Conformations of adducts and kinetics of binding to DNA of the optically pure enantiomers of anti-benzo(a)pyrene diol epoxide

Non-covalent interactions of the carcinogen (+)-anti-BPDE with exon 1 of the human K-ras proto-oncogene

Investigating the complementary, but different, effects of physical (non-covalent) and chemical (covalent) mutagen-DNA and carcinogen-DNA interactions is important for understanding possible mechanisms of development and prevention of mutagenesis and carcinogenesis. A highly mutagenic and carcinogenic metabolite of the polycyclic aromatic hydrocarbon benzo[α]pyrene, namely (+)-anti-BPDE, is known to undergo both physical and chemical complexation with DNA. Previous studies of BPDE-DNA complex formation have focused on processes that require substantial structural reorganization, such as intercalation, and consequently relatively long time scales. However, some initial processes which occur within shorter time scales, such as external non-covalent binding, and which do not require major DNA structural reorganization have not been thoroughly investigated. A detailed computational study of such initial BPDE-DNA interactions is needed to elucidate the temporal and structural origins of the major covalent adduct, a promutagenic, which is known to exist in an external (+)-trans-anti-BPDE-N(2)-dGuanosine configuration. Accordingly, the initial stages of external non-covalent BPDE-DNA binding are studied in this work as well as their relationship to subsequent formation of the major, also external, covalent adduct. To study mechanisms that occur prior to extensive DNA structural reorganization, we present a first and detailed codon by codon computational study of the non-covalent interactions of (+)-anti-BPDE with DNA. In particular, due to its relevance to carcinogenesis, the interaction of (+)-anti-BPDE with exon 1 of the human K-ras gene has been studied. External solvent-exposed non-covalent binding sites have been found which may be precursors of the major external trans adduct and, importantly, are located in codons 12 and 13 of the K-ras gene which are known to be key mutation hotspots. In addition, our study explains and correctly predicts preferential (+)-anti-BPDE binding at minor groove guanosines. A subtle combination of van der Waals and hydrogen bonding interactions has been found to be a primary factor in preferentially positioning (+)-anti-BPDE toward the 5' position of a guanosine's strand, consistent with proton NMR observations for the major trans adduct, and at 5'-TGG-3' sequences which are known to yield high binding probability.

Influence of C-5 substituted cytosine and related nucleoside analogs on the formation of benzo[a]pyrene diol epoxide-dG adducts at CG base pairs of DNA

Endogenous 5-methylcytosine ((Me)C) residues are found at all CG dinucleotides of the p53 tumor suppressor gene, including the mutational 'hotspots' for smoking induced lung cancer. (Me)C enhances the reactivity of its base paired guanine towards carcinogenic diolepoxide metabolites of polycyclic aromatic hydrocarbons (PAH) present in cigarette smoke. In the present study, the structural basis for these effects was investigated using a series of unnatural nucleoside analogs and a representative PAH diolepoxide, benzo[a]pyrene diolepoxide (BPDE). Synthetic DNA duplexes derived from a frequently mutated region of the p53 gene (5'-CCCGGCACCC GC[(15)N(3),(13)C(1)-G]TCCGCG-3', + strand) were prepared containing [(15)N(3), (13)C(1)]-guanine opposite unsubstituted cytosine, (Me)C, abasic site, or unnatural nucleobase analogs. Following BPDE treatment and hydrolysis of the modified DNA to 2'-deoxynucleosides, N(2)-BPDE-dG adducts formed at the [(15)N(3), (13)C(1)]-labeled guanine and elsewhere in the sequence were quantified by mass spectrometry. We found that C-5 alkylcytosines and related structural analogs specifically enhance the reactivity of the base paired guanine towards BPDE and modify the diastereomeric composition of N(2)-BPDE-dG adducts. Fluorescence and molecular docking studies revealed that 5-alkylcytosines and unnatural nucleobase analogs with extended aromatic systems facilitate the formation of intercalative BPDE-DNA complexes, placing BPDE in a favorable orientation for nucleophilic attack by the N(2) position of guanine.

Density Functional Study of the Influence of C5 Cytosine Substitution in Base Pairs with Guanine

The present study employs density-functional electronic structure methods to investigate the effect of chemical modification at the C5 position of cytosine. A series of experimentally motivated chemical modifications are considered, including alkyl, halogen, aromatic, fused ring, and strong σ and π withdrawing functional groups. The effect of these modifications on cytosine geometry, electronic structure, proton affinities, gas phase basicities, cytosine-guanine base-pair hydrogen bond network and corresponding nucleophilicity at guanine are examined. Ultimately, these results play a part in dissecting the effect of endogenous cytosine methylation on the reactivity of neighboring guanine toward carcinogens and DNA alkylating agents.

Translesional synthesis on a DNA template containing a single stereoisomer of dG-(+)- or dG-(-)-anti-BPDE (7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene)

Oligodeoxynucleotides modified site-specifically with dG-(+)-trans- and dG-(+)-cis-anti-BPDE (7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene) or dG-(-)-trans- and dG-(-)-cis-anti-BPDE were used as templates in primer extension reactions catalyzed by the Klenow fragment of Escherichia coli DNA polymerase I. The primer could be extended past the dG-(-)-trans-BPDE adduct with small amounts of dAMP incorporated opposite the lesion. A small amount of base deletions was also observed while, with the dG-(-)-cis-BPDE adduct, one- and two-base deletions predominated. When templates containing dG-(+)-trans-BPDE were used, small amounts of products containing one-base deletions were observed; with dG-(+)-cis-BPDE, substitution of dAMP opposite the lesion was also detected. The frequency of nucleotide insertion for dAMP opposite dG-(-)-trans-BPDE and the frequency of extension from the primer terminus containing the dA:dG-(-)-trans-BPDE pair were much higher than those observed with the other, stereochemically different BPDE adducts. Kinetic studies were in agreement with the results of the primer extension study. When the base flanking the 5' side of dG-BPDE was changed from dC to dT, the frequency of one-base deletions increased. We conclude that the trans- or cis-addition product of dG-(-)-anti-BPDE has a higher miscoding potential than dG-(+)-anti-BPDE in our model system and that G-->T transversions and deletions predominate. These observations are consistent with the types of mutations observed in vivo.

Differences in unwinding of supercoiled DNA induced by the two enantiomers of anti-benzo[a]pyrene diol epoxide

The unwinding of supercoiled phi X174 RFI DNA induced by the tumorigenic (+) and non-tumorigenic (-) enantiomers of trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) has been investigated by agarose slab-gel and ethidium titration tube gel electrophoresis. The differences in adduct conformations were verified by flow linear dichroism techniques. Both enantiomers cause a reversible unwinding by the formation of noncovalent intercalative complexes. The effects of covalently bound BPDE residues on the electrophoretic mobilities of the RF I DNA form in agarose gels were investigated in detail in the range of binding ratios rb approximately 0.0-0.06 (covalently bound BPDE residues/nucleotide). In this range of rb values, there is a striking difference in the mobilities of (+)-BPDE- and (-)-BPDE-adducted phi X174 DNA in agarose slab-gels, the covalently bound (+)-BPDE residues causing a significantly greater retardation than (-)-BPDE residues. Increasing the level of covalent adducts beyond rb approximately 0.06 in the case of the (+)-BPDE enantiomer, leads to further unwinding and a minimum in the mobilities (corresponding to comigration of the nicked form and the covalently closed relaxed modified form) at rb 0.10 +/- 0.01; at still higher rb values, rewinding of the modified DNA in the opposite sense is observed. From the minimum in the mobility, a mean unwinding angle (per BPDE residue) of theta = 12 +/- 1.5 degrees is determined, which is in good agreement the value of theta = 11 +/- 1.8 degrees obtained by the tube gel titration method. Using this latter method, values of theta = 6.8 +/- 1.7 degrees for (-)-BPDE-phi X174 adducts are observed. It is concluded that agarose slab gel techniques are not suitable for determining unwinding angles for (-)-BPDE-modified phi X174 DNA because the alterations in the tertiary structures for rb < 0.06 are too small to cause sufficiently large changes in the electrophoretic mobilities. The major trans (+)-BPDE-N2-guanosine covalent adduct is situated at external binding sites and the mechanisms of unwinding are therefore different from those relevant to noncovalent intercalative BPDE-DNA complexes or to classical intercalating drug molecules; a flexible hinge joint and a widening of the minor groove at the site of the lesion may account for the observed unwinding effects. The more heterogeneous (-)-BPDE-nucleoside adducts (involving cis and trans N2-guanosine, and adenosine adducts) are less effective in causing unwinding of supercoiled DNA for reasons which remain to be elucidated.

Metabolic activation of carcinogens

Most chemical carcinogens are not active in themselves but require bioactivation to electrophiles that bind covalently to DNA and often act by producing mutations. In recent years it has been realized that mutations can be important at many stages of carcinogenesis. A variety of different enzymes are involved in bioactivation reactions, which include oxidation, reduction, thiol conjugation, acetyl transfer, sulfur transfer, methyl transfer, glucuronosyl transfer, and epoxide hydrolysis. These processes often occur in concert with a single carcinogen. Humans vary considerably in activities of these enzymes and this variation may contribute to differences in risk.

Excimer fluorescence of (+)-anti-benzo (a)pyrene diol epoxide covalently bound to poly(dG-dC): structural implications

Fluorescence of (+)-anti-benzo(a)pyrene diol epoxide [(+)-anti-BPDE] covalently bound to poly(dG-dC) has been studied with steady-state and time-resolved techniques. Extensive formation of excimers is found, even at small (0.008) BPDE/nucleotide ratios. This indicates favored covalent binding to bases close to already modified guanines. Both fluorescence excitation spectra and lifetime measurements reveal two populations of (+)-anti-BPDE adducts: one that can form excimers and one that cannot. Three excimer lifetimes (4.5, 29, and 83 ns) are observed. Differently shifted monomer and excimer excitation spectra are discussed in terms of pyrene-pyrene exciton interactions, consistent with a distance shorter than 7 A between the excimer-forming BPDE chromophores.

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.

Orientation of DNA in agarose gels

An orientation of the lambda DNA during the electrophoresis in agarose gels was measured by a microscopic linear dichroism technique. The method involved staining the DNA with the dye ethidium bromide and measuring under the microscope the polarization properties of the fluorescence field around the electrophoretic band containing the nucleic acid. It was first established that the fluorescence properties of the ethidium bromide-DNA complex were the same in agarose gel and in a solution. Then the linear dichroism method was used to measure the dichroism of the absorption dipole of EB dye bound to lambda DNA. In a typical experiment the orientation of two-tenth of a picogram (2 x 10(-13)g) of DNA was measured. When the electric field was turned on, the dichroism developed rapidly and assumed a steady state value which increased with the strength of the field and with the size of DNA. A linear dichroism equation related the measured dichroism of fluorescence to the mean orientation of the absorption dipole of ethidium bromide and to an extent to which the orientation of this dipole deviated from the mean. The observed development of dichroism in the presence of an electric field was interpreted as an alignment of DNA along the direction of the field. The increase in the steady state value of dichroism with the rise in the strength of the field and with the increase of the size of DNA was interpreted as a better alignment of DNA along the direction of the field and as a smaller deviation from its mean orientation.

Interaction of benz[a]pyrene diol epoxide with chromatin studied by flow linear dichroism

Chromatin isolated from Ehrlich ascites cells was incubated with the tumourigenic compound (+)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10- tetrahydrobenz[a]pyrene [(+)-anti-BPDE] at low ionic strength and the modified chromatin was analysed using flow linear dichroism (LD). The results confirm that (+)-anti-BPDE preferentially binds to the DNA in the linker regions, and furthermore show that the long axis of the bound pyrenyl chromophore is oriented parallel or close to parallel to the average orientation of the chromatin fiber axis. The data indicate that the binding geometry of (+)-anti-BPDE in chromatin is similar to that in pure DNA and deoxyguanosine-containing double-helical oligonucleotides.

The orientation of DNA fragments in the agarose gels

A microscopic method of measuring the orientation of nucleic acids in the agarose gels is described. A nucleic acid undergoing electrophoresis is stained with the dye ethidium bromide and is viewed under high magnification with a polarization microscope. A high-numerical-aperture microscope objective is used to illuminate and to collect the fluorescence signal, and therefore the orientation of the minute quantities of nucleic-acid can be measured: in a typical experiment we can detect the orientation of one-tenth of a picogram (10(13)g) of DNA. Polarization properties of the fluorescent light emitted by the separate bands corresponding to different molecular weights of the DNA are examined. A linear dichroism equation relates the measured fluorescence to the mean orientation of the absorption dipole of the ethidium bromide (and therefore DNA) and to the extent to which it is disorganized. As an example, we measured the orientation of phi X174 DNA RF/HaeIII fragments undergoing electrophoresis in a field of 10 V/cm. Ethidium bromide bound to the fragments with an angle of the absorption dipole largely perpendicular to the direction of the electrophoretic current. The dichroism declined as the molecular weight of the fragments decreased which is interpreted as an increase in the degree of disorder for shorter DNA.

Differential efficiency of mutagenesis at three genetic loci in CHO cells by a benzo[a]pyrene diol epoxide

The formation of DNA adducts by the ultimate carcinogen 7r,8t-dihydroxy-9t,10t-oxy-7,8,9,10-tetrahydrobenzo[alpha]pyrene (BPDE-I) has been implicated in the process of carcinogenesis. In a line of Chinese hamster ovary (CHO) cells designated AT3-2 and in two derivative mutant lines, UVL-1 and UVL-10, originally selected for hypersensitivity to UV-irradiation, we have measured the formation of BPDE-I: DNA adducts and the production of biological damage. The quantity and quality of BPDE-I: DNA adducts formed initially in the 3 cell lines are identical over a wide range of BPDE-I doses. However, the UVL lines are unable to remove adducts from their DNA, while the AT3-2 cells remove about 50% of the BPDE-I: DNA adducts in a 24-h incubation. Correlated with this, the UVL lines are more sensitive to the lethal effects of BPDE-I than are the AT3-2 cells. Mutant frequencies were measured at the aprt, hprt and oua loci and were found to increase linearly with BPDE-I: DNA adduct formation at doses which gave greater than 50% survival. At the hprt and oua loci, the efficiency of mutation induction was similar for AT3-2 and UVL-10 cells. UVL-1 cells showed slightly higher (within a factor of 2-3) mutant frequencies in response to BPDE-I compared to AT3-2 at these two loci. However, at the aprt locus the repair-deficient cells were much more highly mutable (9-15-fold) than the repair-proficient AT3-2 cells. Based on the measured average level of adduct formation, it is calculated that 15% of the BPDE-I: DNA adducts in the aprt gene are converted into mutations. However, the possibility exists that the aprt locus is subject to higher levels of modification by BPDE-I than is the bulk DNA, which would lead to an artifactually high apparent conversion frequency.

Reactions of stereoisomeric and structurally related bay region diol epoxide derivatives of benz[a]anthracene with DNA. Conformations of noncovalent complexes and covalent carcinogen-DNA adducts

The modes of reaction of the tumorigenic bay region diol epoxide anti-BADE [+/-)-trans-3,4-diol-anti-1,2-epoxy-1,2,3,4-tetrahydrobenz[a]anthr acene) and the less potent tumor initiating diastereomer syn-BADE [+/-)-trans-3,4-diol-syn-1,2-epoxy-1,2,3,4-tetrahydrobenz[a]anthra cene) with native, double-stranded DNA were compared. The bay-region diol epoxide derived from 3-methylcholanthrene (3-MCDE, racemic trans-9,10-diol-anti-7,8-epoxy-7,8,9,10-tetrahydromethylcholanthrene+ ++) was included in this study in order to assess the effects of the methyl and methylene substituents on the reactivity with DNA. Utilizing linear dichroism and other spectroscopic methods, it is shown that all three diol epoxides forn non-covalent complexes with DNA. The diastereomers anti-BADE and syn-BADE form intercalative physical complexes, but the association constant K of the syn-diastereomer is about 6-7 times smaller than for anti-BADE; this effect is ascribed to the bulky quasi-diaxial conformation of the diol epoxide ring in the syn diastereomer. The value of K (4000 M-1) is similar for anti-BADE and 3-MCDE, although the latter is not intercalated in the classical sense since the short axis of the molecule is tilted closer to the axis of the DNA double helix. The conformations of the covalent DNA adducts are interpreted in terms of a quasi-intercalative conformation (site I), and a conformation in which the long axes of the polycyclic molecules are tilted closer to the axis of the helix (site II). Both tumorigens, anti-BADE and 3-MCDE, undergo a marked re-orientation from a non-covalent site I to a covalent site II conformation upon binding chemically with the DNA bases, although a small fraction of the covalent anti-BADE adducts remains quasi-intercalated; in contrast, the alkyl substituents in 3-MCDE not only prevent the formation of intercalative physical complexes, but also the formation of site I covalent adducts. In the case of the less tumorigenic syn-BADE, both the non-covalent complexes and the covalent adducts are of the site I-type. The bay-region diol epoxide of benz[a]anthracene and of 3-methylcholanthrene display a similar pattern of reactivities and covalent adduct conformations as the bay region diol epoxide derivatives of benz[a]pyrene, suggesting that adduct conformation might be an important factor in determining the levels of mutagenic and tumorigenic activities of this class of compounds.

Effect of duration of exposure to benzo(a)pyrene diol-epoxide on neoplastic transformation, mutagenesis, cytotoxicity, and total covalent binding to DNA of rodent cells

We examined the effect of different durations of exposure (20 sec to 24 hr) to (+/-) 7-beta,8 alpha-dihydroxy-9 alpha, 10 alpha -epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene (BPDE I) on the induction of transformation in C3H/10T 1/2 cells and of mutations to 6-thioguanine resistance in Chinese hamster ovary cells (CHO), as well as on BPDE I-DNA binding in these two cell lines. A 20-sec exposure of the cells to BPDE I was sufficient to induce mutations and morphological transformation in vitro. However, the transformation frequency in CH3 mouse-embryo-derived 10T 1/2 cells increased twofold and the frequency of mutations in CHO cells sixfold when the exposure time to BPDE I was increased from 20 sec to 8 h. Cytotoxicity increased under similar conditions. A large number of BPDE I-DNA adducts were formed in both cell lines within the first 15-min of exposure of the cells to this ultimate carcinogen. The total covalent binding did not increase with longer than 15-min incubation times. These results suggest that in addition to its covalent binding to DNA, BPDE I may influence other cellular mechanism(s) that are responsible for the initiation of transformation and mutagenesis.

A-DNA accommodates adducts derived from diol epoxides of polycyclic aromatic hydrocarbons bound in a "side-stacking" mode

The minor groove of undistorted A-DNA provides a good binding site for planar, hydrophobic moieties such as unmetabolized polycyclic aromatic hydrocarbons (PAHs), and the base pairs at the ends of short oligodeoxynucleotide helices. It also accommodates the chief adduct derived from the metabolically activated form of the carcinogen benzo[a]pyrene. B-DNA lacks such a site. Computerized models have been generated for the major (N2-guanine-linked) adducts formed at this site by both + and - enantiomers of anti-benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide (anti-BPDE) with poly(dG).poly(dC) in the A-DNA conformation. The BPDE adducts lie in the shallow, relatively hydrophobic minor groove of the A-DNA after empirical potential energy minimization using the program AMBER. We term this binding mode "side-stacking." The side-stacked + anti-BPDE may constitute the chief carcinogenic lesion derived from benzo[a]pyrene.

Conformations and selective photodissociation of heterogeneous benzo(a)pyrene diol epoxide enantiomer-DNA adducts

The covalent binding of the tumorigenic (+) enantiomer and the nontumorigenic (-) enantiomer of trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,19-tetrahydrobenzo(a)pyrene (BPDE) to double-stranded native DNA gives rise to heterogeneous adducts, especially in the case of (-)-BPDE. The covalent (+)-BPDE-DNA adducts are predominantly of the external site II type, while the (-)-BPDE-DNA adducts are predominantly of the quasi-intercalative, site I type (65%), with 35% of site II adducts. The site I adducts can be selectively photodissociated with near-ultraviolet light (quantum yields in the range 0.0003-0.005); the external site II adducts (photodissociation quantum yield 3 X 10(-5) are 10-100-times more stable. The photolability of covalent (-)-BPDE-DNA adducts accounts for the discrepancies in the linear dichroism properties of these complexes reported previously. Fluorescence quenching data, previously utilized to assess the degree of solvent exposure of the pyrenyl residues in covalent adducts, were in some cases significantly influenced by the presence of highly fluorescent tetraol dissociation products. After correcting for this effect, it is shown that the fluorescence of the external site II (+)-BPDE-DNA adducts is sensitive to acrylamide, while the fluorescence of the dominant site I (-)-BPDE-DNA adducts is not affected by this fluorescence quencher, as expected for adducts with considerable carcinogen-base stacking interactions.

Kinetic flow dichroism study of conformational changes in supercoiled DNA induced by ethidium bromide and noncovalent and covalent binding of benz[a]pyrene diol epoxide

The dynamic conformational changes due to the noncovalent intercalative binding of ethidium bromide and racemic trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE), and the covalent binding of BPDE to supercoiled phi X174 DNA, have been studied by gel electrophoresis and a novel application of a kinetic flow linear dichroism technique. The magnitude of the linear dichroism (delta A) of the DNA oriented in the flow gradient is sensitive to the hydrodynamic shape of the DNA molecule which is affected by the binding of the drug or the carcinogen BPDE. While the linear dichroism of ethidium bromide supercoiled DNA is time independent, the delta A spectra of BPDE-DNA reaction mixtures vary on time scales of minutes, which correspond to the reaction rate constant of BPDE to form 7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene hydrolysis products and covalent DNA adducts. The rapid noncovalent intercalation of BPDE causes an initial large increase in delta A (up to 250%, corresponding to the dichroism observed with relaxed circular DNA), followed by a slower decrease in the linear dichroism signal. This decrease in delta A is attributed to the removal of intercalated diol epoxide molecules and the resulting reversible increase in the number of superhelical turns. The kinetic flow dichroism spectra indicate that the noncovalent BPDE-DNA complexes are intercalative in nature, while the covalent adducts are characterized by a very different conformation in which the long axes of the pyrenyl residues are oriented at a large angle with respect to the average orientation of the planes of the DNA bases.(ABSTRACT TRUNCATED AT 250 WORDS)

The dynamics of chromatin carcinogen interactions in the human cell

Human lung epithelioid cells were treated with Benzo (a) pyrene diol epoxide (anti) in order to establish the binding and removal of covalent adducts in chromosomal components. Isolating two different classes of mononucleosomes, it was found that their DNA contained different concentrations of B(a)PDE-DNA adducts, while in both these mononucleosomal preparations only histones H2A and H3 contained detectable amounts of the carcinogen. Further analysis showed that in the intact human cell the carcinogen-DNA adduct distribution is constantly changing as a function of differential accessibility and repair. These results emphasize the dynamics of chromatin-carcinogen modifications.

Conformations of complexes derived from the interactions of two stereoisomeric bay-region 5-methylchrysene diol epoxides with DNA

The reaction mechanisms of two isomeric bay-region diol epoxides of 5-methylchrysene (trans-1,2-dihydroxy-anti-3,4-epoxy-1,2,3,4-tetrahydro-5-methylchrysene (DE-I) and trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydro-5-methylchrysene (DE-II) with double-stranded DNA in aqueous solutions were studied utilizing kinetic flow dichroism and fluorescence techniques. As in the case of the previously studied benzo(a)pyrene-7,8-diol-9,10-oxide isomers (BaPDE), both DE-I and DE-II rapidly form intercalation-type complexes (association constants K = 2700 and 1500 M-1 respectively in a neutral 5mM phosphate solution). The physically bound diol epoxide molecules react on time scales of minutes to form predominantly tetraols; a greater fraction (6 +/- 1%) of DE-I than of DE-II (2-3%) molecules react with the DNA to form covalent products. The DE-II isomer is characterized by a greater reactivity than DE-I, and the rates of reaction are markedly accelerated in the presence of DNA in both cases. The linear dichroism spectra of the covalent adducts reveal that the conformations of both types of adducts are similar, with the long axes of the phenanthrenyl chromophores tilted, on the average, at angles of 38-52 degrees with respect to the average orientations of the transition moments (at 260 nm) of the DNA bases. The conformations of the covalently bound DE-I and DE-II molecules resemble those observed in the case of the highly tumorigenic (+) enantiomer of anti-BaPDE.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Reactivity and binding of benzo(a)pyrene diol epoxide to poly(dG-dC).(dG-dC) and poly(dG-m5dC).(dG-m5dC) in the B and Z forms

The physical and covalent binding of the carcinogen benzo(a)pyrene-7,8-diol-9,10-oxide (BaPDE) to poly(dG-dC).(dG-dC) and poly(dG-m5dC).(dG-m5dC) in the B and Z forms were studied utilizing absorbance, fluorescence and linear dichroism techniques. In the case of poly(dG-dC).(dG-dC) the decrease in the covalent binding of BaPDE with increasing NaCl concentration (0.1-4 M) as the B form is transformed to the Z form is attributed to the effects of high ionic strengths on the reactivity and physical binding of BaPDE to the polynucleotides; these effects tend to obscure differences in reactivities with the B and Z forms of the nucleic acids. In the case of poly(dG-m5dC).(dG-m5dC) the B-to-Z transition is induced at low ionic strength (2 mM NaCl + 10 microM Co(NH3)6Cl3) and the covalent binding is found to be 2-3-times lower to the Z form than to the B form. Physical binding of BaPDE by intercalation, which precedes the covalent binding reaction, is significantly lower in the Z form than in the B form, thus accounting, in part, for the lower covalent binding. The linear dichroism characteristics of BaPDE covalently bound to the Z and B forms of poly(dG-m5dC).(dG-m5dC) are consistent with nonintercalative, probably external conformations of the aromatic pyrenyl residues.