Mutagenicity of some commercially available nitro compounds for Salmonella typhimurium

Determination of nitrobenzene potential genotoxic impurities in nifedipine with GC-MS

There is an increasing scientific interest in the detection of genotoxic impurities (GTIs), with nitrobenzene compounds being considered potential genotoxic impurities due to their structural alerts, which demonstrates a threat to drug safety for patient. While current reports on the detection of nifedipine impurity primarily focus on general impurities in nifedipine. In this study, an effective and simple gas chromatography-mass spectrometry (GC-MS) method was established and verified for the separation and quantification of 2-nitrotoluene, 2-nitrobenzyl alcohol, 2-nitrobenzaldehyde, 3-nitrobenzaldehyde, 4-nitrobenzaldehyde, and 2-nitrobenzyl bromide in nifedipine, which have not been previously reported. The validation of this GC-MS method was conducted following the International Conference of Harmonization (ICH) guidelines, exhibiting good linearity within the range of 2-40 μg/g and accuracy between 84.6 % and 107.8 %, the RSD% of intra-day and inter-day precision was in the range of 1.77-4.55 %, stability and robustness also met acceptance criteria. This method filled the gap in detection method for nitrobenzene compounds in nifedipine, offering a novel method and technical support for nifedipine quality control.

Development and application of UPLC-Q-Orbitrap HRMS methods for the determination of eight metabolites of p-chloronitrobenzene in human urine

Accurate, reliable, and sensitive methods for the determination of eight metabolites of p-chloronitrobenzene (p-CNB) were developed based on ultra-performance liquid chromatography - quadrupole - orbitrap high resolution mass spectrometry (UPLC-Q-Orbitrap HRMS). The free and conjugated forms of metabolites were determined before and after urine samples were hydrolyzed with acid. Subsequently, three solid phase extraction steps were used for concentration and purification. The calibration curves of the eight metabolites exhibited good linearity with an R2 of >0.999, and the precision was good as well, with the coefficient of variations of intra-day and inter-day being lower than 7.0 % and 8.5 %, respectively. Analytical accuracy for all metabolites varied within ranges of 76.0-102.9 %, and the limit of detection and limit of quantification of all the metabolites varied within ranges of 0.2-7.7 μg/L and 0.6-25.6 μg/L in human urine, respectively. In addition, the application potential of the proposed methods were evaluated by applying them to the determination of metabolites in the urine of workers exposed to p-CNB, and these results showed that these methods were accurate, reliable, and sensitive, which makes them an excellent choice for detecting the metabolites of p-CNB in the urine of exposed workers.

DprE1 and Ddn as promising therapeutic targets in the development of novel anti-tuberculosis nitroaromatic drugs

Tuberculosis remains the second deadliest infectious disease in humans and a public health threat due to the emergence of multidrug-resistant (MDR-TB) and extensively drug-resistant (XDR-TB) strains. Therefore, it is urgent to identify new anti-tuberculosis treatments and novel therapeutic targets to prevent the emergence of resistance. In recent years, the study of anti-tuberculosis properties of nitroaromatic compounds has led to the identification of two novel biological targets, the deazaflavin (F420)-dependent nitroreductase Ddn and the decaprenylphosphoryl-β-d-ribose 2'-epimerase DprE1. This review aims to show why Ddn and DprE1 are promising therapeutic targets and highlight nitroaromatic compounds interest in developing new anti-tuberculosis treatments active against MDR-TB and XDR-TB. Despite renewed interest in the development of new anti-tuberculosis nitroaromatic compounds, pharmaceutical companies often exclude nitro-containing molecules from their drug discovery programs because of their toxic and mutagenic potential. This exclusion results in missed opportunities to identify new nitroaromatic compounds and promising therapeutic targets.

Expansion of the MutS Gene Family in Plants

The MutS gene family is distributed across the tree of life and is involved in recombination, DNA repair, and protein translation. Multiple evolutionary processes have expanded the set of MutS genes in plants relative to other eukaryotes. Here, we investigate the origins and functions of these plant-specific genes. Land plants, green algae, red algae, and glaucophytes share cyanobacterial-like MutS1 and MutS2 genes that presumably were gained via plastid endosymbiotic gene transfer. MutS1 was subsequently lost in some taxa, including seed plants, whereas MutS2 was duplicated in Viridiplantae (i.e., land plants and green algae) with widespread retention of both resulting paralogs. Viridiplantae also have two anciently duplicated copies of the eukaryotic MSH6 gene (i.e., MSH6 and MSH7) and acquired MSH1 via horizontal gene transfer - potentially from a nucleocytovirus. Despite sharing the same name, "plant MSH1" is not directly related to the gene known as MSH1 in some fungi and animals, which may be an ancestral eukaryotic gene acquired via mitochondrial endosymbiosis and subsequently lost in most eukaryotic lineages. There has been substantial progress in understanding the functions of MSH1 and MSH6/MSH7 in plants, but the roles of the cyanobacterial-like MutS1 and MutS2 genes remain uncharacterized. Known functions of bacterial homologs and predicted protein structures, including fusions to diverse nuclease domains, provide hypotheses about potential molecular mechanisms. Because most plant-specific MutS proteins are targeted to the mitochondria and/or plastids, the expansion of this family appears to have played a large role in shaping plant organelle genetics.

Hazard characterization of carcinogenicity, mutagenicity, and reproductive toxicity for short chain primary nitroalkanes

Nitroalkanes are organic aliphatic hydrocarbon compounds with a nitro moiety that are commonly used as solvents or intermediates to synthesize a variety of organic compounds due to their inherent reactivity. In June 2020, a harmonized classification and labeling (CLH) proposal was submitted to the European Chemicals Agency (ECHA) for the following harmonized carcinogenicity, mutagenicity, and reproductive toxicity ("CMR") classifications for nitromethane (NM), nitroethane (NE), and 1-nitropropane (1-NP): NM Carc. 1B and Repr. 1B; NE Repr. 1B; and 1-NP Repr. 2. In this assessment, a weight of evidence (WoE) evaluation of studies on animal carcinogenicity and reproductive and developmental toxicity, genotoxicity, and mode of action for these three nitroalkanes was performed to critically assess the relevance of the proposed CMR classifications. Overall, the WoE indicates that NM, NE, and 1-NP are not carcinogenic, genotoxic, nor selective reproductive or developmental toxicants. Based on our analysis, classifying NM, NE, and 1-NP as Category 2 reproductive toxicants is most appropriate. Furthermore, not classifying NE and 1-NP with respect to their carcinogenicity is appropriate based on the available studies for this endpoint coupled with negative results in genotoxicity studies, metabolism data, and in silico predictions. We determined that the classification for NM of Carc. 1B is not appropriate, based on the fact that rat mammary and harderian tumors are likely not relevant to humans and lung and liver tumors reported in mice were equivocal in their dose-response and statistical significance.

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.

Formation, Accumulation, and Hydrolysis of Endogenous and Exogenous Formaldehyde-Induced DNA Damage

Formaldehyde is not only a widely used chemical with well-known carcinogenicity but is also a normal metabolite of living cells. It thus poses unique challenges for understanding risks associated with exposure. N(2-)hydroxymethyl-dG (N(2)-HOMe-dG) is the main formaldehyde-induced DNA mono-adduct, which together with DNA-protein crosslinks (DPCs) and toxicity-induced cell proliferation, play important roles in a mutagenic mode of action for cancer. In this study, N(2)-HOMe-dG was shown to be an excellent biomarker for direct adduction of formaldehyde to DNA and the hydrolysis of DPCs. The use of inhaled [(13)CD2]-formaldehyde exposures of rats and primates coupled with ultrasensitive nano ultra performance liquid chromatography-tandem mass spectrometry permitted accurate determinations of endogenous and exogenous formaldehyde DNA damage. The results show that inhaled formaldehyde only reached rat and monkey noses, but not tissues distant to the site of initial contact. The amounts of exogenous adducts were remarkably lower than those of endogenous adducts in exposed nasal epithelium. Moreover, exogenous adducts accumulated in rat nasal epithelium over the 28-days exposure to reach steady-state concentrations, followed by elimination with a half-life (t1/2) of 7.1 days. Additionally, we examined artifact formation during DNA preparation to ensure the accuracy of nonlabeled N(2)-HOMe-dG measurements. These novel findings provide critical new data for understanding major issues identified by the National Research Council Review of the 2010 Environmental Protection Agency's Draft Integrated Risk Information System Formaldehyde Risk Assessment. They support a data-driven need for reflection on whether risks have been overestimated for inhaled formaldehyde, whereas underappreciating endogenous formaldehyde as the primary source of exposure that results in bone marrow toxicity and leukemia in susceptible humans and rodents deficient in DNA repair.

Ecotoxicological assessment of a high energetic and insensitive munitions compound: 2,4-dinitroanisole (DNAN)

The high explosive nitroaromatic 2,4-dinitroanisole (DNAN) is less shock sensitive than 2,4,6-trinitrotoluene (TNT), and is proposed as a TNT replacement for melt-cast formulations. Before using DNAN in munitions and potentially leading to environmental impact, the present study examines the ecotoxicity of DNAN using selected organisms. In water, DNAN decreased green algae Pseudokirchneriella subcapitata growth (EC50 = 4.0mg/L), and bacteria Vibrio fischeri bioluminescence (Microtox, EC50 = 60.3mg/L). In soil, DNAN decreased perennial ryegrass Lolium perenne growth (EC50 =7 mg/kg), and is lethal to earthworms Eisenia andrei (LC50 = 47 mg/kg). At sub-lethal concentrations, DNAN caused an avoidance response (EC50 = 31 mg/kg) by earthworms. The presence of DNAN and 2-amino-4-nitroanisole in earthworms and plants suggested a role of these compounds in DNAN toxicity. Toxicity of DNAN was compared to TNT, tested under the same experimental conditions. These analyses showed that DNAN was equally, or even less deleterious to organism health than TNT, depending on the species and toxicity test. The present studies provide baseline toxicity data to increase the understanding of the environmental impact of DNAN, and assist science-based decision makers for improved management of potential DNAN contaminated sites.

Proposed model for in vitro interaction between fenitrothion and DNA, by using competitive fluorescence, (31)P NMR, (1)H NMR, FT-IR, CD and molecular modeling

In this work we proposed a model for in vitro interaction of fenitrothion (FEN) with calf thymus-DNA by combination of multispectroscopic and two dimensional molecular modeling (ONIOM) methods. The circular dichroism results showed that FEN changes the conformation of B-DNA and caused some changes to C-DNA form. The FT-IR results confirmed a partial intercalation between FEN and edges of all base pairs. The competitive fluorescence, using methylene blue as fluorescence probe, in the presence of increasing amounts of FEN, revealed that FEN is able to release the non-intercalated methylene blue from the DNA. The weak chemical shift and peak broadening of (1)H NMR spectrum of FEN in the presence of DNA confirmed a non-intercalation mode. The (31)P NMR showed that FEN interacts more with DNA via its -NO2 moiety. The ONIOM, based on the hybridization of QM/MM (DFT, 6.31++G (d,p)/UFF) methodology, was also performed by Gaussian 2003 package. The results revealed that the interaction is base sequence dependent, and FEN interacts more with AT base sequences.

A core in vitro genotoxicity battery comprising the Ames test plus the in vitro micronucleus test is sufficient to detect rodent carcinogens and in vivo genotoxins

In vitro genotoxicity testing needs to include tests in both bacterial and mammalian cells, and be able to detect gene mutations, chromosomal damage and aneuploidy. This may be achieved by a combination of the Ames test (detects gene mutations) and the in vitro micronucleus test (MNvit), since the latter detects both chromosomal aberrations and aneuploidy. In this paper we therefore present an analysis of an existing database of rodent carcinogens and a new database of in vivo genotoxins in terms of the in vitro genotoxicity tests needed to detect their in vivo activity. Published in vitro data from at least one test system (most were from the Ames test) were available for 557 carcinogens and 405 in vivo genotoxins. Because there are fewer publications on the MNvit than for other mammalian cell tests, and because the concordance between the MNvit and the in vitro chromosomal aberration (CAvit) test is so high for clastogenic activity, positive results in the CAvit test were taken as indicative of a positive result in the MNvit where there were no, or only inadequate data for the latter. Also, because Hprt and Tk loci both detect gene-mutation activity, a positive Hprt test was taken as indicative of a mouse-lymphoma Tk assay (MLA)-positive, where there were no data for the latter. Almost all of the 962 rodent carcinogens and in vivo genotoxins were detected by an in vitro battery comprising Ames+MNvit. An additional 11 carcinogens and six in vivo genotoxins would apparently be detected by the MLA, but many of these had not been tested in the MNvit or CAvit tests. Only four chemicals emerge as potentially being more readily detected in MLA than in Ames+MNvit--benzyl acetate, toluene, morphine and thiabendazole--and none of these are convincing cases to argue for the inclusion of the MLA in addition to Ames+MNvit. Thus, there is no convincing evidence that any genotoxic rodent carcinogens or in vivo genotoxins would remain undetected in an in vitro test battery consisting of Ames+MNvit.

Databases applicable to quantitative hazard/risk assessment--towards a predictive systems toxicology

The Workshop on The Power of Aggregated Toxicity Data addressed the requirement for distributed databases to support quantitative hazard and risk assessment. The authors have conceived and constructed with federal support several databases that have been used in hazard identification and risk assessment. The first of these databases, the EPA Gene-Tox Database was developed for the EPA Office of Toxic Substances by the Oak Ridge National Laboratory, and is currently hosted by the National Library of Medicine. This public resource is based on the collaborative evaluation, by government, academia, and industry, of short-term tests for the detection of mutagens and presumptive carcinogens. The two-phased evaluation process resulted in more than 50 peer-reviewed publications on test system performance and a qualitative database on thousands of chemicals. Subsequently, the graphic and quantitative EPA/IARC Genetic Activity Profile (GAP) Database was developed in collaboration with the International Agency for Research on Cancer (IARC). A chemical database driven by consideration of the lowest effective dose, GAP has served IARC for many years in support of hazard classification of potential human carcinogens. The Toxicological Activity Profile (TAP) prototype database was patterned after GAP and utilized acute, subchronic, and chronic data from the Office of Air Quality Planning and Standards. TAP demonstrated the flexibility of the GAP format for air toxics, water pollutants and other environmental agents. The GAP format was also applied to developmental toxicants and was modified to represent quantitative results from the rodent carcinogen bioassay. More recently, the authors have constructed: 1) the NIEHS Genetic Alterations in Cancer (GAC) Database which quantifies specific mutations found in cancers induced by environmental agents, and 2) the NIEHS Chemical Effects in Biological Systems (CEBS) Knowledgebase that integrates genomic and other biological data including dose-response studies in toxicology and pathology. Each of the public databases has been discussed in prior publications. They will be briefly described in the present report from the perspective of aggregating datasets to augment the data and information contained within them.

Framework analysis for the carcinogenic mode of action of nitrobenzene

Nitrobenzene (CASRN: 98-95-3) has been shown to induce cancers in many tissues including kidney, liver, and thyroid, following chronic inhalation in animals. However, with a few exceptions, genotoxicity assays using nitrobenzene have given negative results. Some DNA binding/adduct studies have brought forth questionable results and, considering the available weight of evidence, it does not appear that nitrobenzene causes cancer via a genotoxic mode of action. Nitrobenzene produces a number of free radicals during its reductive metabolism, in the gut as well as at the cellular level, and generates superoxide anion as a by-product during oxidative melabolism. The reactive species generated during nitrobenzene metabolism are considered candidates for carcinogenicity. Furthermore, several lines of evidence suggest that nitrobenzene exerts its carcinogenicity through a non-DNA reactive (epigenetic) fashion, such as a strong temporal relationship between non-, pre-, and neoplastic lesions leading to carcinogenesis. In this report, we first describe the absorption, distribution, metabolism, and excretion of nitrobenzene followed by a summary of the available genotoxicity studies and the only available cancer bioassay. We subsequently refer to the mode of action framework of the U.S. Environmental Protection Agency's 2005 Guidelines for Carcinogen Risk Assessment as a basis for presenting possible modes of action for nitrobenzene-induced cancers of the liver, thyroid, and kidney, as supported by the available experimental data. The rationale(s) regarding human relevance of each mode of action is also presented. Finally, we hypothesize that the carcinogenic mode of action for nitrobenzene is multifactorial in nature and reflective of free radicals, inflammation, and/or altered methylation.

Testing strategies in mutagenicity and genetic toxicology: An appraisal of the guidelines of the European Scientific Committee for Cosmetics and Non-Food Products for the evaluation of hair dyes

The European Scientific Committee on Cosmetics and Non-Food Products (SCCNFP) guideline for testing of hair dyes for genotoxic/mutagenic/carcinogenic potential has been reviewed. The battery of six in vitro tests recommended therein differs substantially from the batteries of two or three in vitro tests recommended in other guidelines. Our evaluation of the chemical types used in hair dyes and comparison with other guidelines for testing a wide range of chemical substances, lead to the conclusion that potential genotoxic activity may effectively be determined by the application of a limited number of well-validated test systems that are capable of detecting induced gene mutations and structural and numerical chromosomal changes. We conclude that highly effective screening for genotoxicity of hair dyes can be achieved by the use of three assays, namely the bacterial gene mutation assay, the mammalian cell gene mutation assay (mouse lymphoma tk assay preferred) and the in vitro micronucleus assay. These need to be combined with metabolic activation systems optimised for the individual chemical types. Recent published evidence [D. Kirkland, M. Aardema, L. Henderson, L. Müller, Evaluation of the ability of a battery of three in vitro genotoxicity tests to discriminate rodent carcinogens and non-carcinogens. I. Sensitivity, specificity and relative predictivity, Mutat. Res. 584 (2005) 1-256] suggests that our recommended three tests will detect all known genotoxic carcinogens, and that increasing the number of in vitro assays further would merely reduce specificity (increase false positives). Of course there may be occasions when standard tests need to be modified to take account of special situations such as a specific pathway of biotransformation, but this should be considered as part of routine testing. It is clear that individual dyes and any other novel ingredients should be tested in this three-test battery. However, new products are formed on the scalp by reaction between the chemicals present in hair-dye formulations. Ideally, these should also be tested for genotoxicity, but at present such experiences are very limited. There is also the possibility that one component could mask the genotoxicity of another (e.g. by being more toxic), and so it is not practical at this time to recommend routine testing of complete hair-dye formulations as well. The most sensible approach would be to establish whether any reaction products within the hair-dye formulation penetrate the skin under normal conditions of use and test only those that penetrate at toxicologically relevant levels in the three-test in vitro battery. Recently published data [D. Kirkland, M. Aardema, L. Henderson, L. Müller, Evaluation of the ability of a battery of three in vitro genotoxicity tests to discriminate rodent carcinogens and non-carcinogens. I. Sensitivity, specificity and relative predictivity, Mutat. Res. 584 (2005) 1-256] suggest the three-test battery will produce a significant number of false as well as real positives. Whilst we are aware of the desire to reduce animal experiments, determining the relevance of positive results in any of the three recommended in vitro assays will most likely have to be determined by use of in vivo assays. The bone marrow micronucleus test using routes of administration such as oral or intraperitoneal may be used where the objective is extended hazard identification. If negative results are obtained in this test, then a second in vivo test should be conducted. This could be an in vivo UDS in rat liver or a Comet assay in a relevant tissue. However, for hazard characterisation, tests using topical application with measurement of genotoxicity in the skin would be more appropriate. Such specific site-of-contact in vivo tests would minimise animal toxicity burden and invasiveness, and, especially for hair dyes, be more relevant to human routes of exposure, but there are not sufficient scientific data available to allow recommendations to be made. The generation of such data is encouraged.

In-silico screening of high production volume chemicals for mutagenicity using the MCASE QSAR expert system

Computational screening is suggested as a way to set priorities for further testing of high production volume (HPV) chemicals for mutagenicity and other toxic endpoints. Results are presented for batch screening of 2484 HPV chemicals to predict their mutagenicity in Salmonella typhimurium (Ames test). The chemicals were tested against 15 databases for Salmonella strains TA100, TA1535, TA1537, TA97 and TA98, both with metabolic activation (using rat liver and hamster liver S9 mix test) and without metabolic activation. Of the 2484 chemicals, 1868 are predicted to be completely nonmutagenic in all of the 15 data modules and 39 chemicals were found to contain structural fragments outside the knowledge of the expert system and therefore suggested for further evaluation. The remaining 616 chemicals were found to contain different biophores (structural alerts) believed to be linked to mutagenicity. The chemicals were ranked indescending order according to their predicted mutagenic potential and the first 100 chemicals with highest mutagenicity scores are presented. The screening result offers hope that rapid and inexpensive computational methods can aid in prioritizing the testing of HPV chemicals, save time and animals and help to avoid needless expense.

From mutagenic to non-mutagenic nitroarenes: effect of bulky alkyl substituents on the mutagenic activity of 4-nitrobiphenyl in Salmonella typhimurium. Part I. Substituents ortho to the nitro group and in 2'-position

Eleven alkyl substituted derivatives of 4-nitrobiphenyl (4NBp) and two corresponding nitroso compounds were synthesised and tested for mutagenic potency in strains TA98 and TA100 of Salmonella typhimurium. The mutagenicity of compounds substituted ortho to the nitro group (3-methyl-, 3-ethyl-, 3-isopropyl-, 3-tertbutyl-, 3, 5-diethyl-, 3,5-diisopropyl-, and 3,5-ditertbutyl-4NBp) decreased with growing steric demand of the alkyl substituents in both tester strains. The most sterically hindered compounds were non-mutagenic even at highest concentrations. This reduction of mutagenicity is correlated with deviations from the coplanar orientation of the nitro group relative to the aromatic plane. Since a comparable decrease of mutagenicity for the nitroso compounds (4NOBp and 3-tertbutyl-4NOBp) was not observed, the first reduction step of non-planar nitro groups must be inhibited. Alkyl groups in the 2'-position of 4NBp (2'-methyl-, 2'-ethyl-, 2'-isopropyl-, and 2',4', 6'-trimethyl-4NBp) also reduced mutagenic activity with increasing size and removed it completely for the most sterically hindered species (2'-isopropyl-, 2',4',6'-trimethyl-4NBp). In this case, the co-planarity of the nitro group is not affected but the twisting of the two aromatic rings, which is associated with a less effective charge delocalisation of the nitrenium ion. The experimental mutagenicities of all nitro compounds were compared to predicted values, that are based on recently developed empirical equations. While there was reasonable correspondence for the parent compound (4NBp), its ortho methylated derivative (3-methyl-4NBp) and two highly hydrophobic dialkylated species (3,5-diisopropyl- and 3, 5-ditertbutyl-4NBp), predictions for all other alkyl substituted compounds were too high. Thus, steric parameters should be included to improve the general validity of predictions by means of quantitative structure-activity relationships (QSAR).

Induction of Mutation in Drosophila melanogaster fed a hexane extract of vegetables grown in soil contaminated with particulates from diesel engine exhaust

Trans-heterozygous larvae of the improved high-bioactivation cross Drosophila melanogaster (ORR;flr3/TM3, Ser females mated with mwh males) were fed with medium containing hexane extract of the edible portion of five vegetables grown in three different soil treatments for 48 hours. the wing hairs of the surviving flies were analysed for the frequency and size of single and twin spots. It was found that the clone induction frequency of the wing hairs of flies treated with a hexane extract of leaves of sacred basil and green kuang futsoi was not significantly different from that of the controls. Conclusive results were obtained when larvae were raised on the medium containing hexane extracts of lettuce and water spinach grown in contaminated soils. Interestingly, the extracts of multiply onion, grown both in the treated and in the untreated soils, induced mutation in the wing spot test. It was concluded that some plants grown in soil contaminated with diesel exhaust provoked mutagenic responses, whereas some showed negative results.

Mutagenicities of 2,4- and 2,6-dinitrotoluenes and their reduced products in Salmonella typhimurium nitroreductase- and O-acetyltransferase-overproducing Ames test strains

Mutagenicities of 2,4- and 2,6-dinitrotoluene (2,4-and 2,6-DNT), and reduced metabolites formed by the incubation of 2,4- and 2,6-DNT with Salmonella typhimurium TA98, were tested using S. typhimurium YG strains possessing high level of nitroreductase (NR) and/or O-acetyltransferase (OAT) activities. All compounds tested showed greatest mutagenic activities toward strains YG1041 and YG1042, which possess high levels of NR and OAT activities. The relative mutagenic activities of 2,4-DNT and its related compounds toward YG1041 and YG1042 were aminonitrotoluenes<2,4-DNT<2,2'-dimethyl-5, 5'-dinitroazoxybenzene (2,2'-DM-5, 5'-DNAOB)4-hydroxylamino-2-nitrotoluene (4HA2NT)<<4, 4'-dimethyl-3,3'-dinitroazoxybenzene (4,4'-DM-3,3'-DNAOB), and aminonitrotoluenes (2A4AT, 4A2NT)<2,4-DNT<4HA2NT4,4'-dimethyl-3, 3'-dinitroazoxybenzene (4,4'-DM-3,3'-DNAOB)<2HA4NT, respectively. In addition, the relative mutagenic activities of 2,6-DNT and its related compounds toward YG1041 and YG1042 were 2, 6-DNT<2-hydroxylamino-6-nitrotoluene (2HA6NT)<2,2'-dimethyl-3, 3'-dinitroazoxybenzene (2,2'-DM-3,3'-DNAOB), and 2-amino-6-nitrotoluene (2A6NT)<2,6-DNT<2HA6NT, respectively. These results, together with previous findings, suggested that aminohydroxylamino dimethylazoxybenzenes or aminohydroxylamino dimethylazobenzenes produced either by the reduction of hydroxylaminonitrotoluenes or by the reduction of dimethyl dinitroazoxybenzenes are active metabolites responsible for the mutagenic activities of 2,4- and 2,6-DNT.

Genotoxic activity of important nitrobenzenes and nitroanilines in the Ames test and their structure-activity relationship

The most important commercially available nitro- and aminobenzenes and the explosive trinitrobenzene were tested for mutagenicity in the Salmonella typhymurium TA 98 and TA 100 both in the absence and presence of S 9. Ten of the 14 compounds tested (71%) were mutagenic. All the substances showed positive results in TA 98 and 4 substances were also mutagenic in TA 100. The three diaminobenzenes and 4-nitroaniline were mutagenic only with metabolic activation. All other compounds did not require the addition of S 9. Only nitrobenzene, 1,2-dinitrobenzene, aniline and 2-nitroaniline were negative in both strains. In summary, all substances that are derived from nitrobenzene or aniline by addition of a nitro group in the meta- or para-position were mutagenic, whereas nitrobenzene and aniline themselves and their ortho-derivates were nonmutagenic. The possible relationships between the position of the substituents and the mutagenicity are discussed.

Nitroreductase independent mutagenicity of 1-halogenated-2,4-dinitrobenzenes

The 1-halogen substituted 2,4-dinitrobenzenes have been found to be mutagenic in Salmonella TA98 with an activity order of 1-fluoro > 1-chloro > 1-bromo > 1-iodo. This specific activity was not lowered in the nitroreductase deficient strain TA98NR and O-acetyltransferase deficient strain TA98/1,8-DNP6, indicating that the mutagenicity of these compounds is not dependent upon -NO2 reduction to -NHOH and its subsequent esterification. The activity was, rather, higher in the former and remained almost equal in the latter strain compared to TA98, suggesting these compounds to react directly with bacterial DNA. Further, the reaction of these halodinitrobenzenes with methionine, at physiological pH, resulting in the formation of 1-S-methyldinitrobenzene showed that these compounds have the ability to attack DNA directly through nucleophilic substitution of the halogen atom and the role of the bacterial nitroreductases for production of mutagenic lesions was not considered. It was, further, proposed that these compounds interact with DNA through SNAr mechanism instead of SN2 cited in literature.

Review of mutagenicity of monocyclic aromatic amines: quantitative structure-activity relationships

Monocyclic aromatic amines (MAAs) are environmental pollutants. Many of them are genotoxic and impose hazards to human health. The mutagenicity of more than 80 of these amines was reviewed with primary emphasis on evaluation by the Ames Salmonella/microsome testing system. Many amines are mutagenic in Salmonella tester strains TA98 and TA100, but S9 mix is required for activity for most of the active ones. 2,4-Diaminotoluene, 2,4-diaminoethylbenzene, and a few amines containing a nitro-group are direct mutagens. There are several quantitative structure-activity relationship (QSAR) models which rationalize mutagenicity of many aromatic amines and several parameters, such as the lowest unoccupied molecular orbital energy (ELUMO), highest occupied molecular orbital energy (EHOMO), and hydrophobicity that are important. What factors determine the minimum requirement for the compound to be mutagenic and what factors determine the extent of mutagenicity suggest questions for further study.

Genotoxicity of nitro musks in the micronucleus test with human lymphocytes in vitro and the human hepatoma cell line Hep G2

The nitro musk compounds musk xylene (1-tert.-butyl-3,5-dimethyl-2,4,6-trinitrobenzene), musk ketone (4-tert.-butyl-3,5-dinitro-2,6-dimethylacetophenone), musk ambrette (1-tert.-butyl-4-methyl-6-methoxy-3,5-dinitrobenzene), musk moskene (1,1,3,3,5-pentamethyl-4,6-dinitroindane) and musk tibetene (1-tert.-butyl-3,4,5-trimethyl-2,6-dinitrobenzene) were tested for their genotoxic activity in the micronucleus test (MN) with human lymphocytes in vitro and the human hepatoma cell line Hep G2. Compound concentrations were employed up to cytotoxic doses. Musk xylene, musk ketone, musk ambrette, musk moskene and musk tibetene revealed no genotoxicity in the micronucleus test with human lymphocytes and with the human hepatoma cell line Hep G2.

A comparative study of five nitro musk compounds for genotoxicity in the SOS chromotest and Salmonella mutagenicity

Five nitro musk compounds widely used in cosmetics and detergents were examined for DNA-damaging and mutation-inducing properties. For this purpose two short-time assays were used, the SOS chromotest and the Salmonella/mammalian microsome test. Musk ambrette showed mutagenicity in Salmonella typhimurium TA 100 requiring metabolic activation by rat liver postmitochondrial supernatant (S9) but it lacked mutagenicity in the absence of S9 and genotoxicity in the SOS chromotest. Musk xylene, musk ketone, musk moskene and musk tibetene showed neither mutagenicity nor genotoxicity in the presence and absence of S9.

Genotoxic activity of environmentally important polycyclic aromatic hydrocarbons and their nitro derivatives in the wing spot test of Drosophila melanogaster

The genotoxicity of three polycyclic aromatic hydrocarbons (PAHs) and of three of their nitro derivatives was evaluated in the wing Somatic Mutation And Recombination Test (SMART) in Drosophila melanogaster. Two crosses were used, i.e. the standard cross (ST) and the improved high bioactivation cross (HB) which is characterised by an increased sensitivity to the genotoxic effects of promutagens and procarcinogens. Larvae trans-heterozygous for the two recessive wing cell markers multiple wing hairs (mwh) and flare (flr3) were fed with the test compounds for 48 h. The wings of the surviving flies were analysed for the occurrence of single and twin spots. Naphthalene, 1-nitronaphthalene and 1,5-dinitronaphthalene proved to be more genotoxic in the HB cross than in the ST cross. Anthracene showed a clear genotoxic activity only in the HB cross whereas it was negative in the ST cross. 9-Nitroanthracene gave inconsistent results in both crosses. Phenanthrene was negative in the ST cross, but weakly positive in the HB cross. These results demonstrate that the genotoxic activity of these PAHs and their nitro derivatives can be detected with the somatic cells of the wing imaginal discs of larvae with high bioactivation capacity.

Effect of 5-nitroindole on adenylate energy charge, oxidative phosphorylation, and lipid peroxidation in rat hepatocytes

5-Nitroindole (NI), a mutagenic nitroarene, was assayed for cytotoxic effects on rat hepatocytes. After incubation with 25-100 microM NI, the adenylate energy charge of the hepatocytes decreased significantly as a result of the decrease in ATP and the increase in AMP. ATP depletion correlated well with the effects of NI on mitochondrial electron transfer and energy transduction in hepatocytes. Thus, NI (a) inhibited the antimycin-sensitive hepatocyte respiration; (b) inhibited NADH oxidation by disrupted hepatocyte mitochondria; (c) inhibited L-malate-L-glutamate oxidation by ADP-supplemented mitochondria; (d) in the absence of ADP, stimulated the same substrates and also succinate oxidation by mitochondria; (e) released the latent ATPase activity of mitochondrial F1F0-ATP synthase; (f) shifted the redox level of reduced cytochromes (c + c1) and b towards the oxidized state; (g) inhibited NADH oxidation by disrupted mitochondria in the vicinity of the NADH-dehydrogenase flavoprotein; (h) inhibited Ca2+ uptake by mitochondria using L-malate-L-glutamate as an energy source; (i) inhibited valinomycin-induced, endogenously energized K+ uptake, with little effect on the ATP-induced uptake; and (j) inhibited the MgATP-dependent contraction of Ca(2+)-swollen mitochondria. NI inhibited lipid peroxidation in hepatocytes and also in substrate-supplemented liver microsomes and mitochondria, thus ruling out hydroperoxides as a cause of NI cytotoxicity. Long-term incubation with NI produced loss of hepatocyte viability, as indicated by lactate dehydrogenase leakage.

Enhancement by butylated hydroxytoluene of the in vitro activation of 3,3'-dichlorobenzidne

Mutagenicity of Salmonella TA98 and covalent binding to DNA of 3,3'-dichlorobenzidine (DCB) were used to assess the influence of di-tert.-butylated hydroxytoluene (BHT) on the in vitro activation of the arylamine by rat hepatic S9 metabolic systems. BHT at a concentration of 4 or 20 microM enhanced the mutagenicity of DCB by 32 or 21%, respectively, and the covalent binding of DCB to added DNA by 76 or 328%, respectively. The antioxidant altered the HPLC profile of isolable DCB metabolites, causing a decrease in the formation of three metabolites, an increase in the formation of one metabolite, and the formation of an entirely new metabolite. BHT inhibited the mutagenicity of the promutagen 2-acetylaminofluorene (2-AAF) but had no effect on that of the direct-acting mutagen 2,4- dinitrophenylhydrazine (DNPH). The results show that BHT enhances the mutagenicity of and DNA binding by DCB, in contrast with the predominantly inhibitory effect of the antioxidant on the mutagenicity of other chemicals that require bioactivation.

Metabolism of 2,4-dinitrotoluene by Salmonella typhimurium strains TA98, TA98NR and TA98/1,8-DNP6, and mutagenicity of the metabolites of 2,4-dinitrotoluene and related compounds to strains TA98 and TA100

The products detected in the incubation of 2,4-dinitrotoluene (2,4-DNT) with Salmonella typhimurium strains TA98 and TA98/1,8-DNP6 were nitrosonitrotoluenes, hydroxylaminonitrotoluenes, aminonitrotoluenes and dimethyl dinitroazoxybenzene. The capacity of TA98NR to reduce 2,4-DNT was much lower than that of TA98 and TA98/1,8-DNP6. The bacterial products showed no mutagenic activity in the Ames assay using TA98 and TA100. These results indicate that the lack of mutagenic activity of 2,4-DNT is not due to low reductive metabolism of 2,4-DNT by the bacteria, but to the lack of mutagenic activity of the bacterial reductive products of 2,4-DNT, including dimethyl dinitroazoxybenzene.

Interaction with microsomal lipid as a major factor responsible for S9-mediated inhibition of 1,8-dinitropyrene mutagenicity

1,8-Dinitropyrene (1,8-DNP), present in polluted air, is a rodent carcinogen and a potent, direct-acting mutagen in salmonella typhimurium TA98. This mutagenicity is markedly reduced in the presence of mammalian hepatic S9 or microsomes. We demonstrate that at least a substantial part of this effect is attributable to non-enzymatic processes. The microsomal-dependent inhibition was unaffected by omission of an NADPH-generating system or when the cytochrome P-450 inhibitor, SKF-525A, or the cytochrome P-448 inhibitor, ellipticine, was incorporated in the metabolic activation system, suggesting that mixed function oxidases are not involved. Heat inactivation partially decreased the ability of induced S9 to reduce DNP mutagenicity. Substitution of S9 with a similar concentration of bovine serum albumin did not affect DNP activity. Thus non-specific binding to microsomal protein is not involved. However, when lipids, derived from uninduced microsomes, were added to incubations of DNP and S. typhimurium TA98, mutagenicity was decreased. Furthermore, substitution of microsomal lipids with a suspension of phosphatidylcholine (PC), a major lipid constituent of microsomes, affected DNP mutagenicity similarly. An increase in PC concentration resulted in a greater inhibitory effect. The reduction in DNP mutagenicity observed with microsomal lipids or with PC was less than that detected with uninduced S9, whilst the mutagenicity of 2-nitrofluorene was reduced to an approximately equal extent by lipids and S9. This phenomenon may be responsible for the S9-mediated detoxification of other mutagenic nitroaromatic compounds and may have important implications for mutagenicity testing.

Toxicity of nitrobenzene compounds towards isolated hepatocytes: dependence on reduction potential

1. The cytotoxicity of p-substituted nitrobenzenes towards isolated hepatocytes under aerobic or hypoxic conditions has been determined. The nitrobenzene concentration required to cause 50% cytoxicity in 2 h was a function of the one-electron reduction potential of the nitrobenzene, with the more cytotoxic compounds having the strongest electron-withdrawing substituents. 2. The effectiveness of the nitrobenzenes at causing cytotoxicity under aerobic but not hypoxic conditions was markedly increased if hepatocyte catalase was inhibited with azide. 3. Nitrobenzenes at cytotoxic concentrations induced cyanide-resistant respiration in isolated hepatocytes. Their effectiveness correlated with their cytotoxicity. 4. The rate of oxygen activation of these nitrobenzenes by ascorbate was also a function of the one-electron reduction potential. The nitro compounds with the strongest electron-withdrawing substituents were the most rapidly reduced. 5. Most nitrobenzenes were more cytotoxic under aerobic than hypoxic conditions. Ascorbate enhanced hypoxic, but not aerobic, cytotoxicity. 6. It was concluded that the cytotoxicity of different nitrobenzenes is related to their ease of reduction to nitro radical anions and nitrosobenzenes. Aerobic cytotoxicity is probably initiated by redox cycling and oxygen activation by the nitro radical anions whereas hypoxic cytotoxicity is probably initiated by the alkylation of macromolecules by nitrosobenzene metabolites.

Comparative gastrointestinal enzyme activity and activation of the promutagen 2,6-dinitrotoluene in male CD-1 mice and male Fischer 344 rats

Comparative intestinal nitroreductase, azo reductase, beta-glucuronidase, dechlorinase and dehydrochlorinase activities in young male Fischer 344 rats and young male CD-1 mice were measured in vitro while the comparative biotransformation of 2,6-dinitrotoluene to mutagenic metabolites was determined in vivo. The mice, which exhibit a high spontaneous incidence of hepatomas, had markedly greater nitroreductase activity and metabolized significantly more 2,6-dinitrotoluene to mutagenic metabolites than did Fischer 344 rats, which show a low incidence of liver tumors. Results of this study indicate that species differences in the incidence of hepatomas may be influenced by microbial flora and/or the biotransformation of xenobiotics in the G.I. tract.

Effect of lindane on nitroreductase and dechlorinase enzyme activity in the gastrointestinal tract

A previously reported acceleration of parathion metabolism in the gastrointestinal (GI) tract of lindane-pretreated rats could have been due to either a prolonged residence time of parathion or increased GI nitroreductase activity or both. Thus to determine the effect on GI nitroreductase and dechlorinase activity, 20 mg/kg lindane or 535 mg/kg neomycin were administered daily, by gavage, to weanling F-344 rats. Enzyme activity in the small intestine and cecum were assayed after 2 weeks and 5 weeks of treatment. Neomycin treatment inhibited the activity of both enzymes in the cecum but had no significant effect on enzyme activity in the small intestine, suggesting the presence of mucosal nitroreductase and dechlorinase in the small intestine. In contrast, lindane, which had no effect on enzyme activity in the cecum, significantly increased nitroreductase activity in the small intestine after treatment for 5 weeks. This increased nitroreductase may account for the previously reported lindane-parathion interaction and could influence the metabolism, toxicity, and risk assessment of many other environmental nitro-compounds that become toxic, mutagenic or carcinogenic upon reduction of their nitro-groups.

Effects of dinitrotoluenes on morphological cell transformation and intercellular communication in Syrian hamster embryo cells

The effects of four isomers of dinitrotoluene (DNT) and technical DNT (a mixture of DNT isomers and other compounds, with 2,4-DNT as the major constituent) were studied in two short-term in vitro assays. None of the isomers or technical DNT induced an increase in morphological transformation of Syrian hamster embryo (SHE) cells. Four DNT metabolites (2,4-diaminotoluene, 2-amino-4-nitrotoluene, 2-amino-6-nitrotoluene, and 2,4-dinitobenzoic acid), representing different stages in reduction or oxidation of DNT isomers, were also negative for induction of morphological transformation. The DNT isomers were tested in an intercellular communication assay based on dye transfer. 2,4-DNT, 2,6-DNT, and technical DNT inhibited intercellular communication in the SHE cell line BPNi at toxic concentrations. This may be reminiscent of in vivo data showing promoting activity of these compound. 2,3-DNT and 3,4-DNT did not inhibit communication.

Comparison of in vivo binding of aromatic nitro and amino compounds to rat hemoglobin

The hemoglobin (Hb) binding of five nitroarenes, i.e. nitrobenzene (NB), 4-nitrobiphenyl (4-NBP), 1-nitropyrene (1-NP), 2-nitronaphthalene (2-NN) and 2-nitrofluorene (2-NF), and the corresponding amines, administered p.o. to male S.D. rats, was determined by HPLC, to evaluate the extent of in vivo reductive and oxidative activations of these compounds to N-hydroxylamines, which covalently bind to Hb to form acid-labile sulfinamides. Hb binding of the nitroarenes, except for NB, was significantly lower than that of the corresponding amines. Among the aromatic amines, 4-aminobiphenyl exhibited extremely high Hb binding. Hb binding of NB and 4-NBP decreased markedly after pretreatment with a mixture of antibiotics, but the binding of the others did not decrease appreciably. 1-Aminopyrene and 1-NP bound abundantly to plasma proteins, although the Hb binding was slight. Based on the Hb binding and the in vitro metabolism by liver microsomes and intestinal bacteria, the extent of in vivo reductive activation of nitroarenes is discussed.

Mutagenicity studies on fenitrothion in bacteria and mammalian cells

The mutagenicity of fenitrothion was determined in strains of Salmonella typhimurium and Escherichia coli. Fenitrothion was found to be non-mutagenic in Salmonella typhimurium strains of TA98, TA1535 and TA1537 and in Escherichia coli WP2uvrA both with and without S9 mix, while weak mutagenicity was observed only in Salmonella typhimurium TA100 and enhanced by the addition of S9 mix. The mutagenicity observed in the TA100 strain was not expressed in a nitroreductase-deficient strain, TA100 NR, and decreased in a transacetylase-deficient strain, TA100 1,8-DNP6. The mutagenicity of fenitrothion was also examined by a gene mutation assay using the gene for hypoxanthine-guanine phosphoribosyltransferase (hgprt) in V79 Chinese hamster lung cells. Fenitrothion did not induce any increment of 6-thioguanine-resistant mutant cells at doses ranging from 0.01 to 0.3 mM regardless of the presence or absence of S9 mix. These results suggest that reduction of fenitrothion by a bacterial nitroreductase of TA100 to an active form is essential for the expression of the mutagenicity of fenitrothion in TA100 and that a bacterial transacetylase of TA100 also has an important role in the process of mutagenic activation.

Enterohepatic circulation of 2,4-dinitrobenzaldehyde, a mutagenic metabolite of 2,4-dinitrotoluene, in male Wistar rat

1. The major biliary metabolite of 2,4-dinitrotoluene (2,4-DNT) in male Wistar rat was 2,4-dinitrobenzyl alcohol glucuronide and the minor metabolites were 2,4-dinitrobenzyl alcohol, 2,4-dinitrobenzaldehyde, 2-acetylamino-4-nitrotoluene, 4-amino-2-nitro(2-amino-4-nitro)benzyl alcohol sulphate, 2,4-dinitrobenzoic acid, 2,4-diacetylaminobenzoic acid and 2-amino-4-nitrobenzoic acid. 2. 2,4-Dinitrobenzyl alcohol, 2,4-dinitrobenzaldehyde, 2,4-dinitrobenzyl alcohol glucuronide and 4-amino-2-nitro(2-amino-4-nitro)benzyl sulphate were excreted in the bile of male Wistar rat dosed with 2,4-dinitrobenzyl alcohol. 3. 2,4-Dinitrobenzaldehyde, 2,4-dinitrobenzyl alcohol, 2,4-dinitrobenzyl glucuronide, 4-amino-2-nitro(2-amino-4-nitro)benzyl alcohol sulphate and 2,4-diacetylaminobenzoic acid were excreted in the bile of male Wistar rat dosed with 2,4-dinitrobenzaldehyde. 4. These results indicate that the common biliary metabolites of 2,4-DNT, 2,4-dinitrobenzyl alcohol and 2,4-dinitrobenzaldehyde are 2,4-dinitrobenzyl alcohol and its glucuronide, and 2,4-dinitrobenzaldehyde, and suggest the enterohepatic circulation of 2,4-dinitrobenzaldehyde in the metabolism of 2,4-DNT.

Diazo-positive metabolites in urine from workers exposed to aromatic nitro-amino compounds

The amount of diazo-positive compounds in urine from workers at a chemical plant producing pharmaceuticals and explosives was determined in samples collected after work and after a holiday. Forty-five persons working with aromatic nitro-amino compounds (ANA) showed a statistically significant (p less than 0.01) increase in the exposed samples (1.21 +/- 1.40 mmol/mol creatinine) compared to the unexposed samples (0.56 +/- 0.31 mmol/mol creatinine). No increase in the level of diazo-positive metabolites was found in the 25 workers not exposed to ANA compounds. In a follow-up study, 32 trinitrotoluene (TNT) workers were divided into three exposure categories and seemed to show a dose-dependent increase in the level of urinary diazo-positive metabolites. However, there was a considerable interindividual variation. The method seems suitable for the biological assessment of exposure to ANA compounds--at least on a group level. This may be valuable, especially in situations where significant dermal uptake is expected.

Mutagenic potency of some conjugated nitroaromatic compounds and its relationship to structure

The mutagenicities of 12 conjugated non-fused nitroaromatic compounds and 1 amino analogue were determined in strains TA100 and TA98 of Salmonella typhimurium. Reversions by p-nitroaromatics increased in the order of the acetophenone, benzaldehyde, styrene, chalcone, cinnamic acid and stilbene indicating the importance for mutagenic potency of extended conjugation to the p-nitrophenyl substituent. Highest mutagenicity was found with alpha-substituted 4-nitrostyryl derivatives of which the phenyl derivative (31 revertants per nmole in TA100) was the most active. Generally, the TA100 strain was more sensitive than TA98 to these mutagens and S9 treatment was unnecessary for activity, although 4-nitrochalcone required S9 activation. Para-nitro isomers of the cinnamic acids and chalcones were much more active than the corresponding ortho and meta isomers. The 4-aminocinnamic acid analogue was inactive suggesting that complete reduction in Salmonella of 4-nitrocinnamic acid to an active amino derivative is not response for the high mutagenicity of the former. Mutagenicity of these p-nitrostyryl compounds may be explained by the covalent interaction of the electrophilic benzylic carbon with Salmonella DNA.

Dependence on Salmonella typhimurium enzymes of mutagenicities of nitrobenzene and its derivatives in the presence of rat-liver S9 and norharman

Dependence on S. typhimurium enzymes of mutagenicities of nitrobenzene (NB) and o-, p-chloronitrobenzenes (o-, p-CNBs), which are only mutagenic in the presence of S9 and norharman (NOH), was investigated using a nitroreductase-deficient strain TA98NR and an esterifying enzyme-deficient strain TA98/1,8-DNP6. NB exhibited mutagenicity towards TA98 but did not towards TA98NR strain in spite of the presence of S9 in the assay system. The mutagenicity of o-CNB towards TA98NR was significantly lower than that of o-CNB towards TA98. In contrast to NB and o-CNB, synthesized phenylhydroxylamine (PHA) and o-chlorophenylhydroxylamine (o-CPHA) exhibited approximately the same mutagenicity towards both tester strains. These results indicate that the nitroreduction required for the appearance of mutagenicity of the nitrobenzene derivatives in the presence of S9 and NOH is dependent on the nitroreductase of the tester strain. In addition, the mutagenicities of PHA and p-CPHA were significantly higher towards TA98/1,8-DNP6 than towards TA98, suggesting that the esterification of their hydroxylamines produced inactivation rather than activation. From these results, it was concluded that S9 and NOH play a role in metabolic activation other than the reduction of the nitro group to hydroxylamine and subsequent esterification for the mutagenesis of NB and its derivatives.

Mutagenicity of the C-nitroso analog of fenitrothion

The chemicals fenitrothion, nitroso fenitrothion, amino fenitrothion and 3-methyl-4-nitrophenol were tested for mutagenicity to Salmonella typhimurium strains TA98 and TA100, both in the presence and absence of rat liver S-9 mix. The strong mutagenicity of nitroso fenitrothion to both strains either in the presence or absence of S-9 mix contrasted with the observation that fenitrothion displayed no mutagenicity in these tester strains. The results suggest that the normal nitroreductases present in TA98 and TA100 cannot metabolize fenitrothion to a mutagenic metabolite. This inability of the tester strains to effect partial nitroreduction results in the failure of this screening system to predict the potential genotoxicity of this pesticide.

Mutagenicity of halogenated and other substituted dinitrobenzenes in Salmonella typhimurium TA100 and derivatives deficient in glutathione (TA100/GSH-) and nitroreductase (TA100NR)

In a previous study, it was shown that 1-chloro-2,4-dinitrobenzene (CDNB) was less mutagenic in a glutathione (GSH)-deficient derivative of Salmonella typhimurium TA100 (TA100/GSH-) than in TA100 itself, suggesting that the mutagenicity of the compound is dependent on GSH, possibly mediated by the action of a bacterial nitroreductase(s) on the CDNB-GSH conjugate. In the present study a series of mutagenicity tests were performed to determine how CDNB could be activated after reaction with GSH. In liquid preincubation assays, strains TA100, TA100/GSH- and TA100NR, a nitroreductase-deficient derivative of TA100, were treated with CDNB and its fluoro and bromo analogues (FDNB and BDNB), further with its GSH conjugate (S-GSH-DNB) and possible metabolic products, such as S-cysteine-dinitrobenzene (S-Cys-DNB) and S-methyl-dinitrobenzene (S-methyl-DNB), and with 2 more analogues, O-methyl-dinitrobenzene (O-methyl-DNB) and dinitrobenzene (DNB). CDNB, FDNB and BDNB were found to be mutagenic in TA100 and TA100NR, while TA100/GSH- was much less sensitive to the mutagenic action of these halogenated dinitrobenzenes. DNB, O-methyl-DNB, S-methyl-DNB and S-Cys-DNB induced equal numbers of His+ revertants in TA100 and TA100/GSH-, but were not mutagenic in TA100NR. S-GSH-DNB showed no mutagenic activity in any of the 3 strains under the present experimental conditions. These results suggest that the halogenated aromatics may react with bacterial DNA and produce pre-mutagenic alterations according to 2 mechanisms: direct attack on the DNA through nucleophilic substitution (SN2) of the halogen atoms; activation through GSH conjugation and subsequent nitroreduction of the conjugate or its metabolic products to more reactive intermediates.

Genotoxic activity of m-nitrobenzaldehyde

m-Nitrobenzaldehyde (MNB) was evaluated for mutagenic activity using the Ames microbial mutagenicity test and for its ability to induce DNA single-strand breaks in rat hepatocytes as measured by alkaline elution. MNB was tested in S. typhimurium strains TA1535, TA1537, TA1538, TA98, and TA100, both with and without pretreatment with liver microsomes (S9) isolated from rats pretreated with Aroclor 1254. MNB produced 2-fold or greater increases in revertants in TA1538, both with and without S9, and in TA100 with S9 only. A 2-fold increase in revertants was seen in TA98, but only at the highest dose tested which did not produce inhibition of background growth. MNB caused a greater than 3-fold increase in elution slope, with DNA alkaline elution assay, but only at highly cytotoxic doses and, therefore, is not considered genotoxic in this system. It is concluded that MNB possesses weak genotoxic activity.

Effects of norharman on the mutagenicity of chlorophenylhydroxylamine and its metabolism with rat liver S9

o,p-Chlorophenylhydroxylamines (CPHAs) (10468-16-3, 823-86-9) only demonstrated mutagenicity in the presence of S9 mix and norharman (NOH) (244-63-3), as well as chloronitrobenzenes. The mutagenic activity of o-CPHA was 30 times higher than that of p-CPHA. When o-CPHA was preincubated with S9 mix without NOH, the mutagenic activity disappeared rapidly. The decrease in activity during the preincubation was suppressed by addition of NOH. HPLC analysis revealed that o-CPHA was metabolized to o-chloroaniline (o-CA) (95-51-2) and that the metabolic reduction was inhibited by NOH. When microsomes containing NADPH were used instead of S9 mix, o-CPHA exhibited only very weak mutagenicity. The activity in the microsome system, however, was greatly enhanced by adding cytosol or ascorbic acid (50-81-7). These phenomena were only observed in the conventional plate incorporation method. In the case of the liquid incubation assay, in which test compound metabolism and tester strain mutation only occur in the liquid incubation medium, the mutagenic activity of o-CPHA in the microsome system with NOH was comparable to that in the S9 system, indicating that o-CPHA was activated by an enzyme in microsomes in the presence of NOH. Consequently, it was concluded that NOH not only affects the metabolic inactivation of o-CPHA to o-CA by S9, but also the metabolic activation of o-CPHA by microsomes. No appreciable enhancing effects of cytosol and ascorbic acid were observed in the liquid incubation assay, suggesting that these factors affect the stability of CPHA or an active metabolite. The microsome activation of o-CPHA was dependent on NADPH and oxygen; SKF-525A (62-68-0), metyrapone (54-36-4) and alpha-naphthoflavone (604-59-1) inhibited the mutagenic activity by about 50%, suggesting that cytochrome P-450 was involved in the metabolic activation.

The Salmonella typhimurium/mammalian microsomal assay. A report of the U.S. Environmental Protection Agency Gene-Tox Program

The Salmonella assay has been in use for almost 15 years and can be defined as a routine test for mutagenicity and for predicting potential carcinogenicity. It detects the majority of animal carcinogens and consequently plays an important role in safety assessment. The test is also routinely used as the frontline screen for environmental samples (complex mixtures) isolated from air, water and food. This role will continue to remain an area of growth as or because sample volumes associated with these testing areas are generally very limited and more extensive testing is generally impossible. While this test, like all others, has some limitations, it is recommended that it be regularly included in all genetic testing batteries.

Mutagenicity of mono-nitrobenzene derivatives in the Ames test and rec assay

The mutagenicities of 37 mono-nitrobenzene derivatives, i.e. the ortho, meta, and para isomers of nitrotoluene, nitrophenol, nitroaniline, nitroanisole, nitrobenzaldehyde, nitrobenzyl chloride, nitrobenzonitrile, nitroacetophenone, nitrophenetol, nitrobenzoic acid, nitrophenylacetic acid, and nitrocinnamic acid and p-nitrothiophenol, were tested in the Ames test and rec assay. The rec assay gave more positive results than the Ames test. The para-substituted nitrobenzene derivatives were always positive in the rec assay as were all but 2 in the Ames test, while 5 out of 12 ortho and meta derivatives were mutagenic in the Ames test. The results of the present study, combined with those of the previous study, are discussed with special reference to the structure-mutagenicity relationships.

Structure-activity relationships of nitro and methyl-nitro derivatives of indoline, indole, indazole and benzimidazole in Salmonella typhimurium

The mutagenic activities of eleven nitro derivatives and eleven N-methyl-nitro derivatives of indoline, indole, indazole and benzimidazole were investigated in Salmonella TA98 and TA100. The presence of a nitro group at C4 or C7 resulted in only weakly or nonmutagenic compounds, while a nitro group at C2, C5 or C6 usually resulted in measurable mutagenic activity in the non-N-methylated compounds. Methylation of a ring nitrogen usually reduced the mutagenic activity of these nitroheterocyclics except 2-nitro-benzimidazole, which resulted in a better than 300-fold increase in mutagenic activity. A proposed mechanism for the increased mutagenic activity obtained by methylation of imidazole nitrogens may provide insights into the reasons for the potent mutagenicities observed for several similarly methylated cooked-food mutagens.