Structural basis of the mutagenicity of heterocyclic amines formed during the cooking processes

A knowledge-based expert rule system for predicting mutagenicity (Ames test) of aromatic amines and azo compounds

Cancer is one of the main causes of death in Western countries, and a major issue for human health. Prolonged exposure to a number of chemicals was observed to be one of the primary causes of cancer in occupationally exposed persons. Thus, the development of tools for identifying hazardous chemicals and the increase of mechanistic understanding of their toxicity is a major goal for scientific research. We constructed a new knowledge-based expert system accounting the effect of different substituents for the prediction of mutagenicity (Ames test) of aromatic amines, a class of compounds of major concern because of their widespread application in industry. The herein presented model implements a series of user-defined structural rules extracted from a database of 616 primary aromatic amines, with their Ames test outcomes, aimed at identifying mutagenic and non-mutagenic chemicals. The chemical rationale behind such rules is discussed. Besides assessing the model's ability to correctly classify aromatic amines, its predictivity was further evaluated on a second database of 354 azo dyes, another class of chemicals of major concern, whose toxicity has been predicted on the basis of the toxicity of aromatic amines potentially generated from the metabolic reduction of the azo bond. Good performance in classification on both the amine (MCC, Matthews Correlation Coefficient=0.743) and the azo dye (MCC=0.584) datasets confirmed the predictive power of the model, and its suitability for use on a wide range of chemicals. Finally, the model was compared with a series of well-known mutagenicity predicting software. The good performance of our model compared with other mutagenicity models, especially in predicting azo dyes, confirmed the usefulness of this expert system as a reliable support to in vitro mutagenicity assays for screening and prioritization purposes. The model has been fully implemented as a KNIME workflow and is freely available for downstream users.

Mutagenic potency of food-derived heterocyclic amines

The understanding of mutagenic potency has been primarily approached using "quantitative structure-activity relationships" (QSAR). Often this method allows the prediction of mutagenic potency of the compound based on its structure. But it does not give the underlying reason why the mutagenic activities differ. We have taken a set of heterocyclic amine structures and used molecular dynamic calculations to dock these molecules into the active site of a computational model of the cytochrome P4501A2 enzyme. The calculated binding strength using Boltzman distribution constants was then compared to the QSAR value (HF/6-31G* optimized structures) and the Ames/Salmonella mutagenic potency. Further understanding will only come from knowing the complete set of mutagenic determinants. These include the nitrenium ion half-life, DNA adduct half-life, efficiency of repair of the adduct, and ultimately fixation of the mutation through cellular processes. For two isomers, PhIP and 3-Me-PhIP, we showed that for the 100-fold difference in the mutagenic potency a 5-fold difference can be accounted for by differences in the P450 oxidation. The other factor of 20 is not clearly understood but is downstream from the oxidation step. The application of QSAR (chemical characteristics) to biological principles related to mutagenesis is explored in this report.

Extended quantitative structure-activity relationships for 80 aromatic and heterocyclic amines: structural, electronic, and hydropathic factors affecting mutagenic potency

The mutagenic/carcinogenic heterocyclic amines formed during the cooking of protein foods have been determined to be probable or possible human carcinogens. As part of a comprehensive study of the food mutagens, our laboratory has produced a series of quantitative structure-activity relationships (QSARs) of aromatic and heterocyclic amines, to attempt to elucidate the mechanisms of mutagenesis/carcinogenesis. Amines are genotoxically active only after activation by a series of reactions converting the parent compound to an electrophilic derivative, which is postulated to be a nitrenium ion that covalently binds to and damages DNA. An important agent in this conversion is cytochrome P450. In this report we develop a QSAR for 80 amines of diverse structure and a range of 10 orders of magnitude in mutagenic potency. New structural factors and quantum chemical ab initio and Hückel calculations are included. The results are interpreted to show that a main determinant of mutagenic potency is the extent of the aromatic pi-electron system. Small contributions are made by both the dipole moment and the calculated stability of the nitrenium ion. Multiple linear regression models account for nearly two-thirds of the variance in potency, leaving room for additional unknown factors. The role of cytochrome P450 1A in amine toxification is supported, and further theoretical and experimental research on its reaction mechanisms and modeling of its active site are proposed.

Genotoxic effects of heterocyclic aromatic amines in human derived hepatoma (HepG2) cells

In order to study the mutagenic effects of heterocyclic aromatic amines (HAAs) in cells of human origin, five compounds, namely 2-amino-3-methyl-imidazo[4,5-f]quinoline (IQ), 2-amino-3, 4-dimethyl-imidazo[4,5-f]quinoline (MeIQ), 2-amino-3, 8-dimethyl-imidazo[4,5-f]quinoxaline (MeIQx), the pyridoimidazo derivative 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1), were tested in micronucleus (MN) assays with a human derived hepatoma (HepG2) cell line. All HAAs caused significant, dose-dependent effects. The activities of IQ, MeIQ, MeIQx and PhIP were similar (lowest effective concentrations 25-50 microM), whereas Trp-P-1 was effective at a dose of >/=2.1 microM. In addition, the HAAs were tested in MN assays with Chinese hamster ovary (CHO) cells and in Salmonella strain YG1024 using HepG2 cell homogenates as an activation mix. In the CHO experiments, positive results were obtained with Trp-P-1 and PhIP, whereas the other compounds were devoid of activity under all experimental conditions. The discrepancy in the responsivity of the two cell lines is probably due to differences in their acetylation capacity: enzyme measurements with 2-aminofluorene as a substrate revealed that the cytosolic acetyltransferase activity in the HepG2 cells is approximately 40-fold higher than that of the CHO cells. In the bacterial assays all five HAAs gave positive results but the ranking order was completely different from that seen in the HepG2/MN experiments (IQ > MeIQ > Trp-P-1 >/= MeIQx >> PhIP) and the mutagenic potencies of the various compounds varied over several orders of magnitude. The order obtained in bacterial tests with rat liver S9 mix was more or less identical to that seen in the tests with HepG2 cell homogenates but the concentrations of the amines required to give positive results were in general substantially lower (10(-5)-10(-1) microM). Overall, the results of the present study indicate that MN/HepG2 tests might reflect the mutagenic effects of HAAs more adequately than other in vitro mammalian cell systems due to the presence of enzymes involved in the metabolic conversion of the amines.

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.

SOS chromotest results in a broader context: empirical relationships between genotoxic potency, mutagenic potency, and carcinogenic potency

Environmental monitoring requires that large numbers of samples be processed in a relatively short period of time. While microbioassays facilitate rapid testing, the results are often difficult to interpret in the broader context of human or animal health. Determining the consequences of exposure to genotoxic substances will ultimately require in situ monitoring of exposed organisms. However, it is immediately possible to construct a broad empirical framework within which available microbioassay results can be interpreted. To do this for SOS Chromotest results, we investigated the empirical relationships between SOS genotoxic potency and mutagenic potency (as measured with the Salmonella/microsome assay), as well as between genotoxic potency and carcinogenic potency (as measured using standard, chronic animal bioassays). Strong relationships were identified between; 1) genotoxic potency and mutagenic potency for 268 direct-acting substances (r2=0.76) and 2) genotoxic potency and mutagenic potency for 126 S9-activated substances (r2=0.65). Ordinary least squares regression analyses of the SOS genotoxicity-Salmonella mutagenicity relationship revealed a significant effect of SOS genotoxicity as well as differences in mutagenic potency that can be attributed to the Salmonella strain used to measure mutagenic potency. Analyses of S9-activated substances revealed a significant interaction between the SOS genotoxic potency (SOSIP) effect and the Salmonella strain effect. Two regression models relating SOS genotoxicity and Salmonella mutagenicity were used to predict the mutagenic potency of several industrial effluent extracts previously analyzed for SOS genotoxicity by White et al. [(1996): Environ Mol Mutagen 27:116-139]. Predictions are consistent with published mutagenic potency values for similar industrial waste materials. A consistent relationship was also identified between genotoxic potency and carcinogenic potency for 51 substances. Linear regression analyses revealed an effect of SOS genotoxic potency as well as differences in carcinogenic potency that may be attributable to experimental animal and route of exposure. The correlation between genotoxicity and carcinogenicity was fairly weak (maximum r value = 0.51). Previous studies revealed similar strength of association between Ames mutagenicity and carcinogenicity. Predicted carcinogenic potencies of previously examined genotoxic, industrial effluent extracts are generally low compared to the pure substances included in the data set.

Structural and quantum chemical factors affecting mutagenic potency of aminoimidazo-azaarenes

A set of 16 mutagenic aminoimidazo-azaarenes, including four that have been isolated from cooked foods and identified as bacterial mutagens and rodent carcinogens, was selected from a larger series previously published [Hatch et al. (1991): Environ Mol Mutagen 17:4-19] for an in-depth structure-activity study using computational methods. Structural features believed to affect mutagenic potency were tabulated. Molecular orbital energies and other electronic properties of these compounds were calculated using Huckel, semiempirical AM1, and ab initio quantum mechanical methods. Factor interrelationships were studied by multiple linear regression and canonical correlation analyses. Our goal was an improved understanding of the chemical basis of mutagenicity for this class of heterocyclic amines. The major findings were as follows: 1) mutagenic potency is related to the size of the aromatic ring system; 2) potency is enhanced by the presence and location of an N-methyl group; 3) potency is enhanced by addition of ring nitrogen atoms in pyridine, quinoline, and quinoxaline configurations; 4) potency is inversely related to the energy of the LUMO (lowest unoccupied molecular orbital) of the parent amines; 5) potency is directly, though weakly, related to the LUMO energy of the derived nitrenium ions; and 6) the calculated thermodynamic stability of the nitrenium ions (relative to the parent amine) is directly correlated with nitrenium LUMO energy and with the negative charge on the exocyclic nitrogen atom. Although this study raises several intriguing issues relating mutagenicity to chemical properties, further study will be required to determine the plausibility of the nitrenium ion as the ultimate mutagen for binding to DNA.

Structure-mutagenicity relationships of four amino-imidazonaphthyridines and imidazoquinolines

We tested four isomeric imidazonaphthyridines and one imidazoquinoline compound for mutagenic activity in the Ames/Salmonella mutagenicity assay, using strain TA98 and strain YG1024, an analogue of strain TA98 with elevated O-acetyltransferase levels. Their potency was related to calculated electronic parameters. Five compounds with a linear arrangement of 3 rings showed a positive response in strain YG1024. Compound 2 (1-methylimidazo[4,5-b][1,7]naphthyridin-2-amine) is the most mutagenic in both strains, giving specific activities of about 200 and 30 revertants per microgram in strains YG1024 and TA98, respectively. Three of the compounds were weak mutagens, giving a positive dose-response only in strain YG1024, with 3-5 revertants per microgram. A higher response of all five compounds in strain YG1024 as opposed to TA98 indicates that they require O-acetyltransferase activity for their metabolism. Mutagenic potencies in strain YG1024 were positively correlated to the energy of the LUMO (lowest unoccupied molecular orbital) of the nitrenium ion.