The metabolic activation of 7-methylbenz(a)anthracene: the induction of malignant transformation and mutation in mammalian cells by non-K-region dihydrodiols

Structure, function and carcinogenicity of metabolites of methylated and non-methylated polycyclic aromatic hydrocarbons: a comprehensive review

The Unified Theory of PAH Carcinogenicity accommodates the activities of methylated and non-methylated polycyclic aromatic hydrocarbons (PAHs) and states that substitution of methyl groups on meso-methyl substituted PAHs with hydroxy, acetoxy, chloride, bromide or sulfuric acid ester groups imparts potent cancer producing properties. It incorporates specific predictions from past researchers on the mechanism of carcinogenesis by methyl-substituted hydrocarbons, including (1) requirement for metabolism to an ArCH2X type structure where X is a good leaving group and (2) biological substitution of a meso-methyl group at the most reactive center in non-methylated hydrocarbons. The Theory incorporates strong inferences of Fieser: (1) The mechanism of carcinogenesis involves a specific metabolic substitution of a hydrocarbon at its most reactive center and (2) Metabolic elimination of a carcinogen is a detoxifying process competitive with that of carcinogenesis and occurring by a different mechanism. According to this outlook, chemical or biochemical substitution of a methyl group at the reactive meso-position of non-methylated hydrocarbons is the first step in the mechanism of carcinogenesis for most, if not all, PAHs and the most potent metabolites of PAHs are to be found among the meso methyl-substituted hydrocarbons. Some PAHs and their known or potential metabolites and closely related compounds have been tested in rats for production of sarcomas at the site of subcutaneous injection and the results strongly support the specific predictions of the Unified Theory.

The metabolism of the carcinogen 7-methylbenz[c]acridine by hepatocytes isolated from untreated and induced rats

The metabolic fate of the carcinogenic aza-aromatic hydrocarbon 7-methyl[7-(14)C]benz[c]acridine (14C-7MBAC) was studied in hepatocytes freshly isolated from untreated, phenobarbital-pretreated and 3-methylcholanthrene-pretreated rats. 14C-7MBAC (4-200 microM) was metabolized in a concentration-dependent manner; Michaelis-Menten kinetics were not followed. Using 100 microM 14C-7MBAC, the bulk of the ethyl acetate-extractable metabolites were found in the incubation medium; about 50% of the total metabolites were not extractable into ethyl acetate. The nature of the water-soluble metabolites was examined by enzyme hydrolysis of glucuronides and sulphates, and by glutathione-depletion experiments. Organo-extractable metabolites were examined by reverse-phase h.p.l.c. and quantified by co-chromatography with standards. pretreatment of the rats with mixed-function oxidase inducers, phenobarbital and 3-methylcholanthrene resulted in 2.85- and 5.70-fold increases, respectively, in total metabolism of 14C-7MBAC. Major metabolites for all three hepatocyte preparations co-chromatographed with 7-hydroxymethylbenz[c]acridine, trans-5,6-dihydro-5,6-dihydroxy-7-methylbenz[c]acridine and trans-8,9-dihydro-8,9-dihydroxy-7-methylbenz[c]acridine.

Stereoselective metabolism of 7-methylbenz[a]anthracene: absolute configuration of five dihydrodiol metabolites and the effect of dihydrodiol conformation on circular dichroism spectra

7-Methylbenz[a]anthracene (7-MBA) was metabolized stereoselectively by rat liver microsomes to form five optically active dihydrodiols as the predominant metabolites. The dihydrodiols were purified by a combination of reversed-phase and normal-phase high performance liquid chromatography (HPLC). By comparison of their circular dichroism (CD) spectra with the corresponding benz[a]anthracene (BA) dihydrodiols of known absolute stereochemistry, the major dihydrodiol enantiomers of 7-MBA have been determined to have 1R,2R-, 3R,4R- and 10R , 11R - absolute configurations, respectively. Due to their quasi- diaxial conformations, the absolute configuration of trans-5,6- and trans-8,9-dihydrodiols, the two most abundant metabolites of 7-MBA, could not be determined by simple comparisons of their circular dichroism spectra with those of the quasidi -equatorial BA 5R, 6R - and 8R , 9R -dihydrodiols. The major enantiomers of the quasi- diaxial trans-5,6- and trans-8,9-dihydrodiol metabolites of 7-MBA were determined by comparison to the CD spectrum of 7-bromo-BA 5R, 6R -dihydrodiol and by the exciton chirality method to have R,R absolute stereochemistry. This study also revealed that the circular dichroism Cotton effects of an enantiomeric dihydrodiol of polycyclic aromatic hydrocarbons can be drastically altered if the conformation (quasi- diaxial vs. quasi di-equatorial ) of the dihydrodiol is changed.

Cell transformation by chemical agents--a review and analysis of the literature. A report of the U.S. Environmental Protection Agency Gene-Tox Program

The literature on cell transformation by chemical carcinogens has been critically reviewed. This subject is highly relevant to carcinogenesis in vivo, because the phenotypic changes that are collectively referred to as cell transformation usually involve the acquisition of tumorigenicity on inoculation into suitable rodent hosts. The systems chosen for review fall into 3 categories: cell strains (cells with a limited lifespan); cell lines (cells with an unlimited lifespan); and oncogenic viral-chemical interactions involving cells (Fischer rat embryo cells expressing an endogenous retrovirus, mouse embryo cells expressing the AKR leukemia virus, chemical enhancement of a simian adenovirus, SA7 transformation of Syrian hamster or rat embryo cells). Of the entire literature reviewed, 117 papers have been accepted for data abstraction by pre-defined criteria; these include 41 references to cell strains, 40 in cell lines, and 38 in viral-chemical interactions including cells. Because different systems have been reviewed, it would be meaningless to group all the compounds. The overall summary of the systems is as follows (many compounds have been tested in more than one system and, hence, are duplicated in these totals). (Chart: see text) In general, there is a reasonably good correlation between the results of the cell transformation systems and in vivo carcinogenesis. However, the many deficiencies of the EPA Merged Carcinogen List preclude definitive comparisons. Moreover, a number of 'false negatives' were obtained in systems that did not employ external metabolic activation. Further validation of all systems is required, but it seems very probable that several cell transformation systems will become valuable in assaying (with reasonable time and cost) the carcinogenic potential of environmental chemicals.

Metabolism of 7-methylbenz[a]anthracene and 7-hydroxymethylbenz[a]anthracene by Cunninghamella elegans

The fungal metabolism of 7-methylbenz[a]anthracene (7-MBA) and 7-hydroxymethylbenz[a]anthracene (7-OHMBA) was studied. 7-MBA was metabolized by Cunninghamella elegans to form 7-OHMBA-trans-8,9-dihydrodiol and 7-OHMBA-trans-3,4-dihydrodiol as the predominant metabolites. Other metabolites were identified as 7-OHMBA, 7-MBA-trans-8,9-dihydrodiol and 7-MBA-trans-3,4-dihydrodiol, and 7-MBA-8,9,10,11-tetraol. Incubation of 7-OHMBA with C. elegans cells indicated that 7-OHMBA-trans-8,9-dihydrodiol and 7-OHMBA-trans-3,4-dihydrodiol were major metabolites. The metabolism of 7-MBA by rat liver microsomes from 3-methylcholanthrene-treated rats showed that the metabolites were qualitatively similar to those formed by C. elegans, except additional dihydrodiol metabolites were formed at the 5,6 and 10,11 positions. The metabolites formed were isolated by high-performance liquid chromatography and identified by comparing their chromatographic, UV-visible absorption and mass spectral properties with those of reference compounds.

Mutagenesis by chemical agents in V79 chinese hamster cells: a review and analysis of the literature. A report of the Gene-Tox Program

The report reviews and evaluates the current literature (about 125 primary publications) on chemically induced specific locus mutations in the V79 Chinese hamster lung cell line. The V79 cell is convenient to use for mutagenesis studies since it has a rapid growth rate, high plating efficiency, and a stable karyotype. Mutation can be easily measured at either the hypoxanthine-guanine phosphoribosyl transferase or the Na+/K+ ATPase locus, both of which have been well characterized. Other less-studied markers are also described. We discuss the protocols for quantitative mutation studies including measurements of cytotoxicity, mutant expression times, mutant selection agents, cell densities during selection, and the stability and verification of mutant phenotypes. Mutations in the V79 cells by chemicals that require activation can be tested after their metabolism by cell homogenates or by intact cells, and the results with each type of activation are compared. For purposes of analysis, we classified a compound as mutagenic if it induced a mutation frequency that is at least 3 times higher than the spontaneous mutant frequency reported for that specific experiment. By this criterion two-thirds of the chemicals analyzed were mutagenic--; 11% with and 55% without metabolic activation. Of the 191 chemicals examined; 119 were polycyclic aromatic hydrocarbons; 25 were nitro or nitroso compounds, 9 were alkyl halides; 7 were purine or pyrimidine derivatives and the remaining 31 were from other chemical classes. We also defined mutagenic potency as the concentration of a compound that increases the mutant frequency by 10 times the spontaneous frequency. Mutagenic potencies of the compounds examined varied over a range of 5 X 10(6). We have also found large interlaboratory variations in the mutagenic potencies. Such variation in potency could be reduced by normalizing the results to a standard mutagen such as N-methyl-N'-nitro-N-nitrosoguanidine. The role of the V79 assay in mutagenicity and carcinogenicity testing is discussed and recommendations are suggested for future investigation.

International Commission for Protection against Environmental Mutagens and Carcinogens. ICPEMC working paper 2/5: mutagenesis in mammalian cells

Chemical mutagenesis in animal cells is a complex process. Whereas some chemicals are mutagenic in their original form, others such as the nitrosamines and polycyclic hydrocarbon carcinogens are mutagenic only when enzymatically activated. The active form, or ultimate carcinogen, can interact with proteins and nucleic acids, altering amino acids and producing modified bases in DNA. The modified bases do not usually constitute mutations as produced. Instead they are acted on by the DNA enzymes of the cell, which repair most damaged bases but occasionally insert incorrect base sequences at or near the sites of damage. The frequency at which mutant animal cells are recovered depends upon the selection conditions in culture, upon whether the mutation selected is in a gene present in single or multiple active copies, and upon whether expression is dominant or recessive. Many studies depend on selecting for 8-azaguanine- or 6-thioguanine-resistant mutants, which are due to mutations in the HGPRT locus present in a single active copy on the X-chromosome. Other widely used systems depend on selecting for ouabain resistance, which is dominant and results from a change in the sodium/potassium ATPase activity, or on selecting for thymidine kinase mutants in heterozygous Tk+/Tk- mouse cells. Many other types of mutation including nutritional markers are recessive and express only in cells carrying a single active gene copy, as is sometimes the case in established cell lines. The types of base damage causing mutations have been identified in very few cases only, and little is known about the enzymatic mechanisms of mutagenesis. However, chemical mutagenesis in cultured animal cells provide a practical way of testing chemicals and radiations for mutagenicity directly in animal cells, and much has been learned about the mutagenicity of various carcinogenic substances. To date, there is reasonable qualitative agreement between these results and those obtained in the widely used liver microsome-activated bacterial mutagenesis test systems.

Specificity of chemiluminescence in the metabolism of benzo[a]pyrene to its carcinogenic diol epoxide

The metabolism of 7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene results primarily in the ultimate carcinogenic metabolite of benzo[a]pyrene, 7,8-dihydroxy-9,10-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene and to a lesser extent 7,8-dihydroxy-9,10-dioxetane-7,8,9,10-tetrahydrobenzo[a]pyrene, from which chemiluminescence is observed. This specific microsomal chemiluminescence has been used to establish that the rate-limiting reaction in the metabolism of benzo[a]pyrene to the bay region diol epoxide is the production of the 7,8-diol. The microsome-mediated chemiluminescence of the parent benzo[a]pyrene is therefore an indicator of the activity of the specific sequence of metabolic reactions leading to the ultimate carcinogenic metabolite.

Additional evidence for the involvement of the 3,4-diol 1,2-oxides in the metabolic activation of 7,12-dimethylbenz[a]anthracene in mouse skin

The role of vicinal diol-epoxides in the metabolic activation of 7,12-dimethylbenz[a]anthracene to intermediates that react with nucleic acids was investigated using Sephadex LH-20 column chromatography and high pressure liquid chromatography. The results show that some of the hydrocarbon-DNA products formed in mouse skin treated in vivo with 7,12-dimethylbenz[a]anthracene arise from the reaction of DNA with 3,4-dihydro-3,4-dihydroxy-7,12-dimethylbenz[a]anthracene 1,2-oxides which, on the basis of this and other evidence, appears to be a biologically-active metabolite of 7,12-dimethylbenz[a]anthracene. However, since other nucleic acid-hydrocarbon adducts were also present that have not been identified as resulting from the reaction of the 3,4-diol 1,2-oxides with DNA, other mechanisms may also be involved in the metabolic activation of 7,12-dimethylbenz[a]anthracene in mouse skin.

Biotransformation and bioactivation of 7, 12-dimethylbenz[a]anthracene (7, 12-DMBA)

As the result of rapidly developing technological advances, our understanding of the biotransformation and bioactivation of 7, 12-DMBA has increased markedly in recent years. In terms of the metabolic conversion of this polynuclear aromatic hydrocarbon to reactive mutagen/carcinogens, the "bay region" generalization appears to apply, although the candidacy of a number of other intermediary metabolites as ultimate biologically-active forms still remains viable. Large gaps remain in knowledge concerning the nonoxidative metabolic transformations of 7, 12-DMBA, and these require closing in order to further our understanding of the regulation of mechanics controlling steady-state levels of reactive intermediates. Studies on the photooxidation of the hydrocarbon have allowed a stronger appreciation of its chemical reactivity and instability and promise to help resolve many of the apparently conflicting observations of the past. 7, 12-DMBA remains a highly interesting and valuable tool in investigations of bioactivation processes as they relate to the etiology of several important pathologic conditions, including chemically induced tissue necrosis, mutagenesis, carcinogenesis, teratogenesis, atherogenesis, and, possibly, other pathogenic phenomena as well. It is hoped that this review will serve to benefit research in these areas and hasten the reduction of such pathologic phenomena in our society.

The formation of dihydrodiols in the chemical or enzymic oxidation of dibenz[a,c]anthracene, dibenz[a,h]-anthracene and chrysene

The formation of trans-dihydrodiols from dibenz[a,c]anthracene, dibenz[a,h]anthracene and chrysene by chemical oxidation in an ascorbic acid-ferrous sulphate-EDTA system and by rat-liver microsomal fractions has been studied using a combination of thin-layer (TLC) and high pressure liquid chromatography (HPLC) to separate the mixtures of isomeric dihydrodiols. The 1,2- and 3,4-dihydrodiols of dibenz[a,c]anthracene, the 1,2-,3,4- and 5,6-dihydrodiols of dibenz[a,h]anthracene and the 1,2-, 3,4- and 5,6-dihydrodiols of chrysene were formed in chemical oxidations. These dihydrodiols were also formed when the three parent hydrocarbons were metabolized by rat-liver microsomal fractions and, in addition, dibenz[a,c]anthracene yielded the 10,11-dihydrodiol. The 1,2- and 3,4-dihydrodiols of dibenz[a,c]anthracene have not been reported previously either as metabolites of the hydrocarbon or as products of chemical syntheses and the 5,6-dihydrodiol of chrysene was not detected in earlier metabolic studies.

Fluorescence of hydrocarbon-deoxyribonucleoside adducts

In comparison with the fluorescence emission spectra of 7-methylbenz[a]-anthracene-nucleoside adducts, the fluorescence emission spectra of hydrocarbon-deoxyribonucleoside adducts containing a methyl substituent in the "bay region" lack spectral resolution at room temperature and appear at substantially longer wavelength. This spectral resolution is improved when spectra are measured at 77 K and an irreversible spectral shift to shorter wavelength, accompanied by improved resolution, results from mild acid hydrolysis. These spectral properties peculiar to the "bay region-substituted" adducts presumably result from an intramolecular interaction between the hydrocarbon fluorophore and the attached nucleoside brought about, in the examples studied here, by the presence of the 12-methyl group in 7,12-dimethylbenz[awanthracene (DMBA) and in 7-hydroxymethyl-12-methylbenz[a]anthracene. This interaction suggests that the site of nucleoside attachment is in close proximity to the 12-methyl group and that binding occurs, therefore, through the intermediacy of a 3,4-diol-1,2-oxide, i.e. a "bay region" diol-epoxide in each case.

The formation of dihydrodiols by the chemical or enzymic oxidation of benz[a] anthracene and 7,12-dimethylbenz[a] anthracene

When benz[a] anthracene was oxidised in a reaction mixture containing ascorbic acid, ferrous sulphate and EDTA, the non-K-region dihydrodiols, trans-1,2-dihydro-1,2-dihydroxybenz[a] anthracene and trans-3,4-dihydro-3,4-dihydroxybenz[a] anthracene together with small amounts of the 8,9- and 10,11-dihydrodiols were formed. When oxidised in a similar system, 7,12-dimethylbenz[a] anthracene yielded the K-region dihydrodiol, trans-5,6-dihydro-5,6-dihydroxy-7,12-dimethylbenz[a] anthracene and the non-K-region dihydrodiols, trans-3,4-dihydro-3,4-dihydroxy-7,12-dimethylbenz[a] anthracene, trans-8,9-dihydro-8,9-dihydroxy-7,12-dimethylbenz[a] anthracene, trans-10,11-dihydro-10,11-dihydroxy-7,12-dimethylbenz[a] anthracene and a trace of the 1,2-dihydrodiol. The structures and sterochemistry of the dihydrodiols were established by comparisons of their UV spectra and chromatographic characteristics using HPLC with those of authentic compounds or, when no authentic compounds were available, by UV, NMR and mass spectral analysis. An examination by HPLC of the dihydrodiols formed in the metabolism, by rat-liver microsomal fractions, of benz[a] anthracene and 7,12-dimethylbenz[a] anthracene was carried out. The metabolic dihydriols were identified by comparisons of their chromatographic and UV or fluorescence spectral characteristics with compounds of known structures. The principle metabolic dihydriols formed from both benz[a] anthracene and 7,12-dimethylbenz[a] anthracene were the trans-5,6- and trans-8,9-dihydrodiols. The 1,2- and 10,11-dihydrodiols were identified as minor products of the metabolism of benz [a] anthracene and the tentative identification of the trans-3,4-dihydriol as a metabolite was made from fluorescence and chromatographic data. The minor metabolic dihydriols formed from 7,12-dimethylbenz[a] anthracene were the trans-3,4-dihydrodiol and the trans-10,11-dihydriol but the trans-1,2-dihydrodiol was not detected in the present study.

The formation of dihydrodiols by chemical or enzymic oxidation of 3-methylcholanthrene

The chemical oxidation of 3-methylcholanthrene in an ascorbic acid-ferrous sulphate-EDTA reaction mixture gave all five possible dihydrodiols. The structures and stereochemistry of the dihydrodiols were shown by UV, mass and NMR spectral studies and by chemical examination to be cis-2a,3-dihydroxy-3-methylcholanthrene, trans-4,5-dihydro-4,5-dihydroxy-3-methylcholanthrene, trans-7,8-dihydro-7,8-dihydroxy-3-methylcholanthrene, trans-9,10-dihydro-9,10-dihydroxy-3-methylcholanthrene, cis-11,12-dihydro-11,12-dihydroxy-3-methylcholanthrene and trans-11,12-dihydro-11,12-dihydroxy-3-methylcholanthrene. An examination by HPLC of the dihydrodiols formed in the metabolism of 3-methylcholanthrene by rat-liver microsomal preparations showed the presence of trans-4,5-dihydro-4,5-dihydoxy-3-methylcholanthrene, trans-7,8-dihydro-7,8-dihydroxy-3-methylcholanthrene, trans-9,10-dihydro-9,10-dihydroxy-3-methylcholanthrene and trans-11,12-dihydro-11,12-dihydroxy-3-methylcholanthrene, identified by comparison of their UV and chromatographic characteristics with those of authentic standards. Tentative identification of cis- and trans-1,2-dihydroxy-3-methylcholanthrene, cis-2a,3-dihydroxy-3-methylcholanthrene and cis-11,12-dihydro-11,12-dihydroxy-3-methylcholanthrene as metabolites were made from their mobilities using HPLC. A quantitative comparison of the dihydrodiols formed from 3H-labelled 3-methylcholanthrene by microsomal preparations from the livers of normal and 3-methylcholanthrene-treated rats was carried out. trans-9,10-Dihydro-9,10-dihydroxy-3-methylcholanthrene and cis- and trans-1,2-dihydroxy-3-methylcholanthrene were formed when 3-methylcholanthrene was incubated with mouse skin in organ culture.

Induction of sister-chromatid exchanges in Chinese hamster ovary cells treated in vitro with non-K-region dihydrodiols of 7-methylbenz[a]anthracene and benzo[a]pyrene

Studies were carried out on the incidence of sister-chromatid exchanges induced in Chinese hamster ovary cells by in vitro treatment with the polycyclic aromatic hydrocarbons 7-methylbenz[a]anthracene and benzo[a]pyrene and with related K-region and non-K-region dihydrodiols. Appreciable increased in the incidence of sister-chromatid exchanges were apparent in cells treated with non-K-region dihydrodiols: the most active compounds were 3,4-dihydro-3,4-dihydroxy-7-methylbenz[a]anthracene and 7,8-dihydro-7,8-dihydroxybenzo[a]pyrene and the effects were dose-dependent. The parent hydrocarbons and the related K-region dihydrodiols induced some sister-chromatid exchanges but they were considerably less active than these two non-K-region diols. The results suggest that this system may usefully be applied to studies aimed at determining which dihydrodiols are important in the metabolic activation of the carcinogenic polycyclic hydrocarbons. These and other results also infer that Chinese hamster ovary cells possess some intrinsic ability to metabolize such compounds in the absence of exogenous activation systems.

The preparation of dihydrodiols from 7-methylbenz[a]-anthracene

The products formed when the carcinogenic polycyclic hydrocarbon 7-methylbenz[a] anthracene is oxidized with an ascorbic acid-ferrous sulphate mixture have been investigated. All 5 possible dihydrodiols were formed and the isolation of the 3 non-K-region dihydrodiols, trans-1,2-dihydro-1,2-dihydroxy-7-methylbenz[a]anthracene, trans-3,4-dihydro-3,4-dihydroxy-7-methylbenz[a] anthracene and trans-8,9-dihydro-8,9-dihydroxy-7-methylbenz[a] anthracene is described. The purification of the dihydrodiols was carried out by thin-layer (TLC) followed by preparative high pressure liquid chromatography (HPLC). The ultra-violet, spectral and nuclear magnetic resonance (NMR) characteristics of the dihydrodiols are reported and the data used to assign the proposed structures. An explanation for the unusual preferred conformation which the 8,9-dihydrodiol adopts is advanced.

Some properties of vicinal diol-epoxides derived from benzo(a)anthracene and benzo(a)pyrene

The alkylating properties of pairs of syn- and anti-isomers of 2 diol-epoxides derived from benzo(a)pyrene (BP) and of 1 derived from benz(a)anthracene (BA) have been investigated. Of the anti-diol-epoxides, anti-BP 7,8-diol-9,10-oxide was the most reactive compound towards DNA, towards sodium p-nitrothiophenolate in a non-aqueous solvent system, and towards 4-(p-nitrobenzyl)pyridine in aqueous solution; anti-BP 9,10,-diol-7,8-oxide was of intermediate reactivity and anti-BA 8,9-diol-10,11-oxide was least reactive. The syn-diol-epoxides gave unsatisfactory results with DNA and 4-(p-nitrobenzyl)pyridine because of their rapid solvolysis in aqueous solution, but with sodium p-nitrothiophenolate showed the order of reactivity syn-BP 7,8-diol-9,10-oxide greater than syn-BA 8,9-diol-10,11-oxide greater than syn-BP 9,10-diol-7,8-oxide. The products of the reaction between diol-epoxides and nucleic acids were examined by Sephadex LH-20 chromatography followed by high-pressure liquid chromatography (HPLC) and the diol-epoxides were shown to react principally with the guanosine and adenosine moieties of RNA.

Mutagenicity of isomeric diol-epoxides of benzo[a]pyrene and benz[a]anthracene in S. typhimurium TA98 and TA100 and in V79 Chinese hamster cells

Pairs of isomeric vicinal diol-epoxides derived from benzo[a]pyrene 7,8- and 9,10-dihydrodiols and from benz[a]anthracene 8,9-dihydrodiol were tested for their abilities to revert salmonella typhimurium strains TA98 and TA100 to histidine prototrophy and to induce the formation of 8-azaguanine- or of ouabain-resistant V79 Chinese hamster cells. All six diol-epoxides were active in both bacterial strains, but 7beta,8alpha-dihydroxy-9beta,10beta-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (the syn isomer) was considerably more mutagenic than the other diol-epoxides. Within the three pairs of stereo-isomeric diol-epoxides, the ratio of the mutagenic potencies of the syn over the related anti isomers varied bothwith the chemical structure and the bacterial strain. The half lives of hydration of these diol-epoxides at pH 7.4 were inversely related to their mutagenic potencies in bacteria. In V79 cells, the two benzo[a]pyrene 7,8-diol 9,10-oxides were mutagenic and the anti isomer was more active than the syn isomer; a reversed order of mutagenic potency with these stereo isomers was observed in S. typhimurium. The other four diol-epoxides were non-mutagenic in V79 cells at the concentrations tested.

The activity of 7-methylbenz(a)anthracene metabolites in an in vitro-in vivo carcinogenicity test using mouse lung tissue

7-Methylbenz(a)anthracene (7-MBA) and its 5,6-oxide and trans-5,6- and trans-8,9-dihydrodiol were tested for carcinogenic activity in a system in which mouse lung tissue was incubated in the presence of a test compound for 1 h and then implanted into isologous mice. All four compounds gave small yields of adenomas and in addition the 5,6-oxide gave two carcinomas.

The metabolic activation of 7-methylbenz(a)anthracene in mouse skin

The metabolism of 7-methylbenz(a)anthracene by rat-liver preparations and by mouse skin has been studied using a combination of thin-layer and high pressure liquid chromatography and all five possible trans-dihydrodiols have been detected as metabolites but in different proportions. The roles of these dihydrodiols and of the related vicinal diol-epoxides in the metabolic activation of 7-methylbenz(a)anthracene in mouse skin has been studied using Sephadex LH-20 column chromatography. The results show that the hydrocarbon-nucleic acid products formed in mouse skin in vivo most probably arise from 3,4-dihydro-3,4-dihydroxy-7-methylbenz(a)anthracene 1,2-oxide which, on the basis of this and other evidence, appears to be the reactive intermediate involved in the metabolic activation of 7-methylbenz(a)anthracene in this tissue.