The reactions of horseradish peroxidase, lactoperoxidase, and myeloperoxidase with enzymatically generated superoxide

The Roles of Neutrophil-Derived Myeloperoxidase (MPO) in Diseases: The New Progress

Myeloperoxidase (MPO) is a heme-containing peroxidase, mainly expressed in neutrophils and, to a lesser extent, in monocytes. MPO is known to have a broad bactericidal ability via catalyzing the reaction of Cl- with H2O2 to produce a strong oxidant, hypochlorous acid (HOCl). However, the overproduction of MPO-derived oxidants has drawn attention to its detrimental role, especially in diseases characterized by acute or chronic inflammation. Broadly speaking, MPO and its derived oxidants are involved in the pathological processes of diseases mainly through the oxidation of biomolecules, which promotes inflammation and oxidative stress. Meanwhile, some researchers found that MPO deficiency or using MPO inhibitors could attenuate inflammation and tissue injuries. Taken together, MPO might be a promising target for both prognostic and therapeutic interventions. Therefore, understanding the role of MPO in the progress of various diseases is of great value. This review provides a comprehensive analysis of the diverse roles of MPO in the progression of several diseases, including cardiovascular diseases (CVDs), neurodegenerative diseases, cancers, renal diseases, and lung diseases (including COVID-19). This information serves as a valuable reference for subsequent mechanistic research and drug development.

Covalent Attachment of Horseradish Peroxidase to Single-Walled Carbon Nanotubes for Hydrogen Peroxide Detection

Single-walled carbon nanotubes (SWCNTs) are desirable nanoparticles for sensing biological analytes due to their photostability and intrinsic near-infrared fluorescence. Previous strategies for generating SWCNT nanosensors have leveraged nonspecific adsorption of sensing modalities to the hydrophobic SWCNT surface that often require engineering new molecular recognition elements. An attractive alternate strategy is to leverage pre-existing molecular recognition of proteins for analyte specificity, yet attaching proteins to SWCNT for nanosensor generation remains challenging. Towards this end, we introduce a generalizable platform to generate protein-SWCNT-based optical sensors and use this strategy to synthesize a hydrogen peroxide (H 2 O 2 ) nanosensor by covalently attaching horseradish peroxidase (HRP) to the SWCNT surface. We demonstrate a concentration-dependent response to H 2 O 2 , confirm the nanosensor can image H 2 O 2 in real-time, and assess the nanosensor's selectivity for H 2 O 2 against a panel of biologically relevant analytes. Taken together, these results demonstrate successful covalent attachment of enzymes to SWCNTs while preserving both intrinsic SWCNT fluorescence and enzyme function. We anticipate this platform can be adapted to covalently attach other proteins of interest including other enzymes for sensing or antibodies for targeted imaging and cargo delivery.

Superoxide: The enigmatic chemical chameleon in neutrophil biology

The burst of superoxide produced when neutrophils phagocytose bacteria is the defining biochemical feature of these abundant immune cells. But 50 years since this discovery, the vital role superoxide plays in host defense has yet to be defined. Superoxide is neither bactericidal nor is it just a source of hydrogen peroxide. This simple free radical does, however, have remarkable chemical dexterity. Depending on its environment and reaction partners, superoxide can act as an oxidant, a reductant, a nucleophile, or an enzyme substrate. We outline the evidence that inside phagosomes where neutrophils trap, kill, and digest bacteria, superoxide will react preferentially with the enzyme myeloperoxidase, not the bacterium. By acting as a cofactor, superoxide will sustain hypochlorous acid production by myeloperoxidase. As a substrate, superoxide may give rise to other forms of reactive oxygen. We contend that these interactions hold the key to understanding the precise role superoxide plays in neutrophil biology. State-of-the-art techniques in mass spectrometry, oxidant-specific fluorescent probes, and microscopy focused on individual phagosomes are needed to identify bactericidal mechanisms driven by superoxide. This work will undoubtably lead to fascinating discoveries in host defense and give a richer understanding of superoxide's varied biology.

The inhibition of lactoperoxidase catalytic activity through mesna (2-mercaptoethane sodium sulfonate)

Here, we show that mesna (sodium-2-mercaptoethane sulfonate), primarily used to prevent nephrotoxicity and urinary tract toxicity caused by chemotherapeutic agents such as cyclophosphamide and ifosfamide, modulates the catalytic activity of lactoperoxidase (LPO) by binding tightly to the enzyme, functioning either as a one electron substrate for LPO Compounds I and II, destabilizing Compound III. Lactoperoxidase is a hemoprotein that utilizes hydrogen peroxide (H₂O₂) and thiocyanate (SCN^-) to produce hypothiocyanous acid (HOSCN), an antimicrobial agent also thought to be associated with carcinogenesis. Our results revealed that mesna binds stably to LPO within the SCN^- binding site, dependent of the heme iron moiety, and its combination with LPO-Fe(III) is associated with a disturbance in the water molecule network in the heme cavity. At low concentrations, mesna accelerated the formation and decay of LPO compound II via its ability to serve as a one electron substrate for LPO compounds I and II. At higher concentrations, mesna also accelerated the formation of Compound II but it decays to LPO-Fe(III) directly or through the formation of an intermediate, Compound I*, that displays characteristic spectrum similar to that of LPO Compound I. Mesna inhibits LPO's halogenation activity (IC₅₀ value of 9.08 μM) by switching the reaction from a 2e^- to a 1e^- pathway, allowing the enzyme to function with significant peroxidase activity (conversion of H₂O₂ to H₂O without generation of HOSCN). Collectively, mesna interaction with LPO may serve as a potential mechanism for modulating its steady-state catalysis, impacting the regulation of local inflammatory and infectious events.

Redox properties of myeloperoxidase

The heme-containing enzyme myeloperoxidase (MPO) is secreted from polymorphonuclear leukocytes and monocytes. It is involved in host defence and inflammation by oxidation of numerous small molecules. This review summarises our current results on the determination of redox properties of all intermediates involved in the halogenation and peroxidase cycle of MPO. The standard reduction potentials of the redox couples compound I/native MPO, compound I/compound II of MPO, and compound II/native MPO have been determined to be 1.16 V, 1.35 V, and 0.97 V, respectively, at pH 7 and 25 degrees C. Thus, for the first time, a full description of these important thermodynamic parameters of myeloperoxidase has been performed, allowing a better understanding of its extraordinary reactivity.

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.

Spectral Studies of Intermediate Species Formed in One-electron Reactions of Bovine Liver Catalase at Room and Low Temperatures. A Comparison with Peroxidase Reactions

The reactions of native bovine catalase with superoxide and solvated electrons have been investigated using three different methods for generation of these reducing substrates: gamma-radiolysis of oxygenated or deaerated buffer solutions in the presence of an OH radical scavenger; either xanthine or acetaldehyde with xanthine oxidase; and low-temperature (77 K) gamma-radiolysis of buffered ethylene glycol/water solutions with subsequent annealing of samples at 183 K. The first spectral evidence for catalase compound II formation from native catalase via reaction with superoxide was obtained. The results are compared with results for peroxidase compound II or III formation observed under the same experimental conditions. A scheme is proposed to explain these observations involving intermediate formation of catalase compounds I and III and the ferrous enzyme. The one-electron reduction of catalase and peroxidase by radiolytically-generated solvated electrons was compared. In the present study the first absorption spectrum of a high-spin ferrous catalase which has peaks at 561 and 594 nm is reported, in comparison with a hemochromogen low-spin ferrous peroxidase observed under the same experimental conditions (peaks at 527 and 556 nm). Both spectra were recorded at 77 K. Data presented in this work also provide the first spectral evidence indicating the low temperature (183 K) conversion of high-spin ferrous catalase into compound III (oxycatalase) in the presence of dioxygen. Under the same experimental conditions low-spin ferrous peroxidase was converted into the high-spin ferrous form without oxyperoxidase formation.

Glutathione-induced radical formation on lactoperoxidase does not correlate with the enzyme's peroxidase activity

Lactoperoxidase (LPO) is believed to serve as a mediator of host defense against invading pathogens. The protein is more abundant in body fluids such as milk, saliva, and tears. Lactoperoxidase is known to mediate the oxidation of halides and (pseudo)halides in the presence of hydrogen peroxide to reactive intermediates presumably involved in pathogen killing. More recently, LPO has been shown to oxidize a wide diversity of thiol compounds to thiyl free radicals, which ultimately lead to the formation of a protein radical characterized by DMPO-immunospin trapping. In the same study by our group the authors claimed that a consequence of this protein radical formation was the inactivation of LPO (Guo et al., J. Biol. Chem.279:13272-13283; 2004). Here we demonstrate that although thiyl radical formation does lead to LPO radical production, the formation of this radical is unrelated to the enzyme's activity. We suggest the source of this misleading interpretation to be the binding of GSH to ELISA plates, which interferes with ABTS and guaiacol oxidation. In addition, DMPO-GSH-nitrone adducts bind to ELISA plates, leading to ambiguities of interpretation since we have demonstrated that DMPO-GSH nitrone does not bind to LPO, and only LPO-protein-DMPO-nitrone adducts can be detected by Western blot.

Purification and biochemical characterization of a heme containing peroxidase from the human parasite P. falciparum

A peroxidase (30 kDa) has been purified from the human malaria parasite Plasmodium falciparum to its homogeneity. The protein is a dimer of 15 kDa subunit as evident from SDS-PAGE and MALDI-TOF mass analysis. The antibodies developed against the purified protein cross-react selectively with this protein present in parasite lysate. It is a heme containing peroxidase [R/Z value (A408/A278)=2.33] showing characteristic heme spectra with Soret peak at 408 nm and visible peaks at 536 and 572 nm. Analysis of Soret spectra in presence or absence of cyanide or azide reveals that iron of heme is in Fe-III state. Circular dichroism spectral analysis establishes that this protein contains mainly alpha-helix (60-70%). H2O2 interacts with the heme moiety of the enzyme as evidenced by optical difference spectroscopy and spectral studies indicate the formation of catalytically active peroxidase-H2O2 complex (Soret peak at 413 nm) to exhibit peroxidase activity. During the erythrocytic stages of its life cycle, the parasite is exposed to oxidative stress. As the parasite is susceptible to oxidative stress, this peroxidase may offer antioxidant role by scavenging endogenous H2O2.

Myeloperoxidase: friend and foe

Neutrophilic polymorphonuclear leukocytes (neutrophils) are highly specialized for their primary function, the phagocytosis and destruction of microorganisms. When coated with opsonins (generally complement and/or antibody), microorganisms bind to specific receptors on the surface of the phagocyte and invagination of the cell membrane occurs with the incorporation of the microorganism into an intracellular phagosome. There follows a burst of oxygen consumption, and much, if not all, of the extra oxygen consumed is converted to highly reactive oxygen species. In addition, the cytoplasmic granules discharge their contents into the phagosome, and death of the ingested microorganism soon follows. Among the antimicrobial systems formed in the phagosome is one consisting of myeloperoxidase (MPO), released into the phagosome during the degranulation process, hydrogen peroxide (H2O2), formed by the respiratory burst and a halide, particularly chloride. The initial product of the MPO-H2O2-chloride system is hypochlorous acid, and subsequent formation of chlorine, chloramines, hydroxyl radicals, singlet oxygen, and ozone has been proposed. These same toxic agents can be released to the outside of the cell, where they may attack normal tissue and thus contribute to the pathogenesis of disease. This review will consider the potential sources of H2O2 for the MPO-H2O2-halide system; the toxic products of the MPO system; the evidence for MPO involvement in the microbicidal activity of neutrophils; the involvement of MPO-independent antimicrobial systems; and the role of the MPO system in tissue injury. It is concluded that the MPO system plays an important role in the microbicidal activity of phagocytes.

Morphological changes and oxidative stress in rat prostate exposed to a non-carcinogenic dose of cadmium

Cadmium chloride is an environmental toxicant implicated in human prostate carcinogenesis. The mechanism of its toxicity is far from fully understood. This study evaluates the effect of exposure to an oral non-carcinogenic dose of cadmium (15 ppm in drinking water for three months) on different parameters of the ventral prostatic lobe of normal and exposed rats. We analyzed the histology by optic light microscopy, activities of antioxidant enzymes (CAT, SOD, GPx and G-6-PDH), expression of iNOS and COX-2 by Western blot, expression of MT-I, MT-II, IGF-I, IGF-BP5 and rtert by RT-PCR. Histological changes were found: the height of the cells decreased, acinar lumen were enlarged and they lost the typical invaginations. Lipoperoxidation was increased in the Cd group and the antioxidant enzymes changed their activities: SOD increased, CAT and G-6-PDH decreased and GPx did not show variations. iNOS and COX-2 did not change their expressions. MT-I and IGF-BP5 mRNA increased while MT-II, IGF-I and rtert did not show variations. Cd exposure induces important morphological changes in the prostate, which could be a consequence of lipoperoxidation and oxidative stress, which are not related to iNOS and COX-2. The histology suggests an involution state of the gland, confirmed by the expression of IGF-I, IGF-BP5 and rtert.

High dissociation rate constant of ferrous-dioxy complex linked to the catalase-like activity in lactoperoxidase

Heme reduction of ferric lactoperoxidase (LPO) into its ferrous form initially leads to the accumulation of the unstable form of LPO-Fe(II), which spontaneously converts to a more stable species, the two of which can be identified by Soret peaks at 440 and 434 nm, respectively. Our data demonstrate that both LPO-Fe(II) species are capable of binding O(2) at a similar rate to generate the ferrous-dioxy complex. Its formation with respect to O(2) was first order and monophasic and with rate constants of k(on) = 3.8 x 10(4) m(-1) s(-1) and k(off) = 11.2 s(-1). The dissociation rate constant for the formation of LPO-Fe(II)-O(2) is relatively high, in contrast to hemoprotein model compounds. This high dissociation rate can be attributed to a combination of effects that include the positive trans effect of the proximal ligand, the heme pocket environment, and the geometry of the Fe-O(2) linkage. Our results have also shown that the decay of the LPO-Fe(II)-O(2) complex occurs by two sequential O(2)-independent steps. The first step involves formation of a short-lived intermediate that can be characterized by its Soret absorption peak at 416 nm and may be attributed to the weakening of the Fe(II)-O(2) linkage with a rate constant of 0.5 s(-1). The second step is spontaneous conversion of this intermediate to generate the native enzyme and presumably superoxide as end products with a rate constant of 0.03 s(-1). A comprehensive kinetic model that links LPO-Fe(II)-O(2) complex formation to the LPO catalase-like activity, combined with the classic catalytic cycle, is presented here.

Acetaminophen stimulates the peroxidative metabolism of anthracyclines

Acetaminophen, a common analgesic and antipyretic drug, is frequently administered to individuals undergoing anthracycline chemotherapy. Here, the effect of acetaminophen on the metabolism of daunorubicin and doxorubicin by isolated enzymes lactoperoxidase and myeloperoxidase, and by myeloperoxidase-containing human leukemia HL-60 cells was investigated using spectrophotometric and EPR techniques. We report that at pharmacological concentrations acetaminophen strongly stimulates oxidation of the anthracyclines by lactoperoxidase and myeloperoxidase systems, which results in irreversibly altered (colorless) products. The initial rate and efficacy of daunorubicin oxidation depends on pH. While at pH approximately 7 the oxidation is rapid and extensive, almost no oxidation occurs at pH approximately 5. In the absence of daunorubicin, oxidation of acetaminophen by lactoperoxidase/hydrogen peroxide is only weakly dependent on pH, however, at pH 7.4 it strongly depends on [daunorubicin]. Ascorbate and reduced glutathione strongly inhibited oxidation of anthracyclines by lactoperoxidase and HL-60 systems. Using EPR, a daunorubicin-derived radical was detected in a daunorubicin/acetaminophen/peroxidase/hydrogen peroxide system as a narrow single line (0.175 mT) with g = 2.0047. When daunorubicin was omitted, only an acetaminophen-melanin EPR signal was detected (g = 2.0043, line width approximately 0.5 mT). Similar results were obtained with doxorubicin. We suggest that the stimulation by acetaminophen is primarily due to its preferential oxidation by peroxidases to the corresponding phenoxyl radical, which subsequently reacts with daunorubicin (doxorubicin). Because biological properties of oxidatively transformed anthracyclines will certainly be different from those of their parent compounds, the possible acetaminophen-enhanced degradation of the anthracyclines in vivo is likely to interfere with anticancer and/or cardiotoxic activities of these agents.

Unraveling the catalytic mechanism of lactoperoxidase and myeloperoxidase. A reflection on some controversial features

Although belonging to the widely investigated peroxidase superfamily, lactoperoxidase (LPO) and myeloperoxidase (MPO) share structural and functional features that make them peculiar with respect to other enzymes of the same group. A survey of the available literature on their catalytic intermediates enabled us to ask some questions that remained unanswered. These questions concern controversial features of the LPO and MPO catalytic cycle, such as the existence of Compound I and Compound II isomers and the identification of their spectroscopic properties. After addressing each of these questions, we formulated a hypothesis that describes an integrated vision of the catalytic mechanism of both enzymes. The main points are: (a) a re-evaluation of the role of superoxide as a reductant in the catalytic cycle; (b) the existence of Cpd I isomers; (c) reciprocal interactions between catalytic intermediates and (d) a mechanistic explanation for catalase activity in both enzymes.

Redox properties of the couples compound I/compound II and compound II/native enzyme of human myeloperoxidase

Myeloperoxidase (MPO) is an important component of the neutrophil's antimicrobial armory and has been implicated in promoting tissue damage in numerous inflammatory diseases. For the first time the standard reduction potential of the redox couple compound II/native enzyme has been determined to be (0.97+/-0.01)V at pH 7.0 and 25 degrees C. This was achieved by rapid mixing of preformed compound II with either tyrosine or nitrite by using the sequential-mixing stopped-flow technique and measuring spectrophotometrically the concentrations of the reacting species and products at equilibrium. Using the recently determined standard reduction potential for the couple compound I/native enzyme (1.16 V), the reduction potential of the couple compound I/compound II was calculated to be 1.35 V at pH 7 and 25 degrees C. These data reveal substantial differences between the two known heme peroxidase superfamilies and reflect the dramatic differences observed in the oxidisability of substrates by the MPO redox intermediates compound I and compound II.

Non-oxidative decarboxylation of glycine derivatives by a peroxidase

Under anaerobic, peroxide-free conditions (pH 5.5, 25 degrees C), horseradish peroxidase (HRP) catalyzes the rapid, non-oxidatve decarboxylation of N-alkyl-N-phenylglycine derivatives to the corresponding N-alkyl-N-methylanilines in 100% yield. When the reaction is conducted in D2O buffer, the product contains a single deuterium in the methyl group. The reactions are very fast compared to the oxidative decarboxylation of the same substrates under standard peroxidatic conditions (i.e., hydrogen peroxide added, air present) and in fact are inhibited by peroxide and oxygen. To account for these unprecedented observations, we propose a cyclic mechanism in which ferric HRP abstracts an electron from the substrate, giving an aminium ion intermediate that decarboxylates; protonation of the resulting alpha-aminoradical on carbon gives an aminium ion that is reduced by ferrous HRP to complete the cycle.

Oxidation of melatonin and tryptophan by an HRP cycle involving compound III

We recently described that horseradish peroxidase (HRP) and myeloperoxidase (MPO) catalyze the oxidation of melatonin, forming the respective indole ring-opening product N(1)-acetyl-N(2)-formyl-5-methoxykynuramine (AFMK) (Biochem. Biophys. Res. Commun. 279, 657-662, 2001). Although the classic peroxidatic enzyme cycle is expected to participate in the oxidation of melatonin, the requirement of a low HRP:H(2)O(2) ratio suggested that other enzyme paths might also be operative. Here we followed the formation of AFMK under two experimental conditions: predominance of HRP compounds I and II or presence of compound III. Although the consumption of substrate is comparable under both conditions, AFMK is formed in significant amounts only when compound III predominates during the reaction. Using tryptophan as substrate, N- formyl-kynurenine is formed in the presence of compound III. Both, melatonin and tryptophan efficiently prevents the formation of p-670, the inactive form of HRP. Since superoxide dismutase (SOD) inhibits the production of AFMK, we proposed that compound III acts as a source of O(-*)(2) or participates directly in the reaction, as in the case of enzyme indoleamine 2,3-dioxygenase.

Chorion peroxidase-mediated NADH/O(2) oxidoreduction cooperated by chorion malate dehydrogenase-catalyzed NADH production: a feasible pathway leading to H(2)O(2) formation during chorion hardening in Aedes aegypti mosquitoes

A specific chorion peroxidase is present in Aedes aegypti and this enzyme is responsible for catalyzing chorion protein cross-linking through dityrosine formation during chorion hardening. Peroxidase-mediated dityrosine cross-linking requires H(2)O(2), and this study discusses the possible involvement of the chorion peroxidase in H(2)O(2) formation by mediating NADH/O(2) oxidoreduction during chorion hardening in A. aegypti eggs. Our data show that mosquito chorion peroxidase is able to catalyze pH-dependent NADH oxidation, which is enhanced in the presence of Mn(2+). Molecular oxygen is the electron acceptor during peroxidase-catalyzed NADH oxidation, and reduction of O(2) leads to the production of H(2)O(2), demonstrated by the formation of dityrosine in a NADH/peroxidase reaction mixture following addition of tyrosine. An oxidoreductase capable of catalyzing malate/NAD(+) oxidoreduction is also present in the egg chorion of A. aegypti. The cooperative roles of chorion malate/NAD(+)oxidoreductase and chorion peroxidase on generating H(2)O(2) with NAD(+) and malate as initial substrates were demonstrated by the production of dityrosine after addition of tyrosine to a reaction mixture containing NAD(+) and malate in the presence of both malate dehydrogenase fractions and purified chorion peroxidase. Data suggest that chorion peroxidase-mediated NADH/O(2) oxidoreduction may contribute to the formation of the H(2)O(2) required for chorion protein cross-linking mediated by the same peroxidase, and that the chorion associated malate dehydrogenase may be responsible for the supply of NADH for the H(2)O(2) production.

Heme Peroxidases: Structure, Function, Mechanism and Involvement in Activation of Carcinogens. A Review

Peroxidases are enzymes playing an important role in large and diverse numbers of physiological processes in organisms including human. We have attempted in this article to summarize and review the important structural and catalytic properties of principal classes of heme peroxidases as well as their biological functions. Major reactions catalyzed by these enzymes (a conventional peroxidase cycle, reactions using O2and halogenations) and their mechanism are reviewed, too. Moreover, the reaction mechanisms by which peroxidases are implicated in bioactivation of xenobiotic chemicals are presented. Numerous chemicals including protoxicants and procarcinogens are metabolized by equally numerous chemical reactions catalyzed by peroxidases. The unifying theme is the radical nature of the oxidations. The direct conventional peroxidase reaction forming reactive species is generally responsible for the activation of procarcinogenic substrates of peroxidases. The subsequent formation of a superoxide anion radical and peroxy radicals is necessary for activation of chemicals that are poor substrates for peroxidases. The significance of studies concerning the reactions catalyzed by peroxidases is underlined in the present review article. A review with 166 references.

Roles of reactive oxygen species: signaling and regulation of cellular functions

Reactive oxygen species (ROS) are the side products (H2O2, O2.-, and OH.) of general metabolism and are also produced specifically by the NADPH oxidase system in most cell types. Cells have a very efficient antioxidant defense to counteract the toxic effect of ROS. The physiological significance of ROS is that ROS at low concentrations are able to mediate cellular functions through the same steps of intracellular signaling, which are activated by natural stimuli. Moreover, a variety of natural stimuli act through the intracellular formation of ROS that change the intracellular redox state (oxidation-reduction). Thus, the redox state is a part of intracellular signaling. As such, ROS are now considered signal molecules at nontoxic concentrations. Progress has been achieved in studying the oxidative activation of gene transcription in animal cells and bacteria. Changes in the redox state of intracellular thiols are considered to be an important mechanism that regulates cell functions.

Extracellular superoxide dismutase in the airways of transgenic mice reduces inflammation and attenuates lung toxicity following hyperoxia

Extracellular superoxide dismutase (EC-SOD, or SOD3) is the major extracellular antioxidant enzyme in the lung. To study the biologic role of EC-SOD in hyperoxic-induced pulmonary disease, we created transgenic (Tg) mice that specifically target overexpression of human EC-SOD (hEC-SOD) to alveolar type II and nonciliated bronchial epithelial cells. Mice heterozygous for the hEC-SOD transgene showed threefold higher EC-SOD levels in the lung compared with wild-type (Wt) littermate controls. A significant amount of hEC-SOD was present in the epithelial lining fluid layer. Both Tg and Wt mice were exposed to normobaric hyperoxia (>99% oxygen) for 48, 72, and 84 hours. Mice overexpressing hEC-SOD in the airways attenuated the hyperoxic lung injury response, showed decreased morphologic evidence of lung damage, had reduced numbers of recruited inflammatory cells, and had a reduced lung wet/dry ratio. To evaluate whether reduced numbers of neutrophil infiltration were directly responsible for the tolerance to oxygen toxicity observed in the Tg mice, we made Wt and Tg mice neutropenic using anti-neutrophil antibodies and subsequently exposed them to 72 hours of hyperoxia. Both Wt and Tg neutrophil-depleted (ND) mice have less severe lung injury compared with non-ND animals, thus providing direct evidence that neutrophils recruited to the lung during hyperoxia play a distinct role in the resultant acute lung injury. We conclude that oxidative and inflammatory processes in the extracellular lung compartment contribute to hyperoxic-induced lung damage and that overexpression of hEC-SOD mediates a protective response to hyperoxia, at least in part, by attenuating the neutrophil inflammatory response.

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.

One-electron oxidations by peroxidases

1. Peroxidases typically follow the reaction cycle: native enzyme-->compound I-->compound II-->native enzyme, in which the latter two steps involve hydrogen atom transfer from substrate to enzyme. 2. Exceptions involve (1) very facile, rapidly reacting reducing substrates that transfer an electron rather than a hydrogen atom, resulting in formation of a substrate pi-cation radical; (2) two two-electron transfer steps: native enzyme-->compound I-->native enzyme; and (3) compound III and the reduced form of the enzyme containing iron(II). 3. Prostaglandin H synthase is a peroxidase with some of the properties of a P450 in that compound I can abstract the hydrogen atom from a C-H bond. 4. The so-called cyclooxygenase and peroxidase activities of prostaglandin H synthase are intimately connected and, with the above exception, both are part of a conventional peroxidase cycle.

The role of O2.- in the production of HO.: in vitro and in vivo

In vitro O2.- reduces Fe(III) to Fe(II), which, in turn, reduces the H2O2, yielding Fe(II)O or HO.. In vivo O2.- increases the supply of free iron by oxidatively attacking the [4Fe-4S] clusters of dehydratases such that they release Fe(II), which can then reduce H2O2. In vivo, O2.- also increases the production of H2O2 by acting as an oxidant toward the dehydratases and toward other cellular reductants.

Distinct heme active-site structure in lactoperoxidase revealed by resonance Raman spectroscopy

Low-frequency resonance Raman spectra of the cyanide and carbon monoxide adducts of lactoperoxidase are obtained with Soret excitation. The nu(Fe-CN) and delta(Fe-C-N) modes are detected at 360 and 453 cm-1, respectively. Upon the isotopic substitution of 13C14N, 12C15N, and 13C15N, the band at 453 cm-1 in the natural abundance adduct shifts to 448, 452, and 445 cm-1, while the 360-cm-1 peak shifts to 358, 357, and 356 cm-1, respectively. The 360-cm-1 band is shifted to 355 cm-1 when the pH is changed from 7.0 to 10.5. On the basis of a previous normal-mode analysis of the cyanoferric adduct of myeloperoxidase, a bent Fe-C-N linkage is suggested for the cyanide adduct of lactoperoxidase. The nu(Fe-CN) (374 cm-1) and delta(Fe-C-N) (480 cm-1) modes are observed for the cyanide adduct of reduced lactoperoxidase. For the carbon monoxide adduct, the nu(Fe-CO) (533 cm-1) and delta(Fe-C-O) (578 cm-1) modes at pH 7.0 are observed to shift to 498 and 570 cm-1 as the pH is raised from 7.0 to 10.0. The strong intensity of delta(Fe-C-O) at both acid and alkaline pHs, along with a suggested bent structure of the Fe-C-N moiety, implies a narrow heme pocket for lactoperoxidase.

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.

Electron spin resonance study of peroxidase activity and kinetics

An electron spin resonance (ESR) assay has been developed for peroxidase activity. The assay measures the formation of the paramagnetic nitroxide Tempol from the oxidation of its hydroxylamine derivative (TOLH) by short-lived radicals produced by peroxidase cycle intermediates, Compounds I and II. Using phenol as a peroxidase electron donor, the ESR approach is suitable for measurements of peroxidase activity ( > or = 0.003 U/ml) and micromolar quantities of H2O2 in sample sizes as small as 2 microliters. In addition, the ESR method can be used to continuously monitor activity in cell suspensions and other media that are susceptible to optical artifacts. The high membrane permeability of TOLH also makes it possible to estimate peroxidase activity in membrane-enclosed compartments, provided that TOLH oxidation rates can be stimulated with exogenous peroxidase reductants, e.g., phenol. Analysis of TOLH oxidation rates under conditions of low electron donor concentrations and high concentrations of H2O2 also shows clear indications of substrate-dependent inhibition and increased catalytic activity. Computer simulations indicate that the results obtained are consistent with the peroxidase reaction scheme proposed by Kohler et al. (1988, Arch. Biochem. Biophys. 264, 438-449) modified to correct for a nitroxide dependent stimulation of peroxidase catalytic activity.

Horseradish peroxidase-catalyzed aerobic oxidation and peroxidation of indole-3-acetic acid. I. Optical spectra

A study of the indole-3-acetate reaction with horse-radish peroxidase, in the absence or presence of hydrogen peroxide, has been performed, employing rapid scan and conventional spectrophotometry. We present here the first clear spectral evidence, obtained on the millisecond time scale, indicating that at pH 5.0 and for high [enzyme/substrate] ratios peroxidase compound III is formed. Most, if not all, of the compound III is formed by oxygenation of the ferrous peroxidase. There is an inhibitory effect of superoxide dismutase and histidine on compound III formation which indicates the involvement of the active oxygen species superoxide and singlet oxygen. It is concluded that the oxidation of indole-3-acetate by horseradish peroxidase at pH 5.0 proceeds through compound III formation to the catalytically inactive forms P-670 and P-630. A reaction path in which the enzyme is directly reduced by indole-3-acetate might be involved as an initiation step. Rapid scan spectral data, which indicate differences in the formation and decay of enzyme intermediate compounds at pH 7.0, in comparison with those observed at pH 5.0, are also presented. At pH 7.0 compound II is a key intermediate in oxidation--peroxidation of substrate. Mechanisms of reactions consistent with the experimental data are proposed and discussed.

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.

Reaction of autoxidation products of penicillamine with myeloperoxidase

Spectral evidence is presented which shows that penicillamine is able to initiate the formation of the oxidized intermediates of myeloperoxidase in the absence of exogenous hydrogen peroxide. The autoxidation of penicillamine presumably produces superoxide which dismutates spontaneously to form hydrogen peroxide. Thus, the formation of both compounds II and III of myeloperoxidase was observed. We also report that penicillamine can directly reduce cytochrome c and therefore, it could possibly act as a one-electron donor to myeloperoxidase.