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

Effect of various chemicals on the metabolism of benzo(a)pyrene by cultured rat colon

The effect of various co- and anti-carcinogens of colon carcinogenesis on the metabolism of benzo(a)pyrene (BP) in cultured rat colon is reported. Rat colon enzymatically converted BP into metabolites which bind to cellular macromolecules i.e., DNA and protein. Activity of aryl hydrocarbon hydroxylase (AHH) activity and binding levels of BP to macromolecules were higher in the descending colon when compared to other segments. The major metabolites of BP, extractable with ethylacetate, were quinones, tetrols, 7,8-diol and a peak containing 9,10-dihydroxy-9,10-dihydrobenzo(a)pyrene and 7,8,9-trihydroxy-7,8-dihydrobenzo(a)pyrene. The binding levels of BP to DNA and protein in the explant was lowered by co-incubation with 7,8-benzoflavone (7,8-BF) (3.6 and 18.0 micron), a known inhibitor of AHH, and with disulfiram (100 micron), an anti-oxidant. The absence of vitamin A in the media also resulted in a lower level of BP binding to DNA and protein and in lower activity of AHH. Pretreatment with known inducers of AHH such as phenobarbital (PB) or benz(a)anthracene (BA), did not have any significant effect on the binding levels of BP to DNA or on the AHH activity. Of the bile acids investigated only taurodeoxycholic acid significantly increased the binding level of BP to DNA.

Reaction of arene oxides with nucleosides

Inosine was reacted with phenanthrene-9,10-oxide and 7,12-dimethylbenz(a)anthracene-5,6-oxide (DMBA-5,6-oxide) to yield the corresponding N-1-alkylinosines. They were shown to undergo hydrolysis to 5-amino-4-imidazole-N-alkylcarboxamide ribosides when heated in the presence of sodium carbonate. Guanosine was reacted with DMBA-5,6-oxide to yield a mixture of N-7-alkylguanines along with a smaller amount of an orcine positive product exhibiting an ultraviolet spectrum resembling that of DMBA-5,6-dihydrodiol.

DNA strand scission by benzo[a]pyrene diol epoxides

Syn-and anti-benzo[a]pyrene diol epoxides elicit a concentration-dependent nicking of superhelical Col E1 DNA in an in vitro reaction monitored by agarose gel electrophoresis and electron microscopy. This strand scission represents less than 1 percent of the DNA modification by diol epoxide. Kinetic analysis implicates the formation of unstable phosphotriesters, hydrolysis of which nick the DNA.

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.

Template activity of calf thymus DNA modified by a dihydrodiol epoxide derivative of benzo[a]pyrene

The purpose of the present study was to determine the effects of covalent binding to DNA of a reactive derivative of benzo[a]pyrene on template activity during in vitro transcription with RNA polymerase. Calf thymus deoxyribonucleic acid, modified by reaction with (+/-)-7beta,8alpha-dihydroxy-9alpha, 10alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, was transcribed with Escherichia coli DNA-dependent RNA polymerase. With increasing levels of modification, there was a progressive inhibition of transcription. The inhibition was much greater under conditions where continuous reinitiation of transcription occurred than under conditions where only one RNA chain was synthesized per initiation site. This suggested that the modified sites block the movement of polymerase along the template and prevent recycling of the enzyme. Consistent with this interpretation were analyses of RNA transcripts on sucrose density gradients which showed a progressive decrease in average RNA chain length as the extent of template modification increased. In contrast to the inhibitory effect on chain elongation, evidence was obtained that the modified DNA had an increase in the number of initiation sites for transcription. These results are consistent with separate physical studies indicating that modification of DNA by this benzo[a]pyrene derivative can induce small localized regions of denaturation.

Conformation of DNA modified with a dihydrodiol epoxide derivative of benzo[a]pyrene

The conformation of calf thymus DNA modified by reaction with (+/-)-7beta,8alpha-dihydroxy-9alpha,10alpha-epoxy7,8,9,10-tetrahydrobenzo[a]pyrene, which binds covalently mainly to the 2-amino group of guanosine residues, was studied. With samples in which 1.5 or 2.2% of the bases were modified, there was a slight decrease in Tm during heat denaturation and a slight increase in susceptibility to the single strand specific nuclease S1. In a DNA sample in which 4.5% of the bases were modified, there was an appreciable decrease in Tm and a marked increase in susceptibility to S1 nuclease. The kinetics of the reaction of the modified DNAs with formaldehyde provided evidence for locally destabilized regions ranging from 1 to 7 base plates, depending on the extent of modification. Alkaline and neutral sucrose gradient analyses revealed no evidence for strand breakage in the 1.5 and 2.2% modified samples, although single-strand breaks were found in the 4.5% modified samples. Taken together, these results suggest that DNA molecules containing a covalently bound benzo[a]pyrene derivative have an altered conformation characterized by small localized regions which are destabilized and easily denatured. The conformational changes associated with the covalent binding of the benzo[a]pyrene derivative to native DNA appear to be different from, and less marked, than those associated with the covalent binding of N-2-acetylaminofluorene to native DNA.

Benzo[a]pyrene carcinogenesis: a biochemical selection mechanism

Separation of the metabolic products of benzo[a]pyrene has been readily accomplished by high-pressure liquid chromatography. This technique is uniquely suited for compounds labile to air and light and resolving positional isomers of phenolic or other types of oxygenated metabolites of this carcinogen. This procedure has been utilized to separate and compare benzo[a]pyrene activation and detoxification products between rat, mouse, and hamster hepatic microsomes and mouse and hamster embryo cell cultures. While metabolic profiles exhibited the same types of derivatives, marked quantitative variation was observed. Microsomal preparation produced large amounts of noncarcinogenic phenols, while intact cell metabolism favored diol formation. These results are in agreement with reactivation of metabolic diols as substrates for further activation to a more proximate carcinogenic species of benzo[a]pyrene and caution against extrapolating metabolic results from any single test system to other species or tissues.

Chemical carcinogenesis

The field of chemical carcinogenesis is reviewed, with emphasis on three aspects: 1) environmental chemicals are a major cause of human cancer; 2) most chemical carcinogens require metabolic activation by mixed-function oxidases to electrophilic metabolites that form strong covalent chemical bonds with cellular macromolecules and thereby initiate the carcinogenic process; and 3) several systems are available in which normal cells can be transformed by chemical carcinogens into malignant cancer cells. Much has been learned about the cellular mechanisms of chemical carcinogenesis using these systems. They also have considerable potential to be used as prescreens for environmental carcinogens.

Relationships of quantum mechanical calculations, relative mutagenicity of benzo[a]anthracene diol epoxides, and "bay region" concept of aromatic hydrocarbon carcinogenicity

Evidence supporting the conclusion that 7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrenes are ultimate mutagenic and carcinogenic forms of benzo[a]pyrene (BP) is summarized. Qauntum mechanical calculations that predict reactivity of diol epoxides derived from BP and other polycyclic aromatic hydrocarbons are described. The calculations predict that diol epoxides in which the oxirane ring forms part of a "bay region" of a tetrahydrobenzo ring should be the most reactive for a given aromatic hydrocarbon. Experiments with dihydrodiols and diol epoxides from benzo[a]anthracene (BA) are described. The ability to metabolically activate BA 3,4-dihydrodiol to species much more mutagenic that those obtained from other BA dihydrodiols ant the much greater mutagenicity of the diastereoisomeric 3,4-diol 1,2-epoxides of 1,2,3,4-tetrahydro BA relative to other diol epoxides of BA are in accord with predictions of the quantum mechanical calculations.

Transformation of normal hamster cells by benzo(a)pyrene diol-epoxide

The frequency of cell transformation was determined after treatment of normal hamster embryo cells with benzo(a)pyrene (BP) and six of its metabolites. These metabolites included the trans 4,5-, 7,8- and 9,10-dihydrodiols; the 4,5-epoxide; and two stereoisomers of the non-K-region diol epoxides r-7, t-8-dihydroxy t-9, 10-oxy-7,8,9,10-tetrahydrobenzo(a)pyrene (diol-epoxide 1) and r-7, t-8, dihydroxy c-9, 10-oxy-7,8,9,10-tetrahydrobenzo(a)pyrene (diol-epoxide 11). The trans 7,8-dihydrodiol was more active than the other two dihydrodiols tested and was also more active than the parent hydrocrabon BP. Of the three epoxides tested, the diol-epoxide 1 was more active than the 4,5-epoxide and diol-epoxide 11. The results suggest that diol-epoxide 1 is a major cell-transforming metabolite of BP.

Benzo[a]pyrene diol epoxides: mechanism of enzymatic formation and optically active intermediates

Studies of the mechanism of benzo[a]pyrene metabolism to reactive diol epoxides and of their disposition indicate that the metabolic intermediates of the activation pathways, 7,8-epoxide and trans-7,8-diol, as well as the two stereoisomeric diol epoxides are all optically active. Benzo[a]pyrene is converted to optically active 9,10-epoxides of (-)trans-7,8-diol by three enzymatic steps: (i) stereospecific oxygenation at the 7,8 double bond of benzo[a]pyrene by the mixed-function oxidases to essentially a single enantiomer of 7,8-epoxide, (ii) hydration of the 7,8-epoxide by epoxide hydratase to an optically pure (-)trans-7,8-diol, and (iii) stereoselective oxygenation by the mixed-function oxidases at the 9,10 double bond of the (-) trans-7,8-diol to optically active r-7,t-8-dihydroxy-t-9,10-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene and optically active r-7,t-8-dihydroxy-c-9,10-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene in a ratio of approximately 10 to 1.

Excision of 7-bromomethylbenz[a]anthracene--DNA adducts in replicating mammalian cells

The excision of 7-bromomethylbenz[a]anthracene--DNA adducts was studied in two cell lines (HeLa S-3 and Chinese hamster V-79379A). In both cell lines, carcinogen-modified adenine residues were excised more readily than the modified guanine residues and the percentage of the total products excised decreased after treatment with higher concentrations of carcinogen. At the highest concentrations used in the Chinese hamster cells, neither DNA synthesis nor excision was detected. The lowest concentration used for these cells permitted almost 100% survival and all the DNA was replicated in a 30-h interval even though 50% of the initial damage was still present. The two- to threefold lower sensitivity of the Chinese hamster cells (compared with the Hela cells) to the carcinogen is attributed to this capacity for replication of DNA on a damaged template since the two cell lines' capacities for excision of the chemical damage were found to be comparable.