Nature of the inhibition of horseradish peroxidase and mitochondrial cytochrome c oxidase by cyanyl radical

New dual-responsive fluorescent sensor for hypochlorite and cyanide sensing and its imaging application in live cells and zebrafish

Excessive levels of cyanide (CN-) and hypochlorite (ClO-) anions are the significant threats to the human health and the environment. Thus, great efforts have been to design and synthesize molecular sensors for the simple, instantaneous and efficient detecting environmentally and biologically important anions. Currently, developing a single molecular sensor for multi-analyte sensing is still a challenging task. In our present work, we developed a new molecular sensor (3TM) based on oligothiophene and Meldrum's acid units for detecting cyanide and hypochlorite anions in biological, environmental and food samples. The detecting ability of 3TM has been examined to various testing substances containing amino acids, reactive oxygen species, cations and anions, showing its high selectivity, excellent sensitivity, short response time (ClO-: 30 s, CN-: 100 s), and broad pH working range (4-10). The detection limits were calculated as 4.2 nM for ClO- in DMSO/H2O (1/8, v/v) solution and 6.5 nM for CN- in DMSO/H2O (1/99, v/v) solution. Sensor 3TM displayed sharp turn-on fluorescence increasement (555 nm, 435 nm) and sensitive fluorescence color changes caused by CN-/ClO-, which is ascribed to the nucleophilic addition and oxidation of ethylenic linkage by cyanide and hypochlorite, respectively. Moreover, sensor 3TM was applied for hypochlorite and cyanide detecting in real-world water, food samples and bio-imaging in live cells and zebrafish. To our knowledge, the developed 3TM sensor is the seventh single-molecular sensor for simultaneous and discriminative detecting hypochlorite and cyanide in food, biological and aqueous environments using two distinct sensing modes.

Mitochondrial DNA integrity and function are critical for endothelium-dependent vasodilation in rats with metabolic syndrome

Endothelial dysfunction in diabetes is generally attributed to oxidative stress, but this view is challenged by observations showing antioxidants do not eliminate diabetic vasculopathy. As an alternative to oxidative stress-induced dysfunction, we interrogated if impaired mitochondrial function in endothelial cells is central to endothelial dysfunction in the metabolic syndrome. We observed reduced coronary arteriolar vasodilation to the endothelium-dependent dilator, acetylcholine (Ach), in Zucker Obese Fatty rats (ZOF, 34 ± 15% [mean ± standard deviation] 10^-3 M) compared to Zucker Lean rats (ZLN, 98 ± 11%). This reduction in dilation occurred concomitantly with mitochondrial DNA (mtDNA) strand lesions and reduced mitochondrial complex activities in the endothelium of ZOF versus ZLN. To demonstrate endothelial dysfunction is linked to impaired mitochondrial function, administration of a cell-permeable, mitochondria-directed endonuclease (mt-tat-EndoIII), to repair oxidatively modified DNA in ZOF, restored mitochondrial function and vasodilation to Ach (94 ± 13%). Conversely, administration of a cell-permeable, mitochondria-directed exonuclease (mt-tat-ExoIII) produced mtDNA strand breaks in ZLN, reduced mitochondrial complex activities and vasodilation to Ach in ZLN (42 ± 16%). To demonstrate that mitochondrial function is central to endothelium-dependent vasodilation, we introduced (via electroporation) liver mitochondria (from ZLN) into the endothelium of a mesenteric vessel from ZOF and restored endothelium-dependent dilation to vasoactive intestinal peptide (VIP at 10^-5 M, 4 ± 3% vasodilation before mitochondrial transfer and 48 ± 36% after transfer). Finally, to demonstrate mitochondrial function is key to endothelium-dependent dilation, we administered oligomycin (mitochondrial ATP synthase inhibitor) and observed a reduction in endothelium-dependent dilation. We conclude that mitochondrial function is critical for endothelium-dependent vasodilation.

Involvement of a unique chemodosimeter in the selective estimation of noxious cyanide in common water hyacinth (Eichhornia crassipes): an environmental refinement

A naphthaldehyde-pyridoxal conjugated chemodosimeter (NPLC) was developed and employed for the sensitive and selective detection and estimation of cyanide in common water hyacinth (Eichhornia crassipes), a free floating macrophyte used in the phytoremediation process since ancient times. The non-fluorescent nature of the probe NPLC, directed by the possibility of excited state intramolecular proton transfer process (ESIPT), was promptly changed due to CN^- induced di-deprotonation of the probe. The naked eye color change and turn on vivid fluorescent color of NPLC was attributed to the inhibition of the ESIPT mechanism in the deprotonated NPLC (NPLC-D). The selective detection of cyanide ion in the nanomolar range (81 nM), among other interfering anions, makes it exclusive. The involvement of the probe in a chemodosimetric fashion toward cyanide was elucidated by experimental and computational studies.

A Zr-Based Metal-Organic Framework with a DUT-52 Structure Containing a Trifluoroacetamido-Functionalized Linker for Aqueous Phase Fluorescence Sensing of the Cyanide Ion and Aerobic Oxidation of Cyclohexane

A zirconium (Zr) metal-organic framework having a DUT-52 (DUT stands for Dresden University of Technology) structure with face-centered cubic topology and bearing the rigid 1-(2,2,2-trifluoroacetamido) naphthalene-3,7-dicarboxylic acid (H₂NDC-NHCOCF₃) ligand was prepared, and its solid structure was characterized with the help of the X-ray powder diffraction (XRPD) technique. Other characterization methods like thermogravimetric analysis (TGA) and Fourier transform infrared (FT-IR) spectroscopy were applied to verify the phase purity of the compound. In order to get the solvent-free compound (1'), 1 was stirred with methanol for overnight and subsequently heated at 100 °C overnight under vacuum. As-synthesized (1) and activated (1') compounds are thermally stable up to 300 °C. The Brunsuer Emmett-Teller (BET) surface area of 1' was found to be 1105 m² g^-1. Fluorescence titration experiments showed that 1' exhibits highly selective and sensitive fluorescence turn-on behavior toward cyanide (CN^-) anion. The interference experiments suggested that other anions did not interfere in the detection of CN^-. Moreover, a very short response time (2 min) was shown by probe 1' for CN^- detection. The detection limit was found to be 0.23 μM. 1' can also be effectively used for CN^- detection in real water samples. The mechanism for the selective detection of CN^- was investigated systematically. Furthermore, the aerobic oxidation of cyclohexane was performed with 1' under mild reaction conditions, observing higher activity than the analogous DUT-52 solid under identical conditions. These experiments clearly indicate the benefits of hydrophobic cavities of 1' in achieving higher conversion of cyclohexane and cyclohexanol/cyclohexanone selectivity. Catalyst stability was proved by two consecutive reuses and comparing the structural integrity of 1' before and after reuses by the XRPD study.

Acute toxicity of cyanide in aerobic respiration: Theoretical and experimental support for murburn explanation

The inefficiency of cyanide/HCN (CN) binding with heme proteins (under physiological regimes) is demonstrated with an assessment of thermodynamics, kinetics, and inhibition constants. The acute onset of toxicity and CN's mg/Kg LD50 (μM lethal concentration) suggests that the classical hemeFe binding-based inhibition rationale is untenable to account for the toxicity of CN. In vitro mechanistic probing of CN-mediated inhibition of hemeFe reductionist systems was explored as a murburn model for mitochondrial oxidative phosphorylation (mOxPhos). The effect of CN in haloperoxidase catalyzed chlorine moiety transfer to small organics was considered as an analogous probe for phosphate group transfer in mOxPhos. Similarly, inclusion of CN in peroxidase-catalase mediated one-electron oxidation of small organics was used to explore electron transfer outcomes in mOxPhos, leading to water formation. The free energy correlations from a Hammett study and IC50/Hill slopes analyses and comparison with ligands ( CO/ H 2 S/ N 3 - ) $\left( {\text{CO}}/{{{{\text{H}}_{2}}\text{S}}/{\text{N}_{3}^{\text{-}}}\;}\; \right)$ provide insights into the involvement of diffusible radicals and proton-equilibriums, explaining analogous outcomes in mOxPhos chemistry. Further, we demonstrate that superoxide (diffusible reactive oxygen species, DROS) enables in vitro ATP synthesis from ADP+phosphate, and show that this reaction is inhibited by CN. Therefore, practically instantaneous CN ion-radical interactions with DROS in matrix catalytically disrupt mOxPhos, explaining the acute lethal effect of CN.

Anion Sensing by Novel Triarylboranes Containing Boraanthracene: DFT Functional Assessment, Selective Interactions, and Mechanism Demonstration

Analytical methods often involve expensive instrumentation and tedious sample pretreatment for an analyte detection. Being toxic and detrimental to human health, sensing of cyanide (CN-), fluoride (F-), chloride (Cl-), bromide (Br-), nitrate (NO3 -), acetate (CH3COO-), and bisulfate (HSO4 -) is performed by a boron-based molecular receptor, N,N,N,3,5-pentamethyl-4-{2-thia-9-boratricyclo[8.4.0.03,8]tetradeca-1(10),3(8),4,6,11,13-hexaen-9-yl}anili-nium (1), and the three newly designed receptors from it. Thermodynamics, electronic structure, and photophysical properties are computed by employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT) to explore selective sensing of these anions and its mechanism. Free-energy changes (ΔG) and binding energies (ΔE) suggest that among these anions, only binding of CN- and F- is thermodynamically feasible with a very strong binding affinity with the receptors. Boron atoms containing positive natural charges act as the electrophilic centers to bind the anions involving a 2p-2p orbital overlap resulting in charge transfer. In the receptor-analyte complexes with CN- and F-, fluorescence is quenched due to the intramolecular charge transfer transitions (π-π* transitions in the case of the receptors lead to fluorescence), internal conversion, and associated configurational changes. Among the six tested functionals, CAM-B3LYP/6-31G(d) is found to be the most accurate one. The designed receptors are better fluorescent probes for F- and CN-, demonstrating their importance for the practical utility.

Fluorescence turn-on chemodosimetric sensing of cyanide by cyanovinylterpyridine modified phthalonitrile and subphthalocyanine

Terpyridine-attached phthalonitrile (Pn-TP) linked by cyanovinyl bond has been synthesized and employed for the preparation of subphthalocyanine (SubPc-TP) bearing conjugated terpyridine moieties. Both Pn-TP and SubPc-TP exhibited highly selective fluorescence turn-on in the presence of cyanide anions (CN^-) based on chemodosimetric sensing mechanism. The conjugation of the Pn-TP molecule was interrupted by the addition of CN^- at the cyanovinyl bond, showing the ratiometric fluorescence turn-on behavior. This sensing mechanism was further supported by density functional theory calculation and nuclear magnetic resonance titration studies. Optical and photophysical responses of SubPc-TP towards CN^- were also investigated, in which similar fluorescence enhancement was observed due to the addition of CN^- at the reactive boron trimer. The detection limit was estimated to be 94 nM, much below the World Health Organization-allowed level (1.9 μM) of CN^- in water.

A theoretical study on anion sensing mechanism of multi-phosphonium triarylboranes: intramolecular charge transfer and configurational changes

The binding selectivity and recognition mechanism of a series of mono-, di- and triphosphonium substituted triarylboranes: (4-(dimesitylboryl)-3,5-dimethylphenyl)phosphonium ([Mes₂BAr^P]⁺, 1), 1,1'-mesitylboranediylbis(3,5-dimethylphenyl)phosphonium ([MesBAr^P₂]²⁺, 2) and 1,1',1''-boranetriyltris(3,5-dimethylphenyl)phosphonium ([BAr^P₃]³⁺, 3) where Ar^P = 4-(H₃P)-2,6-Me₂-C₆H₂, for various anions has been investigated by employing density functional theory (DFT) and time dependent-density functional theory (TD-DFT) methods. Natural population analysis indicates the electrophilic nature of the boron centers in 1-3 for the nucleophilic addition of anions. The calculated free energy changes (ΔG) reveal that out of CN^-, F^-, Cl^-, Br^-, NO₃^-, CH₃COO^- and HSO₄^- only the binding of CN^- and F^- with 1, 2 and 3 is thermodynamically feasible. In addition, the calculated binding energies reflect that CN^- shows lesser binding affinity than F^- with 1, 2 and 3. Frontier molecular orbital (FMO) analysis reveals that the first excited states (S₁) of 1-3 are the local excited states with a π → π* transition, whereas the third excited state (S₃), fifth excited state (S₅), fourth excited state (S₄) and fourth excited states (S₄) of [Mes₂BAr^P]⁺F (1F, the fluoro form of 1), [MesBAr^P₂]²⁺F (2F, the fluoro form of 2), [Mes₂BAr^P]⁺CN (1CN, the cyano form of 1) and [MesBAr^P₂]²⁺CN (2CN, the cyano form of 2), respectively, are charge separation states found to be responsible for the intramolecular charge transfer (ICT) process. The partial configuration changes and ICT induce fluorescence quenching in 1F, 2F, 1CN and 2CN synergistically after an internal conversion (IC) from their respective S₃, S₅, S₄ and S₄ to S₁.

Selective Complexation of Cyanide and Fluoride Ions with Ammonium Boranes: A Theoretical Study on Sensing Mechanism Involving Intramolecular Charge Transfer and Configurational Changes

The anion binding selectivity and the recognition mechanism of two isomeric boranes, namely, 4-[bis(2,4,6-trimethylphenyl)boranyl]-N,N,N-trimethylaniline ([p-(Mes₂B)C₆H₄(NMe₃)]⁺, 1, where "Mes" represents mesitylene and "Me" represents methyl) and 2-[bis(2,4,6-trimethylphenyl)boranyl]-N,N,N-trimethylaniline ([o-(Mes₂B)C₆H₄(NMe₃)]⁺, 2) has been investigated using density functional theory (DFT) and time dependent-density functional theory (TD-DFT) methods. Natural population analysis indicates that the central boron atoms in 1 and 2 are the most active centers for nucleophilic addition of anions. The negative magnitude of free energy changes (ΔG) reveals that out of CN^-, F^-, Cl^-, Br^-, NO₃^-, and HSO₄^- only the binding of CN^- and F^- with 1 and 2 is thermodynamically feasible and spontaneous. In addition, the calculated binding energies reveal that the CN^- is showing lesser binding affinity than F^- both with 1 and 2, while other ions, viz. NO₃^-, HSO₄^-, Br^-, and Cl^-, either do not bind at all or show very insignificant binding energy. The first excited states (S₁) of 1 and 2 are shown to be the local excited states with π → σ* transition by frontier molecular orbital analysis, whereas fourth excited states (S₄) of 4-[bis(2,4,6-trimethylphenyl)boranyl]-N,N,N-trimethylaniline cyanide ([p-(Mes₂B)C₆H₄(NMe₃)] CN, 1CN, the cyano form of 1) and 4-[bis(2,4,6-trimethylphenyl)boranyl]-N,N,N-trimethylaniline fluoride ([p-(Mes₂B)C₆H₄(NMe₃)] F, 1F, the fluoro form of 1) and fifth excited state (S₅) of 2-[bis(2,4,6-trimethylphenyl)boranyl]-N,N,N-trimethylaniline fluoride ([o-(Mes₂B)C₆H₄(NMe₃)] F, 2F, the fluoro form of 2) are charge separation states that are found to be responsible for the intramolecular charge transfer (ICT) process. The synergistic effect of ICT and partial configuration changes induce fluorescence quenching in 1CN, 1F, and 2F after a significant internal conversion (IC) from S₄ and S₅ to S_1.

Highly selective colorimetric detection of cyanide anions in aqueous media by triphenylamine and phenanthro(9,10-d)imidazole based probes

Two novel triphenylamine based fluorescent probes TPC and PITP have been synthesized and studied as an effective cyanide selective indicator in aqueous media. Complete colour bleaching and visible colour changes was observed upon addition of cyanide to fluorophores TPC and PITP respectively.

Coumarin-Spiropyran Dyad with a Hydrogenated Pyran Moiety for Rapid, Selective, and Sensitive Fluorometric Detection of Cyanide Anion

We synthesized a coumarin-spiropyran dyad with a hydrogenated pyran moiety (2), behaving as an off-on type fluorescent receptor for rapid, selective, and sensitive detection of cyanide anion (CN(-)) in aqueous media. The receptor itself shows almost no fluorescence with a quantum yield < 0.01, due to the delocalization of π-electrons over the molecule. Selective nucleophilic addition of CN(-) to the spirocarbon of the molecule rapidly promotes spirocycle opening within only 3 min. This leads to localization of π-electrons on the coumarin moiety and exhibits strong light-blue fluorescence at 459 nm with very high quantum yield (0.52). As a result of this, the receptor facilitates rapid, selective, and sensitive fluorometric detection of CN(-) as low as 1.0 μM.

Off–on fluorometric detection of cyanide anions in an aqueous mixture by an indane-based receptor

An indanedione–coumarin conjugate behaves as an off–on type fluorescent receptor for rapid, selective, and sensitive detection of cyanide anions in an aqueous mixture via a nucleophilic interaction with the electrophilic β-carbon of the receptor.

A pyrylium–coumarin dyad as a colorimetric receptor for ratiometric detection of cyanide anions by two absorption bands in the visible region

A pyrylium–coumarin dyad behaves as a colorimetric receptor for ratiometric detection of cyanide anions in aqueous media by two absorption bands in the visible wavelength region.

A simple chemosensor for the dual-channel detection of cyanide in water with high selectivity and sensitivity

By a deprotonated mechanism, the simple chemosensorHYshowed high sensitivity and selectivity for cyanide in aqueous media.

Effects of substituents on fluorometric detection of cyanide anions by indolium–coumarin dyads

Fluorometric detection of cyanide anions was carried out with indolium–coumarin dyads in aqueous media. Substituents on the dyads strongly affect the selectivity, sensitivity, and response to cyanide anions.

Redox equilibration after one-electron reduction of cytochrome c oxidase: radical formation and a possible hydrogen relay mechanism

Kinetic studies using UV/visible and EPR spectroscopy were carried out to follow the distribution of electrons within beef heart cytochrome c oxidase (CcO), both active and cyanide-inhibited, following addition of reduced cytochrome c as electron donor. In the initial one-electron reduced state the electron is shared between three redox centers, heme a, CuA and a third site, probably CuB. Using a rapid freeze system and the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) a protein radical was also detected. The EPR spectrum of the DMPO adduct of this radical was consistent with tyrosyl radical capture. This may be a feature of a charge relay mechanism involved in some part of the CcO electron transfer system from bound cytochrome c via CuA and heme a to the a3CuB binuclear center.

Horseradish peroxidase: modulation of properties by chemical modification of protein and heme

Horseradish peroxidase (HRP) is one of the most studied enzymes of the plant peroxidase superfamily. HRP is also widely used in different bioanalytical applications and diagnostic kits. The methods of genetic engineering and protein design are now widely used to study the catalytic mechanism and to improve properties of the enzyme. Here we review the results of another approach to HRP modification-through the chemical modification of amino acids or prosthetic group of the enzyme. Computer models of HRPs with modified hemes are in good agreement with the experimental data.

Potential role of a quetiapine metabolite in quetiapine-induced neutropenia and agranulocytosis

Clozapine was the first of the atypical antipsychotics to be developed, but its use has been restricted because of toxicity issues, particularly the risk of potentially life-threatening drug-induced neutropenia and agranulocytosis, which occurs in about 1% of patients. Bioactivation of clozapine by peroxidases forms a reactive nitrenium ion, which covalently adducts to protein and leads to neutrophil toxicity. The current generation of clozapine-inspired atypical antipsychotics has reduced toxicity through improved potency/decreased dose or through structural modification to prevent peroxidase-catalyzed nitrenium ion formation. Through the substitution of sulfur for the bridging nitrogen found in clozapine, quetiapine does not directly form a nitrenium ion when incubated with myeloperoxidase/H(2)O(2). We present evidence that cytochrome P450 2D6 catalyzes the formation of 7-hydroxyquetiapine, which can be oxidized by human myeloperoxidase to form a reactive quinone-imine and a reactive radical, which may account for the continued, although reduced, neutrophil toxicity. In the presence of myeloperoxidase/H(2)O(2) and glutathione, covalent 7-hydroxyquetiapine-glutathione adducts were formed. Bioactivation of quetiapine was verified in vivo in rat where three 7-hydroxyquetiapine-mercaptate adducts and a 7-hydroxyquetiapine-glutathione adduct were detected from bile after oral dosing. The decreased incidence of agranulocytosis with quetiapine over clozapine is postulated to be due to the lower exposure of the toxic precursor, 7-hydroxyquetiapine versus clozapine, respectively.

Biphasic modulation of the mitochondrial electron transport chain in myocardial ischemia and reperfusion

Mitochondrial electron transport chain (ETC) is the major source of reactive oxygen species during myocardial ischemia-reperfusion (I/R) injury. Ischemic defect and reperfusion-induced injury to ETC are critical in the disease pathogenesis of postischemic heart. The properties of ETC were investigated in an isolated heart model of global I/R. Rat hearts were subjected to ischemia for 30 min followed by reperfusion for 1 h. Studies of mitochondrial function indicated a biphasic modulation of electron transfer activity (ETA) and ETC protein expression during I/R. Analysis of ETAs in the isolated mitochondria indicated that complexes I, II, III, and IV activities were diminished after 30 min of ischemia but increased upon restoration of flow. Immunoblotting analysis and ultrastructural analysis with transmission electron microscopy further revealed marked downregulation of ETC in the ischemic heart and then upregulation of ETC upon reperfusion. No significant difference in the mRNA expression level of ETC was detected between ischemic and postischemic hearts. However, reperfusion-induced ETC biosynthesis in myocardium can be inhibited by cycloheximide, indicating the involvement of translational control. Immunoblotting analysis of tissue homogenates revealed a similar profile in peroxisome proliferator-activated receptor-γ coactivator-1α expression, suggesting its essential role as an upstream regulator in controlling ETC biosynthesis during I/R. Significant impairment caused by ischemic and postischemic injury was observed in the complexes I- III. Analysis of NADH ferricyanide reductase activity indicated that injury of flavoprotein subcomplex accounts for 50% decline of intact complex I activity from ischemic heart. Taken together, our findings provide a new insight into the molecular mechanism of I/R-induced mitochondrial dysfunction.

Internal binding of halogenated phenols in dehaloperoxidase-hemoglobin inhibits peroxidase function

Dehaloperoxidase (DHP) from the annelid Amphitrite ornata is a catalytically active hemoglobin-peroxidase that possesses a unique internal binding cavity in the distal pocket above the heme. The previously published crystal structure of DHP shows 4-iodophenol bound internally. This led to the proposal that the internal binding site is the active site for phenol oxidation. However, the native substrate for DHP is 2,4,6-tribromophenol, and all attempts to bind 2,4,6-tribromophenol in the internal site under physiological conditions have failed. Herein, we show that the binding of 4-halophenols in the internal pocket inhibits enzymatic function. Furthermore, we demonstrate that DHP has a unique two-site competitive binding mechanism in which the internal and external binding sites communicate through two conformations of the distal histidine of the enzyme, resulting in nonclassical competitive inhibition. The same distal histidine conformations involved in DHP function regulate oxygen binding and release during transport and storage by hemoglobins and myoglobins. This work provides further support for the hypothesis that DHP possesses an external binding site for substrate oxidation, as is typical for the peroxidase family of enzymes.

Characterisation of phenol oxidase and peroxidase from maize silk

Silk of some maize genotypes contains a high level of phenolics that undergo enzymatic oxidation to form quinones, which condense among themselves or with proteins to form brown pigments. Two phenolic oxidizing enzymes, peroxidase (POD; EC 1.11.1.7) and polyphenol oxidase (PPO; EC 1.10.3.1), from maize (Zea mays L.) silk were characterised with respect to their preferred substrate, different isoforms and specific effectors. One browning silk sample with high, and two non-browning samples with low phenolic content were investigated. Although POD oxidizes a wide range of phenolic substrates in vitro, its activity rate was independent of silk phenolic content. PPO activity, detected with o-diphenolic substrates, was abundant only in browning silk, and low or absent in non-browning silk. Pollination increased POD but not PPO activity. Isoelectric-focusing (IEF) and specific staining for POD and PPO showed a high degree of polymorphism that varied with silk origin. The IEF pattern of POD revealed a number of anionic and several cationic isoenzymes, with the most pronounced having neutral pI 7 and a basic isoform with pI 10. Detected isoforms of PPO were anionic, except for one neutral form found only in browning silk, and occupied positions different from those of POD. Different inhibitory effects of NaN(3), EDTA, KCN, and L-cysteine, as well as different impacts of a variety of cations on the oxidation of chlorogenic acid, mediated by PPO or POD, were detected. The findings are discussed in terms of a possible roles of these enzymes in defence and pollination.

Electron paramagnetic resonance oximetry and redoximetry

Reactive oxygen/nitrogen species (ROS/RNS) have been increasingly recognized as important mediators and play a number of critical roles in cell injury, metabolism, disease pathology, diagnosis, and clinical treatment. Electron paramagnetic resonance (EPR) spectroscopy enables the spectral information at certain spatial position, and, from the observed line-width and signal intensity, the localized tissue oxygenation, and tissue redox status can be determined. We applied in vivo EPR oximetry and redoximetry technique and implemented its physiological/pathophysiological applications, along with the use of biocompatible lithium pthalocyanine (liPc) and nitroxide redox sensitive probes, on in vivo tissue oxygenation and redox profile of the ischemic and reperfused heart in living animals. We have observed that the hypoxia during myocardial ischemia limited mitochondrial respiration and caused a shift of tissue redox status to a more reduced state. ROS/RNS generated at the beginning of reperfusion not only caused a shift of redox status to a more oxidized state which may contribute to the postischemic myocardial injury, but also a marked suppression of in vivo tissue O(2) consumption in the postischemic heart through modulation of mitochondrial respiration based on alterations in enzyme activity and mRNA expression of NADH dehydrogenase (NADH-DH) and cytochrome c oxidase (CcO). In addition, ischemic preconditioning was found to be able to markedly attenuate postischemic myocardial hyperoxygenation with less ROS/RNS generation and preservation of mitochondrial O(2) metabolism, due to conserved NADH-DH and CcO activities. These studies have demonstrated that EPR oximetry and redoximetry techniques have advanced to a stage that enables in-depth insight in the process of ischemia reperfusion injury.

The radical trap 5,5-dimethyl-1-pyrroline N-oxide exerts dose-dependent protection against myocardial ischemia-reperfusion injury through preservation of mitochondrial electron transport

Free radicals are important mediators of myocardial ischemia-reperfusion injury. Nitrone spin traps have been shown to scavenge free radicals. The cardioprotective effect of the spin trap, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), was investigated in an isolated heart model of global ischemia and reperfusion. Rat hearts were perfused and subjected to global ischemia for 30 min followed by reperfusion with four treatment groups of varying DMPO concentration (0.5-10 mM) administered before induction of ischemia. DMPO treatment improved the recovery of left ventricular (LV) function and coronary flow over the 30-min period of reperfusion compared with untreated hearts. Enhanced recovery was observed for all doses studied but was highest with 1 mM treatment with 2.4-fold higher recovery of LV developed pressure and 37% reduction in infarct size. Superoxide was measured by tissue fluorometry using the O(2)* probe hydroethidine. Hearts treated with 1 mM DMPO showed a significant reduction in O(2)* production compared with control hearts both over the first 5 min of ischemia and upon reperfusion after 30 min of global ischemia. Studies of mitochondrial function demonstrated that 1 mM DMPO increased the recovery of function of complexes I, II/III, and IV after 30 min of reperfusion. Immunoblotting with antibodies against complexes I, II, and IV further revealed marked up-regulation of mitochondrial proteins, suggesting that DMPO prevents their ischemic degradation via scavenging oxygen radicals generated during ischemia/reperfusion. Thus, DMPO functions as a protective agent against ischemic and postischemic injury via radical scavenging, conferring robust dose-dependent protection with salvage of mitochondrial function and redox homeostasis.

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.

Endothelial nitric oxide synthase (NOS3) knockout decreases NOS2 induction, limiting hyperoxygenation and conferring protection in the postischemic heart

Although it has been shown that endothelial nitric oxide synthase (eNOS)-derived nitric oxide downregulates mitochondrial oxygen consumption during early reperfusion, its effects on inducible NOS (iNOS) induction and myocardial injury during late reperfusion are unknown. Wild-type (WT) and eNOS(-/-) mice were subjected to 30 min of coronary ligation followed by reperfusion. Expression of iNOS mRNA and protein levels and peroxynitrite production were lower in postischemic myocardium of eNOS(-/-) mice than levels in WT mice 48 h postreperfusion. Significantly improved hemodynamics (+/-dP/dt, left ventricular systolic pressure, mean arterial pressure), increased rate pressure product, and reduced myocardial infarct size (18 +/- 2.5% vs. 31 +/- 4.6%) were found 48 h after reperfusion in eNOS(-/-) mice compared with WT mice. Myocardial infarct size was also significantly decreased in WT mice treated with the specific iNOS inhibitor 1400W (20.5 +/- 3.4%) compared with WT mice treated with PBS (33.9 +/- 5.3%). A marked reperfusion-induced hyperoxygenation state was observed by electron paramagnetic resonance oximetry in postischemic myocardium, but Po(2) values were significantly lower from 1 to 72 h in eNOS(-/-) than in WT mice. Cytochrome c-oxidase activity and NADH dehydrogenase activity were significantly decreased in postischemic myocardium in WT and eNOS(-/-) mice compared with baseline control, respectively, and NADH dehydrogenase activity was significantly higher in eNOS(-/-) than in WT mice. Thus deficiency of eNOS exerted a sustained beneficial effect on postischemic myocardium 48 h after reperfusion with preserved mitochondrial function, which appears to be due to decreased iNOS induction and decreased iNOS-derived peroxynitrite in postischemic myocardium.

Shear-induced reactive nitrogen species inhibit mitochondrial respiratory complex activities in cultured vascular endothelial cells

There is evidence that nitric oxide (NO), superoxide (O(2)(*-)), and their associated reactive nitrogen species (RNS) produced by vascular endothelial cells (ECs) in response to hemodynamic forces play a role in cell signaling. NO is known to impair mitochondrial respiration. We sought to determine whether exposure of human umbilical vein ECs (HUVECs) to steady laminar shear stress and the resultant NO production modulate electron transport chain (ETC) enzymatic activities. The activities of respiratory complexes I, II/III, and IV were dependent on the presence of serum and growth factor supplement in the medium. EC exposure to steady laminar shear stress (10 dyn/cm(2)) resulted in a gradual inhibition of each of the complexes starting as early as 5 min from the flow onset and lasting up to 16 h. Ramp flow resulted in inhibition of the complexes similar to that of step flow. When ECs were sheared in the presence of the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME; 100 microM), the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO; 100 microM), or the peroxynitrite (ONOO(-)) scavenger uric acid (UA; 50 microM), the flow-inhibitory effect on mitochondrial complexes was attenuated. In particular, L-NAME and UA abolished the flow effect on complex IV. Increased tyrosine nitration was observed in the mitochondria of sheared ECs, and UA blocked the shear-induced nitrotyrosine staining. In summary, shear stress induces mitochondrial RNS formation that inhibits the electron flux of the ETC at multiple sites. This may be a critical mechanism by which shear stress modulates EC signaling and function.

Superoxide generation from mitochondrial NADH dehydrogenase induces self-inactivation with specific protein radical formation

Mitochondrial superoxide (O(2)(.)) production is an important mediator of oxidative cellular injury. While NADH dehydrogenase (NDH) is a critical site of this O(2)(.) production; its mechanism of O(2)(.) generation is not known. Therefore, the catalytic function of NDH in the mediation of O(2)(.) generation was investigated by EPR spin-trapping. In the presence of NADH, O(2)(.) generation from NDH was observed and was inhibited by diphenyleneiodinium chloride (DPI), indicating involvement of the FMN-binding site of NDH. Addition of FMN increased O(2)(.) production. Destruction of the cysteine ligands of iron-sulfur clusters decreased O(2)(.) generation, suggesting a secondary role of this site. This inhibitory effect was reversed by addition of FMN. However, FMN addition could not reverse the inhibition of NDH by either DPI or heat denaturation, demonstrating involvement of both FMN and its FMN-binding protein moiety in the catalysis of O(2)(.) generation. O(2)(.) production by NDH also induced self-inactivation. Immunospin-trapping with anti-DMPO antibody and subsequent mass spectrometry was used to define the sites of oxidative damage of NDH. A DMPO adduct was detected on the 51-kDa subunit and was O(2)(.)-dependent. Alkylation of the cysteine residues of NDH significantly inhibited NDH-DMPO spin adduct formation, indicating involvement of protein thiyl radicals. LC/MS/MS analysis of a tryptic digest of the 51-kDa polypeptide revealed that cysteine (Cys(206)) and tyrosine (Tyr(177)) were specific sites of NDH-derived protein radical formation. Thus, two domains of the 51-kDa subunit, Gly(200)-Ala-Gly-Ala-Tyr-Ile-Cys(206)-Gly-Glu-Glu-Thr-Ala-Leu-Ile-Glu-Ser-Ile-Glu-Gly-Lys(219) and Ala(176)-Tyr(177)-Glu-Ala-Gly-Leu-Ile-Gly-Lys(184), were demonstrated to be susceptible to oxidative attack, and their oxidative modification results in decreased electron transfer activity.

Spontaneous generation of reactive oxygen species in the mixture of cyanide and glycerol

Reactive oxygen species are involved in tumor promotion or apoptosis. In assaying prooxidant or antioxidant activities, cyanide has been commonly used as an inhibitor of mitochondrial oxidases, peroxidases, or Cu,Zn-superoxide dismutase, which have an influence on intracellular levels of reactive oxygen species. It has also been used to chemically mimic hypoxia. On the other hand, glycerol has been widely used as a stabilizer of various enzymes. In particular, glycerol is required to maintain the enzymatic activities of membrane-bound NAD(P)H oxidases extracted from surrounding phospholipids. Since both cyanide and glycerol are relatively inert, they have been used concomitantly regardless of any mutual interference. In this study, we demonstrate that a mixture of glycerol and cyanide reduced cytochrome c and nitroblue tetrazolium, both of which are superoxide anion indicators. The mixture also enhanced the production of superoxide anion in the presence of redox-cycling compounds. Superoxide production by the mixture was confirmed by electron spin resonance spectra. Moreover, the mixture induced lipid peroxidation and hemolysis in human erythrocytes. These results suggest that cyanide and glycerol should be used carefully in reaction systems used to measure superoxide production or antioxidant activity. However, sucrose and sodium azide in combination do not produce such artifacts and thus may be used as an alternative.

EPR spin trapping of protein radicals

Electron paramagnetic resonance (EPR) spin trapping was originally developed to aid the detection of low-molecular-mass radicals formed in chemical systems. It has subsequently found widespread use in biology and medicine for the direct detection of radical species formed during oxidative stress and via enzymatic reactions. Over the last 15 years this technique has also found increasing use in detecting and identifying radicals formed on biological macromolecules as a result of either radical reactions or enzymatic processes. Though the EPR signals that result from the trapping of large, slowly tumbling radicals are often broad and relatively poor in distinctive features, a number of techniques have been developed that allow a wealth of information to be obtained about the nature, site, and reactions of such radicals. This article summarizes recent developments in this area and reviews selected examples of radical formation on proteins.

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.

Mechanism in the reaction of cytochrome c oxidase with organic hydroperoxides: an ESR spin-trapping investigation

Organic hydroperoxides are of great utility in probing the reaction mechanism and the toxicological consequences of lipid peroxidation. In the present study, ESR spin-trapping was employed to investigate the peroxidation of mitochondrial cytochrome c oxidase (CcO) with t-butyl hydroperoxide (t-BuOOH) and cumene hydroperoxide (CumOOH). The spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was used to detect the radical species formed from the reaction of CcO with t-BuOOH. The presence of t-BuOOH-derived alkoxyl radical (t-BuO*) as the primary radical indicates reductive scission of the O-O bond by CcO. The ESR signal of DMPO/*Ot-Bu can be partially abolished by cyanide, implying that the reductive cleavage involved the haem a(3)Cu(B) binuclear site of CcO. A nitroso spin trap, 2-methyl-2-nitrosopropane (MNP), was used to detect and identify radical species from the reaction of CcO with CumOOH. In addition to the t-BuOOH-derived methyl, hydroxylmethyl and tertiary carbon-centred radicals, a protein-derived radical was detected. The intensity of the ESR signal from the protein radical increased with the CumOOH concentration at low CumOOH/CcO ratios, with maximal intensity at a ratio of 100 mol of CumOOH/mol of CcO. The immobilized protein radical adduct of MNP was stable and persistent after dialysis; it was also resistant to proteolytic digestion, suggesting that it was formed in the transmembrane region, a region that is not accessible to proteases. Its signal was greatly enhanced when CcO cysteine residues were chemically modified by N-ethylmaleimide, when the tryptophan residues in CcO were oxidized by N-bromosuccimide, and when tyrosine residues on the surface of CcO were iodinated, showing that a radical equilibrium was established among the cysteine, tryptophan and tyrosine residues of the protein-centred radical. Pre-treatment of CcO with cyanide prevented detectable MNP adduct formation, confirming that the haem a(3)-Cu(B) binuclear centre was the initial reaction site. When the CcO was pre-treated with 10 mM (100 equivalents) of CumOOH, the enzyme activity decreased by more than 20%. This inhibition was persistent after dialysis, suggesting that the detected protein-centred radical was, in part, involved in the irreversible inactivation by CumOOH. Visible spectroscopic analysis revealed that the haem a of CcO was not affected during the reaction. However, the addition of pyridine to the reaction mixture under alkaline conditions resulted in the destruction of the haem centre of CcO, suggesting that its protein matrix rather than its haem a is the target of oxidative damage by the organic hydroperoxide.

Chronic treatment with azide in situ leads to an irreversible loss of cytochrome c oxidase activity via holoenzyme dissociation

Chronic treatment of cultured cells with very low levels of azide (I(50)<10 microm) leads to slow (t(12) = 6 h), irreversible loss of cytochrome c oxidase (COX) activity. Azide-mediated COX losses were not accompanied by inhibition of other mitochondrial enzymes and were not dependent upon electron flux through oxidative phosphorylation. Although azide treatment also reduced activity (but not content) of both CuZn superoxide dismutase and catalase, a spectrum of pro-oxidants (and anti-oxidants) failed to mimic (or prevent) azide effects, arguing that losses in COX activity were not due to resultant compromises in free radical scavenging. Loss of COX activity was not attributable to reduced rates of mitochondrial protein synthesis or declines in either COX subunit mRNA or protein levels (COX I, II, IV). Co-incubation experiments using copper (CuCl(2), Cu-His) and copper chelators (neocuproine, bathocuproine) indicated that azide effects were not mediated by interactions with either Cu(A) or Cu(B). In contrast, difference spectroscopy and high performance liquid chromatography analyses demonstrated azide-induced losses in cytochrome aa(3) content although not to the same extent as catalytic activity. Differential azide effects on COX content relative to COX activity were confirmed using a refined inhibition time course in combination with blue native electrophoresis, and established that holoenzyme dissociation occurs subsequent to losses in catalytic activity. Collectively, these data suggest that COX deficiency can arise through enhanced holoenzyme dissociation, possibly through interactions with the structure or coordination of its heme moieties.

Identification of hemes and related cyclic tetrapyrroles by matrix-assisted laser desorption/ionization and liquid secondary ion mass spectrometry

Mass spectrometry has proven to be a powerful technique applicable on trace amounts for the identification of known hemes and cyclic tetrapyrroles, and for providing critical information for the structure of new and novel versions. This report describes investigations of the practical limits of detection for such bioinorganic prosthetic groups, primarily by liquid secondary ion mass spectrometry (LSIMS) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS), including a survey of the utility of common matrices. The lower limit of detection under favorable conditions extends to low picomole amounts. Certain derivatization techniques, such as methyl esterification and chelation to zinc, both increase the sensitivity of analyses and provide spectroscopic signatures that enable heme/cyclic tetrapyrrole ions to be identified in the presence of contaminants.