Salmonella typhimurium mutagenicity tester strains that overexpress oxygen-insensitive nitroreductases nfsA and nfsB

Nitroreductase Increases Menadione-Mediated Oxidative Stress in Aspergillus nidulans

Nitroreductases (NTRs) catalyze the reduction of a wide range of nitro-compounds and quinones using NAD(P)H. Although the physiological functions of these enzymes remain obscure, a tentative function of resistance to reactive oxygen species (ROS) via the detoxification of menadione has been proposed. This suggestion is based primarily on the transcriptional or translational induction of an NTR response to menadione rather than on convincing experimental evidence. We investigated the performance of a fungal NTR from Aspergillus nidulans (AnNTR) exposed to menadione to address the question of whether NTR is really an ROS defense enzyme. We confirmed that AnNTR was transcriptionally induced by external menadione. We observed that menadione treatment generated cytotoxic levels of O₂^•-, which requires well-known antioxidant enzymes such as superoxide dismutase, catalase, and peroxiredoxin to protect A. nidulans against menadione-derived ROS stress. However, AnNTR was counterproductive for ROS defense, since knocking out AnNTR decreased the intracellular O₂^•- levels, resulting in fungal viability higher than that of the wild type. This observation implies that AnNTR may accelerate the generation of O₂^•- from menadione. Our in vitro experiments indicated that AnNTR uses NADPH to reduce menadione in a single-electron reaction, and the subsequent semiquinone-quinone redox cycling resulted in O₂^•- generation. We demonstrated that A. nidulans nitroreductase should be an ROS generator, but not an ROS scavenger, in the presence of menadione. Our results clarified the relationship between nitroreductase and menadione-derived ROS stress, which has long been ambiguous. IMPORTANCE Menadione is commonly used as an O₂^•- generator in studies of oxidative stress responses. However, the precise mechanism through which menadione mediates cellular O₂^•- generation, as well as the way in which cells respond, remains unclear. Elucidating these events will have important implications for the use of menadione in biological and medical studies. Our results show that the production of Aspergillus nidulans nitroreductase (AnNTR) was induced by menadione. However, the accumulated AnNTR did not protect cells but instead increased the cytotoxic effect of menadione through a single-electron reduction reaction. Our finding that nitroreductase is involved in the menadione-mediated O₂^•- generation pathway has clarified the relationship between nitroreductase and menadione-derived ROS stress, which has long been ambiguous.

Microbial nitroreductases: A versatile tool for biomedical and environmental applications

Nitroreductases, enzymes found mostly in bacteria and also in few eukaryotes, use nicotinamide adenine dinucleotide (NADH) or nicotinamide adenine dinucleotide phosphate (NADPH) as a cofactor for their activity and metabolize an enormous list of a diverse nitro group-containing compounds. Nitroreductases that are capable of metabolizing nitroaromatic and nitro heterocyclic compounds have drawn great attention in recent years owing to their biotechnological, biomedical, environmental, and human impact. These enzymes attracted medicinal chemists and pharmacologists because of their prodrug selectivity for activation/reduction of nitro compounds that wipe out pathogens/cancer cells, leaving the host/normal cells unharmed. It is applied in diverse fields of study like prodrug activation in treating cancer and leishmaniasis, designing fluorescent probes for hypoxia detection, cell imaging, ablation of specific cell types, biodegradation of nitro-pollutants, and interpretation of mutagenicity of nitro compounds. Keeping in view the immense prospects of these enzymes and a large number of research contributions in this area, the present review encompasses the enzymatic reaction mechanism, their role in antibiotic resistance, hypoxia sensing, cell imaging, cancer therapy, reduction of recalcitrant nitro chemicals, enzyme variants, and their specificity to substrates, reaction products, and their applications.

N-dodecanoyl-homoserine lactone influences the levels of thiol and proteins related to oxidation-reduction process in Salmonella

Quorum sensing is a cell-cell communication mechanism mediated by chemical signals that leads to differential gene expression in response to high population density. Salmonella is unable to synthesize the autoinducer-1 (AI-1), N-acyl homoserine lactone (AHL), but is able to recognize AHLs produced by other microorganisms through SdiA protein. This study aimed to evaluate the fatty acid and protein profiles of Salmonella enterica serovar Enteritidis PT4 578 throughout time of cultivation in the presence of AHL. The presence of N-dodecanoyl-homoserine lactone (C12-HSL) altered the fatty acid and protein profiles of Salmonella cultivated during 4, 6, 7, 12 and 36 h in anaerobic condition. The profiles of Salmonella Enteritidis at logarithmic phase of growth (4 h of cultivation), in the presence of C12-HSL, were similar to those of cells at late stationary phase (36 h). In addition, there was less variation in both protein and fatty acid profiles along growth, suggesting that this quorum sensing signal anticipated a stationary phase response. The presence of C12-HSL increased the abundance of thiol related proteins such as Tpx, Q7CR42, Q8ZP25, YfgD, AhpC, NfsB, YdhD and TrxA, as well as the levels of free cellular thiol after 6 h of cultivation, suggesting that these cells have greater potential to resist oxidative stress. Additionally, the LuxS protein which synthesizes the AI-2 signaling molecule was differentially abundant in the presence of C12-HSL. The NfsB protein had its abundance increased in the presence of C12-HSL at all evaluated times, which is a suggestion that the cells may be susceptible to the action of nitrofurans or that AHLs present some toxicity. Overall, the presence of C12-HSL altered important pathways related to oxidative stress and stationary phase response in Salmonella.

Structure-activity investigation of the potentiating effect of cyano substitution on nitroaniline mutagenicity in the ames test

2,6-Dicyano-4-nitroaniline and 2-cyano-4-nitroaniline (CNNA; 2-amino-5-nitrobenzonitrile) are potent mutagens in the Ames test, even though unsubstituted nitroanilines (NAs) are no more than weak mutagens. These compounds are putative reduction products of many commercial azo dyes, including Disperse Blue 165, Disperse Blue 337, Disperse Red 73, Disperse Red 82, Disperse Violet 33, and Disperse Violet 63. We have examined the mutagenicity in strains TA98 and YG1024 of a series of commercially-available isomers of CNNA, and some related compounds, to probe the relationship between structure and genotoxic activity in this class of compounds. The potentiating effect of the cyano substituent is seen in many cases; e.g. 2-amino-4-nitrobenzonitrile is a much more potent mutagen than 3-NA. 2,4-Dinitrobenzonitrile is also highly mutagenic. Possible mechanisms for the "cyano effect" are considered, with respect to the likely structures of cyanonitroaniline-DNA adducts and the roles of the enzymes (nitroreductase and acetyl CoA:arylamine N-acetyltransferase) believed to be involved in the activation of nitroaromatic compounds. Environ. Mol. Mutagen. 59:114-122, 2018. © 2017 Wiley Periodicals, Inc.

Comparison of in vitro test systems using bacterial and mammalian cells for genotoxicity assessment within the "health-related indication value (HRIV) concept

In numerous cases, the German health-related indication value (HRIV) concept has proved its practicability for the assessment of drinking water relevant trace substances (Umweltbundesamt 2003). The HRIV is based on the toxicological profile of a substance. An open point of the HRIV concept has been the assignment of standardized test procedures to be used for the assessment. The level of the HRIV is at its lowest as soon as the genotoxicity of the substance is detected. As a single test on its own, it is not sufficient enough to assess the human toxicological relevance of a genotoxic effect or exclude it in the case of a negative result; a reasonable test battery was required, technically oriented towards the already harmonized international, hierarchical evaluation for toxicological assessment of chemicals. Therefore, an important aim of this project was to define a strategy for the genotoxicological assessment of anthropogenic trace substances. The basic test battery for genotoxicity of micropollutants in drinking water needs to fulfill several requirements. Although quick test results are needed for the determination of HRIV, a high degree of transferability to human genotoxicity should be ensured. Therefore, an in vitro genotoxicity test battery consisting of the Ames fluctuation test with two tester strains (ISO 11350), the umu test and the micronucleus test, or from the Ames test with five tester strains (OECD 471) and the micronucleus test is proposed. On the basis of selected test substances, it could be shown that the test battery leads to positive, indifferent, and negative results. Given indifferent results, the health authority and the water supplier must assume that it is a genotoxic substance. Genetically modified tester strains are being sensitive to different chemical classes by expression of selected mammalian key enzymes for example nitroreductase, acetyltransferase, and glutathione-S-transferase. These strains may provide valuable additional information and may give a first indication of the mechanism of action. To check this hypothesis, various additional strains expressing specific human-relevant enzymes were investigated. It could be shown that the additional use of genetically modified tester strains can enhance the detectable substance spectrum with the bacterial genotoxicological standard procedures or increase the sensitivity. The additional use provides orienting information at this level as a lot of data can be obtained quite quickly and with little effort. These indications of the mechanism of action should be however verified with a test system that uses mammalian cells, better human cells, to check their actual relevance. The selection of appropriate additional tester strains has to be defined from case to case depending on the molecular structure and also still requires some major expertise.

Potent mutagenicity in the Ames test of 2-cyano-4-nitroaniline and 2,6-dicyano-4-nitroaniline, components of disperse dyes

Genotoxicity data on commercial azo dyes and their components remain sparse, despite their widespread use. We have tested the mutagenicity of 2-cyano-4-nitroaniline (CNNA) and 2,6-dicyano-4-nitroaniline (CNCNNA), components of azo dyes such as Disperse Blue 165 and Disperse Red 73, in Ames test strains. Both compounds are extraordinarily potent frameshift mutagens, with much greater activity than structurally similar dihalonitroanilines and halodinitroanilines. Analysis of the responses of strains over-expressing or deficient in bioactivation enzymes shows that bacterial nitroreductase and acetyl CoA: arylamine N-acetyltransferase are important mediators of the mutagenicity of CNNA and CNCNNA.

Development and analysis of furazolidone-resistant Escherichia coli mutants

Furazolidone-resistant mutants were obtained from four clinical isolates of diarrhoeagenic Escherichia coli. The stability of the resistance and the frequency of mutation were established. The minimal inhibitory concentration of furazolidone, nitrofurantoin, nalidixic acid, ampicillin, chloramphenicol and tetracycline was established both in the presence and absence of the efflux pump inhibitor Phe-Arg-β-Naphtylamyde. The presence of mutations in the nitroreductase genes nfsA and nfsB was analysed by PCR; sequencing and their enzymatic activity was assessed by a spectrophotometric assay. Alterations in outer membrane proteins were studied by SDS-PAGE. The frequency of mutation ranged from <9.6 × 10(-10) to 9.59 × 10(-7) . Neither an effect on efflux pumps inhibited by Phe-Arg-β-Naphtylamyde nor cross-resistance with the antibiotics studied was observed. Nineteen mutants (52.94%) presented mutations in the nitroreductase-encoding genes: 17 in the nfsA gene (15 mutants with an internal stop codon, 2 with amino acid changes), 2 in the nfsB (all amino acid changes). Alterations in the outer membrane proteins OmpA and OmpW were also observed. Although more studies are necessary to find other resistance mechanisms, present data showed the low potential of selecting furazolidone-resistant mutants, together with the lack of cross-resistance with unrelated antimicrobial agents.

Two nitro derivatives of azabenzo[a]pyrene N-oxide: electronic properties and their relation to mutagenic activity

The equilibrium geometries, relative energies, IR and Raman spectra, vertical ionization potentials (IP), vertical electron affinities (EA), dipole moments (μ), electronic dipole polarizabilities (α), and molecular electrostatic potentials (MEP) of two species that show very high mutagenicity, 1-nitro-6-azabenzo[a]pyrene N-oxide (1-N-6-ABPO) and 3-nitro-6-azabenzo[a]pyrene N-oxide (3-N-6-ABPO), are investigated by means of Density Functional Theory (DFT) using B3LYP functional with different basis sets. The 3-N-6-ABPO isomer was estimated to be much more mutagenic in Salmonella typhimurium tester strain TA98 (396,000 revertants/nmol) than 1-N-6-ABPO (36100 revertants/nmol) (Fukuhara et al., 1992). The results show that for both isomers the structural, energetic, and vibrational properties are similar. The orientation of the nitro group with respect to the plane of the aromatic system as well as the nitroreduction and oxidation reaction and polarizability seem not be important for the determination of different mutagenic behavior of these isomers. However, the dipole moment of 3-N-6-ABPO is about 3 times that of 1-N-6-ABPO. The larger dipole moment and the different electronic charge distribution of 3-N-6-ABPO compared to 1-N-6-ABPO imply stronger electrostatic and inductive molecular interactions so that the active site of the enzyme involved in the mutagenic activation can more effectively bind 3-N-6-ABPO compared to 1-N-6-ABPO.

Bacterial chromate reductase, a potential enzyme for bioremediation of hexavalent chromium: a review

Hexavalent chromium is mobile, highly toxic and considered as a priority environmental pollutant. Chromate reductases, found in chromium resistant bacteria are known to catalyse the reduction of Cr(VI) to Cr(III) and have recently received particular attention for their potential use in bioremediation process. Different chromate reductases such as ChrR, YieF, NemA and LpDH, have been identified from bacterial sources which are located either in soluble fractions (cytoplasm) or bound to the membrane of the bacterial cell. The reducing conditions under which these enzymes are functional can either be aerobic or anaerobic or sometimes both. Enzymatic reduction of Cr(VI) to Cr(III) involves transfer of electrons from electron donors like NAD(P)H to Cr(VI) and simultaneous generation of reactive oxygen species (ROS). Based on the steps involved in electron transfer to Cr(VI) and the subsequent amount of ROS generated, two reaction mechanisms, namely, Class I "tight" and Class II "semi tight" have been proposed. The present review discusses on the types of chromate reductases found in different bacteria, their mode of action and potential applications in bioremediation of hexavalent chromium both under free and immobilize conditions. Besides, techniques used in characterization of the Cr (VI) reduced products were also discussed.

The molecular properties of nitrobenzanthrone isomers and their mutagenic activities

The mutagenic activity of five mono-substituted nitrobenzanthrones (NBA) has been determined in the Ames assay (Takamura-Enya et al., 2006). In the present study, a theoretical investigation of the electronic properties of all mono-substituted NBA isomers and their relation to mutagenic activity are presented. Equilibrium geometries, vertical ionization potentials (VIP), vertical electron affinities (VEA), relative energies, dipole moments and electronic dipole polarizabilities, and the IR and Raman spectra of NBA isomers calculated by Density Functional Theory (DFT) methods are presented. The position of the nitro group affects the spectral features of the IR and Raman spectra of the NBA isomers. The results show that a good linear relationship exists between the summation of Raman activities (∑ARaman) over all the 3N-6 vibrational modes and the mutagenic activity of the NBA isomers in Salmonella typhimurium strains. The spectroscopic results suggest that the unknown mutagenic activities of 4-NBA, 5-NBA, 6-NBA, 8-NBA and 10-NBA are predicted to follow the order 4-NBA>10-NBA>5-NBA>8-NBA>6-NBA.

Structure of RdxA--an oxygen-insensitive nitroreductase essential for metronidazole activation in Helicobacter pylori

The RdxA oxygen-insensitive nitroreductase of the human gastric pathogen Helicobacter pylori is responsible for the susceptibility of this organism to the redox active prodrug metronidazole [2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethanol]. Loss-of-function mutations in rdxA are primarily responsible for resistance to this therapeutic. RdxA exhibits potent NADPH oxidase activity under aerobic conditions and metronidazole reductase activity under strictly anaerobic conditions. In the present study, we report the crystal structure of RdxA, which is a homodimer exhibiting domain swapping and containing two molecules of FMN bound at the dimer interface. We have found a gap between the side chain of Tyr47 and the isoalloxazine ring of FMN that appears to be appropriate for substrate binding. The structure does not include residues 97-128, which correspond to a locally unstable part of the NTR from Escherichia coli, and might be involved in cofactor binding. Comparison of H. pylori RdxA with other oxidoreductases of known structure suggests that RdxA may belong to a new subgroup of oxidoreductases in which a cysteine side chain close to the FMN cofactor could be involved in the reductive activity. In this respect, the mutation of C159 to A or S (C159A/S) has resulted in a loss of metronidazole reductase activity but not NADPH oxidase activity. The RdxA structure enables the interpretation of the many loss-of-function mutations described previously, including those affecting C159, a residue whose interaction with FMN is required for the nitroreduction of metronidazole. The present studies provide unique insights into the redox behaviour of the flavin in this key enzyme for metronidazole activation, including a potential use in gene therapy.

DATABASE

Structural data have been deposited in the Protein Data Bank under accession number 3QDL.

Current Status of Short-Term Tests for Evaluation of Genotoxicity, Mutagenicity, and Carcinogenicity of Environmental Chemicals and NCEs

The advent of the industrial revolution has seen a significant increase in the number of new chemical entities (NCEs) released in the environment. It becomes imperative to check the toxic potential of NCEs to nontarget species before they are released for commercial purposes because some of these may exert genotoxicity, mutagenicity, or carcinogenicity. Exposure to such compounds produces chemical changes in DNA, which are generally repaired by the DNA repair enzymes. However, DNA damage and its fixation may occur in the form of gene mutations, chromosomal damage, and numerical chromosomal changes and recombination. This may affect the incidence of heritable mutations in man and may be transferred to the progeny or lead to the development of cancer. Hence, adequate tests on NCEs have to be undertaken for the risk assessment and hazard prediction. Compounds that are positive in tests that detect such damages have the potential to be human mutagens/carcinogens. Only long-term animal bioassays, involving lifetime studies on animals, were used earlier to classify substances as mutagens/carcinogens. These tests were cumbersome and time consuming and required a lot of facilities and personnel. Short-term tests, therefore, were brought into practice. A "battery" of three to four of these short-term tests has been proposed now by a number of regulatory authorities for the classification of compounds as mutagenic or carcinogenic. This review deals with the current status of these short-term tests.

Environmental nitration processes enhance the mutagenic potency of aromatic compounds

This work is an attempt to establish if aromatic nitration processes are always associated with an increase of genotoxicity. We determined the mutagenic and genotoxic effects of Benzene (B), Nitrobenzene (NB), Phenol (P), 2-Nitrophenol (2-NP), 2,4-Dinitrophenol (2,4-DNP), Pyrene (Py), 1-Nitropyrene (1-NPy), 1,3-Dinitropyrene (1,3-DNPy), 1,6-Dinitropyrene (1,6-DNPy), and 1,8-Dinitropyrene (1,8-DNPy). The mutagenic activities were evaluated with umuC test in presence and in absence of metabolic activation with S9 mix. Then, we used both cytokinesis-blocked micronucleus (CBMN) assay, in combination with fluorescent in situ hybridization (FISH) of human pan-centromeric DNA probes on human lymphocytes in order to evaluate the genotoxic effects. Analysis of all results shows that nitro polycyclic aromatic hydrocarbons (PAHs) are definitely environmental genotoxic/mutagenic hazards and confirms that environmental aromatic nitration reactions lead to an increase in genotoxicity and mutagenicity properties. Particularly 1-NPy and 1,8-DNPy can be considered as human potential carcinogens. They seem to be significant markers of the genotoxicity, mutagenicity, and potential carcinogenicity of complex PAHs mixtures present in traffic emission and industrial environment. In prevention of environmental carcinogenic risk 1-NPy and 1,8-DNPy must therefore be systematically analyzed in environmental complex mixtures in association with combined umuC test, CBMN assay, and FISH on cultured human lymphocytes. © 2010 Wiley Periodicals, Inc. Environ Toxicol, 2012.

Theoretical investigations on the formation of nitrobenzanthrone-DNA adducts

3-Nitrobenzanthrone (3-NBA) is a potent mutagen and suspected human carcinogen identified in diesel exhaust. The thermochemical formation cascades were calculated for six 3-NBA-derived DNA adducts employing its arylnitrenium ion as precursor using density functional theory (DFT). Clear exothermic pathways were found for four adducts, i.e., 2-(2'-deoxyadenosin-N(6)-yl)-3-aminobenzanthrone, 2-(2'-deoxyguanosin-N(2)-yl)-3-aminobenzanthrone, N-(2'-deoxyguanosin-8-yl)-3-aminobenzanthrone and 2-(2'-deoxyguanosin-8-yl)-3-aminobenzanthrone. All four have been observed to be formed in cell-free experimental systems. The formation of N-(2'-deoxyadenosin-8-yl)-3-aminobenzanthrone is predicted to be not thermochemically viable explaining its absence in either in vitro or in vivo model systems. However, 2-(2'-deoxyadenosin-8-yl)-3-aminobenzanthrone, can be formed, albeit not as a major product, and is a viable candidate for an unknown adenine adduct observed experimentally. 2-nitrobenzanthrone (2-NBA), an isomer of 3-NBA, was also included in the calculations; it has a higher abundance in ambient air than 3-NBA, but a much lower genotoxic potency. Similar thermochemical profiles were obtained for the calculated 2-NBA-derived DNA adducts. This leads to the conclusion that enzymatic activation as well as the stability of its arylnitrenium ion are important determinants of 2-NBA genotoxicity.

Theoretical Investigations into the Role of Aryl Nitrenium Ions' Stability on Their Mutagenic Potential

By using Mulliken and Natural Bond Orbital (NBO) methods based on the density functional theory (DFT), partial charges of exocyclic nitrogen atoms were calculated for nitrenium ions formed from 201 known drugs and 50 Ames positive (mutagenic) compounds containing aryl amine and nitro moieties. The statistical difference of the partial charges was analysed based on the hypothesis that the mutagens have a more negative charge on their exocyclic nitrogen atom resulting in stable nitrenium ions, and thus a longer lifetime to react selectively with DNA; whereas known drugs are not in general mutagenic and therefore have a relatively more positive partial charge. The nitrenium ions with 1° amine parent compounds did not show a statistical difference between drugs and mutagens based on the Mulliken charges. A slight difference was observed in the NBO data where the drugs have more negative partial charge on their exocyclic nitrogen atoms compared with the mutagens. Interestingly, nitrenium ions with aryl nitro drugs as their parent compounds have more negative charge on the exocyclic nitrogen compared with the other drug classes. Aryl nitro drugs are relatively scarce and are often linked to genotoxicity, which fits with the hypotheses proposed. These results indicate that other physical properties besides the stability of the nitrenium ions are important to determine the mutagenic potential of aryl amine and nitro containing compounds.

Evaluation of in vitro assays for assessing the toxicity of cigarette smoke and smokeless tobacco

BACKGROUND

In vitro toxicology studies of tobacco and tobacco smoke have been used to understand why tobacco use causes cancer and to assess the toxicologic impact of tobacco product design changes. The need for toxicology studies has been heightened given the Food and Drug Administration's newly granted authority over tobacco products for mandating tobacco product performance standards and evaluate manufacturers' health claims about modified tobacco products. The goal of this review is to critically evaluate in vitro toxicology methods related to cancer for assessing tobacco products and to identify related research gaps.

METHODS

PubMed database searches were used to identify tobacco-related in vitro toxicology studies published since 1980. Articles published before 1980 with high relevance also were identified. The data were compiled to examine (a) the goals of the study, (b) the methods for collecting test substances, (c) experimental designs, (d) toxicologic end points, and (e) relevance to cancer risk.

RESULTS

A variety of in vitro assays are available to assess tobacco smoke that address different modes of action, mostly using non-human cell models. However, smokeless tobacco products perform poorly in these assays. Although reliable as a screening tool for qualitative assessments, the available in vitro assays have been poorly validated for quantitative comparisons of different tobacco products. Assay batteries have not been developed, although they exist for nontobacco assessments. Extrapolating data from in vitro studies to human risks remains hypothetical.

CONCLUSIONS

In vitro toxicology methods are useful for screening toxicity, but better methods are needed for today's context of regulation and evaluation of health claims.

Known drug space as a metric in exploring the boundaries of drug-like chemical space

In this work, marketed drug compounds (or known drug space) were used as a metric to test the principles of eliminating parent structures of the nitrenium ion (aryl-amine/nitro compounds) as well as sulphur and halogen containing molecules from screening compound collections. Molecules containing such moieties and/or atoms have biological and physiochemical properties, which possibly make them less attractive as leads in drug development. It was found that precursors to the nitrenium ion were relatively abundant in known drug space at 14%. Thus, their simple elimination from drug-like chemical space is not advisable. Interestingly, the mutagenic potential of the nitrenium ions is linked to their stability and quantum mechanical calculations can be used to estimate it. Furthermore, 24% of drugs investigated contained sulphur atoms and around 28% were halogenated. As some sulphur containing moieties were abundant whilst others were scarce, it was deduced that it would be more effective to eliminate specific molecular scaffolds rather than all sulphur containing molecules. In conclusion, it has been shown that by statistically analysing known drug space a better understanding of the boundaries of drug-like chemical space was established which can help medicinal chemists in finding rewarding regions of chemical space.

RecBCD and RecFOR dependent induction of chromosomal deletions by sodium selenite in Salmonella

RecBCD and RecFOR homologous recombination pathways induced bacterial chromosomal duplication-segregation by sodium selenite (SSe) at sub-inhibitory concentrations. This evidence suggests that SSe induces both, double and single DNA strand damage with a concomitant DNA repair response, however the strong dependence for recombinogenic activity of RecB product suggests that the main DNA repair pathway copes with dsDNA breaks. A role for SSe recombinogenic induction is proposed to explain its effect on DNA instability.

Activation of aminoimidazole carcinogens by nitrosation: mutagenicity and nucleotide adducts

2-Amino-3-methylimidazo[4,5-f]quinoline (IQ) and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline(MeIQx) are heterocyclic amines (HCAs) derived from high temperature cooking of meat and thought to cause colon cancer in humans. Reactive nitrogen oxygen species, which are mediators of the inflammatory response, can convert these amines to the corresponding N-nitrosamines, N-NO-IQ and N-NO-MeIQx. This study was designed to evaluate whether these N-nitrosamines are genotoxic and could be responsible, in part, for the high incidence of colon cancer in individuals with colitis. Such an association would counsel reduced intake of well-done red meat by colitis patients. Mutagenicity was evaluated by reversion of a lacZ frameshift allele in three different E. coli strains. Strains DJ701 and DJ702 express recombinant(S. typhimurium) aromatic amine N-acetyltransferase (NAT); DJ702 also expresses recombinant human cytochrome P450 1A2 and NADPH-P450 reductase; and DJ2002 served as an N-acetyltransferase negative control. In strain DJ701, N-NO-IQ and N-NO-MeIQx elicited dose-dependent mutagenicity,which was not further increased in DJ702. Neither nitrosamine was mutagenic in strain DJ2002. While both N-nitrosamines are stable for >4 h (pH 7.4, 37 degrees C), they react with DNA or 2'-deoxyguanosine 3'-monophosphate at lower pH (5.5) to form adducts. HOCl, a component of the inflammatory response,increased adduct formation, as measured by 32P-postlabeling. Following treatment with nuclease P1and separation by two-dimensional thin-layer chromatography and then HPLC, N-NO-IQ and N-NOMeIQxwere shown to form the same adducts as those formed by N-OH-MeIQx or N-OH-IQ, namely N-(deoxyguanosin-8-yl) adducts. In summary, these N-nitrosamines are genotoxic and might be alternatives to their hydroxylamine analogues as activated intermediates leading to initiation of colon cancer in individuals with colitis.

Tissue-specific metabolic activation and mutagenicity of 3-nitrobenzanthrone in MutaMouse

3-Nitrobenzanthrone (3-NBA) is a mutagen and suspected human carcinogen detected in diesel exhaust, airborne particulate matter, and urban soil. We investigated the tissue specific mutagenicity of 3-NBA at the lacZ locus of transgenic MutaMouse following acute single dose or 28-day repeated-dose oral administration. In the acute high dose (50 mg/kg) exposure, increased lacZ mutant frequency was observed in bone marrow and colonic epithelium, but not in liver and bladder. In the repeated-dose study, a dose-dependent increase in lacZ mutant frequency was observed in bone marrow and liver (2- and 4-fold increase above control), but not in lung or intestinal epithelium. In addition, a concentration-dependent increase in mutant frequency (8.5-fold above control) was observed for MutaMouse FE1 lung epithelial cells exposed in vitro. 1-Nitropyrene reductase, 3-NBA reductase, and acetyltransferase activities were measured in a variety of MutaMouse specimens in an effort to link metabolic activation and mutagenicity. High 3-NBA nitroreductase activities were observed in lung, liver, colon and bladder, and detectable N-acetyltransferase activities were found in all tissues except bone marrow. The relatively high 3-NBA nitroreductase activity in MutaMouse tissues, as compared with those in Salmonella TA98 and TA100, suggests that 3-NBA is readily reduced and activated in vivo. High 3-NBA nitroreductase levels in liver and colon are consistent with the elevated lacZ mutant frequency values, and previously noted inductions of hepatic DNA adducts. Despite an absence of induced lacZ mutations, the highest 3-NBA reductase activity was detected in lung. Further studies are warranted, especially following inhalation or intratracheal exposures.

Mutagenic potential of nitrenium ions of nitrobenzanthrones: correlation between theory and experiment

The mutagenic activity of nine substituted nitrobenzanthrone (NBA) derivatives was recently established in the Ames assay and ranged from near inactivity to extremely high mutagenic activity (Takamura-Enya et al. 2006: Mutagenesis 21:399-404). Using thermochemical and molecular modeling techniques, the activation pathway of these NBA derivatives, namely 1-nitro-, 2-nitro-, 3-nitro-, 9-nitro-, 11-nitro-, 1,9-dinitro-, 3,9-dinitro-, 3,11-dinitro-, and 3,9,11-trinitrobenzanthrone, and the formation of the corresponding aryl-nitrenium ions, were investigated using density functional theory calculations. The calculated properties of the NBA derivatives were systematically compared with their bacterial mutagenic potency. Accommodation of the ligand substrates into the binding pocket of the bacterial nitroreductases was not sterically inhibited for the NBAs. Moreover, electron affinities, water elimination energies, esterification, and solvolysis energies did not reveal any possible links with the observed mutagenic potency of the NBAs. However, a strong negative linear correlation was found when the relative energies of the nitrenium ions of the mono and disubstituted NBAs were plotted against the logarithm of the mutagenic potency of the NBAs found in the different Salmonella typhimurium strains. Therefore, our data clearly indicate that the stability of the nitrenium ions is one critical determinant of the mutagenic potency of NBAs in Salmonella tester strains.

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.

In vitro synthesis of 1,N6-etheno-2'-deoxyadenosine and 1,N2-etheno-2'-deoxyguanosine by 2,4-dinitrophenol and 1,3-dinitropyrene in presence of a bacterial nitroreductase

The formation of covalent nitro-PAH DNA adducts and nitro-PAH mediated oxidative lesions are two possible mechanisms for the initiation of nitro-PAH carcinogenesis. Sixty-minute incubation of 1,3-dinitropyrene (100 microM) or 1,4-dinitrophenol (100 microM) with a mixture of 150 microM NADH, 0.5 units of E. coli nitroreductase, 100 microM linoleic acid, 0.5 mM ferrous iron, and 100 microM 2'-deoxyadenosine (2'-dA) or 100 microM 2'-deoxyguanosine (2'-dG) were analyzed by liquid chromatography multistage mass spectrometry. Mixtures of 1,N(6)-etheno-2'-deoxyadenosine (epsilondA) plus 4-oxo-2-nonenal (4-ONE) and 1,N(2)-etheno-2'-deoxyguanosine (epsilondG) plus 4-ONE could be detected from 2'-dA and 2'-dG, respectively. Addition of 2% propanol inhibited the formation of etheno adducts. Analyses of disappearance kinetics of dA and dG showed that dG was more rapidly eliminated than does dA (t[1/2] = 23.3 min and 98.3 min for dG and dA, respectively). Curves of formation kinetics revealed that the peak of epsilondG was at 55.6 min while that of epsilondA was at 186.9 min. These peaks represented 1.43% and 1.25% of the original dG and dA, respectively. In both cases, the peaks were followed by rapid degradations of etheno adducts. The results, obtained in this system, do not allow any extrapolation to realistic cellular responses; nevertheless, these data questioned the validity of the use of unsubstituted etheno adducts as reliable oxidative stress and nitro-PAH exposure biomarkers.

Nitrocompound activation by cell-free extracts of nitroreductase-proficient Salmonella typhimurium strains

A characterization of nitrocompounds activation by cell-free extracts (CFE) of wild-type (AB(+)), SnrA deficient (B(+)), Cnr deficient (A(+)) and SnrA/Cnr deficient (AB(-)) Salmonella typhimurium strains has been done. The Ames mutagenicity test (S. typhimurium his(+) reversion assay) was used, as well as nitroreductase (NR) activity determinations where the decrease in absorbance generated by nitrofurantoin (NFN) reduction and NADP(H) oxidation in the presence of NFN, nitrofurazone (NFZ), metronidazole (MTZ) and 4-nitroquinoline-1-oxide (4NQO) were followed. Different aromatic and heterocyclic compounds were tested for mutagenic activation: 2-nitrofluorene (2-NF); 2,7-dinitrofluorene (2,7-DNF); 1-nitropyrene (1-NP), 1,3-dinitropyrene (1,3-DNP); 1,6-dinitropyrene (1,6-DNP); and 1,8-dinitropyrene (1,8-DNP). Differential mutagenicity was found with individual cell free extracts, being higher when the wild type or Cnr containing extract was used; nevertheless, depending on the nitrocompound, activation was found when either NR, SnrA or Cnr, were present. In addition, all nitrocompounds were more mutagenic after metabolic activation by CFE of NR proficient strains, although AB(-) extract still showed activation capacity. On the other hand, NR activity was predominantly catalyzed by wild type CFE followed by A(+), B(+) and AB(-) extracts in that order. We can conclude that results from the Ames test indicate that Cnr is the major NR, while NFN and NFZ reductions were predominantly catalyzed by SnrA. The characterization of the residual NR activity detected by the mutagenicity assay and the biochemical determinations in the AB(-) CFE needs further investigation.

A duplicated nitrotienyl derivative with antimycobacterial activity: synthesis, X-ray crystallography, biological and mutagenic activity tests

A duplicated nitrotienyl derivative was obtained as a by-product from the synthesis of a proposed molecular hybrid of a nitrotienyl derivative and isoniazid with an expected dual antimycobacteria mechanism. The structure was shown to be the 5,5'-dinitro-2-(2,3-diaza-4-(2'-tienyl)buta-1,3-dienyl)tiophene by X-ray crystallography. The minimal inhibitory concentration (MIC) determination of this compound proved to be promising against Mycobacterium pathogenic strains such as M. avium and M. kansasii, although it had a high level of mutagenicity, as observed in mutagenic activity tests.

Effect of dried plums on colon cancer risk factors in rats

Dried plums (that is, prunes) are a fruit that show promise as a food to lower colon cancer risk, based on their high content of dietary fiber and polyphenolics. In this study, we have examined the effect of diets containing dried plums on the number of colonic precancerous lesions (aberrant crypts, ACs), fecal bile acid concentration, and cecal bacterial enzyme activities related to colon cancer risk. Rats were fed one of four diets: a basal diet (a modified AIN-93G diet), a low-concentration dried plum diet (LCDP, 4.75% dried plum powder), a high-concentration dried plum diet (HCDP, 9.5% dried plum powder), or a diet matched to the carbohydrate content of the HCDP diet (CH-M) for 10 days. All animals were then administered azoxymethane (15 mg/kg, s.c., given two times, 1 wk apart) and fed their respective diets for 9 additional weeks. The number of AC foci (ACF), large ACF (>3 AC/ACF), or ACF multiplicity (AC/ACF) did not differ among the four groups. When compared with the basal diet, rats fed the LCDP diet had significantly lower concentrations of total fecal bile acids, deoxycholic acid, and hyodeoxycholic acid. Rats fed the HCDP diet had significantly lower fecal concentrations of lithocholic acid and hyodeoxycholic acid. The LCDP and HCDP diets significantly decreased the cecal activity of 7alpha-dehydroxylase, and the LCDP also had lower beta-glucuronidase activity. The LCDP, HCDP, and CH-M groups had significantly greater cecal nitroreductase activities than the basal group. There was a significant correlation between 7alpha-dehydroxylase activity and fecal lithocholic acid concentration. Compared with the basal diet, both the LCDP and HCDP diets greatly increased cecal supernatant oxygen radical absorbance capacity (ORAC). These results suggest that, although dried plums did not reduce ACF number, they favorably altered other colon cancer risk factors.

Frameshift mutations induced by four isomeric nitroacridines and their des-nitro counterpart in the lacZ reversion assay in Escherichia coli

Acridines are well-known as compounds that intercalate noncovalently between DNA base pairs and induce +/-1 frameshift mutations at sites of monotonous repeats of a single base. Reactive derivatives of acridines, including acridine mustards and nitroacridines, form covalent adducts in DNA and exhibit mutagenic properties different from the simple intercalators. We compared the frameshift mutagenicity of the cancer chemotherapy drug nitracrine (1-nitro-9-(3'-dimethylaminopropylamino)-acridine), its des-nitro counterpart 9-(3'-dimethylaminopropylamino)-acridine (DAPA), and its 2-, 3-, and 4-nitro isomers (2-, 3-, and 4-nitro-DAPA) in the lacZ reversion assay in Escherichia coli. DAPA is a simple intercalator, much like the widely studied 9-aminoacridine. It most strongly induced +/-1 frameshift mutations in runs of guanine residues and more weakly induced -1 frameshifts in a run of adenine residues. A nitro group in the 1, 3, or 4 position of DAPA reduced the yield of +/-1 frameshift mutations. DAPA weakly induced -2 frameshifts in an alternating CG sequence. In contrast, nitracrine and its 3-nitro isomer resembled the 3-nitroacridine Entozon in effectively inducing -2 frameshift mutations. The 2- and 4-nitro isomers were less effective than the 1- and 3-nitro compounds in -2 frameshift mutagenesis. The results are interpreted with respect to intercalation, steric interactions, effects of base strength on DNA binding, enzymatic processing, and a slipped mispairing model of frameshift mutagenesis.

Oxidative DNA Damage Induced by Carcinogenic Dinitropyrenes in the Presence of P450 Reductase

Nitropyrenes are widespread in the environment due to mainly diesel engine emissions. Dinitropyrenes (DNPs), especially 1,8-dinitropyrene (1,8-DNP) and 1,6-dinitropyrene (1,6-DNP), are much more potent mutagens than other nitropyrenes. The carcinogenicity of 1,8-DNP and 1,6-DNP is stronger than 1,3-dinitropyrene (1,3-DNP). It is considered that adduct formation after metabolic activation plays an important role in the expression of carcinogenicity of nitropyrenes. However, Djuric et al. [(1993) Cancer Lett.] reported that oxidative DNA damage was also found as well as adduct formation in rats treated with 1,6-DNP. We investigated oxidative DNA damage by DNPs in the presence of NAD(P)H-cytochrome P450 reductase using 32P-5'-end-labeled DNA. After P450 reductase treatment, DNPs induced Cu(II)-mediated DNA damage in the presence of NAD(P)H. The intensity of DNA damage by 1,8-DNP or 1,6-DNP was stronger than 1,3-DNP. We also examined synthetic 1-nitro-8-nitrosopyrene (1,8-NNOP) and 1-nitro-6-nitrosopyrene (1,6-NNOP) as one of the metabolites of 1,8-DNP and 1,6-DNP, respectively, to find that 1,8-NNOP and 1,6-NNOP induced Cu(II)-mediated DNA damage in the presence of NAD(P)H but untreated DNPs did not. In both cases of P450 reductase-treated DNPs and NNOPs, catalase and a Cu(I) specific chelator attenuated DNA damage, indicating the involvement of H2O2 and Cu(I). Using a Clarke oxygen electrode, oxygen consumption by the reaction of NNOPs with NAD(P)H and Cu(II) was measured to find that NNOP was nonenzymatically reduced by NAD(P)H and that the addition of Cu(II) promoted the redox cycle. Therefore, these results suggest that DNPs are enzymatically reduced to NNOPs via nitro radical anion and that NNOPs are further reduced nonenzymatically by NAD(P)H. Subsequently, autoxidation of nitro radical anion and the reduced form of NNOP occurs, resulting in O2- generation and DNA damage. We conclude that oxidative DNA damage in addition to DNA adduct formation may play important roles in the carcinogenesis of DNPs via their metabolites.

Reductive transformation of TNT by Escherichia coli: pathway description

The reductive transformation of 2,4,6-trinitrotoluene (TNT) was studied using aerobically grown Escherichia coli cultures. In the absence of an external carbon or energy source, E. coli resting cells transformed TNT to hydroxylaminodinitrotoluenes (2HADNT, 4HADNT, with 4HADNT as the dominant isomer), aminodinitrotoluenes (4ADNT, with sporadic detection of 2ADNT), 2,4-di(hydroxylamino)-6-nitrotoluene (24D(HA)6NT), 2,4-diamino-6-nitrotoluene (24DA6NT), and an additional compound which was tentatively identified as a (hydroxylamino)aminonitrotoluene isomer via gas chromatography/mass spectroscopy and spectral analysis. The resting cell assay, performed in an oxygen-free atmosphere, avoided formation of azoxy dimers and provided good mass balances. Significant preference for reduction in the para versus ortho position was detected. The formation of 24D(HA)6NT, but not ADNT, appeared inhibited by the presence of TNT. The rate and extent of TNT reduction were significantly enhanced at higher cell densities, or by supplying an exogenous reducing power source, revealing the importance of enzyme concentration and reducing power. Whether the oxygen-insensitive E. coli nitroreductases, encoded by nfsA and nfsB, directly catalyze the TNT reduction or account for the complete TNT transformation pathway, remains to be determined.

Mechanism of chromate reduction by the Escherichia coli protein, NfsA, and the role of different chromate reductases in minimizing oxidative stress during chromate reduction

Chromate [Cr(VI)] is a serious environmental pollutant, which is amenable to bacterial bioremediation. NfsA, the major oxygen-insensitive nitroreductase of Escherichia coli, is a flavoprotein that is able to reduce chromate to less soluble and less toxic Cr(III). We show that this process involves single-electron transfer, giving rise to a flavin semiquinone form of NfsA and Cr(V) as intermediates, which redox cycle, generating more reactive oxygen species (ROS) than a divalent chromate reducer, YieF. However, NfsA generates less ROS than a known one-electron chromate reducer, lipoyl dehydrogenase (LpDH), suggesting that NfsA employs a mixture of uni- and di-valent electron transfer steps. The presence of YieF, ChrR (another chromate reductase we previously characterized), or NfsA in an LpDH-catalysed chromate reduction reaction decreased ROS generation by c. 65, 40, or 20%, respectively, suggesting that these enzymes can pre-empt ROS generation by LpDH. We previously showed that ChrR protects Pseudomonas putida against chromate toxicity; here we show that NfsA or YieF overproduction can also increase the tolerance of E. coli to this compound.

Comparative study of the antioxidant effect of ardisin and epigallocatechin gallate in rat hepatocytes exposed to benomyl and 1-nitropyrene

The objective of this study was to compare the antioxidant effect of ardisin and epigallocatechin 3-O-gallate (EGCG) in hepatocytes exposed to either benomyl or 1-nitropyrene (1-NP). Rat hepatocytes were incubated in a serum-free medium with non-cytotoxic concentrations of either ardisin (0.27 microg/ml) or EGCG (3 microg/ml), and with either benomyl (35 microg/ml) or 1-NP (0.25 microg/ml). The level of malondialdehyde (MDA) as a marker of lipid peroxidation was determined, as well as the content of glutathione (GSH) and the activities of glutathione peroxidase (GPx) and glutathione reductase (GR). In comparison to the control, the concentration of GSH improved 282% (P<0.05) and 260% (P<0.05) after the cells were pre-incubated with ardisin or EGCG and then exposed to benomyl, respectively. The activity of GPx decreased 55% with ardisin (P<0.05) and 51% with EGCG (P<0.05), and MDA decreased 7% and 23% (P<0.05) with the same treatments. The concentration of GSH also improved when the cells were incubated with either EGCG (49%, P<0.05) or ardisin (83%, P<0.05) simultaneously with 1-NP, relative to 1-NP alone. Moreover, ardisin decreased MDA formation by 65% (p<0.05), and enhanced the activity of GR by 137% (P<0.05). These results suggest that ardisin is a better suppressor of lipid peroxidation induced by benomyl and 1-NP than EGCG. It is concluded that ardisin and EGCG are potent antioxidants that can afford protection against free radical mediated diseases.