Correlation of Kinetic Parameters of Nitroreductase Enzymes with Redox Properties of Nitroaromatic Compounds

Metabolic activation of 2,4,6-trinitrotoluene; a case for ROS-induced cell damage

The explosive compound 2,4,6-trinitrotoluene (TNT) is well known as a major component of munitions. In addition to its potential carcinogenicity and mutagenicity in humans, recent reports have highlighted TNT toxicities in diverse organisms due to its occurrence in the environment. These toxic effects have been linked to the intracellular metabolism of TNT, which is generally characterised by redox cycling and the generation of noxious reactive molecules. The reactive intermediates formed, such as nitroso and hydroxylamine compounds, also interact with oxygen molecules and cellular components to cause macromolecular damage and oxidative stress. The current review aims to highlight the crucial role of TNT metabolism in mediating TNT toxicity, via increased generation of reactive oxygen species. Cellular proliferation of reactive species results in depletion of cellular antioxidant enzymes, DNA and protein adduct formation, and oxidative stress. While TNT toxicity is well known, its ability to induce oxidative stress, resulting from its reductive activation, suggests that some of its toxic effects may be caused by its reactive metabolites. Hence, further research on TNT metabolism is imperative to elucidate TNT-induced toxicities.

Enzymatic Redox Properties and Cytotoxicity of Irreversible Nitroaromatic Thioredoxin Reductase Inhibitors in Mammalian Cells

NADPH:thioredoxin reductase (TrxR) is considered a potential target for anticancer agents. Several nitroheterocyclic sulfones, such as Stattic and Tri-1, irreversibly inhibit TrxR, which presumably accounts for their antitumor activity. However, it is necessary to distinguish the roles of enzymatic redox cycling, an inherent property of nitroaromatics (ArNO2), and the inhibition of TrxR in their cytotoxicity. In this study, we calculated the previously unavailable values of single-electron reduction potentials of known inhibitors of TrxR (Stattic, Tri-1, and 1-chloro-2,4-dinitrobenzene (CDNB)) and inhibitors identified (nitrofuran NSC697923 and nitrobenzene BTB06584). These calculations were according to the rates of their enzymatic single-electron reduction (PMID: 34098820). This enabled us to compare their cytotoxicity with that of model redox cycling ArNO2. In MH22a and HCT-116 cells, Tri-1, Stattic, CDNB, and NSC697023 possessed at least 10-fold greater cytotoxicity than can be expected from their redox cycling activity. This may be related to TrxR inhibition. The absence of enhanced cytotoxicity in BTB06548 may be attributed to its instability. Another known inhibitor of TrxR, tetryl, also did not possess enhanced cytotoxicity, probably because of its detoxification by DT-diaphorase (NQO1). Apart from the reactions with NQO1, the additional mechanisms influencing the cytotoxicity of the examined inhibitors of TrxR are their reactions with cytochromes P-450. Furthermore, some inhibitors, such as Stattic and NSC697923, may also inhibit glutathione reductase. We suggest that these data may be instrumental in the search for TrxR inhibitors with enhanced cytotoxic/anticancer activity.

Modified quantum mechanical approach for the estimation of single-electron reduction potential of nitroaromatic compounds in aqueous medium

The midpoint single-electron reduction potential of nitroaromatic compounds in aqueous medium at pH 7.0 (potential of ArNO₂/ArNO₂·^- couple, E_m7) frequently determines their therapeutic and/or toxic properties. However, its estimation remains a complex problem. We propose a modified method of E_m7 estimation by quantum mechanical calculations, based on the use of the dielectric continuum model together with a certain number of H₂O molecules at the vicinity of nitro group. The optimal number of H₂O molecules corresponds to a minimal difference between the experimentally determined and calculated values of E_m7, and/or the most negative value of calculated E_m7. This enabled us to calculate the E_m7 values for a number of ArNO₂ (n = 19) with the average deviation of 0.027 V from the experimentally determined ones. Apart from nitrobenzene derivatives, the application of this approach for the representatives of nitropyridines, nitrofurans, nitrothiophenes, and nitrothiazoles was demonstrated. In this case, nitroimidazole derivatives are an exception, evidently due to a strong proton accepting properties of N3 atom of their free radicals.

Major benznidazole metabolites in patients treated for Chagas disease: Mass spectrometry-based identification, structural analysis and detoxification pathways

Benznidazole is the drug of choice for the treatment of Chagas disease, but its metabolism in humans is unclear. Here, we identified and characterized the major benznidazole metabolites and their biosynthetic mechanisms in humans by analyzing the ionic profiles of urine samples from patients and untreated donors through reversed-phase UHPLC-ESI-QTOF-MS and UHPLC-ESI-QqLIT-MS. A strategy for simultaneous detection and fragmentation of characteristic positive and negative ions was employed using information-dependent acquisitions (IDA). Selected precursor ions, neutral losses, and MS³ experiments complemented the study. A total of six phase-I and ten phase-II metabolites were identified and structurally characterized in urine of benznidazole-treated patients. Based on creatinine-corrected ion intensities, nitroreduction to amino-benznidazole (M1) and its subsequent N-glucuronidation to M5 were the main metabolic pathways, followed by imidazole-ring cleavage, oxidations, and cysteine conjugations. This extensive exploration of benznidazole metabolites revealed potentially toxic structures in the form of glucuronides and glutathione derivatives, which may be associated with recurrent treatment adverse events; this possibility warrants further exploration in future clinical trials. Incorporation of this knowledge of the benznidazole metabolic profile into clinical pharmacology trials could lead to improved treatments, facilitate the study of possible drug-drug interactions, and even mitigation of adverse drug reactions.

Single- and Two-Electron Reduction of Nitroaromatic Compounds by Flavoenzymes: Mechanisms and Implications for Cytotoxicity

Nitroaromatic compounds (ArNO₂) maintain their importance in relation to industrial processes, environmental pollution, and pharmaceutical application. The manifestation of toxicity/therapeutic action of nitroaromatics may involve their single- or two-electron reduction performed by various flavoenzymes and/or their physiological redox partners, metalloproteins. The pivotal and still incompletely resolved questions in this area are the identification and characterization of the specific enzymes that are involved in the bioreduction of ArNO₂ and the establishment of their contribution to cytotoxic/therapeutic action of nitroaromatics. This review addresses the following topics: (i) the intrinsic redox properties of ArNO₂, in particular, the energetics of their single- and two-electron reduction in aqueous medium; (ii) the mechanisms and structure-activity relationships of reduction in ArNO₂ by flavoenzymes of different groups, dehydrogenases-electrontransferases (NADPH:cytochrome P-450 reductase, ferredoxin:NADP(H) oxidoreductase and their analogs), mammalian NAD(P)H:quinone oxidoreductase, bacterial nitroreductases, and disulfide reductases of different origin (glutathione, trypanothione, and thioredoxin reductases, lipoamide dehydrogenase), and (iii) the relationships between the enzymatic reactivity of compounds and their activity in mammalian cells, bacteria, and parasites.

Nitroimidazoles as hypoxic cell radiosensitizers and hypoxia probes: misonidazole, myths and mistakes

Nitroimidazoles have been extensively explored as hypoxic cell radiosensitizers but have had limited clinical success, with efficacy restricted by toxicity. However, they have proven clinically useful as probes for tumour hypoxia. Both applications, and probably much of the dose-limiting toxicities, reflect the dominant chemical property of electron affinity or ease of reduction, associated with the nitro substituent in an aromatic structure. This single dominant property affords unusual, indeed extraordinary flexibility in drug or probe design, suggesting further development is possible in spite of earlier limitations, in particular building on the benefit of hindsight and an appreciation of errors made in earlier studies. The most notable errors were: the delay in viewing cellular thiol depletion as a likely common artefact in testing in vitro; slow recognition of pH-driven concentration gradients when compounds were weak acids and bases; and a failure to explore the possible involvement of pH and ascorbate in influencing hypoxia probe binding. The experience points to the need to involve a wider range of expertise than that historically involved in many laboratories when studying the effects of chemicals on radiation response or using diagnostic probes.

The study of NADPH-dependent flavoenzyme-catalyzed reduction of benzo[1,2-c]1,2,5-oxadiazole N-oxides (benzofuroxans)

The enzymatic reactivity of a series of benzo[1,2-c]1,2,5-oxadiazole N-oxides (benzofuroxans; BFXs) towards mammalian single-electron transferring NADPH:cytochrome P-450 reductase (P-450R) and two-electron (hydride) transferring

NAD(P)H

quinone oxidoreductase (NQO1) was examined in this work. Since the =N+ (→O)O- moiety of furoxan fragments of BFXs bears some similarity to the aromatic nitro-group, the reactivity of BFXs was compared to that of nitro-aromatic compounds (NACs) whose reduction mechanisms by these and other related flavoenzymes have been extensively investigated. The reduction of BFXs by both P-450R and NQO1 was accompanied by O2 uptake, which was much lower than the NADPH oxidation rate; except for annelated BFXs, whose reduction was followed by the production of peroxide. In order to analyze the possible quantitative structure-activity relationships (QSARs) of the enzymatic reactivity of the compounds, their electron-accepting potency and other reactivity indices were assessed by quantum mechanical methods. In P-450R-catalyzed reactions, both BFXs and NACs showed the same reactivity dependence on their electron-accepting potency which might be consistent with an "outer sphere" electron transfer mechanism. In NQO1-catalyzed two-electron (hydride) transferring reactions, BFXs acted as more efficient substrates than NACs, and the reduction efficacy of BFXs by NQO1 was in general higher than by single-electron transferring P-450R. In NQO1-catalyzed reactions, QSARs obtained showed that the reduction efficacy of BFXs, as well as that of NACs, was determined by their electron-accepting potency and could be influenced by their binding mode in the active center of NQO1 and by their global softness as their electronic characteristic. The reductive conversion of benzofuroxan by both flavoenzymes yielded the same reduction product of benzofuroxan, 2,3-diaminophenazine, with the formation of o-benzoquinone dioxime as a putative primary reductive intermediate, which undergoes a further reduction process. Overall, the data obtained show that by contrast to NACs, the flavoenzyme-catalyzed reduction of BFXs is unlikely to initiate their redox-cycling, which may argue for a minor role of the redox-cycling-type action in the cytotoxicity of BFXs.

Chalcones and N-acylhydrazones: direct analogues? Exploratory data analysis applied to potential novel antileishmanial agents

Leishmaniasis is an important health and social problem for which there is limited effective therapy. Chalcones and N-acylhydrazones have been studied as promising antileishmanial agents in enzymatic inhibition and in vitro assays. Since these chemical classes of compounds also resemble each other structurally, it would be useful to investigate whether they share direct analogy. Exploratory data analysis was applied to a library of chalcones and nitrated N-acylhydrazones assayed against Leishmania donovani to investigate their similarity. Under the conditions applied in the present study, the two classes did not present functional or structural analogy.

Significance of nitroimidazole compounds and hypoxia-inducible factor-1 for imaging tumor hypoxia

A tumor-specific microenvironment is characterized by hypoxia, in which oxygen tension is considerably lower than in normal tissues. The hypoxic status of various solid tumors has been attributed as an indicator of adverse prognosis due to tumor progression toward a more malignant phenotype with increased metastatic potential and resistance to treatment. Various exogenous and endogenous markers for hypoxia are currently available and studied in relation to each other, tumor architecture, and tumor microenvironment. Over the last few decades, various methods have been suggested to assess the level of oxygenation in solid tumors. Among them, nitroimidazole compounds have provided promising information on tumor hypoxia. To quantify the extent of hypoxia requires that nitroimidazole binding be primarily dependent on oxygen concentration as well as nitroreductase levels in the tumor cells. Furthermore, recent progress in molecular biology has highlighted a transcription factor, hypoxia-inducible factor (HIF)-1, whose activity is induced by hypoxia. HIF-1 plays a central role in malignant progression by inducing the expression of various genes, whose functions are strongly associated with malignant alteration of the entire tumor. The cellular changes induced by HIF-1 are extremely important therapeutic targets of cancer therapy, particularly in the therapy against refractory cancers. In this review, we will discuss the significance of pimonidazole and HIF-1 as exogenous and endogenous hypoxia markers, respectively, as well as their evaluation and imaging of tumor hypoxia.

Radiation chemistry comes before radiation biology

PURPOSE

This article seeks to illustrate some contributions of radiation chemistry to radiobiology and related science, and to draw attention to examples where radiation chemistry is central to our knowledge of specific aspects. Radiation chemistry is a mature branch of radiation science which is continually evolving and finding wider applications. This is particularly apparent in the study of the roles of free radicals in biology generally, and radiation biology specifically. The chemical viewpoint helps unite the spatial and temporal insight coming from radiation physics with the diversity of biological responses. While historically, the main application of radiation chemistry of relevance to radiation biology has been investigations of the free-radical processes leading to radiation-induced DNA damage and its chemical characterization, two features of radiation chemistry point to its wider importance. First, its emphasis on quantification and characterization at the molecular level helps provide links between DNA damage, biochemical repair processes, and mutagenicity and radiosensitivity. Second, its central pillar of chemical kinetics aids understanding of the roles of 'reactive oxygen species' in cell signalling and diverse biological effects more generally, and application of radiation chemistry in the development of drugs to enhance radiotherapy and as hypoxia-specific cytotoxins or diagnostic agents. The illustrations of the broader applications of radiation chemistry in this article focus on their relevance to radiation biology and demonstrate the importance of synergy in the radiation sciences.

CONCLUSIONS

The past contributions of radiation chemistry to radiation biology are evident, but there remains considerable potential to help advance future biological understanding using the knowledge and techniques of radiation chemistry.

Characterization of Escherichia coli nitroreductase NfsB in the metabolism of nitrobenzodiazepines

Nitrobenzodiazepine (NBDZ) is a sedative-hypnotic drug used in the treatment of anxiety and sleep problems. Overdose of NBDZ may cause severe neurological effects, especially for people in drug abuse or addiction. In the present study, we investigated NBDZ nitroreduction in rat enteric contents and characterized the role of enterobacterial nitroreductase in the reductive pathway. Nitroreduction of flunitrazepam (FZ) was studied in the microsomal membrane fractions of rat liver, jejunum and jejunal microflora using HPLC analysis. In the jejunal microflora, FZ was demonstrated to be significantly reduced to its amino derivative under anaerobic condition. Escherichia coli type I nitroreductase NfsB (EC 1.5.1.34) was found in rat jejunal microflora via PCR technique and Western blotting. The participation of NfsB in FZ nitroreduction was demonstrated from inhibition studies. Kinetic study of the purified recombinant NfsB indicated that nitroreduction of FZ, nitrazepam (NZ) and clonazepam (CZ) are mediated by NfsB, where CZ shows lower k(cat)/K(M) ratio than that of the other two. Finally, two other nitroreductases E. cloacae NR (EC 1.6.99.7) and S. typhimurium Cnr were also found to be responsible for FZ nitroreduction. These results provide that the reduction of NBDZ in normal flora is catalyzed by type I nitroreductase NfsB.

Reduction of polynitroaromatic compounds: the bacterial nitroreductases

Most nitroaromatic compounds are toxic and mutagenic for living organisms, but some microorganisms have developed oxidative or reductive pathways to degrade or transform these compounds. Reductive pathways are based either on the reduction of the aromatic ring by hydride additions or on the reduction of the nitro groups to hydroxylamino and/or amino derivatives. Bacterial nitroreductases are flavoenzymes that catalyze the NAD(P)H-dependent reduction of the nitro groups on nitroaromatic and nitroheterocyclic compounds. Nitroreductases have raised a great interest due to their potential applications in bioremediation, biocatalysis, and biomedicine, especially in prodrug activation for chemotherapeutic cancer treatments. Different bacterial nitroreductases have been purified and their biochemical and kinetic parameters have been determined. The crystal structure of some nitroreductases have also been solved. However, the physiological role(s) of these enzymes remains unclear. Nitroreductase genes are widely spread within bacterial genomes, but are also found in archaea and some eukaryotic species. Although studies on regulation of nitroreductase gene expression are scarce, it seems that nitroreductase genes may be controlled by the MarRA and SoxRS regulatory systems that are involved in responses to several antibiotics and environmental chemical hazards and to specific oxidative stress conditions. This review covers the microbial distribution, types, biochemical properties, structure and regulation of the bacterial nitroreductases. The possible physiological functions and the biotechnological applications of these enzymes are also discussed.

Radical properties governing the hypoxia-selective cytotoxicity of antitumor 3-amino-1,2,4-benzotriazine 1,4-dioxides

Revealing the free radical mechanism by which the anticancer drug tirapazamine (3-amino-1,2,4-benzotriazine 1,4-dioxide) induces hypoxia-selective cytotoxicity, is seen as a way forward to develop clinically useful bioreductive drugs against chemo- and radiation-resistant hypoxic tumor cells. Our previous studies point to the formation of an active benzotriazinyl radical following the one-electron reduction of tirapazamine and its elimination of water from the initial reduction intermediate, and have suggested that this species is a cytotoxin. In this paper we have used pulse radiolysis to measure the one-electron reduction potentials of the benzotriazinyl radicals E(B*,H(+)/B) of 30 analogues of tirapazamine as well as the one-electron reduction potentials of their two-electron reduced metabolites, benzotriazine 1-oxides E(B/B*-). The redox dependencies of the back-oxidation of the one-electron reduced benzotriazine 1,4-dioxides by oxygen, their radical prototropic properties and water elimination reactions were found to be tracked in the main by the one-electron reduction potentials of the benzotriazine 1,4-dioxides E(A/A*-). Multiple regression analysis of published aerobic and hypoxic clonogenic cytotoxicity data for the SCCVII murine tumor cell line with the physical chemistry parameters measured in this study, revealed that hypoxic cytotoxicity is dependent on E(B*, H(+)/B) thus providing strong evidence that the benzotriazinyl radicals are the active cytotoxic species in hypoxia, while aerobic cytotoxicity is dependent on E(B/B*-). It is concluded that maximizing the differential ratio between these two controlling parameters, in combination with necessary pharmacological aspects, will lead to more efficacious anticancer bioreductive drugs.

Liver microsomal biotransformation of nitro-aryl drugs: mechanism for potential oxidative stress induction

Toxic effects of several nitro-aryl drugs are attributed to the nitro-reduction that may be suffered in vivo, a reaction that may be catalysed by different reductases. One of these enzymes is NADPH-cytochrome P450 reductase, which belongs to the cytochrome P450 oxidative system mainly localized in the endoplasmic reticulum of the hepatic cell. This system is responsible for the biotransformation of oxidative lipophilic compounds, so that oxidative and reductive metabolic pathways of lipophilic nitro-aryl drugs can take place simultaneously. Because of the affinity of nitro-aryl drugs (xenobiotics) for the endoplasmic reticulum, we propose this subcellular organelle as a good biological system for investigating the toxicity induced by the biotransformation of these or another compounds. In this work we used rat liver microsomes to assess the oxidative stress induced by nitro-aryl drug biotransformation. Incubation of microsomes of rat liver with nifurtimox and nitrofurantoin in the presence of NADPH induced lipoperoxidation, UDP-glucuronyltransferase activation and an increase in the basal microsomal oxygen consumption. Nitro-aryl-1,4-dihydropyridines did not elicit these prooxidant effects; furthermore, they inhibited lipoperoxidation and oxygen consumption induced by Fe3+/ascorbate. Nifurtimox and nitrofurantoin modified the maximum absorption of cytochrome P450 oxidase and inhibited p-nitroanisole O-demethylation, an oxidative reaction catalysed by the cytochrome P450 system, signifying that oxidation may proceed in a similar way to that described for nitro-aryl-1,4-dihydropyridines. Thus the balance between lipophilic nitro-aryl drug oxidation and reduction may be involved in the potential oxidative stress induced by biotransformation.

Nitroreductase catalyzed biotransformation of CL-20

Previously, we reported that a salicylate 1-monooxygenase from Pseudomonas sp. ATCC 29352 biotransformed CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaaza-isowurtzitane) (C(6)H(6)N(12)O(12)) and produced a key metabolite with mol. wt. 346 Da corresponding to an empirical formula of C(6)H(6)N(10)O(8) which spontaneously decomposed in aqueous medium to produce N(2)O, NH(4)(+), and HCOOH [Appl. Environ. Microbiol. (2004)]. In the present study, we found that nitroreductase from Escherichia coli catalyzed a one-electron transfer to CL-20 to form a radical anion (CL-20(-)) which upon initial N-denitration also produced metabolite C(6)H(6)N(10)O(8). The latter was tentatively identified as 1,4,5,8-tetranitro-1,3a,4,4a,5,7a,8,8a-octahydro-diimidazo[4,5-b:4',5'-e]pyrazine [IUPAC] which decomposed spontaneously in water to produce glyoxal (OHCCHO) and formic acid (HCOOH). The rates of CL-20 biotransformation under anaerobic and aerobic conditions were 3.4+/-0.2 and 0.25+/-0.01 nmol min(-1)mg of protein(-1), respectively. The product stoichiometry showed that each reacted CL-20 molecule produced about 1.8 nitrite ions, 3.3 molecules of nitrous oxide, 1.6 molecules of formic acid, 1.0 molecule of glyoxal, and 1.3 ammonium ions. Carbon and nitrogen products gave mass-balances of 60% and 81%, respectively. A comparative study between native-, deflavo-, and reconstituted-nitroreductase showed that FMN-site was possibly involved in the biotransformation of CL-20.

Reductive activation and thiol reactivity of benzazolo[3,2-a]quinolinium salts

Derivatives of benzazolo[3,2-a]quinolium salts (QSDs) are reductively activated by the enzymatic reducing agents hypoxanthine (or xanthine)/xanthine oxidase and NADH dehydrogenase as evidenced by the increase in rates of ferricytochrome c (Cyt(III)c) reduction and oxygen consumption, respectively. No correlation between Michaelis-Menten parameters and QSDs redox potentials was found regarding anaerobic or aerobic Cyt(III)c reduction, although maximum rates were observed for nitro-containing QSDs. However, oxygen consumption rates correlate with QSDs redox potentials when NADH dehydrogenase is used as reducing agent. QSDs bind covalently to bovine serum albumin (BSA) under anaerobic conditions, in the presence, and less in the absence, of HX/XO and only if the nitro group is present at the QSD. QSDs react with glutathione (GSH) in the presence of HX/XO but not in its absence, under anaerobic conditions. The amount of reacted GSH increases, and the relative amount of GSSG formed decreases, with an increase in the QSD reduction potential, thus indicating that GSH reacts with reduced nitro-containing QSDs mainly in a manner which does not involve the production of GSSG, presumably, through the formation of the nitroso-QSD-GSH conjugate. QSDs are, thus, novel nitro-containing heterocyclic compounds which could be bioreductively activated to react with oxygen and thiols.

Flavoenzyme-catalyzed redox cycling of hydroxylamino- and amino metabolites of 2,4,6-trinitrotoluene: implications for their cytotoxicity

The toxicity of 2,4,6-trinitrotoluene (TNT), a widespread environmental contaminant, is exerted through its enzymatic redox cycling and/or covalent binding of its reduction products to proteins and DNA. In this study, we examined the possibility of another cytotoxicity mechanism of the amino- and hydroxylamino metabolites of TNT, their flavoenzyme-catalyzed redox cycling. The above compounds acted as redox-cycling substrates for single-electron transferring NADPH:cytochrome P-450 reductase (P-450R) and ferredoxin:NADP(+) reductase (FNR), as well as substrates for the two-electron transferring flavoenzymes rat liver NAD(P)H:quinone oxidoreductase (NQO1) and Enterobacter cloacae NAD(P)H:nitroreductase (NR). Their reactivity in P-450R-, FNR-, and NR-catalyzed reactions increased with an increase in their single-electron reduction potential (E(1)(7)) or the decrease in the enthalpy of free radical formation. The cytotoxicity of the amino- and hydroxylamino metabolites of TNT towards bovine leukemia virus-transformed lamb kidney fibroblasts (line FLK) was partly prevented by the antioxidant N,N'-diphenyl-p-phenylene diamine and desferrioxamine, and potentiated by 1,3-bis-(2-chloroethyl)-1-nitrosourea, thus pointing to the involvement of oxidative stress. In general, their cytotoxicity increased with an increase in their electron accepting properties, or their reactivity towards the single-electron transferring FNR and P-450R. Thus, our data imply that the flavoenzyme-catalyzed redox cycling of amino and hydroxylamino metabolites of TNT may be an important factor in their cytotoxicity.

Enzymes associated with reductive activation and action of nitazoxanide, nitrofurans, and metronidazole in Helicobacter pylori

Nitazoxanide (NTZ) is a redox-active nitrothiazolyl-salicylamide prodrug that kills Helicobacter pylori and also many anaerobic bacterial, protozoan, and helminthic species. Here we describe development and use of a spectrophotometric assay, based on nitroreduction of NTZ at 412 nm, to identify H. pylori enzymes responsible for its activation and mode of action. Three enzymes that reduce NTZ were identified: two related NADPH nitroreductases, which also mediate susceptibility to metronidazole (MTZ) (RdxA and FrxA), and pyruvate oxidoreductase (POR). Recombinant His-tagged RdxA, FrxA, and POR, overexpressed in nitroreductase-deficient Escherichia coli, each rapidly reduced NTZ, whereas only FrxA and to a lesser extent POR reduced nitrofuran substrates (furazolidone, nitrofurantoin, and nitrofurazone). POR exhibited no MTZ reductase activity either in extracts of H. pylori or following overexpression in E. coli; RdxA exhibited no nitrofuran reductase activity, and FrxA exhibited no MTZ reductase activity. Analysis of mutation to rifampin resistance (Rif(r)) indicated that NTZ was not mutagenic and that nitrofurans were only weakly mutagenic. Alkaline gel DNA electrophoresis indicated that none of these prodrugs caused DNA breakage. In contrast, MTZ caused DNA damage and was strongly mutagenic. We conclude that POR, an essential enzyme, is responsible for most or all of the bactericidal effects of NTZ against H. pylori. While loss-of-function mutations in rdxA and frxA produce a Mtz(r) phenotype, they do not contribute much to the innate susceptibility of H. pylori to NTZ or nitrofurans.

Reductive activation of terpenylnaphthoquinones

Four terpenylnaphthoquinones were found to enhance the rate of superoxide production in the presence of ascorbate as detected from the superoxide dismutase (SOD)-inhibitable initial oxygen consumption rates. Initial rates of oxygen consumption in the presence of ascorbate plus quinone increase with an increase in the half-wave reduction potentials of the quinones. These quinones also enhance the rate of Cyt(III)c reduction by xanthine/xanthine oxidase (X/XO) in both air- and nitrogen-saturated aqueous solutions at pH 7.4. Maximum rates of Cyt(III)c reduction in nitrogen and oxygen-saturated solutions (V(max)), in the presence of X/XO, increase with an increase in the half-wave reduction potentials of the quinones. SOD inhibits Cyt(III)c reduction rates in the presence of these quinones and X/XO in a manner which is also dependent on the quinone half-wave redox potential. The relative antineoplastic activity of two of these quinones follows the order in rates of oxygen consumption or Cyt(III)c reduction. This is consistent with an antineoplastic action of these quinones through the mechanism of redox cycling or possible interference or inhibition of mitochondrial respiration.

Modifying rates of reductive elimination of leaving groups from indolequinone prodrugs: a key factor in controlling hypoxia-selective drug release

3-(4-Methylcoumarin-7-yloxy)methylindole-4,7-diones were synthesised as model prodrugs in order to investigate the correlation between rates of reductive elimination from the (indolyl-3-yl)methyl position with reductive metabolism by hypoxic tumor cells and NADPH: cytochrome P450. Rates of elimination of the chromophore/fluorophore (7-hydroxy-4-methylcoumarin) following one-electron reduction of indolequinones to their semiquinone radicals (Q*-) was measured by pulse radiolysis utilising spectrophotometric and fluorometric detection. Incorporation of a thienyl or methyl substituent at the (indol-3-yl)CHR-position (where R=thienyl or methyl adjacent to the phenolic ether linking bond) significantly shortened the half-life of reductive elimination from 87 to 6 and 2 ms, respectively. Elimination from the methyl substituted analogue can thus compete effectively with the reaction of the semiquinone radical with oxygen at levels typically present in tumours (half-life approximately 1.8 ms at 0.5% O2). Chemical kinetic predictions were confirmed by metabolism in breast tumour MCF-7 cells between 0-2.1% O2. Rates of reductive release of the fluorophore from the non-fluorescent parent indolequinones (R=H, Me, thienyl) were similar under anoxia ( approximately 1.7 nmol coumarinmin(-1)mg protein(-1)) reflecting the similarity in one-electron reduction potential. Whereas coumarin release from the indolequinone (R=H) was completely inhibited above 0.5% O2, the enhanced rate of reductive elimination when R=thienyl or Me increased the metabolic rate of release to approximately 0.35 and 0.7 nmol coumarinmin(-1)mg protein(-1), respectively at 0.5% O2; complete inhibition occurring by 2.1% O2. Similar 'oxygen profiles' of release were observed with NADPH: cytochrome P450 reductase. In conclusion, it is possible to modify rates of reductive elimination from indolequinones to control the release of drugs over a range of tumour hypoxia.

Revisiting the interaction of the radical anion metabolite of nitrofurantoin with glutathione

There have been several conflicting reports as to the scavenging nature of glutathione toward the nitro radical anion of the drug nitrofurantoin. We produced the radical anion enzymatically using the xanthine oxidase/hypoxanthine system at pH 7.4 and pH 9.0 in the presence of various levels of glutathione from 10 to 100 mM and monitored any changes in the radical concentration via electron spin resonance spectroscopy. Independent of glutathione concentration, there was no decrease in the steady-state concentration of the radical. In fact, there was an average 30% increase in the concentration of the radical anion, which suggests enhanced enzyme activity in the presence of glutathione (GSH). These results, together with observations of the effects of glutathione on the stability of the radical anion generated by radiolysis or dithionite, rule out any detectable reaction between the nitrofurantoin radical anion and GSH under physiologically relevant conditions.

Quantitative structure-activity relationships in enzymatic single-electron reduction of nitroaromatic explosives: implications for their cytotoxicity

The mechanisms of cytotoxicity of polynitroaromatic explosives, an important group of environmental pollutants, remain insufficiently studied so far. We have found that the rate constants of single-electron enzymatic reduction, and the enthalpies of single-electron reduction of nitroaromatic compounds (DeltaHf(ArNO(2)(-*)), obtained by quantum mechanical calculation, may serve as useful tools for the analysis of cytotoxicity of nitroaromatic explosives with respect to the possible involvement of oxidative stress. The single-electron reduction rate constants of a number of explosives including 2,4,6-trinitrotoluene (TNT) and 2,4,6-trinitrophenyl-N-methylnitramine (tetryl), and model nitroaromatic compounds by ferredoxin:NADP(+) reductase (FNR, EC 1.18.1.2) and NADPH:cytochrome P-450 reductase (P-450R, EC 1.6.2.4) increased with a decrease in DeltaHf(ArNO(2)(-*)). This indicates that the reduction rates are determined by the electron transfer energetics, but not by the particular structure of the explosives. The cytotoxicity of explosives to bovine leukemia virus-transformed lamb kidney fibroblasts (line FLK) increased with a corresponding increase in their reduction rate constant by P-450R and FNR, or with a decrease in their DeltaHf(ArNO(2)(-*)). This points to an importance of oxidative stress in the toxicity of explosives in this cell line, which was further evidenced by the protective effects of desferrioxamine and the antioxidant N,N'-diphenyl-p-phenylene diamine, and an increase in lipid peroxidation. DT-diaphorase (EC 1.6.99.2) exerted a minor and equivocal role in the cytotoxicity of explosives to FLK cells.

Quantitative structure-activity relationships in two-electron reduction of nitroaromatic compounds by Enterobacter cloacae NAD(P)H:nitroreductase

Enterobacter cloacae NAD(P)H:nitroreductase (NR; EC 1.6.99.7) catalyzes the reduction of a series of nitroaromatic compounds with steady-state bimolecular rate constants (kcat/Km) ranging from 10(4) to 10(7) M(-1) s(-1). In agreement with a previously proposed scheme of two-step four-electron reduction of nitroaromatics by NR (Koder, R. L., and Miller, A.-F. (1998) Biochim. Biophys. Acta 1387, 395-405), 2 mol NADH per mole mononitrocompound were oxidized. An oxidation of excess NADH by polinitrobenzenes, including explosives 2,4,6-trinitrotoluene (TNT) and 2,4,6-trinitrophenyl-N-methylnitramine (tetryl), has been observed as a slower secondary process, accompanied by O2 consumption. This type of "redox cycling" was not related to reactions of nitroaromatic anion-radicals, but was caused by the autoxidation of relatively stable reaction products. The initial reduction of tetryl and other polinitrophenyl-N-nitramines by E. cloacae NR was analogous to a two-step four-electron reduction mechanism of TNT and other nitroaromatics. The logs kcat/Km of all the compounds examined exhibited parabolic dependence on their enthalpies of single-electron or two-electron (hydride) reduction, obtained by quantum mechanical calculations. This type of quantitative structure-activity relationship shows that the reactivity of nitroaromatics towards E. cloacae nitroreductase depends mainly on their hydride accepting properties, but not on their particular structure, and does not exclude the possibility of multistep hydride transfer.

Quantitative structure activity relationships for the electron transfer reactions of Anabaena PCC 7119 ferredoxin-NADP+ oxidoreductase with nitrobenzene and nitrobenzimidazolone derivatives: mechanistic implications

The steady state single electron reduction of polynitroaromatics by ferredoxin-NADP+ oxidoreductase (EC 1.18.1.2) from cyanobacterium Anabaena PCC 7119 has been studied and quantitative structure activity relationships are described. The solubility of the polynitroaromatics as well as their reactivity towards ferredoxin-NADP+ oxidoreductase are markedly higher than those for previously studied mononitroaromatics and this enabled the independent measurement of the kinetic parameters-k(cat) and Km. Interestingly, the natural logarithm of the bimolecular rate constant, k(cat)/Km, and also the natural logarithm of k(cat) correlate with the calculated energy of the lowest unoccupied molecular orbital of the polynitroaromatic substrates. The minimal kinetic model in line with these quantitative structure activity relationships is a ping-pong mechanism which includes substrate binding equilibria in the second half reaction.

DT-diaphorase catalyzes N-denitration and redox cycling of tetryl

Rat liver DT-diaphorase (EC 1.6.99.2) catalyzed reductive N-denitration of tetryl (2,4,6-tri-nitrophenyl-N-methylnitramine) and 2,4-dinitrophenyl-N-methylnitramine, oxidizing the excess of NADPH. The reactions were accompanied by oxygen consumption and superoxide dismutase-sensitive reduction of added cytochrome c and reductive release of Fe2+ from ferritin. Quantitatively, the reactions of DT-diaphorase proceeded like single-electron reductive N-denitration of tetryl by ferredoxin:NADP+ reductase (EC 1.18.1.2) (Shah, M.M. and Spain, J.C. (1996) Biochem. Biophys. Res. Commun. 220, 563-568), which was additionally checked up in this work. Thus, although reductive N-denitration of nitrophenyl-N-nitramines is a net two-electron (hydride) transfer process, DT-diaphorase catalyzed the reaction in a single-electron way. These data point out the possibility of single-electron transfer steps during obligatory two-electron (hydride) reduction of quinones and nitroaromatics by DT-diaphorase.

Nitroreductase reactions of Arabidopsis thaliana thioredoxin reductase

Arabidopsis thaliana NADPH:thioredoxin reductase (TR, EC 1.6.4.5) catalyzed redox cycling of aromatic nitrocompounds, including the explosives 2,4,6-trinitrotoluene and tetryl, and the herbicide 3,5-dinitro-o-cresol. The yield of nitro anion radicals was equal to 70-90%. Redox cycling of tetryl was accompanied by formation of N-methylpicramide. Bimolecular rate constants of nitroaromatic reduction (kcat/Km) and reaction catalytic constants (kcat) increased upon an increase in oxidant single-electron reduction potential (E(1)7). Using compounds with an unknown E(1)7 value, the reactivity of TR increased parallelly to the increase in reactivity of ferredoxin:NADP+ reductase of Anabaena PCC 7119 (EC 1.18.1.2). This indicated that the main factor determining reactivity of nitroaromatics towards TR was their energetics of single-electron reduction. Incubation of reduced TR in the presence of tetryl or 2,4-dinitrochlorobenzene resulted in a loss of thioredoxin reductase activity, most probably due to modification of reduced catalytic disulfide, whereas nitroreductase reaction rates were unchanged. This means that on the analogy of quinone reduction by TR (D. Bironaite, Z. Anusevicius, J.-P. Jacquot, N. Cenas, Biochim. Biophys. Acta 1383 (1998) 82-92), FAD and not catalytic disulfide of TR was responsible for the reduction of nitroaromatics. Tetryl, 2,4,6-trinitrotoluene and thioredoxin increased the FAD fluorescence intensity of TR. This finding suggests that nitroaromatics may bind close to the thioredoxin-binding site at the catalytic disulfide domain of TR, and induce a conformational change of enzymes (S.B. Mulrooney, C.H. Williams Jr., Protein Sci. 6 (1997) 2188-2195). Our data indicate that certain nitroaromatic herbicides, explosives and other classes of xenobiotics may interfere with the reduction of thioredoxin by plant TR, and confer prooxidant properties to this antioxidant enzyme.

Quantitative structure activity relationships for the conversion of nitrobenzimidazolones and nitrobenzimidazoles by DT-diaphorase: implications for the kinetic mechanism

Quantitative structure activity relationships (QSARs) for the conversion of nitrobenzimidazolones and nitrobenzimidazoles by rat liver DT-diaphorase (EC 1.6.99.2) are described. The parameter used for description of the QSARs is the energy of the lowest unoccupied molecular orbital (E(LUMO)) of the nitroaromatic compounds. Interestingly, correlations with E(LUMO) were observed for both the natural logarithm of kcat, but also for the natural logarithm of kcat/Km. The minimal kinetic model in line with these QSARs is a ping-pong mechanism that includes a substrate binding equilibrium in the second half reaction.

Transformation of nitrobenzene by ferredoxin NADP oxidoreductase from spinach leaves

Nitrobenzene was reduced in a solution containing ferredoxin NADP oxidoreductase (FNR) from spinach leaves and NADPH generating system. The product of nitrobenzene was identified as phenylhydroxylamine (PHA) on 1:1 basis.

Metabolism of 14C-labelled 5-nitro-1,2,4-triazol-3-one by rat liver microsomes--evidence for the participation of cytochrome P-450

In the present study, we synthesized 14C-labelled 5-nitro-1,2,4-triazol-3-one (NTO) and investigated its hepatic metabolism by dexamethasone-induced murine hepatic microsomes. Under the nitrogen atmosphere, 5-amino-1,2,4-triazol-3-one was the only detected metabolite of NTO. The microsomal nitroreductase activity was dependent on NADPH, totally inhibited by carbon monoxide and partially inhibited by oxygen. In aerobic conditions, beside a low amount of amine, the major metabolite formed is the 5-hydroxy-triazolone, urazole. This compound resulted from the oxidative denitrification of NTO, which produced equivalent amount of nitrite. This reaction, like the nitroreductase activity, was dependent on NADPH and totally inhibited by carbon monoxide. Both nitroreduction and oxidative denitrification were inhibited by imidazole-related inhibitors: miconazole and methimazole, and to a less extent by N-octylamine. The microsomal denitrification was induced by the treatment of rats with dexamethasone and phenobarbital. The microsomal reductase activity is present in untreated rat microsomes, and recovered with various inducers. The results of this study indicate the role played by cytochrome P-450 in the metabolism of NTO, supported by its transformation with reconstituted cytochrome P-450 systems.

The toxicity of aromatic nitrocompounds to bovine leukemia virus-transformed fibroblasts: the role of single-electron reduction

Bovine leukemia virus-transformed lamb embryo fibroblasts (line FLK) possess activity of DT-diaphorase of ca. 260 U/mg protein and similar levels of other NADP(H)-oxidizing enzymes: NADH:oxidase, 359 U/mg; NADPH:oxidase, 43 U/mg; NADH:cytochrome-c reductase, 141 U/mg; NADPH:cytochrome-c reductase, 43 U/mg. In general, the toxicity of aromatic nitrocompounds towards FLK cells increases on increase of single-electron reduction potentials (E1(1)) of nitrocompounds or the log of their reduction rate constants by single-electron-transferring enzymes, microsomal NADPH:cytochrome P-450 reductase (EC 1.6.2.4) and mitochondrial NADH:ubiquinone reductase (EC 1.6.99.3). No correlation between the toxicity and reduction rate of nitrocompounds by rat liver DT-diaphorase (EC 1.6.99.2) was observed. The toxicity is not significantly affected by dicumarol, an inhibitor of DT-diaphorase. Nitrocompounds examined were poor substrates for DT-diaphorase, being 10(4) times less active than menadione. Their poor reactivity is most probably determined by their preferential binding to a NADPH binding site, but not to menadione binding site of diaphorase. These data indicate that at comparable activities of DT-diaphorase and single-electron-transferring NAD(P)H dehydrogenases in the cell, the toxicity of nitrocompounds will be determined mainly by their single-electron reduction reactions.

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).

Radiation chemistry applied to drug design

Radiation chemistry can contribute to drug design by quantifying redox properties of drugs (useful parameters in quantitative structure-activity relationships), and where free radicals are suspected intermediates in drug action, radiation can be used to generate these putative species and help characterize relevant reactions. Steady radiolysis produces radicals at a readily-varied but quantified rate; pulse radiolysis with fast spectrophotometric and/or conductimetric detection enables the kinetic properties of radicals to be monitored directly. Using these methods, radical intermediates from drugs with specific cytotoxicity towards hypoxic cells have been shown to react rapidly with oxygen, a reaction probably responsible for the therapeutic differential. Radical oxidants from activated neutrophils include superoxide and hydroxyl radicals, and radiation-chemical methods have an important role to play in rational drug design to exploit such oxidative chemistry. Antioxidants can also be evaluated quantitatively by radiolysis methods; the conjugation reactions of thiyl radicals with thiolate and oxygen are now recognised to be major contributions of pulse radiolysis to thiol biochemistry.

Reductive activation of benzazolo[3,2-a]-quinolinium chlorides

Initial ferricytochrome c (Cyt(III)c) reduction rates occurring in aerobic or anaerobic solutions containing either 3-nitrobenzothiazolo[3,2-a]-(NBQCl), 1-ethyl-3-nitrobenzimidazolo[3,2-a]-(ENBIQCl), 7-ethylbenzimidazolo[3,2-a]quinolinium chloride (EHBIQCL), or nitrofurantoin (NFT) and xanthine/xanthine oxidase were measured. Maximum rates in nitrogen-saturated solutions follow the order NFT > NBQCL > ENBIQCL > EHBIQCL. These rates correlate linearly with the half-wave reduction potentials (E1/2) of these compounds. With the exception of EHBIQCl, smaller rates of Cyt(III)c reduction were obtained in air-saturated than in nitrogen-saturated solutions at the quinolinium salt concentrations used. Larger concentrations of superoxide dismutase (SOD) are needed for 50% inhibition of the Cyt(III)c reduction reaction for heterocyclic compounds with larger E1/2 values. Thus, measurement of the portion of the Cyt(III)c reduction rate under air that is inhibited by SOD does not account solely for the production of superoxide. These observations suggest that NBQCL, ENBIQCl, and less probably EHBIQCl may interfere with mitochondrial energy metabolism or induce DNA damage through reduced intermediates.

The one-electron reduction potential of several substrates can be related to their reduction rates by cytochrome P-450 reductase

The rates of reduction of 27 compounds by purified cytochrome P-450 reductase have been studied and correlated with the one-electron reduction potentials E7(X/X.-). For compounds with reduction potentials between approx. -400 and -165 mV, there was a good correlation between the rates and E7(X/X.-); log rate (mumol/min per mg) = (2.38 +/- 0.08) + (0.0061 +/- 0.0003)E7 (mV). Compounds with potentials more positive than -165 mV were shown to deviate from the correlation. It is shown that under the standard conditions of the assay, these compounds undergo two-electron reduction by NADPH rather than one-electron reduction with the enzyme. As a consequence of the study, the rate constants for the reaction of superoxide radicals with native and acetylated cytochrome c were also determined to be (4.5 +/- 0.5).10(5) and (1.5 +/- 0.15).10(5) M-1 s-1, respectively.

Degradation of 2,4-dinitrotoluene by the lignin-degrading fungus Phanerochaete chrysosporium

Under ligninolytic conditions, the white rot basidiomycete Phanerochaete chrysosporium mineralizes 2,4-dinitrotoluene (I). The pathway for the degradation of I was elucidated by the characterization of fungal metabolites and oxidation products generated by lignin peroxidase (LiP), manganese peroxidase (MnP), and crude intracellular cell extracts. The multistep pathway involves the initial reduction of I to yield 2-amino-4-nitrotoluene (II). II is oxidized by MnP to yield 4-nitro-1,2-benzoquinone (XII) and methanol. XII is then reduced to 4-nitro-1,2-hydroquinone (V), and the latter is methylated to 1,2-dimethoxy-4-nitrobenzene (X). 4-Nitro-1,2-hydroquinone (V) is also oxidized by MnP to yield nitrite and 2-hydroxybenzoquinone, which is reduced to form 1,2,4-trihydroxybenzene (VII). 1,2-Dimethoxy-4-nitrobenzene (X) is oxidized by LiP to yield nitrite, methanol, and 2-methoxy-1,4-benzoquinone (VI), which is reduced to form 2-methoxy-1,4-hydroquinone (IX). The latter is oxidized by LiP and MnP to 4-hydroxy-1,2-benzoquinone, which is reduced to 1,2,4-trihydroxybenzene (VII). The key intermediate 1,2,4-trihydroxybenzene is ring cleaved by intracellular cell extracts to produce, after reduction, beta-ketoadipic acid. In this pathway, initial reduction of a nitroaromatic group generates the peroxidase substrate II. Oxidation of II releases methanol and generates 4-nitro-1,2-benzoquinone (XII), which is recycled by reduction and methylation reactions to regenerate intermediates which are in turn substrates for peroxidase-catalyzed oxidation leading to removal of the second nitro group. Thus, this unique pathway apparently results in the removal of both aromatic nitro groups before ring cleavage takes place.

The rotenone-insensitive reduction of quinones and nitrocompounds by mitochondrial NADH:ubiquinone reductase

The rotenone-insensitive reduction of quinones and aromatic nitrocompounds by mitochondrial NADH: ubiquinone reductase (complex I, EC 1.6.99.3) has been studied. It was found that these reactions proceed via a mixed one- and two-electron transfer. The logarithms of the bimolecular rate constants of oxidation (TN/Km) are proportional to the one-electron-reduction potentials of oxidizers. The reactivities of nitrocompounds are close to those of quinones. Unlike the reduction of ferricyanide, these reactions are not inhibited by NADH. However, they are inhibited by NAD+ and ADP-ribose, which also act as the mixed-type inhibitors for ferricyanide. TN/Km of quinones and nitrocompounds depend on the NAD+/NADH ratio, but not on NAD+ concentration. They are diminished by the limiting factors of 2.5-3.5 at NAD+/NADH greater than 200. It seems that rotenone-insensitive reduction of quinones and nitrocompounds takes place near the NAD+/NADH and ferricyanide binding site, and the inhibition is caused by induced conformational changes after the binding of NAD+ or ADP-ribose.

Metabolism of trinitrobenzene sulfonic acid by the rat colon produces reactive oxygen species

Recent studies have shown that intrarectal administration of 2,4,6-trinitrobenzene sulfonic acid (TNBS) in ethanol or intramural injection of TNBS in saline produces an acute and possibly chronic colitis in rats. It has been assumed that interstitial TNBS initiates the inflammatory response via macrophage-mediated recognition and degradation of TNBS-modified mucosal cells and proteins. However, it is known that certain flavoproteins and/or reductants interact with compounds containing the nitro functional group to generate pro-inflammatory, nitrogen-centered free radicals and reactive oxygen metabolites. The objective of this study was to assess the ability of the rat colon, using either colon homogenates, isolated colonocytes, or intestinal interstitial fluid, to produce reactive oxygen species via enzymatic and/or nonenzymatic metabolism of TNBS. It was found that the addition of TNBS (1 mmol/L) to the 10,000 x g supernatant of rat colon homogenates increased the rate of superoxide production from normally undetectable levels to 2.6 +/- 0.23 nmol.min-1.mg protein-1. Addition of nicotinamide adenine dinucleotide, reduced form (NADH; 1 mmol/L) to colon homogenates containing TNBS significantly enhanced superoxide production to 10.4 +/- 0.9 nmol.min-1.mg-1. Similarly, addition of nicotinamide adenine dinucleotide phosphate, reduced form (NADPH; 1 mmol/L) to colon extracts containing TNBS produced an even further increase in the rate of superoxide formation to 25.2 +/- 1.1 nmol.min-1.mg-1. Addition of NADH or NADPH to the colon homogenate in the absence of TNBS produced no detectable superoxide formation, suggesting that TNBS was required for the enhanced oxidative metabolism. In a separate series of experiments, it was found that isolated colonocytes produced small but significant amounts of superoxide (3.15 +/- 0.6 nmol/2 x 10(6) cells) that were significantly increased in the presence of ethanol to 6.55 +/- 1.14 nmol/2 x 10(6) cells. Using purified preparations of two flavoproteins found in the rat colon, it was shown that the addition of TNBS (1 mmol/L) to purified NADH dehydrogenase or glutathione reductase increased the rate of superoxide formation by these enzymes from normally undetectable levels to 1.6 nmol/min and 1.2 nmol/min, respectively. In addition, it was found that intestinal interstitial fluid (lymph) initiated redox cycling of TNBS such that 28.1 +/- 1.6 nmol of oxygen was consumed per minute per milliliter of lymph. This increase in oxygen consumption was inhibited by the addition of superoxide dismutase and catalase. One possible metabolite involved in both mucosal and lymph-mediated metabolism of TNBS is ascorbic acid.(ABSTRACT TRUNCATED AT 400 WORDS)

Interaction of nitrofurans with glutathione reductase

Nitrofurans inhibit the oxidation of NADPH by glutathione, catalyzed by yeast glutathione reductase (EC 1.6.4.2). acting as uncompetitive incomplete inhibitors for NADPH and glutathione. The quinoline-substituted nitrofurans were the most effective inhibitors. These compounds increased the turnover numbers of enzyme at fixed concentrations of reduced glutathione, in the reverse reaction of glutathione reductase, but in most cases diminished the affinity of the enzyme for NAD+. Nitrofurans are weak one-electron oxidants of glutathione reductase. Their reactivity is close to that of p-quinones possessing the analoguous one-electron reduction potential (Cénas, N.K., Rakauskiené, G.A. and Kulys, J.J. (1989) Biochim. Biophys. Acta 973, 399-404), and reaction is stimulated by NADP+. It is assumed, that nitrofurans bind to the 'regulative' site of glutathione reductase (Karplus, P.A., Pai, E.F. and Schulz, G.E. (1989) Eur. J. Biochem. 178, 693-703).