DNA Adduct Formation by Polycyclic Aromatic Hydrocarbon

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JANUARY 1998 VOLUME 11, NUMBER 1 © Copyright 1998 by the American Chemical Society

Perspective DNA Adduct Formation by Polycyclic Aromatic Hydrocarbon Dihydrodiol Epoxides Jan Szeliga and Anthony Dipple* Chemistry of Carcinogenesis Laboratory, ABL-Basic Research Program, NCI-Frederick Cancer Research and Development Center, Frederick, Maryland 21702 Received August 14, 1997

Introduction Classically, active metabolites of chemical carcinogens were identified by isolating various metabolites produced in experimental animals and determining which of these exhibited carcinogenic activity comparable to, or greater than, that of the parent compound (1). This approach was not successful for the polycyclic aromatic hydrocarbon carcinogens, however, because most metabolites examined with this approach were found to be much weaker carcinogens than the parent hydrocarbon from which they were derived (2). Metabolic activation of polycyclic aromatic hydrocarbons was clarified, however, by initially identifying metabolites that were responsible for covalent binding to DNA in cells or tissues [these were found to be vicinal dihydrodiol epoxides in which the epoxide is in a bay or fjord region (Figure 1) (3-5)] and, only thereafter, demonstrating that these metabolites exhibited high tumorigenic activities (6, 7). It has been shown that this dihydrodiol epoxide route of activation applies to a fairly wide range of hydrocarbon structures (reviewed in refs 8-10), but other mechanisms of metabolic activation have also been investigated. For example, it has been suggested that the active metabolites of some polycyclic aromatic hydrocarbons may be radical cations that induce mutation through the intermediacy of apurinic sites in DNA, rather than through the DNA adducts themselves (11, 12). The proposed radical cations are not stable enough to be isolated, and therefore, there is no direct evidence that these inter-

Figure 1. Generalized structures of the four configurational isomers of bay region or fjord region dihydrodiol epoxides. In this Perspective, the benzylic carbon of the diol function is numbered ‘a’ and the other carbons of the dihydrodiol epoxide function are numbered alphabetically from there. Each configurationally isomeric dihydrodiol epoxide is specified by the sequence of four configurational assignments beginning at the benzylic carbon of the dihydrodiol function, to maintain the sequence used in the nomenclature for the extensively studied benzo[a]pyrene derivatives. When R ) H, the epoxide function is in a bay region, when R ) CH3, the epoxide is in a sterically hindered bay region, and when another ring is present (dashed outline), the epoxide is in a fjord region. Where the benzylic hydroxyl and the epoxide function are trans (structures i and ii) or cis (structures iii and iv), the structures are referred to as anti or syn, respectively.

mediates are carcinogenic. Moreover, the presence of radical cation adducts in DNA from cells or tissues cannot

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be demonstrated because they are unstable and are lost from DNA before detection procedures can be completed. However, it has been reported that substituted purines that have been lost from DNA (by glycosidic bond cleavage) can be found in mouse skin treated with hydrocarbons (13, 14). Since the dihydrodiol epoxides have been shown to be potent carcinogens (6, 15, 16) and to be responsible for adduct formation in DNA in target tissues for carcinogenesis (17-20), this summary of polycyclic aromatic hydrocarbon carcinogen reactions with DNA focuses on the reactions of the configurational isomers of various dihydrodiol epoxides with DNA and its constituents. Overall, the majority of adduct formation has been found to result from covalent linkage of the benzylic carbon of the epoxide, i.e., Cd in Figure 1, with the amino groups of deoxyadenosine and deoxyguanosine residues in the DNA (8, 21), although minor products with the amino group of deoxycytidine and the 7-position of deoxyguanosine have been characterized in some cases (22, 23). The reaction at the amino groups of DNA bases is quite different from the reactions of alkylating agents with DNA, which primarily occur at the ring nitrogen atoms of adenine and guanine residues (24), but follows the chemistry first described for the aralkylating 7-(bromomethyl)benz[a]anthracenes (25-27). It has been shown that four configurational isomers (Figure 1) of some polycyclic aromatic hydrocarbon dihydrodiol epoxides can be formed in microsomal systems in vitro (10), so it is conceivable that all four isomers can also be formed in cellular systems and in animals in vivo. However, in studies where DNA adducts have been measured in cellular systems, adducts from only two of the four configurational isomers, i.e., those with (cS,dR)epoxides (i and iv, Figure 1), predominate (19, 28-34). In several cases, carcinogenicity studies have been conducted with all four synthetic dihydrodiol epoxide configurational isomers, and the reactions of each of these isomers with DNA are of interest in relation to these activities. In the following discussion, an attempt is made to summarize and analyze the findings reported for the chemical reactions of dihydrodiol epoxides from different polycyclic aromatic hydrocarbons with DNA in vitro and to extract, as far as possible, any generalizations that can be deduced from these findings about the chemistry of adduct formation. This Perspective does not examine adduct formation in vivo, the relationship between such adducts and carcinogenicity, nor the conformations of hydrocarbon dihydrodiol epoxide-deoxyribonucleoside adducts in the DNA structure. A thorough review of the latter was published very recently (35).

Characterization of Hydrocarbon Dihydrodiol Epoxide-DNA Adduct Structures Hydrocarbon-deoxyribonucleoside adducts derived from DNA are usually only available in minute quantities, but structural information, obtained from proton NMR and mass spectrometry along with ultraviolet absorbance and circular dichroism studies, has been sufficient to assign structures, especially when spectroscopic data for the corresponding tetrols were also available. Characterizations of tetrol products of hydrolysis of dihydrodiol epoxides have been very rigorous in some cases and involved extensive structural studies that included X-ray

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crystallography and structure-determining routes of synthesis. For example, it is clear that each racemic diastereomeric 7,8-dihydrodiol 9,10-epoxide of benzo[a]pyrene [i.e., the syn and anti diastereomers in which the benzylic hydroxyl and the epoxide ring are cis (corresponding to structures iii and iv in Figure 1) or trans (corresponding to structures i and ii in Figure 1), respectively] can give rise, through hydrolysis, to two racemic isomeric tetrols in which the epoxide ring has been opened either cis or trans by the introduction of a hydroxyl function at the 10-position (corresponding to Cd in Figure 1) (36). This was demonstrated through the structure-determining synthesis of the trans tetrol that would be derived from the syn-dihydrodiol epoxide and of the cis tetrol that would be derived from the anti-dihydrodiol epoxide, by X-ray crystallography of the trans tetrol from the antidihydrodiol epoxide (37), and by studies of the NMR properties of all four tetrols. (Although reaction of the four structures in Figure 1 with a chiral nucleoside gives rise to four pairs of diastereomeric adducts resulting from cis and trans opening of the epoxide in each case, hydrolysis results in four pairs of enantiomers, which, unlike diastereomers, are not separable in an achiral environment.) In all four cases, the Cd-OH substituent in the bay region is in a pseudo-axial conformation to avoid steric hindrance from the proton on the 11-position, and for three of these structures, the 7,8-dihydrodiol function is pseudo-diequatorial (deduced from a fairly large coupling for the two protons on C7 and C8, corresponding to Ca and Cb in Figure 1). Only in the case of the tetrol arising from cis opening of the anti-dihydrodiol epoxide are these hydroxyls pseudo-diaxial (36). For tetrols derived from a fjord region hydrocarbon, benzo[c]phenanthrene, the hydroxyl groups of the original dihydrodiol function are pseudo-diequatorial in all four cases (38) so that the major conformational change seen for tetrols (and for deoxyribonucleoside adducts) from bay region dihydrodiol epoxides and fjord region dihydrodiol epoxides is that the cis products of the anti diastereomers have distinctly different conformations. One problem in visualizing the conformations of these tetrols (and corresponding deoxyribonucleoside adducts) is that, because there is one aromatic double bond remaining in the ring, protons and substituents are neither truly axial nor truly equatorial. In the cases of hydrocarbons that contain fjord regions or methyl substituents in the bay region, conformations are even more distorted from the conventional axial and equatorial positions and the distinction between pseudo-axial and pseudo-equatorial can become somewhat blurred. Structural assignments of cis versus trans opening of the epoxide function in either tetrols or adducts can be readily made from NMR coupling constants for products from both syn- and anti-dihydrodiol epoxides of hydrocarbons with unsubstituted bay regions and for syndihydrodiol epoxides of methyl-substituted bay region or fjord region (i.e., sterically crowded) hydrocarbons. However, as seen in Table 1, for anti-dihydrodiol epoxide adducts of the sterically crowded hydrocarbons, cis and trans assignments are not obvious from coupling constant data. Consequently the empirical relationship, in which the chemical shift of the proton at the benzylic carbon of the original epoxide is further downfield in cis than trans adducts, has been used to make trans and cis assign-

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Table 1. Coupling Constants for Tetrols Derived from a Bay Region Hydrocarbon [Benz[a]anthracene (23)] and a Sterically Hindered Fjord Region Hydrocarbon [Benzo[c]phenanthrene (38)] tetrola cis-syn trans-syn cis-anti trans-anti

benz[a]anthraceneb

benzo[c]phenanthreneb

Ja,b ) 7.5, Jb,c ) 12.5, Jc,d ) 3.5 Ja,b ) 7.15, Jb,c ) 7.15, Jc,d ) 4.0 Ja,b ) 3.2, Jb,c ) 2.8, Jc,d ) 4.6 Ja,b ) 8.5, Jb,c ) 2.5, Jc,d ) 3.5

Ja,b ) 5.4, Jb,c ) 10.5, Jc,d ) 2.4 Ja,b ) 8.1, Jb,c ) 3.2, Jc,d ) 3.8 Ja,b ) 8.6, Jb,c ) 2.3, Jc,d ) 3.9 Ja,b ) 8.4, Jb,c ) 2.6, Jc,d ) 4.1

a Tetrols are described by the epoxide ring opening, i.e., cis or trans, and by the diastereomeric dihydrodiol epoxide from which they were derived, i.e., syn or anti. b J values are given in hertz (Hz).

ments in anti-dihydrodiol epoxide-deoxyribonucleoside adducts from such hindered structures. The observation that the major products from anti-dihydrodiol epoxides are always trans (see later) has also been useful in making these assignments. Overall, the structures of tetrols have been more rigorously defined than the structures of deoxyribonucleoside adducts, but the relationship between spectral data for tetrols and adducts has been an important feature of adduct structural assignments. Circular Dichroism (CD) Spectra for Hydrocarbon-Deoxyribonucleoside Adducts. Along with other spectroscopic techniques, CD has proved useful in the elucidation of polycyclic aromatic hydrocarbon-deoxyribonucleoside adduct structures (39). CD spectra arise from the differential absorption of right and left circularly polarized light and therefore are of value in investigations of the absolute configurations of adducts. Thus, reaction of a racemic anti-dihydrodiol epoxide with deoxyadenylic acid, for example, will give rise to four principal products as a result of cis and trans opening of the epoxide ring of each dihydrodiol epoxide enantiomer by the amino group of the purine. In examining the CD spectra of such adducts, it is found that the four spectra consist of two pairs of spectra where each pair contains spectra that are the mirror images of one another, i.e., the cis adduct from one enantiomeric dihydrodiol epoxide has a spectrum that is the mirror image of the cis adduct from the other enantiomer. The same situation is observed for the trans isomers. The decision as to which pair of adducts is cis and which is trans is usually made by reference to NMR spectral data and/or by consideration of other empirical relationships. The CD spectra for deoxyguanosine and deoxyadenosine adducts of various dihydrodiol epoxide isomers derived from several polycyclic aromatic hydrocarbons can be found in the literature. These data, for the adducts with S-configuration at the site of attachment of the hydrocarbon to the nucleoside, are summarized in Figure 2. The CD spectra for deoxyguanosine adducts are summarized in panels a-d of this figure. Some of these spectra are relatively simple. For example, the spectra for adducts derived from both syn- and anti-dihydrodiol epoxides of benzo[c]phenanthrene, benzo[g]chrysene, and benz[a]anthracene as well as those from the anti-dihydrodiol epoxides of 7-methyl- and 7,12-dimethylbenz[a]anthracene are similar in shape and are summarized in panel a of Figure 2. In this figure, spectra of similar shapes are collected together and limits for the wave-

Figure 2. Summary of CD data for hydrocarbon dihydrodiol epoxide-deoxyguanosine (panels a-d) and -deoxyadenosine (panels e-h) adducts derived from the syn- and anti-dihydrodiol epoxides of BcP (38), BgC (56, 57), and BA (23) and the antidihydrodiol epoxides of 7MBA and DMBA (54) (panels a and e), from the syn- and anti-dihydrodiol epoxides of DBA (22) (panels b and f), from the anti-dihydrodiol epoxide of BaP (40) (panels c and g), and from the syn- and anti-dihydrodiol epoxides of 5MC (59, 60) and 56DMC (44) (panels d and h). In this legend and in other figures, dihydrodiol epoxides are identified as follows: 7MBA, 7-methylbenz[a]anthracene-3,4-dihydrodiol 1,2epoxide; BgC, benzo[g]chrysene-11,12-dihydrodiol 13,14-epoxide; 56DMC, 5,6-dimethylchrysene-1,2-dihydrodiol 3,4-epoxide; 5MC, 5-methylchrysene-1,2-dihydrodiol 3,4-epoxide; DBA, dibenz[a,j]anthracene-3,4-dihydrodiol 1,2-epoxide; BaP, benzo[a]pyrene7,8-dihydrodiol 9,10-epoxide; BA, benz[a]anthracene-3,4-dihydrodiol 1,2-epoxide; BcP, benzo[c]phenanthrene-3,4-dihydrodiol 1,2-epoxide.

lengths where the spectra exhibit minima, maxima, or cross the x-axis are indicated by horizontal bars. Similarly, the ranges of ellipticities exhibited by the collected spectra are indicated by vertical bars. The spectrum shown is a hypothetical spectrum showing the general shape for the group of spectra summarized and is designed to pass through the means of the wavelengths and ellipticities representative of these spectra. The CD spectra for both syn- and anti-dibenz[a,j]anthracene dihydrodiol epoxide adducts (panel b) are very similar in shape to those shown in panel a, but the wavelengths of the maxima and minima are shifted almost 20 nm toward longer wavelength. The CD spectrum for anti-benzo[a]pyrene dihydrodiol epoxide adducts (panel c) is more complex as are the spectra for the 5-methyl- and 5,6-dimethylchrysenes (panel d). Despite variations in the shape and intensity of these spectra, it is clear that all of these deoxyguanosine adducts with S-configuration at the site of nucleoside attachment to the hydrocarbon residue exhibit a positive CD signal for their most intense CD band. However, the wavelength at which the most intense signal is seen

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varies substantially from adduct to adduct (ranging from 248 to 280 nm). Therefore, it is difficult to define a specific wavelength region in which a positive CD signal would be seen for all adducts with S-configuration. The CD spectra for deoxyadenosine adducts (panels e-h) are clearly different from those for deoxyguanosine adducts. However, the hydrocarbon dihydrodiol epoxides that gave the simplest spectra for the deoxyguanosine adducts (panel a) are those that give the simplest spectra for deoxyadenosine adducts (panel e). Similarly, the spectra for deoxyadenosine adducts from other hydrocarbons fall into the same groups as did the spectra for deoxyguanosine adducts. However, the wavelengths at which signals are maximal differ greatly for the two purine nucleoside adducts; e.g., whereas the maximum is around 260 nm in panel d, there are two maxima at 225 and 276 nm in panel h. All data presented in Figure 2 are consistent with previous empirical correlations, based on more limited databases, that negative major CD bands are associated with R-absolute stereochemistry (or positive bands with S-absolute stereochemistry) at the benzylic carbon through which the purine nucleoside is attached (23, 38, 40). However, particularly in the case of deoxyadenosine adducts, where both intense positive and intense negative signals are often found (Figure 2e,f), it is not always clear which band should be used in making assignments. Configuration cannot be reliably determined, therefore, on the basis of CD spectra alone. However, it is possible that by comparison of novel CD spectra to the templates described in Figure 2 some indication of configuration might be developed. NMR Spectra for Hydrocarbon-Deoxyribonucleoside Adducts. In the literature, there is a fairly extensive collection of NMR data for hydrocarbon dihydrodiol epoxide-deoxyribonucleoside adducts. Some of these studies report spectral data for the nucleoside adducts in deuterated methanol (41-43), but mostly, NMR data have been collected on peracetylated deoxyribonucleoside adducts in deuterated acetone. From these latter studies, the chemical shift and coupling constant data for the protons in the partially saturated hydrocarbon ring have been collected together (a tabulation is available upon request). To summarize some of the general features that emerge, the ranges of chemical shifts and coupling constants exhibited by the hydrogens in the partially saturated hydrocarbon ring in the peracetylated adducts from various hydrocarbons have been collected in Figure 3. With only a few exceptions, the NMR data for adduct diastereomers are very similar to one another, and only one set of representative data is given in the figure. To simplify the figure, the ranges of values observed, rather than individual values, are presented. The range seen is influenced by the number of compounds studied since data from only one or two compounds would cover a much narrower range than data from five compounds. For the chemical shift data (upper part of Figure 3), all the available data are summarized together in each case. However, the coupling constant data (lower part of Figure 3) seemed to fall into two separate groups in many cases. Notably, coupling constants for derivatives of hydrocarbons that have methyl-substituted bay regions or fjord regions are often substantially different from those for derivatives of hydrocarbons that are unsubstituted in the bay region. Where notable differences of this nature

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occur, two separate ranges of coupling constants have been indicated, and these are labeled ‘s’ for substituted and ‘u’ for unsubstituted, respectively. Examination of Figure 3 reveals that the NMR spectral properties for deoxyguanosine and deoxyadenosine adducts are remarkably similar overall. There are some differences, however. For example, the chemical shift of the proton on Cd, i.e., the proton at the site of attachment of the purine nucleoside, tends to be further downfield in deoxyadenosine adducts than in deoxyguanosine adducts. If the individual adduct data are examined, Cd-H in deoxyadenosine adducts is further downfield than that in the corresponding deoxyguanosine adducts in both cis and trans adducts from anti-dihydrodiol epoxides and in trans adducts from syn-dihydrodiol epoxides, whereas it can be upfield or downfield for cis-opened adducts of syndihydrodiol epoxides. The Ca-H in deoxyadenosine adducts is usually downfield of its deoxyguanosine counterpart for both trans and cis adducts from the syndihydrodiol epoxide, whereas Cb-H is downfield in both cis- and trans-deoxyadenosine adducts from the antidihydrodiol epoxide. Despite these differences, the overall similarities of the structures of the two purine nucleoside adducts are remarkable. Some important chemical shift differences are seen when adducts that arise from cis opening of the epoxide ring are compared with those arising from trans opening. Thus, for all of the acetylated adducts studied so far, Cd-H is further downfield in the cis adduct than in the corresponding trans adduct. This applies to both deoxyguanosine and deoxyadenosine adducts from either synor anti-dihydrodiol epoxides, but in some cases, notably deoxyadenosine adducts from the anti-dihydrodiol epoxide of 5,6-dimethylchrysene (44), the difference in shift between cis and trans adducts is quite small. Additionally, the Cc-H is usually more upfield and Cb-H more downfield in cis versus trans adducts from the syndihydrodiol epoxides. In comparisons of corresponding syn- with anti-dihydrodiol epoxide adducts, chemical shifts are similar in many cases and no obviously consistent differences are apparent. The coupling constants in Figure 3 are also similar for deoxyguanosine and deoxyadenosine adducts, but as noted above, they do indicate that different conformations are preferred for an adduct from a bay region or fjord region substituted hydrocarbon compared with the corresponding adduct from an unsubstituted hydrocarbon. Thus, the values for Ja,b, i.e., the coupling of the protons of the original diol function, are higher for derivatives of substituted hydrocarbons than for unsubstituted hydrocarbons in trans-opened syn- and cis-opened anti-dihydrodiol epoxide adducts suggesting pseudo-diaxial protons in the substituted cases and pseudo-equatorial protons in the unsubstituted cases. In contrast, this situation is reversed in cis-opened syn-dihydrodiol epoxide adducts. As discussed earlier, coupling constant data are also useful in assigning cis and trans epoxide opening to the various adducts. For syn-dihydrodiol epoxide adducts, cis adducts always exhibit a large coupling for Jb,c, whereas this coupling is much smaller in the trans adducts. For anti-dihydrodiol epoxides, coupling constants readily distinguish cis and trans adducts derived from unsubstituted hydrocarbons because Ja,b is always fairly large in the trans adducts and fairly small in the cis adducts. However, since Ja,b is large in cis adducts from anti-dihydrodiol epoxides from

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Figure 3. Summary of chemical shifts and coupling constants for protons on the partially saturated ring of peracetylated hydrocarbon dihydrodiol epoxide-deoxyribonucleoside adducts. Data for deoxyguanosine adducts are on the left, and data for deoxyadenosine adducts are on the right. The top half of the figure summarizes the range of chemical shift data reported for the protons on Cd, Cc, Cb, and Ca (see Figure 1) of the partially saturated ring, and the lower half of the figure summarizes coupling constant data. The latter is separated into data for bay region substituted (s) hydrocarbons (5MC, DMBA, 56DMC, BcP, and BgC) and unsubstituted (u) hydrocarbons (BA, 7MBA, BaP, and DBA). The hydrocarbon abbreviations are defined in the legend to Figure 2.

substituted hydrocarbons, it is quite difficult to distinguish cis from trans adducts in these cases (Figure 3 and Table 1).

Reactions of Hydrocarbon Dihydrodiol Epoxides with DNA Fraction of Dihydrodiol Epoxide Trapped by DNA in Aqueous Solution. There are numerous literature citations on the reaction of dihydrodiol epoxides with DNA, but only a limited subset of these deal with well-characterized adducts and use conditions for reaction that are fairly similar. Data on 30 dihydrodiol epoxides that are derived from reaction of ∼0.1 mg of dihydrodiol epoxide with about 1 mg of DNA are summarized in Figure 4. The extent to which the dihydrodiol epoxides react with DNA is given in terms of the fraction of dihydrodiol epoxide trapped as DNA adducts under these conditions. This fraction is calculated based on the reasonable assumption that the extinction coefficient of the adducts will not be very different from that of the dihydrodiol epoxide. Additionally, as a rough guide, the

percentages given in the figure will be numerically similar to the value for adducts per 103 nucleotides in DNA, since for a reaction of 0.1 mg of dihydrodiol epoxide with 1 mg of DNA, the number of adducts per 103 nucleotides is calculated as follows:

adducts/103 nucleotides ) % DE trapped × (MWNT/MWDE ) The molecular weight (MW) of the dihydrodiol epoxides (DE) and the molecular weight of an average nucleotide (NT) are roughly similar (∼300). There are some slight variations in the methodology used to obtain the data in different literature studies and the main difference lies in the concentration of DNA used (0.8 mg/mL in some studies, and 1.0 mg/mL in others). These variations require that minor differences in the data for one compound or another not be accorded too much significance and that the focus should be kept on the broad trends seen therein. The data for extents of reaction in Figure 4 are segregated into separate panels for individual configu-

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Figure 4. Relative extents of reaction of various hydrocarbon dihydrodiol epoxides with calf thymus DNA in vitro. These data are derived from the literature where DNA at 1 mg/mL [for 7MBA (ref 54 and unpublished data), BgC (56, 57), 56DMC (44), and 5MC (59, 60)] or 0.8 mg/mL [for DBA (22), BaP (40), BA (23), and BcP (38, 53)] was exposed to the appropriate dihydrodiol epoxide (0.1 mg/mL DNA solution) administered in 0.1 vol of organic solvent. The extent of reaction is measured as the percentage of the dihydrodiol epoxide used that was trapped as DNA adducts. Data are for dihydrodiol epoxides as defined in the legend to Figure 2. Filled columns represent data for dihydrodiol epoxides derived from polycyclic hydrocarbons expected to be substantially planar, whereas open columns represent data for dihydrodiol epoxides derived from nonplanar molecules.

rational isomers, which are identified by the configurations at each chiral carbon atom given in the sequence Ca-Cd (Figure 1). Within each panel, the data for derivatives of planar structures (filled columns) are separated from data for derivatives of nonplanar structures (open columns). [In general, nonplanar hydrocarbons are those with some kind of steric crowding in the bay region. This crowding arises because of the close proximity of hydrogen substituents when the molecule contains a fjord region or has a methyl group in the bay region. 5-Methylchrysene is planar because it accommodates the steric crowding through in-plane distortions (45).] Within each category, the data have been arranged in the order established by increasing extent of reaction of the (aR,bS)-dihydrodiol (cS,dR)-epoxide isomer with DNA. Several observations can be made based on the data in Figure 4. Overall, it seems that the R,S,S,Rconfigurational isomer is trapped more extensively by DNA than are the other isomers. Secondly, the rank order of reactivity toward DNA for dihydrodiol epoxides from different hydrocarbons is fairly similar irrespective of the isomer configuration. Thirdly, in both series derived from planar and nonplanar structures, the fraction of dihydrodiol epoxide trapped by DNA is less for derivatives of hydrocarbons with five aromatic rings (dibenz[a,j]anthracene, benzo[a]pyrene, and benzo[g]chrysene) than for derivatives of hydrocarbons with four rings. Reactions leading to both hydrolysis and covalent reaction with DNA occur through formation of a prereaction noncovalent dihydrodiol epoxide-DNA complex (46, 47). The factors which then determine the relative rates of DNA adduct formation and of hydrolysis are not clearly understood, but the ionization of the dihydrodiol epoxide is thought to be a key step in these reactions (48). With other things being equal, a larger number of aromatic rings and a planar aromatic system should favor this ionization step. Then, if ionization favors hydrolysis

more than DNA adduct formation, a reasonable possibility since water is less nucleophilic than DNA, some aspects of the data shown in Figure 4, i.e., the greater adduct formation for dihydrodiol epoxides derived from nonplanar versus planar structures and from four-ring versus five-ring compounds, can be rationalized by their lower tendency for ionization. In concert with these expectations, it is known that the (R,S,S,R)-dihydrodiol epoxide of the four-ring nonplanar benzo[c]phenanthrene is hydrolyzed some 10-fold more slowly than is the corresponding five-ring planar benzo[a]pyrene derivative (16). It is possible that the slower reaction of the benzo[c]phenanthrene derivative with water allows the more nucleophilic DNA to compete for the dihydrodiol epoxide more effectively in this case (8). This argument suggests that the reactive derivatives of nonplanar four-ring hydrocarbons should be trapped the most effectively by DNA. However, when all the data in Figure 4 are considered, the rather high level of trapping of the planar 5-methylchrysene derivative (45) is not readily explicable on this basis unless its in-plane distortions decrease its ability to stabilize developing charge. Additionally, no satisfactory explanation for the overall variation of reaction extents exhibited in Figure 4 is readily apparent, though they must presumably result from structural differences in the dihydrodiol epoxides compared. Since reaction of these dihydrodiol epoxides with DNA involves noncovalent complex formation prior to reaction, differences in such complex formation, particularly steric differences, need to be considered in order to account for the overall experimental observations. Relative Reactivities with Deoxyadenosine and Deoxyguanosine Residues in DNA. DNA modified by hydrocarbon dihydrodiol epoxides can be hydrolyzed to deoxyribonucleosides, and these adducts can be separated and quantified by HPLC and UV absorbance, respectively. Adduct formation has been found to result primarily from reaction with the amino groups of deoxyadenosine and deoxyguanosine residues in the DNA (8,

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Table 2. Product Distributions (Percentages of Total Adducts) for Configurationally Isomeric Dihydrodiol Epoxides from Several Different Polycyclic Aromatic Hydrocarbons dGuo

dAdo

cis

trans

BaP 5MC 7MBA BA DBA DMBA 56DMC BcP BgC

1.0 4.4 7.0 8.0 19.3 12.3 10.3 3.0 6.4

94.0 82.6 76.7 71.0 60.4 57.9 45.2 30.5 25.8

BaP 5MC 7MBA BA DBA 56DMC BcP BgC

72.7 42.5 47.8 62.0 48.5 20.9 5.1 5.1

10.1 17.5 21.2 16.0 21.0 7.7 5.6 15.2

cis

dGuo cis

trans

4.0 12.6 16.3 21.0 19.3 28.0 42.1 65.0 67.5

22.0 7.2 0.0 6.3 5.1 16.3 5.0 1.7 0.7

63.0 62.1 64.3 56.3 69.6 32.6 25.4 40.0 57.3

2.0 25.0 18.8 16.0 23.0 40.8 68.0 76.6

84.5 50.0 53.0 54.0 31.2 22.0 11.1 3.2

4.1 21.7 4.9 9.0 32.2 13.8 29.6 51.5

R,S,S,R 1.0 0.3 0.0 0.0 1.0 1.8 2.0 1.5 0.3

cis

trans

S,R,R,S

S,R,S,R 15.2 15.0 12.2 6.0 7.5 30.7 21.3 3.1

dAdo

trans

0.0 6.4 7.1 7.5 6.3 44.1 15.7 18.9 2.5

15.0 24.3 28.6 30.0 19.0 7.0 53.8 39.4 39.4

R,S,R,S 9.3 25.0 41.0 35.0 32.2 55.9 51.9 6.1

2.1 3.3 1.1 1.5 4.4 8.4 7.4 39.2

21). In early studies with racemic anti-benzo[a]pyrenedihydrodiol epoxide, it was clear that most of its reaction with DNA (>90%) was accounted for by reaction with deoxyguanosine residues (49). However, studies with the potent carcinogen 7,12-dimethylbenz[a]anthracene indicated that the anti-dihydrodiol epoxide formed in cells led to substantial formation of both deoxyadenosine and deoxyguanosine adducts in DNA (20, 50), indicating that the distribution of hydrocarbon dihydrodiol epoxide between the two purine nucleosides in DNA was dependent on the hydrocarbon structure. An obvious structural difference between benzo[a]pyrene and 7,12-dimethylbenz[a]anthracene is that the 12-methyl group and the hydrogen on the 1-position in the bay region of the latter compound sterically hinder one another and the molecule, therefore, has to be distorted substantially from planarity to accommodate this interaction (51, 52). It seemed possible that this planarity difference might be associated with the different reactivities toward DNA purines, and reactions with dihydrodiol epoxides from another nonplanar polycyclic aromatic hydrocarbon, benzo[c]phenanthrene, seemed to substantiate this view since the anti-dihydrodiol epoxide also exhibited substantial reactivity toward both deoxyguanosine and deoxyadenosine in DNA (28, 38, 53). A fairly extensive data base, summarized in Table 2, is now available, and plots of total deoxyguanosine and deoxyadenosine adducts for different configurational dihydrodiol epoxide isomers from several hydrocarbons as percentages of total deoxyadenosine and deoxyguanosine adducts (Figure 5) show that the ratio of deoxyadenosine:deoxyguanosine adducts in DNA is always greater for dihydrodiol epoxides derived from nonplanar hydrocarbons [7,12-dimethylbenz[a]anthracene (54), 5,6dimethylchrysene (44, 55), benzo[c]phenanthrene (53), and benzo[g]chrysene (56, 57)] than for those derived from planar hydrocarbons [benzo[a]pyrene (40, 58), 5methylchrysene (59, 60), 7-methylbenz[a]anthracene (54, 61), dibenz[a,j]anthracene (22), and benz[a]anthracene (23)]. For individual dihydrodiol epoxide configurational isomers, the rank order of preference for reaction with

either purine in DNA varies among the derivatives of planar and nonplanar hydrocarbons. For the derivatives of planar hydrocarbons, preference for reaction with deoxyadenosine residues seems to be the lowest for the (R,S)-dihydrodiol (S,R)-epoxides. Since there is not an abrupt transition between the deoxyadenosine:deoxyguanosine adduct ratios for dihydrodiol epoxides from nonplanar and planar hydrocarbons, whatever actually determines the preference for reaction with each of the two purine residues in DNA must clearly change gradually through the series of compounds whose reactions are summarized in Figure 5. Further analysis of the reactions with DNA for the dihydrodiol (S,R)-epoxides (i.e., the two dihydrodiol epoxides formed most extensively in cellular systems) shows just the major adduct formed by the derivatives of the planar hydrocarbons and the major adduct formed by the derivatives of the nonplanar hydrocarbons (Figure 6). In all cases the sum of these two adducts accounts for between 67% and 98% of the total adducts. For the (R,S)dihydrodiol (S,R)-epoxides, the two major adducts are the trans-deoxyguanosine and trans-deoxyadenosine adducts (Figure 6, upper panel). However, for the (S,R)-dihydrodiol (S,R)-epoxides, the major adduct for the planar hydrocarbon derivatives is the cis-deoxyguanosine adduct, though the major adduct for the nonplanar hydrocarbon derivatives is again the trans-deoxyadenosine adduct (Figure 6, lower panel). It is interesting that the inversion of the diol function leads to a change in preference from trans to cis epoxide opening for reactions with deoxyguanosine in DNA yet has no effect on the preference for trans opening by deoxyadenosine residues in DNA. This may indicate that the prereaction complex has similar geometry for deoxyadenosine adduct formation irrespective of the diol configuration, whereas in the deoxyguanosine case, the diol configuration strongly influences this geometry. Presently, there is no clear understanding of the factors that direct dihydrodiol epoxides to deoxyadenosine or deoxyguanosine residues in DNA, but there is increasing information about the conformation of these adducts in DNA. The hydrocarbon residue in the trans-deoxyadenosine adducts studied so far is intercalated between the base pairs (35). In relation to denatured DNA, the native DNA structure has been found to promote reaction of a,b-dihydrodiol (cS,dR)-epoxides (i and iv in Figure 1) with deoxyadenosine residues (62) presumably by favoring intercalation adjacent to AT pairs (63, 64). Since deoxyadenosine adduct formation is preferentially trans with all the dihydrodiol (cS,dR)-epoxides [except for the (S,R)-dihydrodiol (S,R)-epoxide (iv in Figure 1) of BaP], intercalation presumably has the same preferred orientation, irrespective of the configuration of the diol function. However, in the case of the benzo[a]pyrene-(S,R)-dihydrodiol (S,R)-epoxide and others where cis-deoxyadenosine adduct formation is substantial, alternative prereaction complex geometry is presumably accessible. In DNA, the hydrocarbon residue of trans-deoxyguanosine adducts largely lies in the narrow groove of the helix, whereas in cis adducts, a base-displaced intercalation is found (35). The switch from preferential transto cis-deoxyguanosine adduct formation on inverting the diol function in dihydrodiol (S,R)-epoxides (Figure 6) suggests that the diol function does affect the prereaction geometry that leads to deoxyguanosine adduct formation.

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Szeliga and Dipple

Figure 5. Different preferences for adduct formation with deoxyadenosine and deoxyguanosine residues in DNA exhibited by the configurational isomers of dihydrodiol epoxides from several hydrocarbons (Table 2). The hydrocarbon abbreviations are defined in the legend to Figure 2, and the data are from BaP (40, 58), BA (23), 7MBA (61), DBA (22), 5MC (60), 56DMC (44), BcP (53), and BgC (57).

Figure 6. Different preferences for adduct formation with deoxyadenosine or deoxyguanosine residues in DNA exhibited by the dihydrodiol (S,R)-epoxides of several hydrocarbons. The hydrocarbon abbreviations are defined in the legend to Figure 2, and the data are from BaP (40, 58), 5MC (59), 7MBA (54), BA (23), DBA (22), DMBA (54), 56DMC (44), BcP (53), and BgC (56), all as summarized in Table 2.

The reactions of the dihydrodiol (cR,dS)-epoxides with DNA were less selective than those of their enantiomers discussed above (see Table 2). However, the reactions of the anti-dihydrodiol (R,S)-epoxides parallel those of their (S,R)-epoxide enantiomers in that trans adducts were preferentially formed, except in the sole case of the deoxyadenosine adducts from DMBA. Thus, cis-deoxy-

guanosine adducts exceeded 10% of total adducts only for benzo[a]pyrene (22%) and 7,12-dimethylbenz[a]anthracene (16%), and cis-deoxyadenosine adducts exceeded 10% of total adducts only for the nonplanar hydrocarbons 7,12-dimethylbenz[a]anthracene (44%), benzo[c]phenanthrene (19%), and 5,6-dimethylchrysene (16%). For the syn-dihydrodiol (S,R)-epoxides, cis-deoxyguanosine adducts were formed in substantial excess over trans-deoxyguanosine adducts except for the derivatives of the fjord region hydrocarbons, benzo[c]phenanthrene and benzo[g]chrysene. With the syn-(R,S)-epoxide enantiomers, a similar situation was obtained except that the dibenz[a,j]anthracene derivative generated similar amounts of cis- and trans-deoxyguanosine adducts. In contrast to the findings for the syn-(S,R)-epoxide derivatives, where trans-deoxyadenosine adduct formation was preponderant, deoxyadenosine adducts from the syndihydrodiol (R,S)-epoxides were preferentially cis in all cases except for benzo[g]chrysene. In summary, for all configurationally isomeric dihydrodiol epoxides, those derived from nonplanar hydrocarbons exhibit the greatest preference for reaction with deoxyadenosine residues and those derived from planar compounds exhibit a greater preference for reaction with deoxyguanosine residues. The greatest selectivity for deoxyadenosine is demonstrated by the (S,R)-dihydrodiol (S,R)-epoxides derived from nonplanar hydrocarbons which modify deoxyguanosine to a very minor extent. In general, both anti-dihydrodiol epoxide enantiomers preferentially generate trans adducts with both deoxyadenosine and deoxyguanosine in DNA, and the selectivity in this regard is greater for the (R,S)-dihydrodiol (S,R)epoxide than for its enantiomer. Again, generalizing for the syn enantiomers, reaction of derivatives of planar hydrocarbons with deoxyguanosine residues primarily gives cis adducts whereas derivatives from the fjord region containing hydrocarbons (benzo[g]chrysene and benzo[c]phenanthrene) primarily give trans adducts. Reactions with deoxyadenosine residues preferentially lead to trans adducts in the (S,R)-dihydrodiol (S,R)epoxide case (benzo[a]pyrene is an exception) and to cis

Perspective

Chem. Res. Toxicol., Vol. 11, No. 1, 1998 9

adducts in the (R,S)-dihydrodiol (R,S)-epoxide case (except for benzo[g]chrysene).

Discussion By now, a fairly substantial number of hydrocarbon dihydrodiol epoxides have been investigated with respect to their reactions with DNA. Although it might have been thought at one time that the extensively studied benzo[a]pyrene derivatives were prototypical of hydrocarbon dihydrodiol epoxides in general, it is quite clear that these compounds display properties characteristic of one end of a spectrum of chemical properties attributable to dihydrodiol epoxides and that the dihydrodiol epoxides of hydrocarbons with fjord regions (benzo[c]phenanthrene and benzo[g]chrysene) or with methyl substituents in the bay region (7,12-dimethylbenz[a]anthracene and 5,6-dimethylchrysene) that are consequently nonplanar lie at the other end of this spectrum. This range of properties is best illustrated in Figure 5, which displays the relative reactivities of configurationally isomeric dihydrodiol epoxides toward deoxyadenosine and deoxyguanosine residues in DNA. Although 5-methylchrysene has a substituent methyl group in the bay region, this parent hydrocarbon is essentially planar because the steric crowding is accommodated by in-plane distortion (45). Interestingly, the dihydrodiol epoxides derived from this hydrocarbon (65, 66) mostly associate with those of the planar unsubstituted hydrocarbon derivatives rather than with those of substituted nonplanar hydrocarbons in Figure 5. The absence or presence of steric crowding in the bay region (or a fjord region) has been known for many years to influence the preferred conformation of the diol functional group in dihydrodiol epoxides. Thus, in such sterically crowded hydrocarbons, the diol group is preferentially pseudo-diequatorial in both syn and anti diastereomers, whereas in noncrowded structures, it is preferentially diequatorial in the anti-dihydrodiol epoxides and diaxial in the syn diastereomers (67). However, there does not seem to be any correlation between the selectivity for reaction with the two DNA purines shown in Figure 5 and any preferential diol conformation in the dihydrodiol epoxides. The presence of steric crowding in the bay region also has distinct effects on the conformation of nucleoside adducts, as illustrated by the different ranges of NMR coupling constants found for the peracetylated derivatives summarized in Figure 3. Interestingly, with respect to these NMR properties, the 5-methylchrysene derivatives associate with the other derivatives of sterically crowded hydrocarbons indicating that the 5-methyl substituent has a profound effect on adduct conformation. A very brief summary of the comparison of cis versus trans epoxide opening for reactions of isomeric dihydrodiol epoxides with DNA is presented in Table 3. Comparison of the two dihydrodiol (S,R)-epoxides indicates that trans opening is preferred in both reactions with deoxyadenosine residues but that inversion of the hydroxyl groups changes the preferred opening from trans to cis for deoxyguanosine. For the two dihydrodiol (R,S)-epoxides, the (S,R)-dihydrodiol function was associated with trans opening for both deoxyadenosine and deoxyguanosine residues and the (R,S)-dihydrodiol function mostly led to cis opening. The same data can be used to compare the effects of inverting the epoxide group.

Table 3. Summary of DNA Reactions of Various Configurationally Isomeric Dihydrodiol Epoxides isomer

adduct

R,S,S,R

dGuo dAdo dGuo

S,R,S,R S,R,R,S R,S,R,S

dAdo dGuo dAdo dGuo dAdo

preferential configuration of major adducts in DNA trans trans cis (except is trans for BgC and trans ) cis for BcP) trans (except is cis for BaP) trans trans (except is cis for DMBA) cis (except is cis ) trans for DBA and is trans for BcP and BgC) cis (except is trans for BgC)

Thus, for (R,S)-dihydrodiols epoxide opening is preferentially trans with (S,R)-epoxides and cis with (R,S)epoxides. For (S,R)-dihydrodiols, deoxyadenosine preferentially opens either epoxide trans but deoxyguanosine residues prefer cis opening for (S,R)-epoxides and trans for (R,S)-epoxides. For three of the four dihydrodiol epoxide isomers, the preferential opening is usually the same for both deoxyguanosine and deoxyadenosine residues, but for the (S,R)-dihydrodiol (S,R)-epoxide the preferred opening differs for the two purine residues. Because preferred opening is different for the two purine residues in the latter case, the direction of opening is not presumably totally controlled by the dihydrodiol epoxide chemistry itself but in some way relates to the interaction of the reactive dihydrodiol epoxide with the DNA structure (47). Three major aspects of dihydrodiol epoxide structure have been found to have profound effects on their reactions with DNA and therefore presumably have either a steric or an electronic effect upon the reaction. These structural features are (1) steric crowding adjacent to the epoxide function that seems to influence selectivity profoundly for reaction with deoxyadenosine versus deoxyguanosine (Figure 5), (2) the configuration of the epoxide group, which influences the direction of epoxide ring opening, and (3) the configuration of the diol functional group, which also changes the preference for cis or trans opening of the epoxide. Since the effect of steric crowding is similar for all dihydrodiol epoxide isomers, i.e., it tends to favor reaction on deoxyadenosine and disfavor reaction on deoxyguanosine, steric crowding may primarily affect the chemical reactivity of the dihydrodiol epoxide or generally disfavor prereaction complex formation leading to deoxyguanosine reaction and favor that leading to deoxyadenosine adduct formation. The latter explanation is preferable because in separate reactions with nucleotides, large differences in reaction preference for the two purine nucleotides are not seen. With respect to the effects of the epoxide and diol configuration on formation of cis versus trans adducts, the direction of opening is presumably not determined by the chemical reactivity of the dihydrodiol epoxides because in the case of the (S,R)-dihydrodiol (S,R)-epoxides, the same compound gives cis products with one purine and trans products with the other. Presumably, at least four prereaction complexes can form, one giving rise to trans opening and one giving rise to cis opening for each purine residue. It would follow then that the preferred orientation of the reactive hydrocarbon residue in such complexes leading to deoxyguanosine adducts would mostly be unchanged by changing from one dihydrodiol epoxide enantiomer to the

10 Chem. Res. Toxicol., Vol. 11, No. 1, 1998

other, though it would have the cis-forming orientation in syn-dihydrodiol epoxide complexes and the transforming orientation in the anti-dihydrodiol epoxide complexes (see Table 3). In other words, the cis- or transforming orientation preference is dependent upon both the epoxide and diol functions together, not on either of these alone. The situation is more complex for deoxyadenosine adducts. Again, anti-dihydrodiol epoxides orient themselves so as to generate trans adducts preferentially. However, whereas one syn enantiomer orients itself to favor cis adducts, the (S,R)-dihydrodiol (S,R)epoxides prefer the orientation leading to trans adduct formation. These observations suggest that the diol and epoxide functions are less prominent determinants of prereaction complexes associated with deoxyadenosine reaction than with those leading to deoxyguanosine reaction, but they play some role in prereaction complex formation because the hydrocarbon residue is presumably inverted in those for the syn-dihydrodiol (S,R)-epoxides compared with those of the other three isomers. Although this analysis does not specify the nature of the prereaction complexes in detail, it does place some limits on these complexes and summarizes properties that need to be accounted for in the formulation of these key reaction-determining events.

Acknowledgment. We wish to thank colleagues who have contributed the literature reviewed herein and also wish to thank Robert C. Moschel, Larry K. Keefer, Bruce Hilton, Thomas Barlow, and Ingrid Ponte´n for their comments on the manuscript. Our research is supported by the National Cancer Institute, DHHS, on a contract with ABL.

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