DNA strand scission by benzo[a]pyrene diol epoxides

Detection of 8-oxodG in Dreissena polymorpha gill cells exposed to model contaminants

Genotoxic end-points are routinely measured in various sentinel organisms in aquatic environments in order to monitor the impact of water pollution on organisms. As a first step towards the evaluation of oxidative DNA damage (8-oxodG) in organisms exposed to chemical water pollution, we have optimized the association between the comet assay and the hOGG1 enzyme for use on zebra mussel (Dreissena polymorpha) gill cells by in vitro exposure to H₂O₂. Firstly, we observed that in vitro exposure of D. polymorpha gill cells to benzo[a]pyrene (B[a]P, 98.4nM) induced an increase of the Olive Tail Moment (OTM) in both the comet-hOGG1 and comet-Fpg assays, indicating that B[a]P causes oxidative DNA damage. By contrast, methylmethane sulfonate (MMS, 33μM) only induced an increase of the Fpg-sensitive sites, indicating that MMS caused alkylating DNA damage and confirming that hOGG1 does not detect alkylating damage. Thus, the hOGG1 enzyme seems to be more specific towards oxidative DNA damage, such as 8-oxodG than Fpg. Secondly, as was observed in vitro, the in vivo exposure of D. polymorpha to B[a]P (24.6 and 98.4nM) increased oxidative DNA damage in gill cells, whereas only Fpg-sensitive sites were detected in mussels exposed to MMS (240μM). These results show that the comet-hOGG1 assay detects oxidative DNA lesions induced in vitro by H₂O₂ and in vivo with BaP. The comet-hOGG1 assay will be used to detect oxidative DNA lesions (8-oxodG) in mussels exposed in situ.

Detection of phosphodiester adducts formed by the reaction of benzo[a]pyrene diol epoxide with 2'-deoxynucleotides using collision-induced dissociation electrospray ionization tandem mass spectrometry

In this study, we investigated the products formed following the reaction of benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide (B[a]PDE) with 2'-deoxynucleoside 3'-monophosphates. The B[a]PDE plus 2'-deoxynucleotide reaction mixtures were purified using solid phase extraction (SPE) and subjected to HPLC with fluorescence detection. Fractions corresponding to reaction product peaks were collected and desalted using SPE prior to analysis for the presence of molecular ions corresponding to m/z 648, 632, 608 and 623 [M-H]- consistent with B[a]PDE adducted (either on the base or phosphate group) 2'-deoxynucleotides of guanine, adenine, cytosine and thymine, respectively, using LC-ESI-MS/MS collision-induced dissociation (CID). Reaction products were identified having CID product ion spectra containing product ions at m/z 452, 436 and 412 [(B[a]Ptriol+base)-H]-, resulting from cleavage of the glycosidic bond between the 2'-deoxyribose and base, corresponding to B[a]PDE adducts of guanine, adenine and cytosine, respectively. Further reaction products were identified having unique CID product ion spectra characteristic of B[a]PDE adduct formation with the phosphate group of the 2'-deoxynucleotide. The presence of product ions at m/z 399 and 497 were observed for all four 2'-deoxynucleotides, corresponding to [(B[a]Ptriol+phosphate)-H]- and [(2'-deoxyribose+phosphate+B[a]Ptriol)-H]-, respectively. In conclusion, this investigation provides the first direct evidence for the formation of phosphodiester adducts by B[a]PDE following reaction with 2'-deoxynucleotides.

Anguilla anguilla L. biochemical and genotoxic responses to benzo[a]pyrene

Eels (Anguilla anguilla L.) were exposed for 2, 4, 6, 8, 16, 24, 48, 72, 144, and 216 h to 0 (control), 0.3, 0.9, and 2.7 microM benzo[a]pyrene (BaP). The biotransformation induced by BaP was measured as liver ethoxyresorufin O-deethylase (EROD) activity and cytochrome P450 content, and compared with the genotoxic effects, such as erythrocytic nuclear abnormalities (ENA), and blood and liver DNA strand breaks. The liver exhibited a highly significant EROD activity increase from 2 up to 216 h exposure to 0.9 and 2.7 microM BaP, whereas 0.3 microM BaP exposure induced a significant liver EROD increase from 2 up to 144 h. Liver cytochrome P-450 content was significantly increased at 8 h to 2.7 microM BaP exposure. Liver DNA integrity was decreased at 16 h, from 8 up to 144 h and 8 up to 72 h exposure to 0.3, 0.9, and 2.7 microM BaP, respectively. A significant decrease in blood DNA integrity was observed at 48, 72, 144 h, from 8 up to 72, and from 6 up to 72 h exposure to 0.3, 0.9, and 2.7 microM BaP, respectively. The A. anguilla L. genotoxic response to BaP, measured as ENA induction, was significantly increased at 144 h exposure to 0.3 microM BaP. The intermediate BaP concentration tested (0.9 microM) induced a significant three fold ENA increase at 48 and 72 h exposure compared to their controls. The highest BaP concentration (2.7 microM) induced a significant increase in ENA frequency at 72, 144 and 216 h exposure.

Characterization and mapping of DNA damage induced by reactive metabolites of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) at nucleotide resolution in human genomic DNA

The nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is an important tobacco-specific carcinogen associated with lung cancer. Its complex enzymatic activation, leading to methyl and pyridyloxobutyl (POB)-modified DNA, makes DNA damage difficult to characterize and quantify. Therefore, we use the NNK analogue 4-[(acetoxymethyl)nitrosamino]-1-(3-pyridyl)-1-butanone (NNKOAc) to induce damage in genomic DNA, and to map the sites and frequency of adducts at nucleotide resolution using ligation-mediated polymerase chain reaction and terminal transferase-dependent polymerase chain reactions (LMPCR and TDPCR). NNKOAc induced single-strand breaks in a concentration-dependent manner. Post-alkylation treatments, including hot piperidine or digestion with the enzymes Escherichia coli 3-methyladenine-DNA glycosylase II, formamidopyrimidine-DNA glycosylase, Escherichia coli endonuclease III, or phage T4 UV endonuclease V did not increase the level of DNA breaks in NNKOAc-treated DNA. Detection of DNA damage using LMPCR was possible only when POB-DNA was 5'-phosphorylated prior to the LMPCR procedure. NNKOAc generated damage at all four bases with the decreasing order guanine>adenine>cytosine>thymine. In contrast to NNKOAc damage distribution patterns, those induced by N-nitroso(acetoxymethyl)methylamine, a methylating NNK analog, induced damage principally at G positions detectable by enzymatic means that did not require phosphorylation. Analysis of damage distribution patterns, reveals a high frequency of damage in the p53 gene in codons 241 and 245 and a lower frequency of damage in codon 248. We analyzed the 3' termini of the NNKOAc induced single-strand breaks using a (32)P-post-labeling assay or a nucleotide exchange reaction at the 3'-termini catalyzed by T4 DNA polymerase combined with endonuclease IV treatment. Both methods indicate that the 3' termini of the single-strand breaks are not hydroxyl groups and are blocked by an unknown chemical structure that is not recognized by endonuclease IV. These data are consistent with POB-phosphotriester hydrolysis leading to strand breaks in DNA. The POB-damage could be mutagenic because NNKOAc produces single-strand breaks with the products being a 5'-hydroxyl group and a 3'-blocking group and strand breaks. These results represent the first step in determining if NNK pyridyloxobutylates DNA with sequence specificity similar to those observed with other model compounds.

Linear dichroism properties and orientations of different ultraviolet transition moments of benzo[a]pyrene derivatives bound noncovalently and covalently to DNA

Linear dichroism and absorption methods are used to study the orientations of transition moments of absorption bands of polycyclic aromatic epoxide derivatives which overlap with those of the DNA band in the 240-300 nm region. Both the short and long axes of the pyrene residues of 1-oxiranylpyrene (1-OP) and the (+) and (-) enantiomers of trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) noncovalently bound to double-stranded native DNA are oriented approximately perpendicular to the axis of the DNA helix, consistent with intercalative modes of binding. The covalent binding of these three epoxide derivatives to DNA is accompanied by reorientations of both the short and long axes of the pyrene residues. Covalent adducts derived from the highly mutagenic (+)-anti-BPDE are characterized by tilts of the short axis within 35 degrees or less, and of the long axis by more than 60-80 degrees, with respect to the planes of the DNA bases. In the adducts derived from the binding of the less mutagenic (-)-anti-BPDE and 1-OP epoxide derivatives to DNA, the long axes of the pyrenyl rings are predominantly oriented within 25 degrees of the planes of the DNA bases; however, in the case of the (-) enantiomer of BPDE, there is significant heterogeneity of conformations. In the case of the 1-OP covalent DNA adducts, the short axis of the pyrene ring system is tilted away from the planes of the DNA bases, and the pyrene ring system is not intercalated between DNA base-pairs as in the noncovalent complexes. The stereochemical properties of the saturated 7,8,9,10-ring in BPDE, or the lack of the 7 and 8 carbon atoms in 1-OP, do not seem to affect noncovalent intercalative complex formation which, most likely, is influenced mainly by the flat pyrenyl residues. These structural features, however, strongly influence the conformations of the covalent adducts, which in turn may be responsible for the differences in the mutagenic activities of these molecules.

The ratio of induced recessive lethals to ring-X loss has prognostic value in terms of functionality of chemical mutagens in Drosophila melanogaster

For 25 mutagens in Drosophila the ratio was determined between the induction of sex-linked recessive lethals (SLRL) and the induction of ring-X loss in male adults. For small monofunctional alkylating agents this ratio increases with decreasing s-value from 1.8 for methyl methanesulfonate (MMS) to 27 for ethylnitrosourea (ENU). For multifunctional cross-linking agents, however, the ratio varies within relatively narrow limits, ranging from 0.15 for cisplatin to 0.07 for tris-(1-aziridinyl)phosphineoxide (TEPA), while for most agents the ratio is around 0.12. The number of reactive groups seems to be of minor importance for compounds with more than one functionality as bi- and tri-functional agents show similar ratios. The systemic difference in the ratios between mono- and multi-functional agents suggests that different mechanisms are involved in the induction of SLRLs and ring-X loss. For ethyleneimine (EI) and ethyleneoxide (EO) low ratios of 0.32 and 0.60 respectively were observed which do not correlate with their s-values. An alternative chromosome-breaking mechanism may be responsible for this deviation, possibly alkylation of the phosphate backbone of DNA, followed by an intramolecular displacement of one of the deoxyribose groups by the beta-amino or the beta-hydroxy group. It is felt that the considerable difference between the ratios for monofunctional and multifunctional agents may be of prognostic value and can be used to obtain information on the mechanisms of mutagens with 'unknown' action, provided that structural features are taken into account. Hexamethylphosphoramide (HMPA), hexamethylmelamine (HEMEL), tetramethylurea (TMU) and dimethylpropyleneurea (DMPU) all show SLRL: ring-X loss ratios that match those of multifunctional agents, 0.08, 0.12, 0.08, and 0.16, respectively. The ratios for the pyrrolizidine alkaloids monocrotalin and seniciphilline, 0.053 and 0.24 respectively, also correspond with this group of mutagens. The low ratios for formaldehyde, 2-chloro-acetaldehyde and 2-chloroethyl methanesulfonate, 0.30, 0.052 and 0.36 respectively, are indicative that cross-linking may attribute considerably to their mutagenic action in Drosophila. On the other hand, not all mutagens containing 2 reactive groups act as cross-linking agents. The ratio for 1,2-dibromoethane, 2.7, indicates that it may act as a monofunctional agent. This is in accordance with the proposed activation mechanism by glutathione S-transferase, producing a monofunctional half-mustard derivative (Rannug, 1980; van Bladeren et al., 1981).

Electron microscopic visualization of DNA single strand breaks

DNA single strand breaks (ssb) have been induced in FLC/C cells in culture. They have been visualized in the electron microscope after decoration with biotin-avidin-ferritin complexes and spreading as monomolecular mixed films. This allowed one to determine the average number of decorated ssbs per unit of DNA length applying straight-forward and simple evaluation methods. This method has been used to investigate the DNA alterations by benzo[a]pyrene (B[a]P) on FLC/C culture cells. Thus a B[a]P-DNA damage curve can be constructed as a regression with a correlation coefficient of r = 0.97, while its isomer benzo[e]pyrene (B[e]P) known to have only low mutagenicity under the same experimental conditions is virtually without effect. The method has further informational potential regarding damage distribution and repair of DNA.

Origins of stereospecificity in DNA damage by anti-benzo[a]pyrene diol-epoxides. A molecular modelling study

A general computational procedure for the modelling of intercalated DNA-ligand complexes has been developed, and is used here to model intercalated complexes of the (+)-anti and (-)-anti enantiomers of benzo[a]pyrene diol-epoxide (BPDE) with cytosine-3',5'-guanosine double-stranded DNA sequences (dCpG). Results are presented indicating differences between the behaviours of the two enantiomers which have implications for the understanding of the stereospecificity of DNA strand breakage by benzo[a]pyrene diol-epoxides.

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.

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.

Covalent binding of (+)- and (-)-trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyr ene to B and Z DNAs

Circular dichroism (CD) as well as absorption spectral measurements reveals that poly(dG-m5dC).poly(dG-m5dC) suffers more extensive covalent modification by (+)-dihydroxy-anti-epoxybenzo[a]pyrene [(+)-anti-BPDE] than its unmethylated counterpart and that the covalently attached pyrenyl moiety exhibits stronger stacking interactions with the bases in the methylated polymer as suggested by the much larger pyrenyl spectral red shifts, most likely the consequence of intercalation. Stereoselective binding properties of these polymers are evidenced by the much reduced preference for the (-) enantiomer. Modifications due to (+)-anti-BPDE on the 50 microM hexaamminecobalt induced Z DNAs are much less pronounced and much less stereoselective, with the pyrenyl spectral characteristics being distinct from those of the B form. Salt titrations on the (+)-anti-BPDE modified poly(dG-dC).poly(dG-dC) and poly(dG-m5dC).poly(dG-m5dC) indicate much reduced cooperativity on the B to Z transition when compared to the unmodified counterparts. Evidence also suggests that covalent modification by anti-BPDE inhibits the B to Z conversion of base pairs in its immediate vicinity, presumably through intercalative stabilization of the B conformer at high salt. In contrast to stabilizing the B conformation for the proximal base pairs, covalent lesion by (+)-anti-BPDE appears to destabilize distal base pairs with the consequence of kinetic facilitation of B to Z transformation for these regions. Interesting differential effects on the reverse Z to B transforming abilities of these two enantiomers are observed with the covalent binding of the (-) isomer showing higher potency for inducing such conversion.

Depurination of benzo[a]pyrene-diolepoxide treated DNA

Rat liver DNA was treated in vitro with benzo[a]pyrene-diolepoxide (BPDE), the ultimate carcinogenic metabolite derived from the polycyclic hydrocarbon benzo[a]pyrene. On incubation of the reacted DNA, apurinic sites developed which gave rise to strand breakage in alkaline solution. The reduction in molecular weight produced by these breaks was measured by analytical ultracentrifugation. In the case of anti-BPDE this depurination was shown to occur in two stages. The first was mainly due to attack at the 7-position of guanine, to yield an adduct which was lost from the DNA within a few hours. The second stage was due to much slower loss of the major N2-guanine adduct. The separated enantiomers, (+)- and (-)-anti-BPDE, and syn-BPDE all caused depurination to various extents. It is argued that although these processes are important in a study of the action of BPDE on DNA in vitro, their contribution to the biological activity of BPDE is probably negligible.

Quantitation of chemically induced DNA strand breaks in human cells via an alkaline unwinding assay

DNA strand breaks induced in human CCRF-CEM cells by electrophilic chemicals (carcinogens/mutagens) can be readily quantitated via a facile alkaline unwinding assay. This procedure estimates the number of chemically induced DNA strand breaks on the basis of the percentage DNA converted from double-stranded to single-stranded form during an exposure to the alkaline unwinding conditions. The assay is based on the assumption that each strand break serves as a strand unwinding point during the alkaline denaturation. The extent of strand separation can be standardized with respect to the initial level of induced strand breaks by the use of X-rays, which produce known levels of DNA strand breaks per rad in mammalian cells. Subsequent to the alkaline exposure, the single- and double-stranded DNA were separated by use of thermostated hydroxylapatite columns (60 degrees C), and the DNA was quantitated via a fluorescence assay (Hoechst 33258 compound). A correlation was shown between mammalian DNA strand-breaking potential (as measured in this procedure) and the propensity of these chemicals to revert Salmonella typhimurium TA100.

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)

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.

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.

The absolute configuration of precocene I dihydrodiols produced by metabolism of precocene I by corpora allata of Locusta migratoria, in vitro

The corpora allata from adult female Locusta migratoria metabolize precocene I (7-methoxy-2,2-dimethyl-2H-benzo [b]pyran to cis- & trans-precocene I dihydrodiols (3,4-dihydro-7-methoxy-2,2-dimethyl-2H-benzo [b]pyran-3,4-diol). Derivatization of the dihydrodiols with (-)menthoxy acetyl chloride allowed complete resolution of all four optical isomers. When [4-3H]-precocene I was incubated in vitro with Locusta migratoria corpora allata, it was metabolized stereospecifically to (-)trans-(3R,4S) and (+)cis-(3R,4R) dihydrodiols. Approximately half the precosyl residues bound to cellular macromolecules were discharged by heating to 95 degrees C at neutral pH, as dihydrodiols of the same stereochemistry.

Strand-break formation in DNA modified by benzo[alpha]pyrene diolepoxide. Quantitative cleavage by Escherichia coli uvrABC endonuclease

Covalently closed circular plasmid DNA was modified by benzo[alpha]pyrene diolepoxide and incubated with partially purified fractions of the Escherichia coli uvr+ gene products. Strand breaks were introduced into the modified DNA by the uvrABC endonuclease; on average, one break was formed for each bound benzo[alpha]pyrene residue in the DNA. These results are direct evidence that benzo[alpha]pyrene adducts in DNA are acted upon by the same repair enzyme as those that handle UV-induced lesions in DNA.

DNA damage by PAH and repair in a marine sponge

The sponge Tethya lyncurium from the Northern Adriatic has been used as an experimental species. A method is outlined for preparation of DNA which yields a highly purified DNA with a double-strand (ds) molecular weight of 25 M-dalton between single-strand (ss) breaks, which when properly damaged can be cut opposite to ss-breaks with nuclease S1. The molecular weights of the resulting ds-DNA pieces and their distribution has been evaluated by electron microscope photographs. Sponges exposed to benzo[a]pyrene (BaP) in the dark only incorporate BaP-derivatives (BaPD) in small amounts, if any. However, in the presence of light, derivatization to BaP derivatives enables effective coupling to occur, as shown previously (R.K. Zahn et al., 1981). Sponges were exposed to radiolabeled BaP in the presence of light. Coupling of BaPD to the DNA as well as the induction of ss-breaks were measured. Light-mediated coupling is concentration dependent from 0.01-20 ppb BaP with a correlation coefficient of r = 0.84. Under conditions of possible repair, ss-breaks completely disappear from sponge DNA in the course of three weeks while a substantial fraction of the BaP derivatives persists. Double label experiments show that substantial DNA synthesis occurs during this time. Pollution causes a decrease of the molecular weight of unnicked DNA, re-incubation in clean water an increase. A DNA species of 24 M-dalton seems to play a critical role. If its percentage in the DNA population drops below a critical level, recovery is not longer possible. DNA damage by PAH and repair in sponges seems to differ from that of most eucaryotes.

Induction and repair of DNA strand breaks in cultured human fibroblasts exposed to various phenols and dihydrodiols of benzo[a]pyrene

Cultured human fibroblasts from healthy donors were incubated for 30 min with nine different benzo[a]pyrene (BP) derivatives in the presence or absence of liver microsomes from 3-methylcholanthrene treated rats. The induction and repair of DNA strand breaks were analysed by alkaline unwinding and separation of double and single stranded DNA (SS-DNA) by hydroxylapatite chromatography immediately after the incubation or at various times after the treatment. In the absence of microsomes DNA stand breaks were detected in fibroblasts exposed to 30 microM of each of the six BP phenols (1-, 2-, 3-, 7-, 9- or 11-OH-BP) and the three BP dihydrodiols (BP-4,5-, BP-7,8- or BP-9,10-dihydrodiol). After removal of the BP derivatives from the medium the DNA strand breaks disappeared within 24 h. alpha-Naphthoflavone (alpha-NF) caused a decrease in the induction of strand breaks by 1-, 3- and 9-OH-BP but did not affect the induction of strand breaks in cells exposed to BP-7,8-dihydrodiol. In the presence of microsomes DNA strand breaks were found after exposure to 30 microM of each of the six BP phenols (1-, 2-, 3-, 7-, 9- or 11-OH-BP), as well as BP-7,8- and 9,10-dihydrodiol. In contrast BP-4,5-dihydrodiol did not induce strand breaks under these conditions. The induction of strand breaks by BP-7,8-dihydrodiol was enhanced in the presence of cytosine-1-beta-D-arabinofuranoside (AraC). In all cases the DNA strand breaks had disappeared 24 h after removal of the BP derivatives and microsomes except after treatment with BP-7,8-dihydrodiol.

Carcinogenic epoxides of benzo[a]pyrene and cyclopenta[cd]pyrene induce base substitutions via specific transversions

We have determined the spectrum of base-pair substitution mutations induced in the lacI gene of a uvrB- strain of Escherichia coli by two polycyclic aromatic hydrocarbons--(+/-)7 alpha,8 beta-dihydroxy-9 beta,10 beta-epoxy-7,8,9,10 tetrahydrobenzo[a]pyrene (BPDE), and 3,4-epoxycylopenta[cd]pyrene (CPPE). Approximately 10% of all lacI mutations induced by either BPDE or CPPE are nonsense mutations, suggesting that base-pair substitutions are a large fraction of the mutational events induced by these agents in the uvrB- bacteria. Both carcinogens specifically induced the G . C leads to T . A and, to a lesser extent, the A . T leads to T . A transversions. One possible mechanism for transversion induction at G . C sites by BPDE might involve carcinogen binding to the exocyclic amino group of guanine in the template strand followed by a rotation of the modified base around its glycosylic bond from the anti to the syn conformation. This could allow specific pairing of modified bases with an imino tautomer of adenine.

In vivo formation of benzo(alpha)pyrene diol epoxide-deoxyadenosine adducts in the skin of mice susceptible to benzo(alpha)pyrene-induced carcinogenesis

The hydrocarbon-deoxyribonucleoside adducts formed in mouse skin DNA have been determined following topical application of an initiating dose of benzo(a)pyrene to Swiss mice, a strain shown to be susceptible to benzo(a)pyrene-induced skin carcinogenesis. Several DNA-bound products were formed, of which the major one (60% of total adducts), in agreement with other workers' findings, was derived from reaction of (+/-) 7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo(alpha)pyrene (BDE) with the exocyclic aminogroup of deoxyguanosine. A further product (9-10% of total adducts), previously observed only after microsomal activation of benzo(a)pyrene, was observed and co-chromatographed with a further metabolite of 9-hydroxybenzo(alpha)pyrene bound to an uncharacterized base in the DNA. Two otherr products (2-3% of total adducts) were also found in the in vivo studies which co-chromatographed with BPDE-deoxyadenosine adducts and arose from cis and trans addition of the exocyclic amino group of deoxyadenosine to the 7R form, but not the 7S form, of BPDE. In contrast to this, the major in vitro deoxyadenosine-bound products, formed following reaction of BPDE with calf-thymus DNA, were derived from the 7S form of BPDE, suggesting either stereoselective formation or reaction of the 7R form of BPDE in mouse skin in vivo. Similar amounts of BPDE-deoxyguanosine and BPDE-deoxyadenosine adducts, as well as those derived from further metabolism of 9-hydroxybenzo(alpha)pyrene were formed in three strains of mice reported to have widely differing susceptibilities to polycyclic hydrocarbon-induced skin carcinogenesis. The relevance of these different hydrocarbon-DNA adducts to carcinogenesis requires further investigation.

Arylhydroxylamine-induced ribonucleic acid chain cleavage and chromatographic analysis of arylamine-ribonucleic acid adducts

Reaction of N-hydroxy-2-aminofluorene (N-OH-AF) with rRNA at pH 5.0 decreased the molecular weight of the polynucleotide. Toluene-soluble aryl derivatives were released on hydrolysis of fluorenylamine- and biphenylamine-substituted RNA by treatment with venom phosphodiesterase and alkaline phosphatase. These data suggested that arylhydroxylamines, activated by incubation at pH 5.0 or by enzymatic O-acetylation, might react with the phosphate group of RNA to give unstable phosphate triesters. Spontaneous hydrolysis of these triesters would result in cleavage of the polynucleotide chain. Further enzymatic hydrolysis of the phosphate esters would yield nonpolar arylamine derivatives. Enzymatically degraded 4-aminobiphenyl(ABP)-RNA adducts were examined by high performance liquid chromatography (HPLC) for the presence of a putative phosphorylated adduct. Synthetic standards of the C-8-guanosine monophosphate-ABP adduct (ABP-GMP) and o-aminobiphenyl-O-phosphate were used as markers in the analysis of the digested RNA. A phosphate adduct of ABP was undetectable by these methods. The data also indicated that the ABP-GMP formed in the acyltransferase-mediated binding of N-hydroxy-4-acetylaminobiphenyl (N-OH-AABP) to RNA is readily degraded during the enzymatic digestion of the RNA adduct.

Estimation of apurinic/apyrimidinic sites and phosphotriesters in deoxyribonucleic acid treated with electrophilic carcinogens and mutagens

The number of apurinic/apyrimidinic (AP) sites in supercoiled SV40 deoxyribonucleic acid (DNA) after treatment with several electrophilic mutagens was quantitated by electrophoretic analysis of the DNA after cleavage of the phosphodiester bonds adjacent to AP sites by a specific endonuclease. The compounds studied, in order of increasing yields of AP sites obtained on incubation with the DNA for 5 h at 37 degrees C, were dimethylcarbamoyl chloride, ethyl methanesulfonate, N-ethyl-N-nitrosourea, 2-(N-acetoxyacetylamino)fluorene, beta-propiolactone, N-methyl-N-nitrosourea, methyl methanesulfonate, 1'-acetoxyestragole, 4-(N-acetoxyacetylamino)stilbene, (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, N-(benzoyloxy)-N-methyl-4-aminoazobenzene, and 1-pyrenyloxirane. After a 5-h incubation at 37 degrees C and extraction of unreacted compound, further incubation at 70 degrees C generally increased the yield of AP sites; an exception was N-(benzoyloxy)-N-methyl-4-aminoazobenzene-reacted DNA. Except for DNA treated with N-ethyl-N-nitrosourea and N-methyl-N-nitrosourea, which are known to bind to a significant extent to DNA phosphates, the number of alkali-labile lesions in the treated DNA was similar to the number of AP sites. For the compounds studied there was no direct correlation between the number of AP sites produced and missense mutagenic activity, as measured in Salmonella typhimurium strain TA100.

A transversion mutation hypothesis for chemical carcinogenesis by N2-substitution of guanine in DNA

Several carcinogenic aromatic amines and polycyclic hydrocarbons react covalently with the exocyclic amino group (N2) of guanine in DNA. In this study, space-filling molecular models of DNA containing N2-guanyl adducts of 2-acetylaminofluorene (AAF) or benzo[a]pyrene (BP) were constructued. From these models and from available physico-chemical data, it is suggested that the N2 adducts may be easily converted from the normal anti to a syn conformation (base/deoxyribose). This confuguration causes minimal distortion of the DNA model with only a 2--3 A shift in the helical axis of symmetry. Such an alteration may account for the persistence of these adducts in DNA and for the frameshift mutations induced by these carcinogens. Additionally, the syn N2-guanyl configuration places the N-7 and O6 atoms of the modified syn guanine in the base pairing region such that, duration replication, mispairing with N-1 and N2 of an opposite guanine may occur. This would then represent a carcinogen-induced transversion mutation and may lead to neoplastic transformation.

Mechanism of benzo[a]pyrene diol epoxide induced deoxyribonucleic acid strand scission

Approximately 1% of (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BaP-diol epoxide) DNA alkylation sites rearrange with strand scission at neutral pH. Phosphotriester hydrolysis and depurination/depyrimidination strand scission were critically examined as possible mechanisms for this phenomenon. The catalysis of nicking by alkali and the inhibition of nicking by counterions were consistent with either mechanism. The kinetics of nicking, however, were characteristic of a multistep reaction such as depurination/depyrimidination strand scission and the detection of apurinic sites in BaP-diol epoxide alkylated DNA strongly supported this mechanism. The number of such sites, especially at lower reaction levels, was probably sufficient to account for strand scission. No direct evidence was obtained for nicking occurring through phosphotriester hydrolysis. Studies with model substrates, including dibutyl phosphate, DNA homopolymers, and TMV RNA, indicated that if BaP-diol epoxide forms phosphotriesters in DNA or RNA, they do not hydrolyze with strand scission. Besides apurinic/apyrimidinic sites, a second alkali-sensitive rearrangement product was present in BaP-diol epoxide modified DNA. These latter sites accumulated with time and after 24 h accounted for as much as 4% of the initial alkylation events. Although relatively stable at neutrality, they spontaneously nicked the DNA backbone at high pH. It is possible that these sites represent a rearrangement of the major N2 guanine adduct.

DNA alkylation and unwinding induced by benzo[a]pyrene diol epoxide: modulation by ionic strength and superhelicity

Superhelical and partially relaxed DNAs of simian virus 40 were allowed to react in vitro with (+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BaP diol epoxide). The modified DNA contained N2 guanine and N6 adenine hydrocarbon adducts in the ratio 86:14. Superhelical simian virus 40 DNA was approximately 6% more susceptible to modification than was partially relaxed viral DNA. Counterions inhibited DNA alkylation by up to 90%, Mg2+ being 50-fold more effective than Na+. The sensitivity of covalent binding to helix stability is consistent with a reaction complex in which BaP diol epoxide is intercalated. The superhelical density of the modified DNA substrates was determined electrophoretically relative to partially relaxed standards, and an unwinding angle for the hydrocarbon adducts was calculated. The angle was dependent upon the superhelicity of the DNA molecule and ranged from 330 degrees to 30 degrees. These data indicate that the modified base pairs are disrupted and, in the presence of torsional strain, act as centers for the further denaturation of up to eight adjacent base pairs. In the absence of such strain the alkylation sites have an ordered structure, with the attached hydrocarbon probably oriented in the minor or major groove of the helix.

DNA damage, cell survival and DNA repair in Chinese hamster ovary cells after in vitro metabolic activation of dimethylnitrosamine and benzo[a]pyrene

Both benzo[a]pyrene (BP) and dimethylnitrosamine (DMN) cause a dose-related increase in single-strand DNA lesions in CHO cells after in vitro metabolic activation, as detected by alkaline sucrose gradients (ASG) sedimentation analysis. This type of DNA damage and cell survival are inversely related with DMN, whereas there is negligible cell killing with BP even at concentrations producing maximal DNA damage. However, high concentrations of BP do result in a reduction in colony size, suggesting a diminished growth rate rather than increased lethality. Up to 80% of the DNA lesions induced by BP and detectable by alkaline sucrose gradient sedimentation may be repaired within 6 h, whereas less than 30% of the DNA damage caused by DMN is apparently removed 24 h after treatment, implying that DNA repair may be involved in the differential lethality of CHO cells exposed to DMN and BP.

Esolation of 3-(2-carboxyethyl)thymine following in vitro reaction of beta-propiolactone with calf thymus DNA

3-(2-Carboxyethyl)thymine (3-CET) was synthesized from beta-propiolactone (BPL) and dThd 5'P at pH 9.0--9.5 via the intermediate 3-(2-carboxyethyl)-thymidine-5'-monophosphoric acid (3-CEdThd5'P). 3-CEdThd5'P was converted to 3-CET by hydrolysis in 1.5 N HCl at 100 degrees C for 2 h. The structure of 3-CET was assigned on the basis of UV spectra, electron impact (EI) and isobutane chemical ionization mass spectra and the EI mass spectrum of a trimethylsilyl derivative of 3-CET. BPL was reacted in vitro with calf thymus DNA at pH 7.5. 100 A units of BPL-reacted DNA yielded, following perchloric acid hydrolysis and preparative paper chromatography, 3 A units of 3-CET. Reaction of BPL with the phosphodiester thymidylyl-(3'-5')-thymidine gave 3-(2-carboxyethyl)thymidylyl-(3'-5')-3-(2-carboxyethyl)-thymidine (approximately 3%). Phosphotriester formation was not detected.

A benzo[alpha]pyrene-7,8-dihydrodiol-9,10-epoxide is the major metabolite involved in the binding of benzo[alpha]pyrene to DNA in isolated viable rat hepatocytes

Benzo[alpha]pyrene is metabolised by isolated viable hepatocytes from both untreated and 3-methylcholanthrene pretreated rats to reactive metabolites which covalently bind to DNA. The DNA from the hepatocytes was isolated, purified and enzymically hydrolysed to deoxyribonucleosides. The hydrocarbon-deoxyribonucleoside products after initial separation, on small columns of Sephadex LH-20, from unhydrolysed DNA, oligonucleotides and free bases, were resolved by high pressure liquid chromatography (HPLC). The qualitative nature of the adducts found in both control and pretreated cells was virtually identical; however pretreatment with 3-methylcholanthrene resulted in a quantitatively higher level of binding. The major hydrocarbon-deoxyribonucleoside adduct, found in hepatocytes co-chromatographed with that obtained following reaction of the diol-epoxide, (+/-) 7 alpha,8 beta-dihydroxy-9 beta,10 beta-epoxy-7,8,9,10-tetrahydrobenzo[alpha]pyrene with DNA. Small amounts of other adducts were also present including a more polar product which co-chromatographed with the major hydrocarbon-deoxyribonucleoside adduct formed following microsomal activation of 9-hydroxybenzo[alpha]-pyrene and subsequent binding to DNA. In contrast to the results with hepatocytes, when microsomes were used to metabolically activate benzo[alpha]-pyrene, the major DNA bound-product co-chromatographed with the more polar adduct formed upon further metabolism of 9-hydroxybenzo[alpha]pyrene. These results illustrate that great caution must be exercised in the extrapolation of results obtained from short-term mutagenesis test systems, utilising microsomes, to in vivo carcinogenicity studies.

Characterization of DNA damages by filtration through nitrocellulose filters: a simple probe for DNA-modifying agents

A simple technique for the detection of DNA-modifying agents is described. The double-stranded covalently closed circular DNA of phage PM2 is exposed to the modifying agent and then analysed for DNA damages by assays involving only incubation steps and filtration through nitrocellulose filters. The technique described allows the measurement of DNA modifications which lead to local denaturation of the DNA double helix, interstrand cross-links, single- and double-strand breaks, damages which render the phosphodiester bonds of the DNA sensitive to hydrolysis and damages which labilise the glycosylic bond between base and sugar moiety.