Mutagenicity testing in Salmonella typhimurium strains possessing both the His reversion and Ara forward mutation systems and different levels of classical nitroreductase or O-acetyltransferase activities

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.

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.

Mutagenic activation of arylamines by subcellular fractions of Chamaelea gallina clams exposed to environmental pollutants

Biochemical measurements in the sentinel clam Chamaelea gallina have been used as biomarkers of marine pollution. In this study, S9, cytosolic fractions (CF), and microsomal fractions (MF) prepared from unexposed clams and clams exposed to model pollutants were used to activate 2-aminoanthracene (2-AA) and 2-acetylaminofluorene (AAF) to mutagens in Salmonella typhimurium strain BA149, which overexpresses O-acetyltransferase. Arylamine activation was similar with subcellular fractions from unexposed and Aroclor 1254-exposed clams, but decreased with fractions from As(III)- and Cu(II)-exposed clams. Bioactivation of arylamines by CF was higher than by MF, and higher with NADH than with NADPH as the reducing agent. alpha-Naphthoflavone inhibited AAF activation by CF and MF, but increased 2-AA activation nearly twofold. In contrast to the results with arylamine activation, benzo[a]pyrene hydroxylase (BPH) activity increased twofold in fractions from Aroclor 1254- and Cu(II)-exposed clams. Activation of 2-AA was also evaluated using S9 fractions from clams sampled at littoral sites with different pollutant levels. Compared to a reference site, lower 2-AA bioactivation and higher BPH activity were found in clams containing high levels of copper and organic contaminants, although the differences were not statistically significant. While these findings agree with the results of the model Cu(II) exposure, the effects of other pollutants cannot be ruled out. The results of the study demonstrate that arylamine activation by clams is not preferentially catalyzed by microsomal monooxygenases but by unknown cytosolic system(s), and that bioactivation of 2-AA and AAF appears to occur by different pathways.

Mutagen content and metabolic activation of promutagens by molluscs as biomarkers of marine pollution

Organisms combat pollutants by inducing biotransformation pathways, which can be used for biomonitoring. Several parameters--biomarkers--change in stressed organisms or populations at different organisation levels. Molecular or cellular biomarkers are early-warning indicators of pollution. Xenobiotics are first biotransformed by phase I enzymes and then conjugated with endogenous metabolites by phase II enzymes. Many organic xenobiotics are initially biotransformed by cytochrome P4501A1; in mammals, it is induced by pollutants via Ah receptor, although in marine invertebrates, its inducibility is much more equivocal. Metallothioneins are small Cys-rich proteins which bind transition metals; they detoxicate pollutant metals and are clearly induced in metal-exposed marine invertebrates. Some pollutants are genotoxins or can be converted into them. Determination of mutagens in bivalve molluscs following extraction with solvents and assay of mutagenicity with bacterial tests is a useful biomarker for marine pollution. While some pollutants are directly mutagenic, others are only mutagenic after they are activated to mutagenic derivatives by monooxygenases or conjugative enzymes. Many of these catalysts are induced by xenobiotics; thus, increased activation of known promutagens can be used as biomarker of environmental pollution. Bioactivation of promutagens requires the simultaneous action of different pathways, thus, reproducing more closely the in vivo situation than the specific assay of individual biotransforming enzymes. Study of molluscs with different pollution levels indicates that polluted animals have higher capacity to activate 2-aminoanthracene and contain more apolar mutagens than those from reference areas.

Microbial photodegradation of aminoarenes. Metabolism of 2-amino-4-nitrophenol by Rhodobacter capsulatus

The phototrophic bacterium Rhodobacter capsulatus photoreduces 2,4-dinitrophenol to 2-amino-4-nitrophenol, which is further metabolized by an aerobic pathway that is also light-dependent. The catabolism of 2-amino-4-nitrophenol requires O2 and the presence of alternative carbon (C) and nitrogen (N) sources, preferably acetate and ammonium. Rhodobacter capsulatus B10, a bacterium unable to assimilate nitrate, releases negligible amounts of nitrite when growing with 2-amino-4-nitrophenol, thus suggesting that an oxygenase, nitrite-producing activity is not involved in the metabolization of the compound. The diazotrophic growth of R. capsulatus increases in the presence of 2-amino-4-nitrophenol, but growth with ammonium is clearly inhibited by the compound. Mutant strains of R. capsulatus B10, which are affected in nifHDK, nifR1, or nifR4 genes, unable to fix dinitrogen, do not grow with 2-amino-4-nitrophenol as the sole N source. This indicates that the compound cannot be used as a N source. The nif mutants degrade 2-amino-4-nitrophenol to the same extent as the wild-type in the presence of ammonium. The compound is not used as a C source by the bacterium, either. Aromatic stable intermediates, such as 2,4-diaminophenol or 4-nitrocatechol, are not detectable in microaerobic cultures of R. capsulatus growing with 2,4-dinitrophenol or 2-amino-4-nitrophenol.

Synthesis, metabolism and structure-mutagenicity relationships of novel 4-nitro-(imidazoles and pyrazoles) in Salmonella typhimurium

A new series of 4-nitro-(imidazoles and pyrazoles) were synthesized as novel antimycotics and tested for their activation to mutagenic forms using Salmonella typhimurium TA98 and TA100, in the presence and in the absence of metabolic activation. TA100NR, TA100/1,8-DNP6, YG1026 and YG1029 strains were employed to identify a specific metabolic reaction which governs the mutagenic potency. Derivatives in the pyrazole group were generally found to be non mutagenic and active imidazoles were weak-direct-acting mutagens. For most of the compounds the mutagenic responses in TA98 were absent or 12- to 22-fold lower compared to TA100. The presence of a methyl or a benzylic group on the imidazole ring and substituents on the N1 and N3 positions were determinant for mutagenicity. Metabolism by bacterial enzyme systems was important to the expression of genotoxicity. Active compounds showed no mutagenicity toward the strain defective in classical nitroreductase and increased mutagenicity, from 2- to 7-fold depending on the test compound, toward the corresponding overproducing bacteria. On the other hand, compounds displayed reduced mutagenicity to the O-acetyltransferase strain without having increased activity in the corresponding overproducing bacteria, YG1029.

Mutagenic activation of aromatic amines by molluscs as a biomarker of marine pollution

Mutagenic activation of arylamines by mollusc S9 fractions was evaluated as a biomarker for marine pollution. Two bivalve species were used as bioindicators, the common mussel (Mytilus edulis) and the striped venus (Chameleo gallina). A strain of Salmonella typhimurium overproducing O-acetyltransferase was used as indicator of mutagenicity. Mussels from an area of the North Atlantic Spanish zone that was exposed to an accidental crude oil spill were compared to bivalves from a reference area. C. gallina samples were from low polluted and highly polluted areas of the South Atlantic Spanish littoral. The promutagen 2-aminoanthracene (2-AA) was activated to mutagenic derivative(s) by S9 fractions from both C. gallina and M. edulis. Animals from contaminated sites showed higher arylamine activation capabilities than reference animals. This was further correlated with the mutagenic activities of corresponding cyclopentone-dichloromethane animal extracts. 2-AA activation by mollusc S9 was potentiated by alpha-naphthoflavone (ANF), known to inhibit PAH-inducible CYP1A cytochromes from vertebrates, but inhibited by methimazole (MZ), a substrate of the flavin monooxygenase (FMO) system. 2-AA-activating enzymes were mainly cytosolic; this localization clearly suggests that such activity could be attributed to soluble enzymes, different from the CYP1A or FMO systems. In conclusion, mutagenic activation of arylamines by mollusc S9, using as indicator a strain of Salmonella typhimurium that overproduces O-acetyltransferase, could be a reliable biomarker for marine pollution.

Targeted disruption of the gene encoding the classical nitroreductase enzyme in Salmonella typhimurium Ames test strains TA1535 and TA1538

The gene encoding the 'classical nitroreductase' (CNR) of Salmonella typhimurium was disrupted. In this manner, cnr null mutant derivatives of strains TA1535 and TA1538 were constructed, and named YG7131 and YG7127, respectively. In both strain backgrounds, cnr gene disruption reduced nitrofurazone-reductase activity. This reduction almost completely eliminated the nitroreductase activity of strain TA1538. In contrast, the nitroreductase activity of strain TA1535 was much higher than that in TA1538. In this background, cnr gene disruption resulted in a reduction in nitroreductase activity by a similar absolute amount as in TA1538, but representing only about one-quarter of the original activity of TA1535. The results suggest that S. typhimurium has originally at least two distinct nitroreductases, one of which is already deficient in strain TA1538; the CNR is present in both TA1535 and TA1538. Also, these two strains (including their derivatives, TA98 and TA100) are not isogenic with regard to nitroreductase activity. After the introduction of plasmid pKM101, the sensitivities of the strains YG7132 and YG7128, the cnr-null mutants of TA98 and TA100, respectively, against several nitro compounds were compared with those of the conventional cnr-deficient strains TA98NR and TA100NR and the wild-type strains TA98 and TA100. The mutagenicities of 2-nitrofluorene and 1-nitropyrene in YG7132 or TA98NR were ten-fold lower than those of the compounds in TA98. Similarly, the mutagenicity of 2-(2-furyl)-3-(5-nitro-2-furyl) acrylamide in strain YG7128 or TA100NR was substantially lower than that of the compound in TA100. However, the mutagenicity of 2-nitronaphthalene in YG7128 was between those observed with TA100 and TA100NR, suggesting that a nitroreductase in S. typhimurium other than CNR is involved in the metabolic activation of this compound. The cnr gene of S. typhimurium positively hybridized with DNA at 13 min on the E. coli chromosome where the nfsB and nfnB genes of E. coli are mapped. These results suggest that the cnr gene of S. typhimurium is a counterpart of the nfsB and nfnB genes of E. coli, and that the newly constructed cnr-deletion strains are useful to assess the role of nitroreductases in the metabolic activation of mutagenic nitro compounds.

Role of classical nitroreductase and O-acetyltransferase on the mutagenicity of nifurtimox and eight derivatives in Salmonella typhimurium

This study investigates the mutagenicity of nifurtimox (NFX) and eight analogues in Salmonella typhimurium indicator strains that possess different levels of classical nitroreductase or O-acetyltransferase activities. The NFX analogues tested replace the 3-methyl-4-yl-tetrahydro-1,4-thiazine-1,1-dioxide group of the parent compound with the following other groups: indazol-1-yl (1G); pyrazol-1-yl (1B); benzimidazol-1-yl (1E); 1,2,4-triazol-4-yl (1D); 1-methyl-3-methylthio-1,2,4-triazol-4-yl-5-thione (1I); 3,5-bis(methylthio)-1,2,4-triazol-4-yl (1H); 1-adamantyl (ADA); and 4,6-diphenylpyridin-1-yl-2-one (1K). In the genetic backgrounds of the standard Ames tester strains TA98 and TA100, these bacteria combine the L-arabinose resistance forward mutation assay (Ara test) with a deficiency or overproduction of either nitroreduction or O-acetylation. The Ara test revealed, in agreement with previous findings, important differences between TA98 and TA100 and demonstrated, moreover, that these genetic differences are of significance in mutagenicity testing with nitrofuran compounds. The Ara test also indicated dissimilarities between the metabolic activation of NFX and its analogues, these compounds being classified in three different groups according to their mutagenicity toward strain BA14 (genetic background of TA98) and its derivatives. The first group included analogues (1G, 1E, 1I, and ADA) that showed similar mutagenic potency in all bacterial strains. These compounds are considered not to be substrates for both classical nitroreductase and O-acetyltransferase. The second group included compounds (analogues 1B and 1K, and the reference drug NFX) with increased mutagenicity toward the strain overproducing the classical nitroreductase, and/or reduced mutagenicity toward the corresponding deficient bacteria. These compounds are considered to be activated by the classical nitroreductase. The third group (analogues 1D and 1H) was activated by bacterial O-acetyltransferase, and consequently showed increased and decreased mutagenicity with the particular overproducer or deficient bacterial strain, as compared to their isogenic parentals. Previous reports have pointed out interest in NFX analogue 1H as a promising candidate for the replacement of NFX. The present study further enhances the putative interest of compound 1H, based on the different metabolic activation pathway exhibited by this analogue as compared to the parental drug, NFX.

Metabolic activation of carcinogenic aromatic amines by fish exposed to environmental pollutants

Activation of arylamines to mutagenic metabolites by hepatic S9 fractions has been evaluated as a biomaker of fish exposure to pollutants, using gilthead seabream (Sparus aurata), a valuable fish species from the Spanish South Atlantic littoral, as model organism. To obtain maximal sensitivity to the mutagenic action of aromatic amines, a strain of Salmonella typhimurium overproducing O-acetyltransferase was used. Fish were treated with Aroclor 1254, pesticides (malathion and dieldrin), or copper(II), and compared to Aroclor 1254-treated rats. The promutagen activation capabilities of the S9 fractions were further characterized by studying the effect of two monooxygenase inhibitors, alpha-naphthoflavone, a well known inhibitor of aromatic hydrocarbon-inducible forms of cytochrome P450, and methimazole, a substrate for the flavin monooxygenase (FMO) system. This study shows that 2-aminoanthracene (2-AA) and 2-acetylaminofluorene (AAF) activation by gilthead liver is enhanced by treatment of fish with different xenobiotics. The catalyst responsible for this enhanced activation appears to be different for each promutagen and, at least for 2-AA, dependent on the type of xenobiotic. The data presented indicate further that treatment of gilthead with some compounds, such as malathion and dieldrin, enhances the activation of aromatic amines in liver, without inducing ethoxyresorufin-O-deethylase activity. The use of acetyltransferase-overproducing bacteria appears to be a useful tool in the study of arylamine activation by fish liver, where biotransformation capability is lower than in mammals.