How relevant is the reoxidation of ferrocytochrome c by hydrogen peroxide when determining superoxide anion production?

Transposable elements expression in Rhinella marina (cane toad) specimens submitted to immune and stress challenge

Transposable elements (TEs) are important components of eukaryotic genomes and compose around 30% of the genome of Rhinella marina, an invasive toad species. Considering the possible role of TEs in the adaptation of populations, we have analyzed the expression of TEs in publicly available spleen tissue transcriptomic data generated for this species after immune and stress challenge. By analyzing the transcriptome assembly, we detected a high number of TE segments. Moreover, some distinct TE families were differentially expressed in some conditions. Our result shows that several TEs are capable of being transcribed in R. marina and they could help to generate a rapid response of specimens to the environment. Also, we can suggest that these TEs could be activated in the germinative cells as well producing variability to be selected and shaped by the evolutionary processes behind the success of this invasive species. Thus, the TEs are important targets for investigation in the context of R. marina adaptation.

Impacts of vesicular environment on Nox2 activity measurements in vitro

BACKGROUND

The production of superoxide anions (O₂^•-) by the phagocyte NADPH oxidase complex has a crucial role in the destruction of pathogens in innate immunity. Majority of in vitro studies on the functioning of NADPH oxidase indirectly follows the enzymatic reaction by the superoxide reduction of cytochrome c (cyt c). Only few reports mention the alternative approach consisting in measuring the NADPH consumption rate. When using membrane vesicles of human neutrophils, the enzyme specific activity is generally found twice higher by monitoring the NADPH oxidation than by measuring the cyt c reduction. Up to now, the literature provides only little explanations about such discrepancy despite the critical importance to quantify the exact enzyme activity.

METHODS

We deciphered the reasons of this disparity in studying the role of key parameters, including. cyt c and arachidonic acid concentrations, in conjunction with an ionophore, a detergent and using Clark electrode to measure the O₂ consumption rates.

RESULTS

Our results show that the O₂^•- low permeability of the vesicle membrane as well as secondary reactions (O₂^•- and H₂O₂ disproportionations) are strong clues to shed light on this inconsistency.

CONCLUSION AND GENERAL SIGNIFICANCE

These results altogether indicate that the cyt c reduction method underestimates the accurate Nox2 activity.

Mycobacterium tuberculosis FasR senses long fatty acyl-CoA through a tunnel and a hydrophobic transmission spine

Mycobacterium tuberculosis is a pathogen with a unique cell envelope including very long fatty acids, implicated in bacterial resistance and host immune modulation. FasR is a TetR-like transcriptional activator that plays a central role in sensing mycobacterial long-chain fatty acids and regulating lipid biosynthesis. Here we disclose crystal structures of M. tuberculosis FasR in complex with acyl effector ligands and with DNA, uncovering its molecular sensory and switching mechanisms. A long tunnel traverses the entire effector-binding domain, enabling long fatty acyl effectors to bind. Only when the tunnel is entirely occupied, the protein dimer adopts a rigid configuration with its DNA-binding domains in an open state, leading to DNA dissociation. The protein-folding hydrophobic core connects the two domains, and is completed into a continuous spine when the effector binds. Such a transmission spine is conserved in a large number of TetR-like regulators, offering insight into effector-triggered allosteric functional control.

FasR is a TetR-like transcriptional activator that plays a central role in sensing mycobacterial long-chain fatty acids and regulating lipid biosynthesis in Mycobacterium tuberculosis. Here authors present crystal structures of M. tuberculosis FasR in complex with acyl effector ligands and with DNA, uncovering its molecular sensory and switching mechanisms.

Quantification of reactive oxygen species generation by photoexcitation of PEGylated quantum dots

Photocatalytic generation of reactive oxygen species (ROS) from quantum dots (QDs) has been widely reported yet quantitative studies of ROS formation and their quantum yields are lacking. This study investigates the generation of ROS by water soluble PEGylated CdSe/ZnS QDs with red emission. PEGylation of QDs is commonly used to confer water solubility and minimise uptake by organs of the reticuloendothelial system; therefore studies of ROS formation are of biomedical relevance. Using non-photolytic visible wavelength excitation, the superoxide anion radical is shown to be the primary ROS species generated with a quantum efficiency of 0.35%. The yield can be significantly enhanced in the presence of the electron donor, nicotinamide adenine dinucleotide (NADH), as demonstrated by oxygen consumption measurements and electron paramagnetic resonance spectroscopy with in situ illumination. Direct production of singlet oxygen is not detectable from the QDs alone. A comparison is made with ROS generation by the same QDs complexed with a sulfonated phthalocyanine which can generate singlet oxygen via Förster resonance energy transfer between the QDs and the phthalocyanine.

Application of a superoxide (O(2)(-)) thermal source (SOTS-1) for the determination and calibration of O(2)(-) fluxes in seawater

Superoxide (O(2)(-)) is an important short lived transient reactive oxygen species (ROS) in seawater. The main source of O(2)(-) in the ocean is believed to be through photochemical reactions though biological processes may also be important. Sink terms for O(2)(-) include redox reactions with bioactive trace metals, including Cu and Fe, and to a lesser extent dissolved organic matter (DOM). Information on the source fluxes, sinks and concentration of superoxide in the open ocean are crucial to improving our understanding of the biogeochemical cycling of redox active species. As O(2)(-) is a highly reactive transient species present at low concentrations it is not a trivial task to make accurate and precise measurements in seawater. In this study we developed the appropriate numerical analysis tools and investigated a number of superoxide sources and methods for the purposes of calibrating O(2)(-) concentrations and/or fluxes specifically in seawater. We found the superoxide thermal source bis(4-carboxybenzyl)hyponitrite (SOTS)-1 easy to employ as a reliable source of O(2)(-) which could be successfully applied in seawater. The thermal decomposition of SOTS-1 in seawater was evaluated over a range of seawater temperatures using both a flux based detection scheme developed using two spectrophotometric methods: (i) 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) and (ii) ferricytochrome c (FC), or a concentration based detection scheme using a chemiluminescence flow injection method based on the Cypridina luciferin analog 2-methyl-6-(p-methoxyphenyl)3-7-dihydroimidazol[1,2-alpha]pyrazin-3-one (MCLA) as reagent. Our results suggest SOTS-1 is the best available O(2)(-) source for determining concentrations and fluxes, all detection systems tested have their pros and cons and the choice of which to use depends more on the duration and type of experiment that is required.

Recognition and modulation of cytochrome c's redox properties using an amphiphilic homopolymer

An amphiphilic homopolymer scaffold has been used to bind to the protein, cytochrome c. This interaction is analyzed using cyclic voltammetry, native gel electrophoresis, UV-visible absorption, and circular dichroism spectroscopy. The polymer binds to cytochrome c with micromolar affinity and the association of polymer with cytochrome c leads to a structural change of the protein. This conformational change exposes the heme unit of the protein, which affords an opportunity to reversibly modulate its electron-transfer properties. We have also shown that the electrostatic binding of polymer to cytochrome c can be used to disrupt its interaction with its natural partner, cytochrome c peroxidase.

Oxidation-state-dependent reactions of cytochrome c with the trioxidocarbonate(•1−) radical: a pulse radiolysis study

The reaction of the trioxidocarbonate(*1-) radical (CO (3) (*-) , "carbonate radical anion") with cytochrome c was studied by pulse radiolysis at alkaline pH and room temperature. With iron(III) cytochrome c, CO (3) (*-) reacts with the protein moiety with rate constants of (5.1 +/- 0.6) x 10(7) M(-1) s(-1) (pH 8.4, I approximately 0.27 M) and (1.0 +/- 0.2) x 10(8) M(-1) s(-1) (pH 10, I = 0.5 M). The absorption spectrum of the haem moiety was not changed, thus, amino acid radicals produced on the protein do not reduce the haem. The pH-dependent difference in rate constants may be attributed to differences in ionization states of amino acids and to the change in the conformation of the protein. With iron(II) cytochrome c, CO (3) (*-) oxidizes the haem quantitatively, presumably via electrostatic guidance of the radical to the solvent-accessible haem edge, with a different pH dependence: at pH 8.4, the rate constant is (1.1 +/- 0.1) x 10(9) M(-1) s(-1) and, at pH 10, (7.6 +/- 0.6) x 10(8) M(-1) s(-1). We propose that CO (3) (*-) oxidizes the iron center directly, and that the lower rate observed at pH 10 is due to the different charge distribution of iron(II) cytochrome c.

Identification of a Cd2+- and Zn2+-Binding Site in CytochromecUsing FTIR Coupled to an ATR Microdialysis Setup and NMR Spectroscopy†

Fourier transform infrared (FTIR) difference spectroscopy allows the study of molecular changes occurring at active sites in proteins with high sensitivity. Reactions are triggered by light, potential, or temperature steps and more recently by the diffusion of buffers containing effectors above membrane proteins deposited as films on ATR crystals. We have adapted a microdialysis system to an ATR, to study metal sites in soluble proteins. In this study, we identified a Cd(2+)- or Zn(2+)-binding site in cytochrome c with dissociation constants of 17 and 42 microM, respectively, which affects the oxidation rate of ferrocytochrome c by hydrogen peroxide. Using the microdialysis ATR-FTIR setup, we determined that a histidine and the carboxylate group of a glutamate are involved in Zn(2+) binding. The implication of His 33 and Glu 104 in the binding site was deduced from the comparison of FTIR data recorded with horse heart and the variant tuna cytochrome c lacking these two amino acids. A two-dimensional NMR analysis of the Zn(2+)-binding site in horse heart cytochrome c confirmed that His 33 and residues close to the C terminus are sensitive to Zn(2+) binding. This study demonstrates that the microdialysis ATR-FTIR setup is promising for the analysis of metal sites in proteins. From H(2)O/(2)H(2)O exchange experiments, we concluded that the impact of Zn(2+) and Cd(2+) binding on the oxidation kinetics of ferrocytochrome c by H(2)O(2) is associated to the perturbation of a hydrogen-bonding network involving His 33 that is sensitive to the redox state of cytochrome c.

Mechanical loading and injury induce human myotubes to release neutrophil chemoattractants

The purpose of this study was to 1) test the hypothesis that skeletal muscle cells (myotubes) after mechanical loading and/or injury are a source of soluble factors that promote neutrophil chemotaxis and superoxide anion (O(2)(-).) production and 2) determine whether mechanical loading and/or injury causes myotubes to release cytokines that are known to influence neutrophil responses [tumor necrosis factor-alpha (TNF-alpha), IL-8, and transforming growth factor-beta1 (TGF-beta1)]. Human myotubes were grown in culture and exposed to either a cyclic strain (0, 5, 10, 20, or 30% strain) or a scrape injury protocol. Protocols of 5, 10, and 20% strain did not cause injury, whereas 30% strain and scrape injury caused a modest and a high degree of injury, respectively. Conditioned media from strained myotubes promoted chemotaxis of human blood neutrophils and primed them for O(2)(-). production in a manner that was dependent on a threshold of strain and independent from injury. Neutrophil chemotaxis, but not priming, progressively increased with higher magnitudes of strain. Conditioned media only from scrape-injured myotubes increased O(2)(-). production from neutrophils. Concentrations of IL-8 and total TGF-beta1 in conditioned media were reduced by mechanical loading, whereas TNF-alpha and active TGF-beta1 concentrations were unaffected. In conclusion, skeletal muscle cells after mechanical loading and injury are an important source of soluble factors that differentially influence neutrophil chemotaxis and the stages of neutrophil-derived reactive oxygen species production. Neutrophil responses elicited by mechanical loading, however, did not parallel changes in the release of IL-8, TGF-beta1, or TNF-alpha from skeletal muscle cells.

Nitric oxide inactivates NADPH oxidase in pig neutrophils by inhibiting its assembling process

The effects of nitric oxide (NO) on superoxide (O-2) generation of the NADPH oxidase in pig neutrophils were studied. NO dose-dependently suppressed O-2 generation of both neutrophil NADPH oxidase and reconstituted NADPH oxidase. Effects of NO on NADPH-binding site and the redox centers including FAD and low spin heme in cytochrome b558 and the electron transfer rates from NADPH to heme via FAD were examined under anaerobic conditions. Both reaction rates and the Km value for NADPH were unchanged by NO. Visible and EPR spectra of cytochrome b558 showed that the structure of heme was unchanged by NO, indicating that NO does not affect the redox centers of the oxidase. In reconstituted NADPH oxidase system, NO did not inhibit O-2 generation of the oxidase when added after activation. The addition of NO to the membrane component or the cytosol component inhibited the activity by 24.0 +/- 5.3 or 37.4 +/- 7.1%, respectively. The addition of NO during the activation process or to the cytosol component simultaneously with myristate inhibited the activity by 74.0 +/- 5.2 or 70.0 +/- 8.3%, respectively, suggesting that cytosol protein(s) treated with myristate becomes susceptible to NO. Peroxynitrite did not interfere with O-2 generation.

Resveratrol has no effect on lipoprotein profile and does not prevent peroxidation of serum lipids in normal rats

Trans-resveratrol, one of the antioxidants found in red wine, has been the subject of controversial reports regarding its protective role against cardiovascular diseases. In this study we synthesized trans-resveratrol and injected it to rats (20 and 40 mg/kg body weight, once a day for 21 days, i.p.) to determine its effect on the serum lipid profile. Synthetic trans-resveratrol was an effective antioxidant in vitro against hydroxyl radical (I50 = 33 microM). Resveratrol treatment, however, did not have any effect on either the lipid profile or on Cu+2-dependent formation of thiobarbituric-acid-reactive substances (TBARS) from protein-associated lipids. Since the amount of resveratrol used in these experiments was orders of magnitude higher than the amounts found in wine, these results suggest that if resveratrol has any effect against coronary heart diseases, it is not related to its antioxidant role on lipids or to changes in lipoprotein profile.

Do nitroxide antioxidants act as scavengers of O2-. or as SOD mimics?

Stable nitroxide radicals were reported to act as SOD mimics and catalyze the dismutation of O2-. through two different catalytic pathways including reductive and oxidative reaction mechanisms (Samuni, A., Krishna, C. M., Riesz, P., Finkelstein, E. & Russo, A. (1988) J. Biol Chem. 263, 17921-17924). Recent studies directly monitoring O2-. and employing kinetics analysis did not reveal SOD activity of nitroxides (Weiss, R. H., Flickinger, A. G., Rivers, W. J., Hardy, M. M., Aston, K. W., Ryan, U. S. & Riley, D. P. (1993) J. Biol. Chem. 268, 23049-23054). Such discrepancy may result in cases where distinction of stoichiometric scavengers from catalytic detoxifiers of O2-. is not readily feasible. Nitroxides are effective antioxidants that protect against oxidative injury in various pathological processes. The distinction of their SOD mimic activity from O2-. scavenging was established by examining the validity of direct and indirect methods employed to assay SOD-like catalytic activity. Kinetics analysis along with direct EPR monitoring were used to study the mechanism underlying nitroxide reactions with O2-.. The nitroxide EPR signal decayed in the presence of NADH but otherwise did not decrease with time, thus substantiating its catalytic role in O2-. dismutation. The catalytic rate constants for O2-., dismutation, determined for the nitroxides tested, were found to increase with [H+], indicating that .OOH rather than O2-. is oxidizing the nitroxide. The results demonstrate the limitations associated with direct kinetics analysis in evaluating SOD mimic activity, underscoring the need for independent assays for valid discrimination of SOD mimics from stoichiometric scavengers of O2-..

ESR spin-trapping of a protein-derived tyrosyl radical from the reaction of cytochrome c with hydrogen peroxide

The reaction of horse heart cytochrome c with hydrogen peroxide was investigated using the ESR spin-trapping technique and the nitroso spin traps 3,5-dibromo-4-nitrosobenzenesulfonic acid (DBNBS) and 2-methyl-2-nitrosopropane (MNP). The ESR spectra obtained using both spin traps were typical of an immobilized nitroxide and indicated that the adduct was a macromolecule. The intensity of the ESR spectrum corresponding to the DBNBS/*cytochrome c radical adduct was greatly enhanced by performing the reaction under anaerobic conditions, which suggested that the spin trap was competing with O2 for reaction with the radical site(s). Nonspecific proteolysis of either the DBNBS or the MNP adducts revealed isotropic three-line spectra. In addition, a high resolution ESR spectrum for the protease-treated MNP cytochrome c-derived protein radical adduct was obtained. The superhyperfine couplings detected in this spectra were identical to those detected from an authentic MNP/tyrosyl adduct. Carbon-13 labeling of the aromatic ring positions of tyrosine yielded additional hyperfine coupling, demonstrating that the radical site was definitely located on the ring of tyrosine. Mass spectrometry detected as many as four DBNBS/.cytochrome c-derived adducts from the reaction of cytochrome with H2O2. Thus, it would appear four radical sites are formed during the reaction, at least one of which is tyrosine.

Measurement of superoxide anion production using maximal rate of cytochrome (III) C reduction in phorbol ester stimulated neutrophils, immobilised to microtiter plates

In the present investigation, a method of studying the maximal rate of superoxide anion (O2.-) production in immobilised human neutrophils using a microtiter plate technique has been developed. The rate of O2.- production was determined from the rate of reduction of cytochrome (III) C, studied as the increase in absorbance at 550 nm. The protein kinase C activator, phorbol 12-myristate 13-acetate, was used to stimulate O2.- production. Neutrophils were evenly immobilised as a monolayer to microtiter culture plates to provide a reproducible exposure to the medium. Phorbol ester stimulated O2.- production was inhibited by staurosporine, a well-known inhibitor of protein kinase C, and by diphenylene iodonium, a potent NADPH-oxidase inhibitor, with IC50-values in this assay of 20 and 220 nm, respectively. The extracellularly produced O2.- was removed by superoxide dismutase with a half maximal effect of 0.6 microgram/mL. The maximal production rate of O2.- could therefore be estimated by addition of 20 micrograms/mL superoxide dismutase. Several antioxidants, including butylated hydroxytoluene, nordihydroguairetic acid, probucol and alpha-tocopherol, were studied and showed neither an effect on O2.- production nor a scavenging effect. This new method was highly reproducible, and the continuous measurement of O2.- production was very useful for validating the effect of inhibitors. The developed microtiter technique using immobilised cells has a large capacity and allows different compounds to be tested under comparable conditions, since they are exposed to the cells in a similar way. This is also the first test model which describes O2.- production as the maximal rate of cytochrome (III) C reduction.

A comparative study of EPR spin trapping and cytochrome c reduction techniques for the measurement of superoxide anions

Superoxide anions (O2.-) generated by the reaction of xanthine with xanthine oxidase were measured by the reduction of cytochrome c and by electron paramagnetic resonance (EPR) spectroscopy using the spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). Studies were performed to determine the relative sensitivities of these two techniques for the measurement of O2.-. Mixtures of xanthine, xanthine oxidase, and DMPO generated two adducts, a transient DMPO-OOH and a smaller but longer-lived DMPO-OH. Both adducts were inhibited by superoxide dismutase (SOD), demonstrating they originated from O2.-, and were also significantly decreased when the experiments were performed using unchelated buffers, suggesting that metal ion impurities in unchelated buffers alter the formation or degradation of DMPO-adducts. O2.-, generated by concentrations of xanthine as low as 0.05 microM, were detectable using EPR spin trapping. In contrast, mixtures of xanthine, xanthine oxidase, and cytochrome c measured spectrophotometrically at 550 nm demonstrated that concentrations of xanthine above 1 microM were required to produce measurable levels of reduced cytochrome c. These studies demonstrate that spin trapping using DMPO was at least 20-fold more sensitive than the reduction of cytochrome c for the measurement of superoxide anions. However, at levels of superoxide generation where cytochrome c provides a linear measurement of production, EPR spin trapping may underestimate radical production, probably due to degradation of DMPO radical adducts.

Caloric restriction, aging, and antioxidant enzymes

The basic mechanisms of aging and its retardation by caloric restriction (CR) remain unclear. One suggested means by which CR could retard aging is based on production of mitochondrial free radicals, and efficiency of their subsequent metabolism. Currently, there is little information concerning the influences of age and CR on the rates of in vivo mitochondrial free radical production. However, evidence for CR-induced modulation of free radical detoxification capacities is mounting. The direction of the influence of CR on free radical detoxification is tissue-specific. These effects are broad and appear to provide positive advantage.

In vitro superoxide production by hyaline cells of the shore crab Carcinus maenas (L.)

This study examines the phagocytic hyaline cells of the shore crab, Carcinus maenas, for a respiratory burst in vitro. Following stimulation of the cells with phorbol myristate acetate (PMA), ferricytochrome c was reduced at 550 nm. Addition of superoxide dismutase (SOD) to the reaction mixture decreased this reduction, confirming that superoxide ions are produced by the stimulated hemocytes. Phytohemagglutinin, con A, and LPS were also shown to stimulate the cells although laminarin failed to elicit a burst. This is the first demonstration of a respiratory burst for crustacean hemocytes.

Evidence that cellobiose oxidase from Phanerochaete chrysosporium is primarily an Fe(III) reductase. Kinetic comparison with neutrophil NADPH oxidase and yeast flavocytochrome b2

Kinetic measurements were made for purified cellobiose oxidase in 100 mM acetate (pH 4.0) at 30 degrees C, with excess cellobiose as substrate and O2 or Fe(III) as acceptor. With O2 at 230 microM as sole electron acceptor, the O2 uptake rate corresponded to a one-electron turnover number of 0.13 +/- 0.01 s-1. Measurements at different O2 concentrations indicated Km(O2) greater than 120 microM. In separate experiments, the reduction of Fe(III) acetate was monitored at 340 nm in the absence of oxygen. The maximum velocity of Fe(III)-acetate reduction (Vmax) was 4.5 +/- 0.7 s-1, while Km[Fe(III) acetate] was 34 +/- 12 microM. With ferricyanide in place of Fe(III) acetate, the corresponding values were 6.9 +/- 0.7 s-1 and 23 +/- 5 microM. Redox titrations established the potential of the haem prosthetic group of the oxidase at pH 4.0 as +165 mV. The midpoint potential for Fe(III)/Fe(II) acetate at pH 4.0 is much higher, a value of +535 mV being obtained with 200 microM Fe. Cellobiose oxidase resembles yeast flavocytochrome b2 and differs from the neutrophil NADPH oxidase in having the potential of its haem group far above the potential for one-electron reduction of O2 to superoxide (Em,4 = -110 mV). A kinetic comparison led to the conclusion that the role of cellobiose oxidase is as an Fe(III) reductase. Fe(II) may have a biological importance as a component of Fenton's reagent [Fe(II)/H2O2]. The concentration of cellobiose oxidase in the growth medium at harvest (0.3 microM) can provide a far higher flux of Fe(II) than a non-enzymic proposal in the literature.

Transferrin: a potential source of iron for oxygen free radical-mediated endothelial cell injury

The ability of transferrin to potentiate oxygen free radical-mediated endothelial cell injury was assessed. 51Cr-labeled endothelial cells derived from rat pulmonary arteries (RPAECs) were incubated with hydrogen peroxide (H2O2) in the presence and absence of holosaturated human transferrin, and the effect of transferrin on H2O2-mediated endothelial cell toxicity was determined. Addition of holosaturated transferrin potentiated H2O2-mediated RPAEC cytotoxicity at concentrations of H2O2 greater than 10 microM, suggesting that transferrin may provide a source of iron for free radical-mediated endothelial cell injury. Free radical-mediated injury is dependent on non-protein-bound iron. The ability of RPAECs to facilitate the release of iron from transferrin was assessed. We determined that RPAECs facilitate the release of transferrin-derived iron by reduction of transferrin-bound ferric iron (Fe3+) to ferrous iron (Fe2+). The reduction and release of transferrin-derived Fe2+ were inhibited by apotransferrin and chloroquine, indicating a dependence on receptor-specific binding of transferrin to the RPAEC cell surface, with subsequent endocytosis, acidification, and reduction of transferrin-bound Fe3+ to Fe2+. The release of transferrin-derived Fe2+ was potentiated by diethyldithiocarbamate, an inhibitor of intracellular superoxide dismutase (SOD). In contrast, exogenous SOD did not alter iron release, suggesting that intracellular superoxide anion (O2-) may play an important role in mediating the reduction and release of transferrin-derived iron. Results of this study suggest that transferrin may provide a source of iron for oxygen free radical-mediated endothelial cell injury and identify a novel mechanism by which endothelial cells may mediate the reduction and release of transferrin-derived iron.

Superoxide anion scavenging capacity measured by a polarographic method. Comparison with a colourimetric method

A polarographic method to assess the scavenging capacity of a molecule for O2-. is proposed. This method is based on the fact that O2-. is not detected by the Clark electrode and that a scavenger competes with spontaneous dismutation of O2-. So, the reduction of O2 into O2-. and the decomposition of H2O2 by catalase, releasing O2, show a biphasic kinetic. Various kinetic parameters can be used to calculate the nmol of O2-. scavenged and also supply data on the reaction mechanisms (oxidation or reduction of O2-.) involved in scavenging. This method presents several other advantages: scavenging capacity can be assayed without added indicators which themselves behave as scavengers (as demonstrated for NBT), the presence of scavengers which interfere with the O2-. generating system (xanthine-xanthine oxidase) does not invalidate the measurements made.

Cytochrome c-catalyzed membrane lipid peroxidation by hydrogen peroxide

Cytochrome c(3+)-catalyzed peroxidation of phosphatidylcholine liposomes by hydrogen peroxide (H2O2) was indicated by the production of thiobarbituric acid reactive substances, oxygen consumption, and emission of spontaneous chemiluminescence. The iron chelator diethylenetriaminepentaacetic acid (DTPA) only partially inhibited peroxidation when H2O2 concentrations were 200 microM or greater. In contrast, iron compounds such as ferric chloride, potassium ferricyanide, and hemin induced H2O2-dependent lipid peroxidation which was totally inhibitable by DTPA. Cyanide and urate, which react at or near the cytochrome-heme, completely prevented lipid peroxidation, while hydroxyl radical scavengers and superoxide dismutase had very little or no inhibitory effect. Changes in liposome surface charge did not influence cytochrome c3+ plus H2O2-dependent peroxidation, but a net negative charge was critical in favoring cytochrome c(3+)-dependent, H2O2-independent lipid auto-oxidative processes. These results show that reaction of cytochrome c with H2O2 promotes membrane oxidation by more than one chemical mechanism, including formation of high oxidation states of iron at the cytochrome-heme and also by heme iron release at higher H2O2 concentrations. Cytochrome c3+ could react with mitochondrial H2O2 to yield "site-specific" mitochondrial membrane lipid peroxidation during tissue oxidant stress.

Cytochrome c-catalyzed oxidation of organic molecules by hydrogen peroxide

Cytochrome c catalyzed the oxidation of various electron donors in the presence of hydrogen peroxide (H2O2), including 2-2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS), 4-aminoantipyrine (4-AP), and luminol. With ferrocytochrome c, oxidation reactions were preceded by a lag phase corresponding to the H2O2-mediated oxidation of cytochrome c to the ferric state; no lag phase was observed with ferricytochrome c. However, brief preincubation of ferricytochrome c with H2O2 increased its catalytic activity prior to progressive inactivation and degradation. Superoxide (O2-) and hydroxyl radical (.OH) were not involved in this catalytic activity, since it was not sensitive to superoxide dismutase (SOD) or mannitol. Free iron released from the heme did not play a role in the oxidative reactions as concluded from the lack of effect of diethylenetriaminepentaacetic acid. Uric acid and tryptophan inhibited the oxidation of ABTS, stimulation of luminol chemiluminescence, and inactivation of cytochrome c. Our results are consistent with an initial activation of cytochrome c by H2O2 to a catalytically more active species in which a high oxidation state of an oxo-heme complex mediates the oxidative reactions. The lack of SOD effect on cytochrome c-catalyzed, H2O2-dependent luminol chemiluminescence supports a mechanism of chemiexcitation whereby a luminol endoperoxide is formed by direct reaction of H2O2 with an oxidized luminol molecule, either luminol radical or luminol diazoquinone.

Reaction products of 1-naphthol with reactive oxygen species prevent NADPH oxidase activation in activated human neutrophils, but leave phagocytosis intact

Activation of human neutrophils with opsonized particles in the presence of a nontoxic dose of 1-naphthol resulted in inhibition of superoxide anion production but not of the phagocytotic activity of the cells. In this study we have investigated the mechanism of action of 1-naphthol. The inhibition is not at the level of cellular activation since the FMLP-induced rise of intracellular free calcium was unaffected. Our results show that the (metabolic) activation of 1-naphthol to 1,4-naphthoquinone by reaction with H2O2 from the oxidative burst is a necessary event for the inhibition to occur. The study provides evidence that by its reactivity with essential thiol groups 1,4-naphthoquinone (1,4-NQ) prevents the assembly of a functional NADPH-oxidase in the neutrophil membrane.

Comparison of the effects of superoxide dismutase and cytochrome c on luminol chemiluminescence produced by xanthine oxidase-catalyzed reactions

Superoxide dismutase (superoxide: superoxide oxidoreductase, EC 1.15.1.1) (SOD) and ferricytochrome c are used to check the effects on luminol chemiluminescence induced by a xanthine or hypoxanthine/xanthine oxidase/oxygen system. Luminol chemiluminescence has been attributed to superoxide anion radical (O2.-) in this system. From kinetic studies on the light intensity vs. time curves it is demonstrated that addition of SOD into the system does not affect the mechanism of O2.- generation, whilst ferricytochrome c dramatically alters the time-course of the reaction. This is interpreted as the effect of cytochrome c redox cycling by reaction with H2O2, modifying oxy-radical generation in the reaction medium. Also, an alternative mechanism for luminol chemiexcitation is proposed under certain experimental conditions.