Benzo(a)pyrene diol epoxides as intermediates in nucleic acid binding in vitro and in vivo

Binding of enantiomers of trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydro-benzo[a]pyrene to polynucleotides

DNA covalent binding studies with enantiomers of trans-7,8-dihydroxy- anti-9,10-epoxy-7,8,9,10-tetrahydro-benzo[a]pyrene (anti-BPDE) have been carried out by means of spectroscopic techniques (UV, CD, and fluorescence). Synthetic polynucleotides are employed to investigate binding differences between the G.C and A.T base pairs and to elucidate the bases for the stereoselective covalent binding of DNA toward anti-BPDE. The results indicate that of all the polynucleotides studied, only poly(dA-dT).poly(dA-dT) exhibits predominant intercalative covalent binding towards (+)-anti-BPDE and suffers the least covalent modification. Only minor intercalative covalent contributions are found in alternating polymer poly(dA-dC).poly(dG-dT). These observations parallel the DNA physical binding results of anti-BPDE and its hydrolysis products. They support the hypothesis that intercalative covalent adducts derive from intercalative physical binding while the external covalent adducts derive from external bimolecular associations. In contrast to the A.T polymers, the guanine containing polymers exhibit pronounced reduction in covalent modification by (-)-anti-BPDE. The intercalative covalent binding mode becomes relatively more important in the adducts formed by the (-) enantiomer as a consequence of decreased external guanine binding. These findings are consistent with the guanine specificity, stereoselective covalent binding at dG, the absence of stereoselectivity at dA for anti-BPDE, and the enhanced binding heterogeneity for the (-) enantiomer as found in the native DNA studies. The possible sequence and/or conformational dependence of such stereoselective covalent binding is indicated by the opposite pyrenyl CD sign exhibited by (+)-anti-BPDE bound to polynucleotides with pyrimidine on one strand and purine on another vs. that bound to polymers containing alternating purine-pyrimidine sequences.

Clotrimazole, an inhibitor of benzo[a]pyrene metabolism and its subsequent glucuronidation, sulfation, and macromolecular binding in BALB/c mouse cultured keratinocytes

The effect of the antifungal imidazole compound, clotrimazole, on the metabolism of benzo[a]pyrene (BP) was studied in cultured keratinocytes prepared from BALB/c mouse epidermis. Varying concentrations of clotrimazole added to the cultured keratinocytes resulted in a dose-dependent inhibition of the activities of the microsomal cytochrome P-450-dependent monooxygenases aryl hydrocarbon hydroxylase and 7-ethoxycoumarin O-deethylase. The major organic solvent-soluble metabolites of BP identified in the cultured cells were trans-7,8-dihydro-7,8-dihydroxybenzo[a]pyrene (BP-7,8-diol), 9-hydroxybenzo[a]pyrene (9-OH-BP), and 3-hydroxybenzo[a]pyrene (3-OH-BP), although small amounts of trans-4,5-dihydro-4,5-dihydroxybenzo[a]pyrene, BP-quinones, and trans-9,10-dihydroxybenzo[a]pyrene were also present. The major organic solvent-extractable metabolites of BP found in the extracellular culture medium were primarily the diols with smaller quantities of phenols and quinones. The major water-soluble metabolites of BP present both intracellularly and extracellularly were glucuronide conjugates of 3-OH-BP, 9-OH-BP, and benzo[a]pyrene-3,6-dione and to a lesser extent sulfate conjugates (primarily of the BP-7,8-diol). Clotrimazole inhibited the generation of organic solvent-soluble and water-soluble conjugates in a dose-dependent manner. The in vitro metabolism of BP by microsomes prepared from control and benz[a]anthracene (BA)-induced cultured keratinocytes was also inhibited by clotrimazole with greater inhibitory effect on BA-induced keratinocytes especially with respect to the formation of diols and quinones. The enzyme-mediated covalent binding of BP to mouse keratinocyte DNA and protein was also substantially diminished by clotrimazole in a dose-dependent fashion. These results indicate that clotrimazole, a widely used drug for the management of a variety of superficial dermatophyte infections of the skin, is a potent inhibitor of cytochrome P-450-dependent transformation of polycyclic aromatic hydrocarbons in cultured murine keratinocytes. This system offers a convenient approach for studies as inhibitors of carcinogen metabolism in the epidermis.

Ellagic acid binding to DNA as a possible mechanism for its antimutagenic and anticarcinogenic action

Ellagic acid (EA), a plant phenol, is reported to possess antimutagenic and anticarcinogenic activity. In the present study, explants of esophagus, trachea, colon, forestomach and bladder from young male Sprague-Dawley rats were incubated in medium containing [3H]EA (4.5 mu Ci/ml) for 24 h at 37 degrees C. DNA from these explants was extracted, purified and quantitated to determine [3H]EA binding to the DNA. Significant covalent binding of [3H]EA to DNA occurred in all the explants. Calf thymus DNA incubated in 0.05 M sodium phosphate buffer containing [3H]EA covalently bound [3H]EA in a concentration dependent manner. Furthermore covalent binding of [3H]EA to calf thymus DNA was inhibited by the addition of unlabeled EA that was concentration dependent over a range of 50-150 microM and by the addition of unlabeled adenosine, cytidine, guanosine or thymidine at a concentration of 1.0 mM. These results suggest that one of the mechanisms by which EA inhibits mutagenesis and carcinogenesis is by forming adducts with DNA, thus masking binding sites to be occupied by the mutagen or carcinogen.

Induction of alkaline-labile sites in DNA by benzo [a]pyrene and the repair of those lesions in cultured Chinese hamster cells

Formation of alkaline-labile sites in DNA by S9-activated benzo [a]pyrene (B [a]P) and the repair of those lesions were investigated using the technique of alkaline elution in cultured Chinese hamster V79 cells. When the cells were treated with B [a]P (1-5 micrograms/ml) there was negligible increase in DNA elution at pH 12.1 as compared to untreated controls. However, the elution of DNA increased at pH 12.6 with a concentration dependency, thereby indicating formation of alkaline-labile sites in DNA by B [a]P. After 4 h of repair incubation the elution of DNA at pH 12.6 of B [a]P (5 micrograms/ml) treated cells returned to the control levels. The half-life of alkaline-labile sites formed by B [a]P was approximately 1.5 h. Inhibitors of DNA-repair synthesis, hydroxyurea (HU) and 1-beta-D-arabinofuranosyl cytosine (ara-C) when added simultaneously with S9-activated B [a]P for 3 h showed an increase in elution of DNA at pH 12.1, indicating that a population of B [a]P-induced DNA lesions could be removed by a rapid DNA-repair process. These results indicate that at least two kinds of DNA lesions, repairable alkaline-labile sites and rapidly repairable DNA single-strand breaks, are detected after B [a]P treatment by the use of the alkaline elution procedure, by changing elution pH.

Chemical reactivities and mutagenicities of a series of chloromethylbenzo[a]pyrenes

The chemical and mutagenic properties of a series of chloromethylbenzo[a]pyrenes (chloromethyl-BaP) (chloromethyl groups in position 1-, 4-, 5-, 6-, 10-, 11- or 12-) were studied in order to address the question of the importance of arylmethyl carbocations as possible ultimate carcinogens of methylated polycyclic aromatic hydrocarbons (PAH). The rates of solvolysis of the series of chloromethyl-BaP in 50% aqueous acetone decrease in the order: 6 greater than 1 much greater than 4 greater than 12 greater than 5 greater than 10 greater than 11. There is a rough correlation (r = -0.80, P less than 0.05) between rates of solvolysis and the carbon chemical shifts of the methylene carbons. There is a good correlation (r = 0.98, P less than 0.001) between the rates of solvolysis and the gas phase stabilities of the carbocations, (M+ -35), obtained from mass spectral analysis. The mutagenicities of the series of chloromethyl-BaP in the Ames assay with strains TA98 and TA100 showed strong to very strong mutagenicities for the 4-, 5-, 10-, 11- and 12-isomers and weak mutagenicities for the 1- and 6-isomers. The corresponding hydroxymethyl-BaP were not mutagenic. The mutagenicities of some of the chloromethyl-BaP are among the highest reported for direct-acting (not requiring microsomal activation) mutagens in the Ames assay.

A comparison of the intercalative binding of non-reactive benzo[a]pyrene metabolites and metabolite model compounds to DNA

The reversible DNA physical binding of a series of non-reactive metabolites and metabolite model compounds derived from benzo[a]pyrene (BP) has been examined in UV absorption and in fluorescence emission and fluorescence lifetime studies. Members of this series have steric and pi electronic properties similar to the highly carcinogenic metabolite trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) and the less potent metabolite 4,5-epoxy-4,5-dihydrobenzo(a)pyrene (4,5-BPE). The molecules examined are trans-7,8-dihydroxy-7,8-dihydrobenzo[a]-pyrene (7,8-di(OH)H2BP), 7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene (tetrol) 7,8,9,10-tetrahydrobenzo[a]pyrene (7,8,9,10-H4BP), pyrene, trans-4,5-dihydroxy-4,5-dihydrobenzo[a]pyrene (4,5-di(OH)H2BP) and 4,5-dihydrobenzo[a]pyrene (4,5-H2BP). In 15% methanol at 23 degrees C the intercalation binding constants of the molecules studied lie in the range 0.79-6.1 X 10(3) M-1. Of all the molecules examined the proximate carcinogen 7,8-di(OH)-H2BP is the best intercalating agent. The proximate carcinogen has a binding constant which in UV absorption studies is found to be 2.8-6.0 times greater than that of the other hydroxylated metabolites. Intercalation is the major mode of binding for 7,8-di(OH)H2BP and accounts for more than 95% of the total binding. Details concerning the specific role of physical bonding in BP carcinogenesis remain to be elucidated. However, the present studies demonstrate that the reversible binding constants for BP metabolites are of the same magnitude as reversible binding constants which arise from naturally occurring base-base hydrogen bonding and pi stacking interactions in DNA. Furthermore, previous autoradiographic studies indicate that in human skin fibroblasts incubated in BP, pooling of the unmetabolized hydrocarbons occurs at the nucleus. The high affinity of 7,8-di(OH)H2BP for DNA may play a role in similarly elevating in vivo nuclear concentrations of the non-reactive proximate carcinogen.

Selective alkylation of carcinogenic 9-anthryloxirane at the N-3 position of adenine in DNA

Carcinogenic 9-anthryloxirane binds covalently to calf thymus DNA and poly(dA-dT). Application of the technique for DNA sequence determination shows that acid cleavage of the modified DNA frees approximately half of the anthryl groups from the DNA. HPLC analysis indicates that an adenine adduct and the glycol derived from 9-anthryloxirane are the major acid-labile products. Spectroscopic analyses establish that the adenine adduct is the N-3 adduct of 9-anthryloxirane to adenine. Similar analyses of modified poly(dA-dT) indicate that the binding of 9-anthryloxirane takes place selectively at the N-3 position of adenine. The significance of this finding is briefly discussed.

Benzo[a]pyrene metabolism in mouse liver. Association of both 7,8-epoxidation and covalent binding of a metabolite of the 7,8-diol with the Ah locus

The 7,8-epoxidation of benzo[a]pyrene, and the 9,10-epoxidation of benzo[a]-pyrene trans-7,8-dihydrodiol coupled with covalent binding of the highly reactive diol-epoxide, are two key P-450-mediated reactions believed to be important in cancer initiation, mutagenesis and teratogenesis. New assays for these two reactions were developed with mouse liver microsomes. These two activities have apparent Km values (approximately 6 microM) similar to that of aryl hydrocarbon hydroxylase activity. Twenty-six individual 3-methylcholanthrene-treated Ahb/Ahd and Ahd/Ahd progeny of the (C57BL/6N)(DBA/2N) F1 X DBA/2N backcross were studied. Both of the newly described activities appear to represent P-450 protein(s) that are responsible for aryl hydrocarbon hydroxylase activity and that are coordinately controlled by the Ahb allele.

Covalent binding of BP-metabolites to DNA of cultured human hair follicle keratinocytes

Primary cultures of human hair follicle keratinocytes were established by using a basement membrane-like growth substrate, the bovine eye lens capsule. A method was adapted for the isolation of 3H-benzo(a)pyrene (BP)-modified DNA from the cellular outgrowth of only one hair follicle (approximately 2 X 10(5) cells). In a routine procedure hair follicle keratinocytes were incubated with 0.5 microM 3H-BP for 24 h. The purified DNA was subjected to enzymic hydrolysis and the adducts were analyzed by Sephadex LH-20 column chromatography followed by HPLC. Only one major adduct, which represented 60-80% of the total radioactivity which can be confined to modified nucleosides in the LH-20 chromatograph, could be identified. This adduct co-chromatographed with the marker adducts resulting from the trans-addition of the N-2-amino group of guanine to the 10-position of (+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene. Co-incubation with 7,8-benzoflavone (0.3 microM), an inhibitor of cytochrome P-448, and with 1,1,1-trichloropropene-2,3-oxide (0.2 microM), an inhibitor of epoxide hydrolase, resulted in a marked inhibitory effect (15% of the control binding) and a large increase (300% of the control value) in BP-DNA binding respectively. Induction of aryl hydrocarbon hydroxylase activity in the cultures with 5,6-benzoflavone (10 microM) or benz(a)anthracene (10 microM) caused a decrease (75 and 46% of the control value respectively) in BP-DNA binding.(ABSTRACT TRUNCATED AT 250 WORDS)

DNA modification by chemical carcinogens

The chemistry and molecular biology of DNA adducts is only one part of the carcinogenic process. Many other factors will determine whether a particular chemical will exert a carcinogenic effect. For example, the size of particles upon which a carcinogenic may be adsorbed will influence whether or not, and if so where, deposition within the lung will occur. The simultaneous exposure to several different agents may enhance or inhibit the metabolism of a chemical to its ultimate carcinogenic form (Rice et al., 1984; Smolarek and Baird, 1984). The ultimate carcinogenic metabolites may be influenced in their ability to react with DNA by a number of factors such as internal levels of detoxifying enzymes, the presence of other metabolic intermediates such as glutathione with which they could react either enzymatically or non-enzymatically, and the state of DNA which is probably most heavily influenced by whether or not the cell is undergoing replication or particular sequences being expressed. Replicating forks have been shown to be more extensively modified than other areas of DNA. Another critical factor which can influence the final outcome of the DNA damage is whether or not the modifications can be repaired. If this occurs with high fidelity and the cell has not previously undergone replication then the effect of the damage by the carcinogen is likely to be minimal. The major area in which progress is needed is an understanding of what this damage really does to the cell such that after an additional period of time, which may be as long as twenty or more years, these prior events are expressed and cell proliferation occurs. Clearly additional stimulatory factors, for example tumor promoting agents such as the phorbol esters or phenobarbital, are often needed. After such prolonged periods it seems likely that the DNA adducts would no longer be present. However, the way in which their earlier presence is remembered is not clear. Simple mutations do not explain all the characteristics of tumor progression and, when it occurs, regression. Even if a specific site mutation does occur then its expression must be under other types of control. Any explanation of the action of DNA modification at the molecular level also requires that account be taken of the diverse nature of the DNA adducts from simple modifications such as methylation to bulkier adducts such as benzo[a]pyrene, aflatoxin or aromatic amines.(ABSTRACT TRUNCATED AT 400 WORDS)

Absolute configuration of trans-7,8-dihydroxy-7,8-dihydro-7-methylbenzo[a]pyrene enantiomer and the unusual quasidiequatorial conformation of the diacetate and dimenthoxyacetate derivatives

The enantiomers of trans-7,8-dihydroxy-7,8-dihydro-7-methylbenzo[a]pyrene (7-MBaP 7,8-dihydrodiol) and of trans-7,8-dihydroxy-7,8,9,10-tetrahydro-7-methylbenzo[a]pyrene (7-MBaP 7,8-tetrahydrodiol) were directly resolved by high-performance liquid chromatography (HPLC) using a commercially available column packed with an (R)-N-(3,5-dinitrobenzoyl)-phenylglycine derivative of gamma-aminopropylsilanized silica. The absolute configurations of the resolved enantiomers were determined by the exciton chirality method. Circular dichroism (CD) spectral analysis of the quasidiequatorial benzo[a]pyrene 7R,8R-dihydrodiol enantiomer and its diacetate and dimenthoxyacetate derivatives indicated conformational changes were induced upon derivatization. However, the characteristic CD Cotton effects of the quasidiequatorial 7-MBaP 7,8-dihydrodiol and its diacetate and dimenthoxyacetate derivatives were similar indicating that the conformation of 7-MBaP trans-7,8-dihydrodiol was not altered upon derivatization. Proton nuclear magnetic resonance (NMR) spectral analyses confirmed that 7-MBaP 7,8-dihydrodiol, its diacetate and dimenthoxyacetate derivatives all have quasidiequatorial conformations. The results indicate that the methyl substituent of 7-MBaP 7,8-dihydrodiol maintains a quasiaxial position regardless of the size of the acyl derivatives linked to the hydroxyl groups.

Intercalative DNA binding of model compounds derived from metabolites of 7,12-dimethylbenz[a]anthracene

The DNA binding of nonreactive model compounds of metabolites of 7,12-dimethylbenz[a]-anthracene (DMBA)1 was studied in fluorescence quenching and fluorescence lifetime experiments. The model compounds examined were DMA and 8,9,10,11-tetrahydro-BA. DMA is a pi electron model of a highly carcinogenic bay region epoxide of DMBA, 8,9,10,11-tetrahydro-BA is a model compound of a less carcinogenic DMBA epoxide. The results indicate that the binding of DMA occurs primarily via intercalation. In 15% methanol the binding constant is 3.1 x 10(3) M-1. In 15% methanol and at DNA phosphate levels of 5.0 x 10(-4) M the intercalative binding of DMA is reduced by a factor of 6.2 when 5.0 x 10(-4) M Mg+2 is added. The DMA binding constant for intercalation is reduced by more than a factor of 4 when the methanol content of the solvent is increased from 0% to 20%. Finally DMA binding arising from pi interactions with the DNA bases is reduced more than 15 times when the DNA is denatured. For 8,9,10,11-tetrahydro-BA in 15% methanol the binding constant for intercalation is 6 times lower than that for DMA. These results along with previously reported binding data on other model compounds suggest that bay region metabolites of DMBA readily participate in physical pi stacking interactions with DNA.

Structural effects in reactivity and adduct formation of polycyclic aromatic epoxide and diol epoxide derivatives with DNA: comparison between 1-oxiranylpyrene and benzo[a]pyrenediol epoxide

Reaction of 1-oxiranylpyrene (1-OP) with DNA and the structures of the covalent and noncovalent complexes formed were studied in aqueous media (5 mM phosphate buffer with 0.1 M NaCl, pH 7) by utilizing the techniques of absorption, fluorescence and linear dichroism spectroscopy in order to gain an understanding of possible structure-activity relationships for polycyclic aromatic hydrocarbon epoxides in tumorigenesis and carcinogenesis, and the results were compared with those obtained for the highly active benzo[a]pyrene diol epoxide (BaPDE). Like BaPDE, 1-OP undergoes acid-catalyzed hydrolysis with the pseudo-first-order rate constant k = 4.6 X 10(-4) s-1 in the absence of DNA, which is about 10 times slower than in the case of BaPDE. In DNA solutions, this hydrolysis is catalyzed by a rapid formation of a physically bound complex of 1-OP-DNA, which subsequently undergoes either (1) hydrolysis to a diol derivative or (2) formation of a covalent adduct of 1-OP-DNA. The same value of the noncovalent binding constant (K = 4000 M-1 is obtained for both 1-OP and for BaPDE, which suggests that the pi-electron interaction between the pyrenyl moiety and the nucleic acid bases is the dominant factor in the formation of the physical complexes and that the two extra OH groups in BaPDE do not play a significant role in determining the value of the physical binding constant.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of reaction of benzo(a)pyrene-7,8-diol-9,10-epoxide with DNA in aqueous solutions

The physical and chemical reaction pathways of the metabolite model compound benzo(a)pyrene-7,8-diol-9,10-epoxide (BPDE) in aqueous (double-stranded) DNA solutions was investigated as a function of temperature (0-30 degrees C), pH (7.0-9.5), sodium chloride concentration (0-1.5M) and DNA concentration in order to clarify the relationships between the multiple reaction mechanisms of this diol epoxide in the presence of nucleic acids. The reaction pathways are (1) noncovalent intercalative complex formation with DNA, characterized by the equilibrium constant K, and Xb the fraction of molecules physically bound; (2) accelerated hydrolysis of BPDE bound to DNA; (3) covalent binding to DNA; and (4) hydrolysis of free BPDE(kh). The DNA-induced hydrolysis of BPDE to tetraols and the covalent binding to DNA are parallel pseudo-first-order reactions. Following the rapid (millisecond time scale) noncovalent complex formation between BPDE and DNA, a much slower (approximately minutes) H+-dependent (either specific or general acid catalysis) formation of a DNA-bound triol carbonium ion (rate constant k3) occurs. At pH 7.0 the activation energy of k3 is 8.7 +/- 0.9 kcal/mol, which is lower than the activation energy of hydrolysis of free BPDE in buffer solution (14.2 +/- 0.7 kcal/mol), and which thus partially accounts for the acceleration of hydrolysis of BPDE upon complexation with DNA. The formation of the triol carbonium ion is followed by a rapid reaction with either water to form tetraols (rate constant kT), or covalent binding to DNA (kc). The fraction of BPDE molecules which undergo covalent binding is fcov approximately equal to kc/(kc + kT) = 0.10 and is independent of the overall BPDE reaction rate constant k = kh(1 - Xb) + k3Xb if Xb----1.0, or is independent of Xb as long as k3Xb much greater than kh(1 - Xb). Thus, at Xb = 0.9, fcov is independent of pH (7.0-9.5) even though k exhibits a 70-fold variation in this pH range and k----kh above pH 9 (k3 = kh). Similarly, fcov is independent of temperature (0-30 degrees C), while k varies by a factor of approx. 3. In the range of 0-1.5 M NaCl, fcov decreases from 0.10 to 0.04. These variations are attributed to a combination of salt-induced variations in the factors k3, Xb and the ratio kc/kT.

Epidermal benzo[a]pyrene metabolism and DNA-binding in Balb/C mice: inhibition by ellagic acid

Topical application of ellagic acid, a common plant phenol, to control or to 3-methylcholanthrene (3-MC) pretreated Balb/C mice, resulted in significant inhibition of hepatic and epidermal microsomal aryl hydrogen hydroxylase activity, and of benzo[a]pyrene (BP) binding to epidermal and hepatic DNA in vivo. In vitro addition of ellagic acid (0.25 mM) to epidermal microsomal incubation systems from either control or 3-MC-treated animals resulted in 62-75% inhibition of BP binding to calf thymus DNA. These studies suggest that ellagic acid could prove useful in understanding and/or modulating polyaromatic hydrocarbon carcinogenesis.

Stereoselective covalent binding of anti-benzo(a)pyrene diol epoxide to DNA conformation of enantiomer adducts

The conformation of adducts derived from the reactions and covalent binding of the (+) and (-) enantiomers of 7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene (anti-BaPDE) with double-stranded calf thymus DNA in vitro were investigated utilizing the electric linear dichroism technique. The linear dichroism and absorption spectra of the covalent DNA complexes are interpreted in terms of a superposition of two types of binding sites. One of these conformations (site I) is a complex in which the plane of the pyrene residue is close to parallel (within 30 degrees) to the planes of the DNA bases (quasi-intercalation), while the other (site II) is an external binding site; this latter type of adduct is attributed to the covalent binding of anti-BaPDE to the exocyclic amino group of deoxyguanine (N2-dG), while site I adducts are attributed to the O6-deoxyguanine and N6-deoxyadenine adducts identified in the product analysis of P. Brookes and M.R. Osborne (Carcinogenesis (1982) 3, 1223-1226). Site II adducts are dominant (approximately 90% in the covalent complexes derived from the (+) enantiomer), but account for only 50 +/- 5% of the adducts in the case of the (-)-enantiomer. The orientation of site II complexes is different by 20 +/- 10 degrees in the adducts derived from the binding of the (+) and the (-) enantiomers to DNA, the long axis of the pyrene chromophore being oriented more parallel to the axis of the DNA helix in the case of the (+) enantiomer. These findings support the proposals by Brookes and Osborne that the difference in spatial orientation of the N2-dG adducts of (-)-anti-BaPDE together with their lower abundance may account for the lower biological activity of the (-) enantiomer. The external site II adducts, rather than site I adducts, appear to be correlated with the biological activity of these compounds.

Biochemical studies of the tracheobronchial epithelium

Tracheobronchial epithelium has been a focus of intense investigation in the field of chemical carcinogenesis. We have reviewed some biochemical investigations that have evolved through linkage with carcinogenesis research. These areas of investigation have included kinetics of carcinogen metabolism, identification of carcinogen metabolites, levels of carcinogen binding to DNA, and analysis of carcinogen-DNA adducts. Such studies appear to have provided a reasonable explanation for the susceptibilities of the respiratory tracts of rats and hamsters to carcinogenesis by benzo(a)pyrene. Coinciding with the attempts to understand the initiation of carcinogenesis in the respiratory tract has also been a major thrust aimed at effecting its prevention both in humans and in animal models for human bronchogenic carcinoma. These studies have concerned the effects of derivatives of vitamin A (retinoids) and their influence on normal cell biology and biochemistry of this tissue. Recent investigations have included the effects of retinoid deficiency on the synthesis of RNA and the identification of RNA species associated with this biological state, and also have included the effects of retinoids on the synthesis of mucus-related glycoproteins. Tracheal organ cultures from retinoid-deficient hamsters have been used successfully to indicate the potency of synthetic retinoids by monitoring the reversal of squamous metaplasia. Techniques applied to this tissue have also served to elucidate features of the metabolism of retinoic acid using high pressure liquid chromatography. In brief, formidable strides have been made in biochemistry specific to this important target tissue, despite the inability to acquire tracheobronchial epithelium in large quantities.

Hydrolysis of benzo[a]pyrene diol epoxide and its covalent binding to DNA proceed through similar rate-determining steps

The mutagenic and carcinogenic metabolite of benzo[a]pyrene, (7R,8S)-dihydroxy-(9R,10R)-epoxy-7,8, 9,10-tetrahydrobenzo[a]pyrene, undergoes two major reactions in the presence of DNA: (i) hydrolysis and (ii) covalent binding. We report that hydrolysis and covalent binding are specific and general acid-catalyzed reactions with the same or similar rate-determining steps. To account for the similarity of rate-determining steps in covalent binding and hydrolysis we propose and test two models. In each model, the rate-determining step results in formation of a carbonium ion, which serves as a precursor for both tetrol and adduct. In model A the carbonium ion is partitioned between two domains (1 and 2), while in model B there is only one domain. Measurements of pseudo-first-order rate constants, product ratios, and rate ratios support model A, while kinetic results are inconsistent with model B. Domain 1 most likely represents activated benzo[a]pyrenes that are intercalated into DNA, while domain 2 hydrocarbons are physically bound to the outside of the DNA helix.

Survival and mutagenesis of bacterial plasmids with localized carcinogen adducts

Restriction fragments from nonessential gene regions in bacterial plasmids were covalently modified with benzo[a]pyrene-7,8-dihydrodiol-9,10-oxide (BPDE) and ligated to the remaining unmodified fragment to construct the plasmid with a localized patch of BPDE adducts. These preparations were introduced into appropriate strains of E. coli and the effect of the carcinogen adducts on plasmid survival and mutagenesis determined. Survival as a function of adduct concentration of randomly modified plasmids was the same as that of plasmids with localized adducts in both repair-proficient and -deficient strains, indicating that the simple presence of the carcinogen is the main factor in plasmid mortality. The results of these and other experiments indicate that plasmid survival is a function of the adduct/plasmid molecule ratio not the adduct/nucleotide ratio. Plasmids with mutations produced in the region (tetracycline resistance) containing the localized adducts were selected and the nature of the mutations determined by direct sequence analysis. The mutations included frameshifts, transitions, and transversions.

The metabolic activation of dibenzo[a,e]fluoranthene in vitro. Evidence that its bay-region and pseudo-bay-region diol-epoxides react preferentially with guanosine

Dibenzo[a,e]fluoranthene ( DBF ), a non- alternant carcinogenic polycyclic aromatic hydrocarbon (PAH), binds covalently to DNA. The main adducts were characterized as covalent additions of its bay-region and pseudo-bay-region diol-epoxides. The structure of these 2 adducts was analyzed by mass spectrometry using their persilyl derivatives. 3,4-Dihydroxy-1,2-epoxy-1,2,3,4-tetrahydro- DBF (3,4-diol-1,2-epoxy- DBF ) and 12,13-dihydroxy-10,11-epoxy-10,11,12,13-tetrahydro- DBF (12, 13-diol-10,11-epoxy- DBF ) obtained by synthesis were allowed to react in vitro with calf thymus DNA or with poly(G). The comigration of DNA and poly(G) adducts isolated after acid hydrolysis of DNA and poly(G) was in good agreement with mass spectroscopic results: both bay-region and pseudo-bay-region DBF diol-epoxides reacted with guanine residues.

Rat liver homogenate (S9)-mediated binding of benzo[alpha]pyrene to DNA in V79 cells, V79 cell nuclei and aqueous solution

The role of the target cell in determining the structures and the amounts of hydrocarbon-DNA adducts formed after hydrocarbon activation by an exogenous metabolic activation system was investigated by exposing intact cells of the Chinese hamster lung cell line V79, V79 cell nuclei and calf thymus DNA to benzo[a]pyrene (B[a]P) in the presence of a rat liver homogenate activation system (S9). The DNA was isolated, enzymatically degraded to deoxyribonucleosides and the B[a]P-deoxyribonucleoside adducts analyzed by high-performance liquid chromatography. Two major adducts were present in all samples; one formed by reaction of r-7, t-8-dihydroxy-t-9, 10-epoxy-7, 8, 9, 10-tetrahydro-B[a]P (anti-B[a]PDE) with the 2-amino group of deoxyguanosine, the other formed by reaction of a metabolite of 9-hydroxybenzo[a]pyrene (9-OH-B[a]P) with an unidentified deoxyribonucleoside. The ratios of the anti-B[a]PDE-DNA adduct to the 9-OH-B[a]P-DNA adduct were: calf thymus DNA, 3 to 1: DNA from V79 nuclei, 8 to 1; DNA from intact V79 cells, 11 to 1. Similar several-fold increases in the proportion of anti-B[a]PDE-DNA adducts in V79 cells over those in calf thymus DNA were observed for a dose range of 1-10 micrograms B[a]P per ml. The relative extent of binding of the activated metabolite of 9-OH-B[a]P to DNA was also much lower in intact V79 cells than in calf thymus DNA after exposure to 9-OH-B[a]P in the presence of the S9 activation system. These results demonstrate that the relative abilities of various reactive benzo[a]pyrene metabolites formed by an exogenous activation system to reach cellular DNA differ substantially. Therefore, assessment of the biological activity of hydrocarbons in mutation assays using exogenous activation systems must take into account not only the amounts of different reactive hydrocarbon metabolites formed but also the relative abilities of these metabolites to reach the DNA of the target cell.

Diethylstilbestrol (DES) quinone: a reactive intermediate in DES metabolism

The quinone of E-diethylstilbestrol (DES), a postulated metabolic intermediate derived from DES, has been synthesized by oxidation of DES in chloroform using silver oxide. The reaction product was structurally characterized by infrared, ultraviolet, nuclear magnetic resonance, and mass spectrometry. The product of oxidation of DES by hydrogen peroxide, catalyzed by horseradish peroxidase and also by rat uterine peroxidase, was shown to be identical with synthetic DES quinone based on identical u.v. spectra and on identical decomposition products. DES quinone was stable only in non-protic solvents such as chloroform. In acids, bases or protic solvents, DES quinone rearranged to Z,Z-dienestrol (beta-DIES). The half-life of DES quinone in water was approximately 40 min; in methanol it was approximately 70 min. Bacterial mutagenicity (Ames) tests did not indicate that DES quinone had mutagenic or genotoxic activity. However, DES quinone was found to bind to calf thymus DNA without any enzyme mediation at levels significantly above the binding of DES under the same conditions. Based on the binding of DES quinone to DNA, this intermediate must be considered as a possible carcinogenic metabolite of DES.

Interactions of molecules with nucleic acids. X. Covalent intercalative binding of the carcinogenic BPDE I(+) to kinked DNA

A theoretical model is proposed for the covalent binding of (+) 7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10- tetrahydrobenzo[a]pyrene denoted by BPDE I(+), to N2 on guanine. The DNA must kink a minimum of 39 degrees to allow proper hybrid configurations about the C10 and N2 atoms involved in bond formation and to allow stacking of the pyrene moiety with the non-bonded adjacent base pair. Conservative (same sugar puckers and glycosidic angles as in B-DNA) and non-conservative (alternating sugar puckers as in intercalation sites) conformations are found and they are proposed structures in pathways connecting B-DNA, an intercalation site, and a kink site in the formation of a covalently intercalative bound adduct of BPDE I(+) to N2 on guanine. Stereographic projections are presented for (3') and (5') binding in the DNA. Experimental data for bending of DNA by BPDE, orientation of BPDE in DNA and unwinding of superhelical DNA is explained. The structure of a covalent intercalative complex is predicted to result from the reaction. Also, an anti----syn transition of guanine results in a structure which allows the DNA to resume its overall B-form. The only change is that guanine has been rotated by 200 degrees about its glycosidic bond so that the BPDE I(+) is bound in the major groove. The latter step may allow the DNA to be stored with an adduct which may produce an error in the genetic code.

Binding of pyrene to DNA, base sequence specificity and its implication

Solubilization as well as spectral studies of pyrene in natural DNA and synthetic deoxypolynucleotide solutions at neutral pH reveal at least two binding modes. Sites I are predominant in native DNA and in poly(dA-dT): poly(dA-dT) whereas sites II are found with denatured DNA and other polynucleotides such as poly(dA):poly(dT) and three different types of guanine containing copolymers which solubilize pyrene to a lesser extent. Spectral comparison with the covalent adducts of trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10- tetrahydro-benzo(a)pyrene (anti-BPDE) and the physical complexes of its tetraols lead to the suggestion of a base sequence specific binding model for this carcinogenic metabolite to account for the puzzling fact that although its physical binding is predominantly intercalative, the covalent adducts appear not to be intercalated. It is speculated that in neutral solutions, intercalation may have little, if any, to do with the chemical lesion of this metabolite to the guanine base of the DNA and may, on the contrary, provide an efficient pathway for detoxification.

The interaction of chromium with nucleic acids

Native and denatured calf thymus DNA, and homopolyribonucleotides were compared with respect to chromium and protein binding after an in vitro incubation with rat liver microsomes, NADPH, and chromium (VI) or chromium (III). A significant amount of chromium bound to DNA when chromium (VI) was incubated with the native or the denatured form of DNA in the presence of microsomes and NADPH. For both native and denatured DNA the amount of protein bound to DNA increased with the amount of chromium bound to DNA. Denatured DNA had much higher amounts of chromium and protein bound than native DNA. There was no interaction between chromium(VI) and either form of DNA in the absence of the complete microsomal reducing system. The binding of chromium(III) to native or denatured DNA was small and relatively unaffected by the presence of microsomes and NADPH. The binding of chromium and protein to polyriboadenylic acid (poly(A], polyribocytidylic acid (poly(C], polyriboguanylic acid (poly(G] and polyribouridylic acid (poly(U] was determined after incubation with chromium(VI) in the presence of microsomes and NADPH. The magnitude of chromium and protein binding to the ribopolymers was found to be poly(G) much greater than poly(A) approximately equal to poly(C) approximately equal to poly(U). These results suggest that the metabolism of chromium(VI) is necessary in order for chromium to interact significantly with nucleic acids. The metabolically-produced chromium preferentially binds to the base guanine and results in DNA-protein cross-links. These findings are discussed with respect to the proposed scheme for the carcinogenicity of chromium(VI).

Effects of dietary cabbage, Brussels sprouts, Illicium verum, Schizandra chinensis and alfalfa on the benzo[alpha]pyrene metabolic system in mouse liver

Male C57B16 mice were fed on diets containing either 20% cabbage, 20% Brussels sprouts, 20% alfalfa, 5% Schizandra chinensis or 5% Illicium verum (two Chinese medicinal herbs) or on a chow or purified basal diet for 14 days after a 1-wk equilibration period on the basal diet. Liver microsomal fractions were assayed for cytochrome P-450 content, aryl hydrocarbon hydroxylase (AHH) and epoxide hydrolase (EH). Liver microsome-mediated benzo[a]pyrene (BP) metabolism (with and without an EH inhibitor, 1,2-epoxy-3,3,3-trichloropropane) was analysed by HPLC. Liver weights of the animals fed on Brussels sprouts and I. verum were significantly increased compared with those of the animals fed on basal diets. S. chinensis induced a 3-fold increase in cytochrome P-450 (P less than 0.05). Although P-450 induction in the other groups was as high as 1.8-fold (for chow), statistical significance was not established. Chow induced AHH activity 2.2-fold (P less than 0.05), while S. chinensis and alfalfa induced 1.6-fold and 1.7-fold increases, respectively, in AHH activity, although neither increase was statistically significant. EH was stimulated significantly in the following order: I. verum (2.1-fold) greater than chow (1.7-fold) greater than S. chinensis (1.6-fold) greater than Brussels sprouts (1.4-fold). Total levels of BP metabolism and phenol II (primarily 3-hydroxybenzo[a]pyrene) formation were closely associated for each dietary treatment. Total BP metabolism was significantly increased (2.1-fold) in the chow-fed group and increased 1.6-fold in the S. chinensis group (P greater than 0.05). No increase was seen with the other diets. Phenol II formation relative to total metabolites was significantly increased for the S. chinensis and I. verum groups compared to the basal group. Diet-related variations in phenol production relative to total metabolism were eliminated by addition of the EH inhibitor to the incubation media.

Interaction of (+/-)-7r,8t-dihydroxy-9t,10t-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene with relaxed circular pBR322 DNA

The interaction of (+/-)-BPDE (1) with DNA at neutral pH was investigated by the application of relaxed circular pBR322 DNA. (+/-)-BPDE causes a rapid positive supercoiling of this DNA followed by a slower spontaneous relaxation. The results indicate that there are two clearly discernible types of chemical interactions between 1 and DNA, a rapid intercalative covalent binding and a slower strand breakage. The implications of these findings are discussed.

Base sequence selectivity in binding of aromatic hydrocarbons with synthetic polynucleotides

The interactions between DOBP (3) and calf thymus DNA as well as four synthetic polynucleotides, poly(dA-dT), polydA:polydT, poly(dG-dC), and polydG:polydC, were investigated by spectroscopic techniques. It was found that the binding of 3 with poly(dA-dT) is favored appreciably over other synthetic polynucleotides and DNA. The results suggest that the initial association of carcinogenic BPDE (2) with DNA may take place preferentially at certain specific base sequences in DNA.

Structure of the mouse cytochrome P1-450 genomic gene

Clone 46 was previously shown to represent mouse cytochrome P1-450 cDNA by both translation arrest experiments and segregation of induced P1-450 mRNA with induced aryl hydrocarbon hydroxylase activity among individual 3-methylcholanthrene-treated offspring of the (C57BL/6N)(DBA/2N)F1 X DBA/2N backcross. With clone 46 as a probe, a MOPC 41 mouse genomic-DNA library was screened. lambda 3NT12, a 16 X 10(3)-base-pair insert of genomic DNA grown in a recombinant Charon 4A lambda vector phage, was isolated and characterized. It was determined that clone 46 hybridizes to the extreme 5' end of lambda 3NT12. pMJE12, a 3.0 X 10(3)-base-pair fragment in the 5' region of lambda 3NT12, was subcloned in plasmid pBR322 and used as a probe to screen again the same mouse-DNA library; recombinant phages lambda 3NT13, lambda 3NT14, and lambda AhP-1 were isolated and characterized. The relative orientation of each of the four genomic clones on the mouse chromosome was determined. Only lambda AhP-1 contains the entire P1-450 genomic gene, which by R-loop analysis spans about 46 X 10(2) base pairs and contains at least five exons. Clone 46 is shown to be a 3' unique sequence of the genomic P1-450 gene. The lambda AhP-1 genomic-DNA clone from the MOPC 41 plasmocytoma is shown by a series of restriction enzymes to be the same as genomic DNA from normal mouse liver. With a subclone in the 5' portion of the P1-450 gene, two and three hybridizable fragments are found with mouse genomic DNA that has been digested with EcoRI and BamHI, respectively.

Transition-state alkylation geometries of 7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene enantiomeric isomers with nucleic acid dimers

The steric contact spaces associated with the reaction of the enantiomeric isomers of 7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (I) with the exocyclic amino group of guanine of dinucleoside dimer structures were examined for a fixed transition-state geometry. This reaction is sterically prohibited for the B form DNA conformation. If, however, the nucleic acid structure is deformed, such that the distance between two adjacent base pairs (one containing guanine and cytosine) is maximized, sterically allowed transition-state geometries can be identified. It was not possible to uniquely identify the preferred transition-state complex with respect to nucleic acid structure or isomer of I. However, two types of general transition-state geometries were observed. In one, I was located "outside" the nucleic acid structure; in the other geometry, I was intercalated between adjacent base pairs in the transition state. The intercalation process might serve as a physical catalyst for the alkylation of NH2-guanine by I.

A molecular orbital treatment on the reactivity of some polycyclic aromatic hydrocarbon diol-epoxides

The total energies of several aromatic hydrocarbon diol-epoxides and their carbonium cations were calculated by using the semi-empirical CNDO/2 SCF MO calculations. The reactivity of the diol-epoxides toward electrophilic reagents was evaluated from the energy difference between the diol-epoxide and its carbonium cation. Next, the total energies of the diol-epoxides and their carbonium cations were divided into several components, which represent the contributions of parts of molecules to the total energy. That is, the total energy was represented as the sum of the block energies and inter-block energies, which were defined in relation to the parts of the molecules. By using this concept of the block and the inter-block energies the reactivities of the diol-epoxides were interpreted in connection with the molecular structure.

Electro-optical fluorescence studies on the DNA binding of medically active drugs

By measuring the transient changes in each of the polarized components of fluorescence during the application of a pulsed electric field to a solution of macromolecules, a method has been developed for evaluating the binding characteristics of fluorescent chemotherapeutic agents to DNA. It is shown that whereas quinacrine and berberine intercalate the DNA helix, hydroxystilbamidine does not. Furthermore, measurements on both the native and the diol-epoxide forms of benzo(a)pyrene show that the former is consistent with an intercalation type of binding, whilst the latter appears to be more inclined to the major DNA axis and may possibly be associated with the external helical grooves. The significance of these findings is discussed.

Fluorescence and photoelectron studies of the intercalative binding of benz(a)anthracene metabolite models to DNA

DNA binding of nonreactive metabolite models derived from benz(a)anthracene was studied. The molecules investigated include 1,2,3,4-tetrahydrobenz(a)anthracene (1), 5,6-dihydrobenz(a)anthracene (2), and 8,9,10,11-tetrahydrobenz(a)anthracene (3), as well as anthracene and phenanthrene. Measurements of the effects of DNA binding upon fluorescence intensities and fluorescence lifetimes indicate that molecules 1 and 3 (KA = 1.5 - 2.5 x 10(3) M-1) bind more strongly to native DNA than does molecule 2 (KA congruent to 0.5 x 10(3) M-1). Furthermore, molecules 1 and 3 bind to DNA much more effectively than do the two less sterically hindered pi electron metabolite models, anthracene and phenanthrene. Photoelectron data suggests that the enhanced binding of molecules 1 and 3 is due to increases in polarizability. Experiments carried out with denatured DNA indicate that the binding of molecule 1 entails the greatest intercalation.

Effects of pH and salt concentration on the hydrolysis of a benzo[alpha]pyrene 7,8-diol-9,10-epoxide catalyzed by DNA and polyadenylic acid

The time-dependent absorbance change that occurs when benzo[alpha]pyrene 7,8-diol-9,10-epoxide is added to solutions of calf thymus DNA has been shown, by an unequivocal chromatographic method, to correspond to DNA-catalyzed hydrolysis of the diol-epoxide. At 25 degrees C and mu = 0.10, the kinetics of the reaction of the diol-epoxide with polyadenylic acid or DNA are consistent with preequilibrium formation of a non-covalent complex between the diol-epoxide and the polynucleotide or DNA, followed by hydrolysis of the bound epoxide by a process that is first-order in hydronium ions. Cacodylic acid also catalyzes the hydrolysis of the epoxide bound to polyadenylic acid. The rate of the DNA-catalyzed hydrolysis exhibits little or no enantiomeric selectivity for the diol-epoxide. DNA catalyzed hydrolysis of the diol-epoxide is extraordinarily sensitive to the salt concentration in the reaction medium: the rate of hydrolysis of the bound epoxide at pH 7 is retarded by a factor of approximately 45 in the presence of 0.1 M sodium chloride compared to a 1 mM buffer containing no added salt. Thus, studies of the interactions of DNA with carcinogenic diol-epoxides must take into account the ionic environment of DNA within the cell.

Stereoselective metabolism of benzo[a]pyrene and 7-methylbenzo[a]pyrene by liver microsomes from sprague-Dawley rats pretreated with polychlorinated biphenyls

The dihydrodiols formed from the metabolism of benzo[a]pyrene (BaP) and 7-methylbenzo[a]pyrene (7-MBaP) by liver microsomes from male Sprague-Dawley rats treated with polychlorinated biphenyls (PCBs, Aroclor 1254) have been examined by circular dichroism (CD) spectropolarimetry. Comparisons with optically pure enantiomers obtained via high performance liquid chromatography (HPLC) resolution of diastereomeric di-(-)-menthoxyacetates indicated that the trans-4,5-, 7,8- and 9,10-dihydrodiols formed from BaP metabolism are predominantly R,R-enantiomers with optical purities greater than 98%. The major enantiomers of the metabolically formed 7-MBaP-trans-4,5- and 9,10-dihydrodiols and 7-hydroxymethyl-BaP-trans-9,10-dihydrodiol have Cotton effects very similar to those of BaP-4R,5R- and 9R,10R-dihydrodiols, respectively. These 7-MBaP-trans-4,5- and 9,10-dihydrodiol metabolites therefore contain predominantly the R,R-enantiomers. The optical purity of metabolically formed 7-MBaP-trans-4,5-dihydrodiol was determined to be 30.8% enriched in (-)-enantiomer. The optical purity of the 9,10-dihydrodiol was not determined due to the lack of synthetic standards. The major trans-7,8-dihydrodiol enantiomer formed from 7-MBaP metabolism is a (+)-enantiomer (optical purity 60.4%) which has Cotton effects opposite in sign to that of the (-)-7R,8R-dihydrodiol formed from BaP metabolism. The results indicate that a methyl substituent on a polycyclic aromatic hydrocarbon may alter the stereoselective properties of the microsomal drug-metabolizing enzyme systems toward the substrate molecule.

Carcinogen induced DNA damage in isolated rat liver nuclei

Alkali-labile DNA lesions were detected by a hydroxylapatite batch assay following incubation of isolated rat liver nuclei with direct and indirect-acting alkylating agents. N-Methyl-N'-nitro-N-nitrosoguanidine (MNNG) (20-200 microM) produced a steep, concentration-related decrease in double stranded DNA. In contrast, only minimal effects on DNA integrity were observed with methylmethane sulfonate (200 microM). The bifunctional alkylating agent, mechlorethamine produced an apparent increase in double-stranded DNA in accord with its ability to induce interstrand DNA crosslinks. The indirect-acting alkylating agent, dimethylnitrosamine (DMN) induced DNA damage only when an NADPH generating system was included in the incubation mixture. These results suggest that isolated rat liver nuclei may be a useful model for studying the effects of mutagenic and carcinogenic chemicals on the integrity of chromosomal DNA.