likely. These observations reveal a novel, peroxidase- and nitrite-dependent mechanism for the oxidative transformation of the anticancer anthracyclines, which may be pertinent to their biological activities in vivo.