Kinetics of the reaction of superoxide anion with ferric horseradish peroxidase

Effect of ionizing radiation on human myeloperoxidase: Reaction with hydrated electrons

Myeloperoxidase (MPO) is a myeloid-lineage restricted enzyme largely expressed in the azurophilic granules of neutrophils. It catalyses the formation of reactive oxygen species, mainly hypochlorous acid, contributing to anti-pathogenic defense. Disorders in the production or regulation of MPO may lead to a variety of health conditions, mainly of inflammatory origin, including autoimmune inflammation. We have studied the effect of ionizing radiation on the activity of MPO, as measured by the capacity retained by the enzyme to produce hypochlorous acid as reactive oxygen species after exposure to successive doses of solvated electrons, the strongest possible one-e- reducing agent in water. Chlorination activity was still present after a very high irradiation dose, indicating that radiation damage does not take place at the active site, hindered in the core of MPO structure. Decay kinetics show a dependence on the wavelength, supporting that the process must occur at peripheral functional groups situated on external and readily accessible locations of the enzyme. These results are relevant to understand the mechanism of resistance of our innate anti-pathogenic defense system and also to get insight into potential strategies to regulate MPO levels as a therapeutic target in autoimmune diseases.

Direct Electrochemical Generation of Catalytically Competent Oxyferryl Species of Classes I and P Dye Decolorizing Peroxidases

This work introduces a novel way to obtain catalytically competent oxyferryl species for two different dye-decolorizing peroxidases (DyPs) in the absence of H2O2 or any other peroxide by simply applying a reductive electrochemical potential under aerobic conditions. UV-vis and resonance Raman spectroscopies show that this method yields long-lived compounds II and I for the DyPs from Bacillus subtilis (BsDyP; Class I) and Pseudomonas putida (PpDyP; Class P), respectively. Both electrochemically generated high valent intermediates are able to oxidize ABTS at both acidic and alkaline pH. Interestingly, the electrocatalytic efficiencies obtained at pH 7.6 are very similar to the values recorded for regular catalytic ABTS/H2O2 assays at the optimal pH of the enzymes, ca. 3.7. These findings pave the way for the design of DyP-based electrocatalytic reactors operable in an extended pH range without the need of harmful reagents such as H2O2.

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.

The reaction of oxygen with radicals from oxidation of tryptophan and indole-3-acetic acid

The oxidation of tryptophan and indole-3-acetic acid (IAA) by the dibromine radical anion or peroxidase from horseradish in aqueous solution was investigated and compared, especially with respect to the involvement of oxygen and superoxide. Using EPR with spin-trapping, the tryptophanyl radical, generated by either method was found to react with oxygen, although this reaction is too slow to be observed by pulse radiolysis (k < 5 x 10(6) dm3 mol-1 s-1). No superoxide results from this reaction, thus excluding an electron-transfer mechanism and suggesting the formation of a tryptophan peroxyl radical, possibly in a reversible process. These observations imply that in proteins where the tryptophanyl radical exists as a stable species it must either have its reactivity modified by the protein environment or be inaccessible to oxygen. The related molecule LAA is oxidized by either peroxidase or Br2.- to a radical cation that decarboxylates to yield a skatolyl radical. The latter reacts with oxygen to give a peroxyl radical that does not release superoxide. However, O2.- is formed during the peroxidase-catalyzed oxidation of indoleacetic acid. This supports the hypothesis that the peroxidase can act in an oxidase cycle involving ferrous enzyme and compound III, with superoxide as a product.

The effect of ultrasound on heme enzymes in aqueous solution

The effect of cavitating 22 kHz ultrasound on aqueous solutions of hydrogen peroxide-consuming enzymes, catalase and peroxidases, both plant (horseradish peroxidase) and animal (lactoperoxidase) was studied. Catalase did not undergo inactivation during sonication, whereas activity of peroxidases decreased with increased duration of sonication. It is suggested, basing on the absorption spectra, that some conformational changes occur in peroxidases upon sonolysis. It is concluded from the experiments with free radical scavengers that partial enzyme inactivation and modification has not a chemical but a mechanical basis.

Conversion of metmyoglobin to NO myoglobin in the presence of nitrite and reductants

Formation of NO myoglobin through the reaction of horse heart metmyoglobin with NADH in the presence of nitrite was observed optically at pH 5.5. Superoxide generation during the reaction was demonstrated using the ESR spin trap, 5,5-dimethyl-1-pyrroline-1-oxide. A weak optical spectrum corresponding to oxymyoglobin appeared transiently and the spectrum of NO myoglobin then developed. The conversion to NO myoglobin was eliminated in the presence of catalase, SOD or 5,5-dimethyl-1-pyrroline-1-oxide. The kinetics of NADH oxidation and oxygen consumption catalyzed by myoglobin showed an initial lag phase, indicating a chain reaction. When the oxygen was exhausted, the NO form emerged. The duration of the lag phase was prolonged by an increase in the concentration of catalase, SOD or 5,5-dimethyl-1-pyrroline-1-oxide, whereas it disappeared in the presence of H2O2. The spectral change from metmyoglobin to NO myoglobin was also observed under anaerobic conditions though the rate was slower than that obtained under aerobic conditions, while the spectral change was accelerated in the presence of H2O2. Nitric oxide (NO) was derived through the reaction of nitrite with NADH. The formation of NO myoglobin from metmyoglobin is explained in terms of the NADH-oxidase reaction catalyzed by myoglobin. Ascorbate and GSH also serve as reductants though NO myoglobin was formed slowly.

Transient and steady-state kinetics of the oxidation of scopoletin by horseradish peroxidase compounds I, II and III in the presence of NADH

Scopoletin, a naturally occurring fluorescent component of some plants and a proven plant growth inhibitor, is a known reactant with peroxidase. However, the kinetics of the elementary steps of the reaction have never been investigated, nor has the quantitative effect of interfering substances ever been explored in detail, despite the fact that scopoletin is widely used in a peroxidase assay for H2O2. In this work, we employed both transient-state and steady-state methods to determine the second-order rate constants for the oxidation of scopoletin by the horseradish peroxidase (HRP) intermediate compounds I and II: (3.7 +/- 0.1) x 10(6) M-1 s-1 and (8.5 +/- 0.5) x 10(5) M-1 s-1 at 20 degrees C, pH 6.0 and ionic strength of 0.1 M. We investigated the possible inhibitory effect of NADH on the reaction of scopoletin with HRP and also the effect of scopoletin on the NADH reaction. In the presence of NADH the rate constant for the reaction between HRP-I and scopoletin decreased slightly to (2.8 +/- 0.1) x 10(6) M-1 s-1. Thus, although NADH is also a peroxidase substrate, it cannot compete effectively for the oxidized forms of the enzyme. On the other hand, scopoletin stimulates the oxidation of NADH by the HRP/H2O2 system, apparently by forming a phenoxyl radical which then oxidizes NADH to NAD. radicals. We present spectral evidence showing that in the aerobic reaction between HRP and NADH at pH 7.0 (without exogenously added H2O2) HRP-II is the dominant enzyme intermediate with HRP-III also detectable. Addition of scopoletin to the HRP/NADH system leads to a biphasic reaction in which HRP-II and HRP-III disappear. The rate constants for both phases are linearly dependent on scopoletin concentration. We attribute the faster phase to the HRP-II reaction with scopoletin with a rate constant of (6.2 +/- 0.1) x 10(5) M-1 s-1 and the slower phase to the HRP-III reaction with scopoletin with rate constant (5.0 +/- 0.4) x 10(4) M-1 s-1. Our present work not only provides rate constants for the oxidation of scopoletin by HRP-I, II and III but also elucidates the interactions that possibly occur physiologically during NADH oxidation in the presence of scopoletin.

Reactions of radiolytically-generated superoxide anion with higher oxidation states of lactoperoxidase

The time-courses of absorption changes after pulse radiolysis of oxygen-saturated solution of lactoperoxidase have been studied. Radiation-generated superoxide reduces compounds I to II. The results suggest that superoxide anion reacts also with compound II of lactoperoxidase; however, the reduction of the heme iron has not been observed. A possible reaction pathway of compound II in the presence of superoxide anion is discussed.

Reactions of hydrated electrons with horseradish peroxidase: a pulse radiolysis study

Kinetics of the reactions of hydrated electrons with horseradish peroxidase (HRP) have been studied by pulse radiolysis. Hydrated electrons reduce HRP to its ferrous form with a yield of 40%. At higher dose (250 Gy) the formation of compound I followed by its reduction to compound II was also observed.

Reactions of the NAD radical with higher oxidation states of horseradish peroxidase

The reactions of the NAD radical (NAD.) with ferric horseradish peroxidase and with compounds I and II were investigated by pulse radiolysis. NAD. reacted with the ferric enzyme and with compound I to form the ferrous enzyme and compound II with second-order rate constants of 8 X 10(8) and 1.5 X 10(8) M-1 s-1, respectively, at pH 7.0. In contrast, no reaction of NAD. with native compound II at pH 10.0 nor with diacetyldeutero-compound II at pH 5.0-8.0 could be detected. Other reducing species generated by pulse radiolysis, such as hydrated electron (eaq-), superoxide anion (O2-), and benzoate anion radical, could not reduce compound II of the enzyme to the ferric state, although the methylviologen radical reduced it. The results are discussed in relation to the mechanism of catalysis of the one-electron oxidation of substrates by peroxidase.

Spectroscopic and equilibrium studies of ligand and organic substrate binding to indolamine 2,3-dioxygenase

The binding of a number of ligands to the heme protein indolamine 2,3-dioxygenase has been examined with UV-visible absorption and with natural and magnetic circular dichroism spectroscopy. Relatively large ligands (e.g., norharman) which do not readily form complexes with myoglobin and horseradish peroxidase (HRP) can bind to the dioxygenase. Except for only a few cases (e.g., 4-phenylimidazole) for the ferric dioxygenase, a direct competition for the enzyme rarely occurs between the substrate L-tryptophan (Trp) and the ligands examined. L-Trp and small heme ligands (CN-,N3-,F-) markedly enhance the affinity of each other for the ferric enzyme in a reciprocal manner, exhibiting positive cooperativity. For the ferrous enzyme, L-Trp exerts negative cooperativity with some ligands such as imidazoles, alkyl isocyanides, and CO binding to the enzyme. This likely reflects the proximity of the Trp binding site to the heme iron. Other indolamine substrates also exert similar but smaller cooperative effects on the binding of azide or ethyl isocyanide. The pH dependence of the ligand affinity of the dioxygenase is similar to that of myoglobin rather than that of HRP. These results suggest that indolamine 2,3-dioxygenase has the active-site heme pocket whose environmental structure is similar to, but whose size is considerably larger than, that of myoglobin, a typical O2-binding heme protein. Although the L-Trp affinity of the ferric cyanide and ferrous CO enzyme varies only slightly between pH 5.5 and 9.5, the unligated ferric and ferrous enzymes have considerably higher affinity for L-Trp at alkaline pH than at acidic pH. L-Trp binding to the ferrous dioxygenase is affected by an ionizable residue with a pKa value of 7.3.