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

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.

Flexible use of QSAR models in predictive toxicology: a case study on aromatic amines

Over the last years, predictive toxicology approaches based on Structure-Activity Relationships have emerged as fundamental tools in the regulatory assessments of chemicals, especially in those programs where regulatory constraints and assessment schemes limit the amount of data available from experimental test methods. Both the qualitative (e.g., Structural Alerts) and the quantitative (Quantitative Structure-Activity Relationships, QSAR) approach can play important roles. However, the two approaches are not familiar to the same extent to the regulators that most often use only the qualitative approach, so that the potentiality of the more sophisticated QSAR approach is neglected. In fact, QSAR is a very flexible tool that allows the user to modulate its response according to different goals and requirements. Here, we present a non-naïve approach to the use of a QSAR relative to a dichotomous biological activity (such as mutagen/nonmutagen), and we show how the user can maximize alternatively the reliability of the prediction of negative compounds (i.e., safe chemicals) or that of positive chemicals (i.e., chemicals that pose high hazard). Because of the environmental and industrial importance of the class of aromatic amines, we apply the approach to a previously published QSAR on the Salmonella typhimurium mutagenicity of these chemicals.

Synthesis and Decomposition of an Ester Derivative of the Procarcinogen and Promutagen, PhIP, 2-Amino-1-methyl-6-phenyl-1H-imidazo[4,5-b]pyridine: Unusual Nitrenium Ion Chemistry

The food-derived heterocyclic amine (HCA) carcinogen 2-amino-1-methyl-6-phenyl-1H-imidazo[4,5-b]pyridine, PhIP, is often generated in the highest concentration of the HCAs formed during broiling and frying of meat and fish. Although it is considered to be an important contributor to human cancer risk from exposure to HCAs, the chemistry of PhIP metabolites that presumably react with DNA to initiate carcinogenesis has received only cursory attention. We have synthesized the ester derivative N-pivaloxy-2-amino-1-methyl-6-phenyl-1H-imidazo[4,5-b]pyridine, 1b, and investigated its chemistry in aqueous solution. Although 1b was too unstable to isolate, we could characterize it by NMR methods in DMF-d7, a solvent in which it is stable at -40 degrees C. It decomposed rapidly in aqueous solution, but its conjugate acid, 1bH+, is not reactive. The nitrenium ion, 2, was trapped by N(3)(-) to form the unusual tetrazole adduct, 16. In the absence of N3-, the expected hydration products of 2 were not detected, but the reduction product, 12, was detected. Although such products are often taken as evidence of triplet nitrenium ions, the efficient trapping of 2 by N(3)(-) indicates that it is a ground state singlet species. The product 12 appears to be generated by reduction of an initially formed hydration product of 2. An alternative addition-elimination mechanism for the formation of 12 does not fit the available kinetic data. The selectivity of 2, measured as kaz/ks, the ratio of the second-order rate constant for its reaction with N(3)(-) and the first-order rate constant for its reaction with the aqueous solvent, is (2.3 +/- 0.6) x 10(4) M(-1), a value that is in the middle of the range of k(az)/k(s) of 10-10(6) M(-1) observed for nitrenium ions derived from other HCAs. The mutagenicity of aromatic amines (AAs) and HCAs, measured as the log of histidine revertants per nanomole of amine, log m, in Salmonella typhimurium TA 98 and TA 100 correlates with log(k(az)/k(s)) for a wide variety of carbocyclic and heterocyclic amine mutagens including PhIP. Previously developed linear regression models for mutagenicity that include log(k(az)/k(s)) as an independent variable predict log m for PhIP with good accuracy in both TA 98 and TA 100. Quantitative carcinogenicity data are less strongly correlated with log(k(az)/k(s)), so prediction of the carcinogenicity of PhIP and other HCAs or AAs based primarily on log(k(az)/k(s)) is less successful.

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.

A QSAR for the mutagenic potencies of twelve 2-amino-trimethylimidazopyridine isomers: structural, quantum chemical, and hydropathic factors

An isomeric series of heterocyclic amines related to one found in heated muscle meats was investigated for properties that predict their measured mutagenic potency. Eleven of the 12 possible 2-amino-trimethylimidazopyridine (TMIP) isomers were tested for mutagenic potency in the Ames/Salmonella test with bacterial strain TA98, and resulted in a 600-fold range in potency. Structural, quantum chemical, and hydropathic data were calculated on the parent molecules and the corresponding nitrenium ions of all of the tested isomers to establish models for predicting the potency of the unknown isomer. The principal determinants of higher mutagenic potency in these amines are: (1) a small dipole moment, (2) the combination of b-face ring fusion and N3-methyl group, (3) a lower calculated energy of the pi electron system, (4) a smaller energy gap between the amine HOMO and LUMO orbitals (Pearson "softness"), and (5) a more stable nitrenium ion. Based on predicted potency from the average of six regression models, the isomer not yet synthesized and tested is expected to have a mutagenic potency of 0.77 revertants/microg in tester strain TA98, which is near the low end of the potency range of the isomers.

Computer-modeling-based QSARs for analyzing experimental data on biotransformation and toxicity

Over the past decades the description of quantitative structure-activity relationships (QSARs) has been undertaken in order to find predictive models and/or mechanistic explanations for chemical as well as biological activities. This includes QSAR studies in toxicology. In an approach beyond the classical QSAR approaches, attempts have been made to define parameters for the QSAR studies on the basis of quantum mechanical computer calculations. The conversion of relatively small xenobiotics within the active sites of biotransformation enzymes can be expected to follow the general rules of chemistry. This makes the description of QSARs on the basis of only one parameter, chosen on the basis of insight in the mechanism, feasible. In contrast, toxicological endpoints can very often be the result of more than one physico-chemical interaction of the compound with the model system of interest. Therefore the description of quantitative structure-toxicity relationships often does not follow a one-descriptor mechanistic approach but starts from the other end, describing QSARs by multi-parameter approaches. The present paper focuses on the possibilities and restrictions of using computer-based QSAR modeling for analyzing experimental toxicological data, with emphasis on examples from the field of biotransformation and toxicity.

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.

Creatine and creatinine metabolism

The goal of this review is to present a comprehensive survey of the many intriguing facets of creatine (Cr) and creatinine metabolism, encompassing the pathways and regulation of Cr biosynthesis and degradation, species and tissue distribution of the enzymes and metabolites involved, and of the inherent implications for physiology and human pathology. Very recently, a series of new discoveries have been made that are bound to have distinguished implications for bioenergetics, physiology, human pathology, and clinical diagnosis and that suggest that deregulation of the creatine kinase (CK) system is associated with a variety of diseases. Disturbances of the CK system have been observed in muscle, brain, cardiac, and renal diseases as well as in cancer. On the other hand, Cr and Cr analogs such as cyclocreatine were found to have antitumor, antiviral, and antidiabetic effects and to protect tissues from hypoxic, ischemic, neurodegenerative, or muscle damage. Oral Cr ingestion is used in sports as an ergogenic aid, and some data suggest that Cr and creatinine may be precursors of food mutagens and uremic toxins. These findings are discussed in depth, the interrelationships are outlined, and all is put into a broader context to provide a more detailed understanding of the biological functions of Cr and of the CK system.

Quantitative structure-activity relationship of flavonoids for inhibition of heterocyclic amine mutagenicity

The mutagenic/carcinogenic heterocyclic amines formed during the cooking of protein foods have been determined to be a potential risk to human health. Therefore, mitigation measures are beginning to be studied. A recent finding is that the induction of mutation in Salmonella by these amines can be inhibited by the addition of flavonoids to the assay. This study combines data on the inhibitory process with structural, ab initio quantum chemical, hydropathic, and antioxidant factors to develop a quantitative structure-activity relationship (QSAR) database and statistical analysis. For 39 diverse flavonoids the inhibitory potency varied approximately 100-fold. Three predictive variables, in order of decreasing contribution to variance, are: (1) a large dipole moment; (2) after geometric minimization of energy, a small departure from planarity (i.e., small dihedral angle between the benzopyran nucleus and the attached phenyl ring), and a low rotational energy barrier to achieving planarity; and (3) fewer hydroxyl groups on the phenyl ring. However, these variables account for less than half of the variance in inhibitory potency of the flavonoids. Frontier orbital energies and antioxidant or radical scavenging properties showed little or no relationship to potency. We conclude that interference by the flavonoids with cytochrome P450 activation of the promutagens is the probable mechanism for inhibition of mutagenesis, and suggest avenues for further research. Environ. Mol. Mutagen. 35:279-299, 2000 Published 2000 Wiley-Liss, Inc.

Heterocyclic amine formation and the impact of structure on their mutagenicity

The occurrence and formation of heterocyclic amines in foods is discussed in light of the consistent finding of a new class of imidazopyridines. In addition, a quantitative structure-activity relationship will be presented correlating the potency of these imidazopyridines to predicted chemical properties. Although no strong linear correlation is found between the potency and the chemical properties, a low dipole moment is found to be a qualitative predictor of high mutagenic potency.

Frontier orbital energies, hydrophobicity and steric factors as physical QSAR descriptors of molecular mutagenicity. A review with a case study: MX compounds

A review on QSARs (Quantitative Structure-Activity Relationships) in modelling molecular mutagenicity is given. The importance of hydrophobicity, frontier orbital (HOMO and LUMO) energies and steric factors as physical descriptors of mutagenicity is emphasized. In addition, some possible connections between QSAR models and the general electrophilic theory of genotoxic activity are discussed. As a detailed example, QSARs for the Ames Salmonella typhimurium TA100 mutagenicity of halogenated hydroxyfuranones including MX, one of the most potent bacterial mutagens ever identified, are discussed and a plausible mechanism for their mutagenic activity is proposed.

Chemical and biological factors affecting mutagen potency

This review surveys the chemical and biological factors that are correlated with the mutagenic activity of the aromatic and heterocyclic amines. Particular attention is given to the predicted quantum chemical properties of the parent amines and their metabolites. A number of chemical properties have been found to correlate well with measured mutagenic potency, including log P, the energies of the frontier orbitals of the parent amines, and the thermodynamic stability of the nitrenium ion, possibly the ultimate DNA-binding species. These correlations are intriguing clues to the mutagenic activity of the aromatic amines; however, many factors still await final explanation, including the exact mechanisms of the metabolic enzymes, the identity(s) of the ultimate DNA-binding species, the reaction mechanism in the DNA-adduction, the role of sequence context in the covalent and non-covalent binding of the adducts, and the role of DNA repair.

Deprotonation and hydride shifts in nitrenium and iminium forms of aminoimidazole-azaarene mutagens

The mutagenicity of many 2-aminoimidazole-azaarenes (AIA) is thought to be mediated by the nitrenium form of the exocyclic amine. This hypothesis is supported by the numerous correlations found between calculated and experimentally-measured chemical properties for the nitreniums and the mutagenic potencies of the nitreniums and their parent amines. One factor favoring high mutagenic potency is the presence of a methyl substituent in the 1- or 3-imidazole position. In this paper, we investigate both the deprotonation of the imidazole ring nitrogens in non-N-methylated AIA mutagens and the plausibility of a chemical pathway involving a 1-4 hydride shift to form an iminium ion, thereby stabilizing the cationic N-methyl substituted AIA mutagens. It has been widely noted that factors that stabilize the nitrenium moiety lead to significantly higher mutagenic potency; hence, the transformation of the nitrenium to a more stable species might be expected to increase the potency, provided that it does not eliminate the electrophilic reactivity of the compound. Using ab initio quantum chemistry and polarizable continuum solvation models, we find that the imidazole ring nitrogens of the nitrenium ions are extremely acidic. This suggests that upon formation of the exocyclic nitrenium these sites will deprotonate to form a neutral imine. We have also studied the 1-4 hydride shift from an imidazole ring methyl to the exocyclic nitrenium to form an iminium. We predict that for AIA mutagens with just two fused rings the resulting iminium species are more stable in the gas phase than the corresponding nitreniums. For mutagens with larger conjugated systems, the nitrenium is stabilized by resonance and is more stable than the corresponding iminium. In the aqueous phase, however, the iminium form is predicted to be more stable than the nitreniums for all polycyclic compounds studied. Although equilibrium calculations favor the iminium form, these have been experimentally shown to be short-lived and their actual concentration will depend on the complex kinetics of AIA mutagen metabolism. The quantum chemical results also show a strong correlation between the relative iminium-nitrenium energy difference and the charge on the exocyclic nitrogen.

Quantitative structure-activity (QSAR) relationships of mutagenic aromatic and heterocyclic amines

We extended our previous studies of mutagenic/carcinogenic heterocyclic aromatic amines formed during the cooking of foods to 66 aromatic and 99 heterocyclic amines for which mutagenic potency data are available. The amines require activation by enzymes to form metabolites reactive with DNA and exhibit an enormous range of potency as frameshift mutagens in the Ames/Salmonella assay. To ascertain factors that might influence potency, structural features and quantum mechanical parameters calculated by the Hückel method (and, for a subset of 20 amines, by semi-empirical AM1, and ab initio methods) were analyzed by multiple linear regression. The major findings were: (1) earlier findings on cooked food mutagens and their synthetic congeners can be extended to other amines; (2) mutagenic potency is directly related to the number of fused aromatic rings (size of the aromatic system), the number of ring nitrogen atoms (participation of lone electron pairs in the pi-cloud), and presence of a methyl substituent on a ring nitrogen; (3) potency is inversely related to the energy of the lowest unoccupied molecular orbital (LUMO) of the parent amine. Ford and Griffin (1992) and Sabbioni and Wild (1992) showed that the LUMO energy of the derived nitrenium ion is closely related to its stability (calculated with reference to aniline). Increased stability has been hypothesized to enhance the probability of adduct formation with DNA by avoiding detoxifying side reactions and increasing the lifetime of the ion. In the large heterogeneous series of amines in our present study the Hückel method energy of the highest occupied molecular orbital (HOMO), rather than the LUMO energy, of the nitrenium ion was marginally related to the potency of the parent amine. However, in the selected subset of 20 amines with ab initio calculation, the LUMO energy of the ion confirmed the previous reports. The contribution of quantum chemical factors to mechanistic insight on the mutagenicity and carcinogenicity of aromatic and heterocyclic amines is still under development.