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

Comparison of in vivo and in vitro binding of polycyclic hydrocarbons to DNA

The in vivo binding of [3H]benzo(a)pyrene (BP) and 3-[3H]methylcholanthrene (3MC) to liver and lung DNA was studied in A/J mice. Only in liver was there any reduction in total DNA-bound radioactivity between 4 h and 24 h after administration of the hydrocarbon. DNA was fractionated on Sephadex LH-20 after enzymatic digestion. A single deoxyribonucleoside-BP adduct was detected whereas two major 3MC-adducts were observed. Wtih both BP and 3MC, three additional peaks of radioactivity eluted rapidly in the lung DNA experiments while a fourth was noted with liver DNA. The nucleoside-bound adducts from lung represented a much larger proportion of the total radioactivity than with liver. In vitro analysis of 3MC binding to DNA showed the nucleoside-bound adducts to be predominantly deoxyguanosine-dependent but that the early peaks were independent of base suggesting binding to another part of the DNA molecule, perhaps phosphate, i.e, phosphotriesters.

Effects of benzo(a)pyrene adducts of DNA synthesis in vitro

Two diol epoxides of benzo(a)pyrene (BP), and benzo(a)pyrene 4,5-oxide, have been used to make adducts in the homopolymers polyribocytidylic acid, (rC); polyriboadenylic acid (rA), polydeoxycytidylic acid (dC) and polydeoxyadenylic acid (dA). With appropriate oligomers as primers these modified and unmodified polynucleotides were used as templates for DNA synthesis with avian myeloblastosis virus DNA polymerase (AMV) or E. coli Pol I DNA polymerase. We have found that: (1) the size of the DNA product is not markedly decreased by the presence of these these polycyclic aromatic hydrocarbon adducts in the templates; (2) the presence of adducts does not lead to increased incorporation of erroneous bases. These results, supported by kinetic data, suggest that these polymerases can bypass a site containing an adduct on the template without leaving a gap or causing misincorporation of a base and they imply that mutagenesis by BP may not be attributable to either of these mechanisms.

Benzo(a)pyrene 7,8-dihydrodiol-9,10-oxide modification of DNA: relation to chromatin structure and reconstitution

Purified duck reticulocyte DNA was incubated in vitro with a 7,8-dihydrodiol-9,10-oxide derivative of benzo(a)pyrene (BPDE). The carcinogen-modified DNA was somewhat more susceptible to partial digestion by the single strand specific endonuclease S1 than unmodified DNA, suggesting slight denaturation of the helix at sites of modification. Chromatin was reconstituted in vitro utilizing this carcinogen-modified DNA and unmodified-chromatin associated proteins. This reconstituted chromatin showed the same kinetics and extent of digestion by Staphylococcal nuclease, and similar nucleosome profiles on sucrose density gradient centrifugation, as those obtained with native chromatin or chromatin reconstituted with unmodified DNA. Moreover, polyacrylamide gel electrophoresis of DNA fragments obtained from nuclease digests gel electrophoresis of DNA fragments obtained from nuclease digests of the reconstituted chromatins suggested that the chromatin containing carcinogen-modified DNA had the same subnucleosome structure as that reconstituted with unmodified DNA. In a separate set of studies intact duck reticulocyte chromatin was reacted directly with BPDE. Nuclease digestion studies indicated that 65% of the carcinogen was bound to the 'open' regions of chromatin, and 35% to 'closed' regions. These results indicate that although convalent binding of a benzo(a)pyrene (BP) derivative to DNA produces local distortions in conformation of the helix, this modification does not appear to interfere with the ability of the DNA to associate with histones to form nucleosome structures. In addition, although DNA in the open regions of chromatin is more susceptible to reaction with the BP derivative, there is appreciable reaction with the DNA associated with histones.

Carcinogen-induced DNA repair in nucleotide-permeable Escherichia coli cells. Analysis of DNA repair induced by carcinogenic K-region epoxides and 1,2,3,4-diepoxybutane

Ether-permeabilized (nucleotide-permeable) Escherichia coli cells exhibited DNA excision repair when exposed to the following carcinogenic K-region epoxides: 7-methyl- and 7,12-dimethyl-benz[a]anthracene-5,6-oxide, chrysene-5,6-oxide and benzo[a]pyrene-4,5-oxide. This DNA excision repair was missing in uvr A and uvr B mutant cells. The K-region epoxide phenanthrene-9,10-oxide was ineffective in all E. coli strains tested. In contrast to the K-region epoxides which where found active only in wild type cells, 1,2,3,4-diepoxybutane and the 6,7-epoxides of the tumor promoter TPA (12-O-tetradecanoyl-phorbol-13-acetate) elicited DNA repair in uvrA, uvrB mutant cells as well. Enzymic activities catalyzing particular repair steps were identified by determining a) repair polymerization and b) size reduction of denatured DNA. A) An easily quantifiable effect in E. coli wild type cells was epoxide-induced repair polymerization. None of the K-region epoxides tested stimulated DNA repair synthesis in uvrA, uvrB mutant cells, indicating that the uvrA-, uvrB-controlled UV-endonuclease initiated excision repair by cleaving epoxide-damaged DNA. 1,2,3,4-Diepoxybutane and the TPA-6,7-oxides induced DNA repair polymerization in uvr-deficient cells, although to a lesser extent than in wild type cells, suggesting the involvement of uvr-independent incision steps. None of the epoxides induced repair polymerization in a mutant (polA107) lacking the 5'--3'exonucleolytic activity of DNA polymerase I (exonuclease VI). The absence of any repair polymerization in the polA107 mutant indicates that the exonuclease VI plays a central role in removing epoxide-damaged nucleotides. As evidenced by greatly reduced levels of repair polymerization measured in polA1 cells, DNA polymerase I was the main polymerizing enzyme. b) As a consequence of treatment with 7-methyl-benz[a]anthracene-5,6-oxide, DNA from wild type cells, contrary to uvrA mutant cells, showed size reduction after denaturation and sedimentation in alkaline sucrose gradients. This is explained by repair-specific endonucleolytic cleavage of damaged DNA. The incision required the presence of ATP indicating that functional UV-endonuclease needs ATP as a cofactor.

Reaction of 7,12-dimethylbenz(a)anthracene with DNA of fetal and maternal rat tissues in vivo

Pregnant BD-IX rats (21st day of gestation) received a single IV injection (15 mg/kg) of tritiated 7,12-dimethylbenz(a)anthracene (DMBA), A DOSE KNOWN TO INduce a high incidence of nervous-system tumors in the offspring. The animals were killed 12 h later and hydrocarbon-deoxyribonucleoside products from DNA of maternal and fetal tissues were separated on Sephadex LH-20 columns eluted with a 20-100% methanol gradient. Concentrations of the major DMBA-DNA adduct varied considerably, with highest values in maternal intestine, liverand lung, followed by spleen, kidney and brain. In fetal intestine and liver, concentrations were 34% and 16% lower than in the respective maternal organs whereas the reaction with cerebral DNA was 2 1/2 times higher in fetuses than in the pregnant mother. This indicates that there is no significant placental barrier to DMBA or DMBA metabolites involved in DNA binding and that rat fetuses participate in the metabolic formation of the ultimate carcinogen.

Comparative tumor initiating activity of 10-methylbenzo-[a]pyrene, 7,10-dimethylbenzo[a]pyrene and benzo[a]pyrene

The tumor initiating activity on mouse skin of benzo[a]pyrene (BaP), 7,10-dimethylBaP, and 10-methylBaP was determined. Each compound was tested at initiating doses of 50 microgram and 100 microgram with promotion by application 3 times weekly of 2.5 microgram tetradecanoylphorbol acetate. BaP induced tumors in 40% (100 microgram) and 25% (50 microgram) of the animals. No tumors were observed in either group treated with 7,10-dimethylBaP. In the groups treated with 10-methyl-Bap, the incidence of tumor bearing animals was 20% at both doses. These results and the results of previous studies on other methylated BaP derivatives suggest that the mechanism of activation of these compounds is similar to that observed for the parent hydrocarbon and probably involves formation of an angular ring diol-epoxide or epoxide.

Computer-generated graphic models of the N2-substituted deoxyguanosine adducts of 2-acetylaminofluorene and benzo[a]pyrene and the O6-substituted deoxyguanosine adduct of 1-naphthylamine in the DNA double helix

Computer models of three deoxyguanosine-carcinogen adducts in double-helical DNA are presented. The carcinogen moiety is rotated and the best fit within the double helix is evaluated. The 2-acetylaminofluorene (AAF) derivative, 3-(deoxyguanosin-N2-yl)-AAF, is found to be situated within the minor groove, has very little freedom of rotation and causes little helical distortion. The (+)-anti-benzo[a]-pyrene (BP)-diol epoxide-N2 adduct, 10beta-(deoxyguanosin-N2-yl)-7beta, 8alpha,9alpha-trihydroxy-7,8,9,10-tetrahydro-BP, has a similar fit with a greater degree of steric interaction, suggesting that this adduct could cause some local destabilization. The 1-naphthylamine (NA) derivative, N1-(deoxyguanosine-O6-yl)-1-NA, resides within the major groove, does not perturb the helix and has considerable freedom of movement.

Conformation of dinucleoside monophosphates modified with benzo[a]pyrene-7,8-dihydrodiol 9,10-oxide as measured by circular dichroism

The conformational properties of GpU modified with the reactive derivative of benzo[a]pyrene, (+/-)-7beta,8alpha-dihydroxy-9alpha,10alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, has been investigated utilizing circular dichroism spectroscopy. Binding of this carcinogen to the N2 of G residues in GpU resulted in the formation of four compounds (I to IV) representing two pairs of diastereoisomers. The molar ellipticity values of the modified dimers were approximately twofold higher than those of the modified guanosine monomers. These values were decreased appreciably when the spectra of the dimers were obtained at 80 degrees C or in methanol rather than at 25 degrees C in water, suggesting that under the latter conditions there is a stacking interaction between the carcinogen and the neighboring uridine residue. Based on these results, a conformation is proposed for modified GpU. It includes insertion of the benzo[a]pyrene moiety, by rotation of the modified guanine residue about its glycoside bond, coplanar to the neighboring uridine and perpendicular to the phosphodiester backbone.

Toxic agents resulting from the oxidative metabolism of steroid hormones and drugs

The oxidative metabolism of some exogenous compounds, and possibly some endogenous compounds as well, can lead to the formation of reactive metabolites. These intermediates react as electrophiles, and they lead in some instances to cell death or cell transformation. Three routes (other routes are also known) of toxicity are discussed. These are the epoxide/dihydrodiol pathway, the catechol/o-quinone pathway, and the alkylation pathway. The possible formation of electrophiles from diethylstilbestrol, from natural estrogens, and from ethynylestradiol is discussed in terms of protein binding. Protein binding is presumptive evidence of electrophile formation, but it does not necessarily indicate that the parent compound is highly cytotoxic, mutagenic, or carcinogenic. Mutagenic and carcinogenic activity is presumed to require reaction of an electrophile with nuclear material. There is evidence for protein binding for these estrogens (diethylstilbestrol, natural estrogens, ethnylestradiol) as a consequence of oxidative metabolism.

Binding of benzo[a]pyrene 7,8-diol-9,10-epoxides to DNA, RNA, and protein of mouse skin occurs with high stereoselectivity

The formation, stereostructure, and cellular reactions of the 7,8-diol-9,10-epoxide metabolites of the carcinogen benzo[a]pyrene have been examined after topical application of benzo[a]pyrene to the skin of mice. In this known target tissue, polymer adducts from diastereomeric diol epoxides, (+)-(7S, 8R, 9R, 10R) and (+)-(7R, 8S, 9R, 10R), were formed stereospecifically from their corresponding 7,8-dihydrodiols. Both diol epoxides bind with proteins, RNA, and DNA in vivo. For the nucleic acids, binding occurs preferentially at the 2-amino group of guanine in cellular RNA and DNA in vivo. Methods for establishing the structure of the cellular adducts as well as the possible biological implications of their formation are discussed.

Formation of 7,12-dimethylbenz[alpha] anthracene-DNA adducts in 7,8-benzoflavone-treated hamster embryo cells

Pretreatment of secondary cultures of Syrian hamster embryo cells with 7,8-benzoflavone (7,8-BF) inhibited both the metabolism of 7,12-dimethylbenz[alpha] anthracene (DMBA) and the formation of DMBA-DNA adducts. The DMBA-deoxyribonucleoside adducts from 7,8-BF-treated cultures had the same elution profiles on Sephadex LH-20 columns as those from cultures exposed to DMBA alone, but 7,8-BF-treated cultures contained smaller amounts of DMBA-DNA adducts per mg DNA. As the concentration of 7,8-BF was increased, the decrease in the amount of DMBA-DNA adducts per mg DNA was logarithmic with respect to the decrease in the amount of DMBA metabolized. The results suggest that more than one metabolic step is required for the binding of DMBA to DNA in hamster embryo cells.

In vitro metabolism of 4-chlorobiphenyl by control and induced rat liver microsomes

The in vitro metabolism, mechanism of metabolism, and macromolecular binding of a monochlorobiphenyl component of commercial polychlorinated biphenyls (PCB) have been investigated. 4-Chlorobiphenyl was metabolized by rat liver microsomes in the presence of NADPH to yield a major metabolite, 4'-chloro-4-biphenylol, and a number of minor metabolites. The metabolism of deuterium-labeled 4-chlorobiphenyl proceeded with the NIH shift of the isotope and no observed isotope effect thus indicating the intermediacy of an arene oxide. Noninduced rat liver microsomes mediated the covalent binding between the 4-chlorobiphenyl and 4'-chloro-4-biphenylol substrates and endogenous microsomal protein. Prior in vivo administration of a commericial PCB preparation, Aroclor 1248 (Monsanto Chemical Co., containing 48 percent by weight of chlorine), resulted in an induced microsomal preparation which significantly increased the substrate-protein binding. The effect of various inhibitors on protein binding was investigated. Aroclor 1248 induced microsomes mediated binding of 4-chlorobiphenyl to endogenous and exogenous nucleic acids, indicating a possible mechanism for the previously reported mutagenic action of this chlorobiphenyl. The spectral properties of Aroclor 1248 induced cytochrome P-450 were investigated and compared with the pentobarbital-induced cytochrome fraction.

beta-Glucuronidase catalyzed hydrolysis of benzo(a)pyrene-3-glucuronide and binding to DNA

beta-Glucuronidase catalyzes the hydrolysis of benzo[a]pyrene-3-glucuronide to 3-hydroxybenzo[a]pyrene. During the enzymatic hydrolysis, a benzo[a]pyrene derivative is formed which binds to DNA to a far greater extent than either the 3-hydroxybenzo[a]pyrene or its glucuronide. These results suggest that conjugates of benzo(a)pyrene may be converted by beta-glucuronidase at intracellular and organ sites distal to the initial sites of oxygenation and conjugation of benzo(a)pyrene to activated intermediates that are possibly carcinogenic.

Mutagenicity to mammalian cells in culture by (+) and (-) trans-7,8-dihydroxy-7,8-dihydrobenzo(a)pyrenes and the hydrolysis and reduction products of two stereoisomeric benzo(a)pyrene 7,8-diol-9,10-epoxides

The mutagenicity for mammalian cells of benzo(a)pyrene (BP) and 9 of its derivatives was tested by resistance to ouabain in Chinese hamster V78 cells. The derivatives included the (-) and (+) enantiomers of trans-7,8-diol; the racemic (+/-)trans-7,8-diol; two triols, (7/8,9)-triol and (7,9/8)-triol; and four tetrols, (7,10/8,9)-tetrol, (7/8,9,10)-tetrol, (7,9/8,10-triol and (7,9,10/8)-tetrol. Since V78 cells do not metabolize polycyclic hydrocarbons, mutagenesis was tested both in the presence and in the absence of Golden hamster cells capable of metabolizing polycyclic hydrocarbons. Neither BP nor any of its 9 tested derivatives showed mutagenicity for V78 cells in the absence of normal Golden hamster cells. However, in the presence of these cells, BP and the optically active and racemic trans-7,8-diols exhibited a mutagenic response that was dose-dependent. All other derivatives were inactive. The most active mutagenic hydrocarbon was (-) trans-7,8-diol, and activity decreased in the order (+/-)trans-7,8-diol, (+) trans-7,8-diol and BP.

Metabolic activations of polycyclic hydrocarbons. Structure-activity relationships

Considerable evidence now points to 7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrenes as ultimate mutagenic and carcinogenic forms of benzo(a)-pyrene. Quantum mechanical calculations have been performed to assess the possible general role of diol epoxides in polycyclic aromatic hydrocarbon (PAH) mutagenesis and carcinogenesis. The calculations enable a prediction of relative reactivity (ease of carbonium ion formation) for diol epoxides derived from a single PAH and also for diol epoxides from different PAHs. The calculated reactivity has so far been found to provide a good estimate of diol epoxide mutagenicity. Results of the metabolic activation of benzo(a)anthracene dihydrodiol derivatives and of the mutagenicity of benzo(a)anthracene diol epoxides are reported. Limitations inherent in predictions of polycyclic aromatic hydrocarbon carcinogenicity using a model based upon the calculated reactivity of a potential metabolite are discussed.

The binding of benzo(alpha)pyrene and N-methyl-N'-nitro-N-nitrosoguanidine to subnuclear fractions of AKR mouse embryo cells in culture

The marked localization of a carcinogenic polycyclic aromatic hydrocarbon, benzo(alpha)pyrene, and its metabolites and a carcinogenic alkylating agent, N-methyl-N'-nitro-N-nitrosoguanidine, to a specific subnuclear fraction (fraction I) from AKR-2B mouse embryo cells in culture is described. Fraction I is isolated by sucrose gradient centrifugation of sheared nuclei from cells exposed to the carcinogens. The association of tritiated benzo(alpha)-pyrene to fraction I consisted of loosely associated radioactivity which is extractable by organic solvents, and of tightly bound (termed "covalently" bound) radioactivity which is not extractable by organic solvents. Increases in the extent of metabolism of benzo(alpha)pyrene and in the amount of "covalently" bound radioactivity occur with increasing periods of incubation of the cells with the labelled carcinogen. This observation, together with the fact that these increases are dramatically reduced by inhibiting polycyclic aromatic hydrocarbon metabolism (using the inhibitor 7,8-benzo-flavone), suggests that a time-dependent metabolism of benzo(alpha)pyrene is required for "covalent" binding to muclear material. Data are presented suggesting that a two-step reaction may be involved in the binding of benzo(alpha)pyrene to subnuclear macromolecules. The fraction I localization of such structurally diverse chemical carcinogens as benzo(alpha)pyrene and N-methyl-N'-nitro-N-nitrosoguanidine suggests that this fraction may localize all species of chemical carcinogens and that this localization may be involved in the chemically induced malignant transformation of cells.

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.