The reduction potential of benzyl viologen: an important reference compound for oxidant/radical redox couples

A Versatile Aldehyde: Ferredoxin Oxidoreductase from the Organic Acid Reducing Thermoanaerobacter sp. Strain X514

Aldehyde:ferredoxin oxidoreductases (AORs) have been isolated and biochemically-characterized from a handful of anaerobic or facultative aerobic archaea and bacteria. They catalyze the ferredoxin (Fd)-dependent oxidation of aldehydes to acids. Recently, the involvement of AOR in the reduction of organic acids to alcohols with electrons derived from sugar or synthesis gas was demonstrated, with alcohol dehydrogenases (ADHs) carrying out the reduction of the aldehyde to the alcohol (AOR-ADH pathway). Here, we describe the biochemical characterization of an AOR of the thermophilic fermentative bacterium Thermoanaerobacter sp. strain X514 (AORX514). The putative aor gene (Teth514_1380) including a 6x-His-tag was introduced into the genome of the genetically-accessible, related species Thermoanaerobacter kivui. The protein was purified to apparent homogeneity, and indeed revealed AOR activity, as measured by acetaldehyde-dependent ferredoxin reduction. AORX514 was active over a wide temperature (10 to 95 °C) and pH (5.5 to 11.5) range, utilized a wide variety of aldehydes (short and branched-chained, aliphatic, aromatic) and resembles archaeal sensu stricto AORs, as the protein is active in a homodimeric form. The successful, recombinant production of AORX514 in a related, well-characterized and likewise strict anaerobe paves the road towards structure-function analyses of this enzyme and possibly similar oxygen-sensitive or W/Mo-dependent proteins in the future.

Structural basis for bacterial energy extraction from atmospheric hydrogen

Diverse aerobic bacteria use atmospheric H₂ as an energy source for growth and survival¹. This globally significant process regulates the composition of the atmosphere, enhances soil biodiversity and drives primary production in extreme environments^2,3. Atmospheric H₂ oxidation is attributed to uncharacterized members of the [NiFe] hydrogenase superfamily^4,5. However, it remains unresolved how these enzymes overcome the extraordinary catalytic challenge of oxidizing picomolar levels of H₂ amid ambient levels of the catalytic poison O₂ and how the derived electrons are transferred to the respiratory chain¹. Here we determined the cryo-electron microscopy structure of the Mycobacterium smegmatis hydrogenase Huc and investigated its mechanism. Huc is a highly efficient oxygen-insensitive enzyme that couples oxidation of atmospheric H₂ to the hydrogenation of the respiratory electron carrier menaquinone. Huc uses narrow hydrophobic gas channels to selectively bind atmospheric H₂ at the expense of O₂, and 3 [3Fe-4S] clusters modulate the properties of the enzyme so that atmospheric H₂ oxidation is energetically feasible. The Huc catalytic subunits form an octameric 833 kDa complex around a membrane-associated stalk, which transports and reduces menaquinone 94 Å from the membrane. These findings provide a mechanistic basis for the biogeochemically and ecologically important process of atmospheric H₂ oxidation, uncover a mode of energy coupling dependent on long-range quinone transport, and pave the way for the development of catalysts that oxidize H₂ in ambient air.

Electrochemical Studies of CO2 -Reducing Metalloenzymes

Only two enzymes are capable of directly reducing CO₂ : CO dehydrogenase, which produces CO at a [NiFe₄ S₄ ] active site, and formate dehydrogenase, which produces formate at a mononuclear W or Mo active site. Both metalloenzymes are very rapid, energy-efficient and specific in terms of product. They have been connected to electrodes with two different objectives. A series of studies used protein film electrochemistry to learn about different aspects of the mechanism of these enzymes (reactivity with substrates, inhibitors…). Another series focused on taking advantage of the catalytic performance of these enzymes to build biotechnological devices, from CO₂ -reducing electrodes to full photochemical devices performing artificial photosynthesis. Here, we review all these works.

Photogeneration and reactivity of flavin anionic semiquinone in a bifurcating electron transfer flavoprotein

Electron transfer bifurcation allows production of a strongly reducing carrier at the expense of a weaker one, by redistributing energy among a pair of electrons. Thus, two weakly-reducing electrons from NADH are consumed to produce a strongly reducing ferredoxin or flavodoxin, paid for by reduction of an oxidizing acceptor. The prevailing mechanism calls for participation of a strongly reducing flavin semiquinone which has been difficult to observe with site-certainly in multi-flavin systems. Using blue light (450 nm) to photoexcite the flavins of bifurcating electron transfer flavoprotein (ETF), we demonstrate accumulation of anionic flavin semiquinone in excess of what is observed in equilibrium titrations, and establish its ability to reduce the low-potential electron acceptor benzyl viologen. This must occur at the bifurcating flavin because the midpoint potentials of the electron transfer (ET) flavin are not sufficiently negative. We show that bis-tris propane buffer is an effective electron donor to the flavin photoreduction, but that if the system is prepared with the ET flavin chemically reduced, so that only the bifurcating flavin is oxidized and photochemically active, flavin anionic semiquinone is formed more rapidly. Thus, excited bifurcating flavin is able to draw on an electron stored at the ET flavin. Flavin semiquinone photogenerated at the bifurcation site must therefore be accompanied by additional semiquinone formation by oxidation of the ET flavin. Consistent with the expected instability of bifurcating flavin semiquinone, it subsides immediately upon cessation of illumination. However comparison with yields of semiquinone in equilibrium titrations suggest that during continuous illumination at pH 9 a steady state population of 0.3 equivalents of bifurcating flavin semiquinone accumulates, and then undergoes further photoreduction to the hydroquinone. Although transient, the population of bifurcating flavin semiquinone explains the system's ability to conduct light-driven electron transfer from bis-tris propane to benzyl viologen, in effect trapping energy from light.

Coordination chemistry of the CuZ site in nitrous oxide reductase and its synthetic mimics

Atmospheric nitrous oxide (N2O) has garnered significant attention recently due to its dual roles as an ozone depletion agent and a potent greenhouse gas. Anthropogenic N2O emissions occur primarily through agricultural disruption of nitrogen homeostasis causing N2O to build up in the atmosphere. The enzyme responsible for N2O fixation within the geochemical nitrogen cycle is nitrous oxide reductase (N2OR), which catalyzes 2H+/2e- reduction of N2O to N2 and H2O at a tetranuclear active site, CuZ. In this review, the coordination chemistry of CuZ is reviewed. Recent advances in the understanding of biological CuZ coordination chemistry is discussed, as are significant breakthroughs in synthetic modeling of CuZ that have emerged in recent years. The latter topic includes both structurally faithful, synthetic [Cu4(µ4-S)] clusters that are able to reduce N2O, as well as dicopper motifs that shed light on reaction pathways available to the critical CuI-CuIV cluster edge of CuZ. Collectively, these advances in metalloenzyme studies and synthetic model systems provide meaningful knowledge about the physiologically relevant coordination chemistry of CuZ but also open new questions that will pose challenges in the near future.

Determination and Environmental Implications of Aqueous-Phase Rate Constants in Radical Reactions

Interests in the kinetics of radical-induced reactions in aqueous solution have grown remarkably due to their water engineering significance (e.g., advanced oxidation processes). Although compilations of the rate constants (k) for various radicals have been documented, surprisingly a systematic review has yet to be reported on the development of reliable methods for determining k values. A knowledge gap exists to critically evaluate and screen the various methods to measure them. In this review, we summarize the direct and indirect methods under steady-state and non-steady-state conditions, followed by critical evaluations on their advantages and disadvantages. The radicals of ·OH, [Formula: see text] , [Formula: see text] , and Cl· were chosen based on their significant aquatic environmental relevance. MS excel spreadsheets that demonstrate the determination processes were provided allowing one to reproduce the data and/or to analyze the unprocessed raw data as a "template". We formulated a standard operation procedure for the k determination, although there is simply no "versatile" method fitting for all radical reactions. Finally, existing challenges and future research focus are discussed. This is the first review covering methodological approaches and considerations, aiming to provide a holistic and fundamental basis to choose an appropriate method for determining the k values for bimolecular reactions between target compounds and radicals in the aqueous phase.

Viologen-Based Electrochromic Materials: From Small Molecules, Polymers and Composites to Their Applications

Organic materials have gained considerable attention for electrochromic (EC) applications owing to improved EC performance and good processability. As a class of well-recognized organic EC materials, viologens have received persistent attention due to the structural versatility and property tunability, and are major active EC components for most of the marketed EC devices. Over the past two decades, extensive efforts have been made to design and synthesize different types of viologen-based materials with enhanced EC properties. This review summarizes chemical structures, preparation and EC properties of various latest viologen-based electrochromes, including small viologen derivatives, main-chain viologen-based polymers, conjugated polymers with viologen side-chains and viologen-based organic/inorganic composites. The performance enhancement mechanisms are concisely discussed. The current marketed viologens-based electrochromic devices (ECDs) are briefly introduced and an outlook on the challenges and future exploration directions for viologen-based materials and their ECDs are also proposed.

Chemical, Biological and Medical Controversies Surrounding the Fenton Reaction

A critical evaluation is made of the role of the Fenton reaction (Fe2+ + H2O2 → Fe3+ + •OH + OH-) in the promotion of oxidative damage in mammalian systems. Following a brief, historical overview of the Fenton reaction, including the formulation of the Haber–Weiss cycle as a mechanism for the catalysis of hydroxyl radical production, an appraisal is made of the biological relevance of the reaction today, following recognition of the important role played by nitric oxide and its congers in the promotion of biomolecular damage. In depth coverage is then given of the evidence (largely from EPR studies) for and against the hydroxyl radical as the active oxidant produced in the Fenton reaction and the role of metal chelating agents (including those of biological importance) and ascorbic acid in the modulation of its generation. This is followed by a description of the important developments that have occurred recently in the molecular and cellular biology of iron, including evidence for the presence of ‘free’ iron that is available in vivo for the Fenton reaction. Particular attention here is given to the role of the iron-regulatory proteins in the modulation of cellular iron status and how their functioning may become dysregulated during oxidative and nitrosative stress, as well as in hereditary haemochromatosis, a common disorder of iron metabolism. Finally, an assessment is made of the biological relevance of ascorbic acid in the promotion of hydroxyl radical generation by the Fenton reaction in health and disease.

Reaction fronts of the autocatalytic hydrogenase reaction

We have built a model to describe the hydrogenase catalyzed, autocatalytic, reversible hydrogen oxidation reaction where one of the enzyme forms is the autocatalyst. The model not only reproduces the experimentally observed front properties, but also explains the found hydrogen ion dependence. Furthermore, by linear stability analysis, two different front types are found in good agreement with the experiments.

Antagonism in effectiveness of evofosfamide and doxorubicin through intermolecular electron transfer

Hypoxic cells pose a problem in anticancer chemotherapy, in which often drugs require oxygen as an electron acceptor to bring about the death of actively cycling cells. Bioreductive anticancer drugs, which are selectively activated in the hypoxic regions of tumours through enzymatic one-electron reduction, are being developed for combination with chemotherapy-, radiotherapy- and immunotherapy-containing regimens to kill treatment-resistant hypoxic cells. The most clinically-advanced bioreductive drug, evofosfamide (TH-302), which acts by releasing a DNA-crosslinking mustard, failed to extend overall survival in combination with doxorubicin, a topoisomerase II inhibitor, for advanced soft tissue sarcoma in a pivotal clinical trial. However, the reasons for the lack of additive efficacy with this combination are unknown. Here, we show that the radical anion of evofosfamide undergoes electron transfer to doxorubicin in kinetic competition to fragmentation of the radical anion, thus suppressing the release the cytotoxic mustard. This electron transfer process may account, at least in part, for the lack of overall survival improvement in the recent clinical trial. This study underlines the need to consider both redox and electron transfer chemistry when combining bioreductive prodrugs with other redox-active drugs in cancer treatment.

The catalytic cycle of nitrous oxide reductase - The enzyme that catalyzes the last step of denitrification

The reduction of the potent greenhouse gas nitrous oxide requires a catalyst to overcome the large activation energy barrier of this reaction. Its biological decomposition to the inert dinitrogen can be accomplished by denitrifiers through nitrous oxide reductase, the enzyme that catalyzes the last step of the denitrification, a pathway of the biogeochemical nitrogen cycle. Nitrous oxide reductase is a multicopper enzyme containing a mixed valence CuA center that can accept electrons from small electron shuttle proteins, triggering electron flow to the catalytic sulfide-bridged tetranuclear copper "CuZ center". This enzyme has been isolated with its catalytic center in two forms, CuZ*(4Cu1S) and CuZ(4Cu2S), proven to be spectroscopic and structurally different. In the last decades, it has been a challenge to characterize the properties of this complex enzyme, due to the different oxidation states observed for each of its centers and the heterogeneity of its preparations. The substrate binding site in those two "CuZ center" forms and which is the active form of the enzyme is still a matter of debate. However, in the last years the application of different spectroscopies, together with theoretical calculations have been useful in answering these questions and in identifying intermediate species of the catalytic cycle. An overview of the spectroscopic, kinetics and structural properties of the two forms of the catalytic "CuZ center" is given here, together with the current knowledge on nitrous oxide reduction mechanism by nitrous oxide reductase and its intermediate species.

Pyranopterin Coordination Controls Molybdenum Electrochemistry in Escherichia coli Nitrate Reductase

We test the hypothesis that pyranopterin (PPT) coordination plays a critical role in defining molybdenum active site redox chemistry and reactivity in the mononuclear molybdoenzymes. The molybdenum atom of Escherichia coli nitrate reductase A (NarGHI) is coordinated by two PPT-dithiolene chelates that are defined as proximal and distal based on their proximity to a [4Fe-4S] cluster known as FS0. We examined variants of two sets of residues involved in PPT coordination: (i) those interacting directly or indirectly with the pyran oxygen of the bicyclic distal PPT (NarG-Ser(719), NarG-His(1163), and NarG-His(1184)); and (ii) those involved in bridging the two PPTs and stabilizing the oxidation state of the proximal PPT (NarG-His(1092) and NarG-His(1098)). A S719A variant has essentially no effect on the overall Mo(VI/IV) reduction potential, whereas the H1163A and H1184A variants elicit large effects (ΔEm values of -88 and -36 mV, respectively). Ala variants of His(1092) and His(1098) also elicit large ΔEm values of -143 and -101 mV, respectively. An Arg variant of His(1092) elicits a small ΔEm of +18 mV on the Mo(VI/IV) reduction potential. There is a linear correlation between the molybdenum Em value and both enzyme activity and the ability to support anaerobic respiratory growth on nitrate. These data support a non-innocent role for the PPT moieties in controlling active site metal redox chemistry and catalysis.

Copper-catalyzed hydroquinone oxidation and associated redox cycling of copper under conditions typical of natural saline waters

A detailed kinetic model has been developed to describe the oxidation of Cu(I) by O2 and the reduction of Cu(II) by 1,4-hydroquinone (H2Q) in the presence of O2 in 0.7 M NaCl solution over a pH range of 6.5-8.0. The reaction between Cu(I) and O2 is shown to be the most important pathway in the overall oxidation of Cu(I), with the rate constant for this oxidation process increasing with an increasing pH. In 0.7 M NaCl solutions, Cu(II) is capable of catalyzing the oxidation of H2Q in the presence of O2 with the monoanion, HQ(-), the kinetically active hydroquinone form, reducing Cu(II) with an intrinsic rate constant of (5.0 ± 0.4) × 10(7) M(-1) s(-1). Acting as a chain-propagating species, the deprotonated semiquinone radical (SQ(•) (-)) generated from both the one-electron oxidation of H2Q and the one-electron reduction of 1,4-benzoquinone (BQ) also reacts rapidly with Cu(II) and Cu(I), with the same rate constant of (2.0 ± 0.5) × 10(7) M(-1) s(-1). In addition to its role in reformation of Cu(II) via continuous oxidation of Cu(I), O2 rapidly removes SQ(•) (-), resulting in the generation of O2(•) (-). Agreement between half-cell reduction potentials of different redox couples provides confirmation of the veracity of the proposed model describing the interactions of copper and quinone species in circumneutral pH saline solutions.

Cyclic transmembrane charge transport mediated by low-potential pyrylium ions

We have investigated the capacity of a series of N-dialkylaminophenyl-substituted pyrylium and thiopyrylium ions to act as photosensitizers and redox mediators between reactants separated by bilayer membranes. These studies were prompted by earlier results indicating that simple trimethy- and triphenyl-substituted analogues could promote efficient photosensitized transmembrane redox between vectorially organized reactants by an electroneutral e(-)/OH(-) antiport mechanism. Unlike the dyes used in the earlier studies, the ions investigated herein absorb strongly throughout the visible absorption region and are therefore potentially useful in solar photoconversion processes. We demonstrate that these ions can carry out cyclic electron transport between phase-separated electron donors and occluded Co(bpy)(3)(3+) in several transversely organized vesicles. The quantum yields obtained were relatively low, but were independent of the membrane microviscosity, suggesting that transmembrane diffusion was not rate-limiting. Triphenylpyrylium and triphenylthiopyrylium ions were shown to be capable of acting as combined photosensitizers/redox relays, apparently by direct oxidation of either solvent (water) or buffer (acetate) ions from their triplet-excited state. These reactions did not require addition of separate photosensitizers and electron donors; as such, they represent a minimal photochemical scheme for effecting transmembrane charge separation. The low-potential visible-absorbing pyrylium ions were unable to function in this dual capacity, consistent with thermodynamic limitations. However, redox titrations established that the pyranyl radicals of these dyes should be capable of reducing H(+) to H(2) in weakly acidic solutions. Consistent with their strongly reducing nature, these dyes were shown to be capable of forming methyl viologen radical in photoinitiated transmembrane redox reactions.

Redox properties and thiol reactivity of geldanamycin and its analogues in aqueous solutions

Geldanamycin (GM), a benzoquinone ansamycin antibiotic, is a natural product inhibitor of Hsp90 with potent and broad anticancer properties, but with unacceptable levels of hepatotoxicity. Less toxic C17-substituted analogues have been synthesizedincluding 17-(allylamino)-17-demethoxygeldanamycin (17-AAG) and the water-soluble 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin (17-DMAG). Redox properties and thiol reactivity are central to the therapeutic and toxicologic effects of quinones, and the question arises as whether the extent of toxicity of GM, 17-AAG, and 17-DMAG is related to their redox potentials. Using pulse radiolysis, the one-electron redox potentials (vs NHE) at pH 7.0 of GM and 17-AAG have been determined to be -62 ± 7 mV and -273 ± 8 mV, respectively, whereas a value of -194 ± 6 mV has been previously published for 17-DMAG. The rate constants of the reaction of GM and its analogues with glutathione, cysteine, or dithiothreitol under anoxia at pH 7.4 followed the order GM > 17-DMAG > 17-AAG, which correlates with the order of the redox potential of the quinone/semiquinone couple. Thus, GM reacts much faster with thiols compared to the less toxic 17-DMAG and 17-AAG, and is also expected to be more readily reduced by reductases to the respective semiquinone radical, which either decomposes to yield the respective hydroquinone or reduces oxygen to superoxide. Because both redox cycling and thiol reactivity have been associated with quinone toxicity, it is concluded that the toxicity of benzoquinone ansamycins is directly related to the redox potential of the quinone/semiquinone couple.

Thermodynamic and kinetic considerations for the reaction of semiquinone radicals to form superoxide and hydrogen peroxide

The quinone/semiquinone/hydroquinone triad (Q/SQ(*-)/H(2)Q) represents a class of compounds that has great importance in a wide range of biological processes. The half-cell reduction potentials of these redox couples in aqueous solutions at neutral pH, E degrees ', provide a window to understanding the thermodynamic and kinetic characteristics of this triad and their associated chemistry and biochemistry in vivo. Substituents on the quinone ring can significantly influence the electron density "on the ring" and thus modify E degrees' dramatically. E degrees' of the quinone governs the reaction of semiquinone with dioxygen to form superoxide. At near-neutral pH the pK(a)'s of the hydroquinone are outstanding indicators of the electron density in the aromatic ring of the members of these triads (electrophilicity) and thus are excellent tools to predict half-cell reduction potentials for both the one-electron and two-electron couples, which in turn allow estimates of rate constants for the reactions of these triads. For example, the higher the pK(a)'s of H(2)Q, the lower the reduction potentials and the higher the rate constants for the reaction of SQ(*-) with dioxygen to form superoxide. However, hydroquinone autoxidation is controlled by the concentration of di-ionized hydroquinone; thus, the lower the pK(a)'s the less stable H(2)Q to autoxidation. Catalysts, e.g., metals and quinone, can accelerate oxidation processes; by removing superoxide and increasing the rate of formation of quinone, superoxide dismutase can accelerate oxidation of hydroquinones and thereby increase the flux of hydrogen peroxide. The principal reactions of quinones are with nucleophiles via Michael addition, for example, with thiols and amines. The rate constants for these addition reactions are also related to E degrees'. Thus, pK(a)'s of a hydroquinone and E degrees ' are central to the chemistry of these triads.

Production, characterization and determination of the real catalytic properties of the putative 'succinate dehydrogenase' from Wolinella succinogenes

Both the genomes of the epsilonproteobacteria Wolinella succinogenes and Campylobacter jejuni contain operons (sdhABE) that encode for so far uncharacterized enzyme complexes annotated as ‘non-classical’ succinate:quinone reductases (SQRs). However, the role of such an enzyme ostensibly involved in aerobic respiration in an anaerobic organism such as W. succinogenes has hitherto been unknown. We have established the first genetic system for the manipulation and production of a member of the non-classical succinate:quinone oxidoreductase family. Biochemical characterization of the W. succinogenes enzyme reveals that the putative SQR is in fact a novel methylmenaquinol:fumarate reductase (MFR) with no detectable succinate oxidation activity, clearly indicative of its involvement in anaerobic metabolism. We demonstrate that the hydrophilic subunits of the MFR complex are, in contrast to all other previously characterized members of the superfamily, exported into the periplasm via the twin-arginine translocation (tat)-pathway. Furthermore we show that a single amino acid exchange (Ala86→His) in the flavoprotein of that enzyme complex is the only additional requirement for the covalent binding of the otherwise non-covalently bound FAD. Our results provide an explanation for the previously published puzzling observation that the C. jejuni sdhABE operon is upregulated in an oxygen-limited environment as compared with microaerophilic laboratory conditions.

Mechanism of the antioxidant to pro-oxidant switch in the behavior of dehydroascorbate during LDL oxidation by copper(II) ions

Oxidised low density lipoprotein (LDL) may be involved in the pathogenesis of atherosclerosis. We have therefore investigated the mechanisms underlying the antioxidant/pro-oxidant behavior of dehydroascorbate, the oxidation product of ascorbic acid, toward LDL incubated with Cu(2+) ions. By monitoring lipid peroxidation through the formation of conjugated dienes and lipid hydroperoxides, we show that the pro-oxidant activity of dehydroascorbate is critically dependent on the presence of lipid hydroperoxides, which accumulate during the early stages of oxidation. Using electron paramagnetic resonance spectroscopy, we show that dehydroascorbate amplifies the generation of alkoxyl radicals during the interaction of copper ions with the model alkyl hydroperoxide, tert-butylhydroperoxide. Under continuous-flow conditions, a prominent doublet signal was detected, which we attribute to both the erythroascorbate and ascorbate free radicals. On this basis, we propose that the pro-oxidant activity of dehydroascorbate toward LDL is due to its known spontaneous interconversion to erythroascorbate and ascorbate, which reduce Cu(2+) to Cu(+) and thereby promote the decomposition of lipid hydroperoxides. Various mechanisms, including copper chelation and Cu(+) oxidation, are suggested to underlie the antioxidant behavior of dehydroascorbate in LDL that is essentially free of lipid hydroperoxides.

The oxidizing power of the glutathione thiyl radical as measured by its electrode potential at physiological pH

The oxidizing power of the thiyl radical (GS*) produced on oxidation of glutathione (GSH) was determined as the mid-point electrode potential (reduction potential) of the one-electron couple E(m)(GS*,H+/GSH) in water, as a function of pH over the physiological range. The method involved measuring the equilibrium constants for electron-transfer equilibria with aniline or phenothiazine redox indicators of known electrode potential. Thiyl and indicator radicals were generated in microseconds by pulse radiolysis, and the position of equilibrium measured by fast kinetic spectrophotometry. The electrode potential E(m)(GS*,H+/GSH) showed the expected decrease by approximately 0.06 V/pH as pH was increased from approximately 6 to 8, reflecting thiol/thiolate dissociation and yielding a value of the reduction potential of GS*=0.92+/-0.03 V at pH 7.4. An apparently almost invariant potential between pH approximately 3 and 6, with potentials significantly lower than expected, is ascribed at least in part to errors arising from radical decay during the approach to the redox equilibrium and slow electron transfer of thiol compared to thiolate.

Chemical radiosensitizers for use in radiotherapy

Radiosensitizers are intended to enhance tumour cell killing while having much less effect on normal tissues. Some drugs target different physiological characteristics of the tumour, particularly hypoxia associated with radioresistance. Oxygen is the definitive hypoxic cell radiosensitizer, the large differential radiosensitivity of oxic vs hypoxic cells being an attractive factor. The combination of nicotinamide to reduce acute hypoxia with normobaric carbogen breathing is showing clinical promise. 'Electron-affinic' chemicals that react with DNA free radicals have the potential for universal activity to combat hypoxia-associated radioresistance; a nitroimidazole, nimorazole, is clinically effective at tolerable doses. Hypoxia-specific cytotoxins, such as tirapazamine, are valuable adjuncts to radiotherapy. Nitric oxide is a potent hypoxic cell radiosensitizer; variations in endogenous levels might have prognostic significance, and routes to deliver nitric oxide specifically to tumours are being developed. In principle, many drugs can be delivered selectively to hypoxic tumours using either reductase enzymes or radiation-produced free radicals to activate drug release from electron-affinic prodrugs. A redox-active agent based on a gadolinium chelate is being evaluated clinically. Pyrimidines substituted with bromine or iodine are incorporated into DNA and enhance free radical damage; fluoropyrimidines act by different mechanisms. A wide variety of drugs that influence the nature or repair of DNA damage are being evaluated in conjunction with radiation; it is often difficult to define the mechanisms underlying chemoradiation regimens. Drugs being evaluated include topoisomerase inhibitors (e.g. camptothecin, topotecan), and the hypoxia-activated anthraquinone AQ4N; alkylating agents include temozolomide. Drugs involved in DNA repair pathways being investigated include the potent poly(ADP ribose)polymerase inhibitor, AG14,361. Proteins involved in cell signalling, such as the Ras family, are attractive targets linked to radioresistance, as are epidermal growth factor receptors and linked kinases (drugs including vandetanib [ZD6,474], cetuximab and gefitinib), and cyclooxygenase-2 (celecoxib). The suppression of radioprotective thiols seems to offer more potential with alkylating agents than with radiotherapy, although it remains a strategy worthy of exploration.

Reaction pathways and mechanisms of photodegradation of pesticides

The photodegradation of pesticides is reviewed, with particular reference to the studies that describe the mechanisms of the processes involved, the nature of reactive intermediates and final products. Potential use of photochemical processes in advanced oxidation methods for water treatment is also discussed. Processes considered include direct photolysis leading to homolysis or heterolysis of the pesticide, photosensitized photodegradation by singlet oxygen and a variety of metal complexes, photolysis in heterogeneous media and degradation by reaction with intermediates generated by photolytic or radiolytic means.

The reaction of superoxide radicals with S-nitrosoglutathione and the products of its reductive heterolysis

Generation of superoxide radicals (0.01-0.1 microm s(-1)) by radiolysis of aqueous solutions containing S-nitrosoglutathione (45-160 microm, pH 3.8-7.3) resulted in loss of this solute at rates varying with solute concentration, radical generation rate, and pH. The results were quantitatively consistent with the loss being attributed to competition between reaction of superoxide with S-nitrosoglutathione (rate constant 300 +/- 100 m(-1) s(-1)) and the pH-dependent disproportionation of superoxide/hydroperoxyl. This rate constant is much lower than previous estimates and seven orders of magnitude lower than the rate constants between superoxide and superoxide dismutase or superoxide and nitric oxide. This indicates that interaction between superoxide and S-nitrosoglutathione is unlikely to be biologically important, contrary to previous suggestions that reaction could serve to prevent the rapid reaction between superoxide and nitric oxide. Reductive homolysis of S-nitrosoglutathione by the carbon dioxide radical anion, a model for biological reductants such as disulfide radical anions, occurred with a rate constant of 7.4 x 10(8) m(-1) s(-1) and produced nitric oxide stoichiometrically. Thiyl radicals were not produced, indicating the alternative homolysis route to generate nitroxyl did not occur.

Modifying the in vitro accumulation of BMS181321, a technetium-99m-nitroimidazole, with unlabelled nitroaromatics

BMS181321, [99mTc]oxo[[3,3,9,9-tetramethyl-1-(2-nitro-1H-imidazol-1-yl)-4,8-diaz aundecane-2,10-dione dioximato]-(3)-N,N',N",N"']technetium, is a 99mTc-nitroimidazole that is being investigated as a hypoxic marker in tumors. Due to the high specific activity of 99mTc, the concentration of BMS181321 used in its applications is very low. Metabolic depletion and non-specific binding of the drug may limit its ability to fully map out hypoxic regions. An attempt has been made to modify the in vitro accumulation of BMS181321 in hypoxic Chinese hamster ovary (CHO) cells with unlabelled nitroaromatics. The 2-nitroimidazole etanidazole (0.08 to 8 mM) caused a concentration-dependent decrease in BMS181321 accumulation to 70-28% and metabolism to 70-40% of the control level in hypoxic cells at 4 hr. In contrast, the 5-nitroimidazole tinidazole (0.09 to 9 mM) caused a concentration-dependent increase in BMS181321 accumulation to 110-170% and metabolism to 100-150% of the control level in hypoxic cells at 4 hr. Nitroaromatics with an electron affinity similar to or greater than that of BMS181321 inhibited its accumulation and metabolism, and 5-nitroimidazoles, which have an electron affinity lower than that of BMS181321, enhanced its accumulation and metabolism. The enhanced accumulation with the addition of metronidazole was not observed in the presence of low oxygen levels or of a nitrofuran of higher electron affinity than BMS181321. These results suggest that a competition for reducing equivalents and/or for the BMS181321 radical anion itself can occur in cells, leading to the inhibition of BMS181321 reduction in the presence of nitroaromatics of similar or greater electron affinity. A transfer of electrons from the radical anion form of the reduced 5-nitroimidazole to the more electron affinic BMS181321 compound may occur, causing increased hypoxic accumulation of BMS181321.

Hypoxia-selective antitumor agents. 15. Modification of rate of nitroreduction and extent of lysosomal uptake by polysubstitution of 4-(alkylamino)-5-nitroquinoline bioreductive drugs

Studies have shown that 4-(alkylamino)-5-nitroquinolines possess high selectivity (20-60-fold) for hypoxic tumor cells in vitro, but are not active as hypoxia-selective cytotoxins (HSCs) in vivo. The compounds show inadequate rates of extravascular diffusion, likely due both to sequestration of the bisbasic compounds into lysosomes and rapid nitroreduction. A further series of analogues, designed to counteract these limitations, has been synthesized and evaluated. Analogues bearing one to three electron-donating substituents on the quinoline have one-electron reduction potentials up to 100 mV lower than that of the unsubstituted compound (5), but do not have improved biological activity. The relationship between hypoxic selectivity and rates of metabolic reduction suggests at least two mechanisms of cytotoxicity for this series of 5-nitroquinolines. Compounds with high rates of reduction are toxic via oxygen-sensitive net bioreduction, while compounds which are poor substrates for nitroreduction are toxic through an oxygen-insensitive non-bioreductive mechanism. As rates of metabolic reduction are lowered, the non-bioreductive mechanism of toxicity becomes dominant and hypoxic selectivity is lost. A small series of analogues bearing hydrophilic but neutral side chains were also prepared. Compounds with a dihydroxypropyl side chain retained cytotoxic potency and hypoxic cell selectivity in cell culture assays, and had lowered uptake into lysosomes, but none of three analogues evaluated against KHT tumors in mice showed activity as an HSC in vivo.

The one-electron reduction potential of 3-amino-1,2,4-benzotriazine 1,4-dioxide (tirapazamine): a hypoxia-selective bioreductive drug

The one-electron reduction potential of 3-amino-1,2,4-benzotriazine 1,4-dioxide, tirapazamine (SR 4233) in aqueous solution has been determined by pulse radiolysis. Reversible electron transfer was achieved between radiolytically-generated one-electron reduced radicals of tirapazamine (T), and quinones or benzyl viologen as redox standards. The reduction potential Em7(T/T-) was -0.45 +/- 0.01 V vs. NHE at pH 7 From the pH dependence of the reduction potential, pKa = 5.6 +/- 0.2 was estimated for the tirapazamine radical, a value similar to the pKa determined by other methods.

Efficient redox cycling of nitroquinoline bioreductive drugs due to aerobic nitroreduction in Chinese hamster cells

Nitroquinoline bioreductive drugs with 4-alkylamino substituents undergo one-electron reduction in mammalian cells, resulting in futile redox cycling due to oxidation of the nitro radical anion in aerobic cultures, and eventual reduction to the corresponding amines in the absence of oxygen. Rates of drug-induced oxygen consumption (R) due to redox cycling in cyanide-treated AA8 cell cultures were determined for 17 nitroquinolines. There was a linear dependence of log R on the one-electron reduction potential at pH 7 (E(7)1 with a slope of 7.1 V-1, excluding compounds with substituents ortho to the nitro group. The latter had anomalously low rates of oxygen consumption relative to E(7)1, suggesting that interaction with the active site of nitroreductases is impeded sterically for such compounds. Absolute values of R (and the observed E(7)1 dependence) were well predicted by a simple kinetic model that used rates of net nitroreduction to the amines under anoxia as a measure of the rates of one-electron reduction in aerobic cells. This indicates that redox cycling of 4-alkylaminonitroquinolines occurs at high efficiency in aerobic cells, suggesting that there are no quantitatively significant fates of nitro radical anions in cells other than their reaction with oxygen (or their spontaneous disproportionation under hypoxia).

Temperature effects on the one-electron reduction potentials of nitroaryl compounds

Pulse radiolysis was used to establish one-electron transfer equilibria between radical cations of methyl or benzyl viologens (V2+) and nitroaryl compounds (ArNO2): a nitroimidazole (misonidazole or metronidazole), 4-nitrobenzoate or nitrofurazone. The equilibrium constants in water at pH 8 were estimated over the temperature range approximately 5 to 75 degrees C. The difference delta E in mid-point one-electron reduction potentials between the nitro compounds and the viologens varied with temperature T; increasing temperature made the nitro compounds apparently less electron-affinic compared to the effects of temperature on the viologen potential. Values of delta(delta E)/delta T were in the range -0.7 to -1.1 mV K-1 at 25 degrees C. If delta[E(V2+/V.+)]/delta T = -0.9 mV K-1 for methyl viologen then delta[E(ArNO2/ArNO2.-)]/delta T is about -2 mV K-1 for these compounds.

Effects of superoxide dismutase on the autoxidation of 1,4-hydroquinone

During autoxidation of 1,4-hydroquinone (H2Q, less than 1 mM) at pH 7.4 and 37 degrees C, stoichiometric amounts of 1,4-benzoquinone (Q) and hydrogen peroxide were formed during the initial reaction. The reaction kinetics showed a significant induction period which was abolished by minute amounts of Q. Hydrogen peroxide and catalase were without effect on the autoxidation process. Transition metals apparently were not involved, since chelators like EDTA, DETAPAC, and desferrioxamine or FeSO4 had no influence on the autoxidation kinetics. Superoxide dismutase (SOD) did not abolish the induction period but dramatically enhanced the autoxidation rate by more than two orders of magnitude. The stimulatory effect was first-order in SOD concentration but showed saturation kinetics. The dependence of Q and hydrogen peroxide formation rates on H2Q concentration shows a biphasic behaviour: dependence on the square at low H2Q, but on the square root at high H2Q concentration. As revealed by calculatory simulations the results can be adequately described by the known reaction rate constants. The reaction starts with the comproportionation of H2Q and Q to yield two semiquinone molecules which autoxidize to give two superoxide radicals and two molecules of Q which enter into a new cycle of comproportionation. Because of unfavourable equilibria the autocatalytic reaction soon comes to steady state, and the further reaction is governed by the rate of superoxide removal. At excess SOD, the comproportionation reaction is rate-limiting, thus explaining the saturation effects of SOD. The experiments do not allow a decision between the two functions of SOD; the conventional action as a superoxide:superoxide oxidoreductase or as a semiquinone:superoxide oxidoreductase. In the latter reaction SOD is thought to be reduced by semiquinone with Q formation. In the second step the reduced enzyme would be re-oxidized by a superoxide radical which is formed during autoxidation of the second semiquinone molecule generated in the comproportionation reaction. From thermodynamic considerations, the latter function of SOD appears to be plausible.