Chloride ions catalyze the formation of cis adducts inthe binding of anti-benzo[a]pyrene diol epoxide to nucleicacids.

Abstract

The alkylation of DNA by racemic 7r,8t-dihydroxy-9t,10t-epoxy-7,8,9,10-tetrahydrobenzo[alpyrene (ant-BPDE) exhibits a strong preference for formation of trans adducts between the N2 deoxyguanosine alkylation site and the (+)-enantiomer of anli-BPDE. In the presence of 10 mM buffer with no added salt, 98% of the adducts formed with native calf thymus DNA result from trans opening ofthe epoxide ring. The strong selectivity for trans adduct formation obtained with duplex DNA at low salt concentration is found to a lesser degree with poly(G) but is nearly absent with dAMP. When DNA adducts are formed in 10 mM MgCl2 or 1 M NaCl, the proportion of cis adducts increases to %-7 and ""26%, respectively. At low salt, 10 mM MgCl2, and 1 M NaCl, deoxyguanosine adducts are approximately 1%, 6%, and 24% cis, whereas deoxyadenosine adducts are approximately 11%, 14%, and 37% cis, respectively. NaCI also increases the proportion of cis adducts formed with poly(G) and dAMP. It is proposed that the increase in cis-adduct formation due to salt results from SN1 attack of chloride ion on the BPDE carbocation, forming a trans chlorohydrin, followed by SN2 attack of DNA.Benzo [a]pyrene is a potent carcinogen and ubiquitous environmental pollutant which is metabolically activated to form several diastereomeric diol epoxide intermediates capable of alkylating DNA, RNA, and protein (1-7). The most mutagenic and carcinogenic of these diastereomers is 7R,8S-dihydroxy-9S, 10R-epoxy-7,8,9,10-tetrahydrobenzo[alpy-rene [(+)-anti-BPDE] (7-9). The primary alkylation site of the diol epoxides in DNA is the N2 position of deoxyguanosine (N2-dGuo) (5)(6)(7)(10)(11)(12), and the principal adduct formed at this site in vivo is due to (+)-anti-BPDE (4). The purine is attached to the C-10 position of the hydrocarbon and is trans to the C-9 hydroxy substituent of the benzo[a]pyrene moiety (a trans adduct). Although trans adducts predominate in DNA, those with the 9 and 10 substituents ofthe hydrocarbon moiety in the cis configuration (cis adducts) also occur (12-15). The interaction of anti-BPDE with DNA involves physical association and a DNA-catalyzed hydrolysis reaction that competes with the alkylation reaction. The mechanisms of these reactions have been reviewed (16,17), as have the characteristics of the covalent adducts (18).The conformations of the trans N2-dGuo adducts of (+)-anti-BPDE and 7S,8R-dihydroxy-9R, 8,9,pyrene [(-)-anti-BPDE] in DNA have recently been determined by NMR, and both adducts have been found to be situated in the minor groove, causing minimal distortion of the duplex (19,20). The hydrocarbon moieties of the two adducts point in opposite directions [toward the 5' end of the modified strand in the (+)-BPDE adduct, and toward the 3' end in the (-)-BPDE adduct]. The cis (+)-anti-BPDE N2-dGuo adduct has an intercalated conformation, leading to disruption of a base pair (21). The conformation of the cis (-)-anti-BPDE N2-dGuo adduct is believed, on the basis of less direct spectroscopic evidence, to be similar (22). A var...

Genome-Wide Functional and Stress Response Profiling Reveals Toxic Mechanism and Genes Required for Tolerance to Benzo[a]pyrene in S. cerevisiae

“…In both yeast and people, these solute carriers are specific to various organic anion molecules including glutathione, sulfate, and glucuronide conjugates, which are the ultimate products of BaP metabolism prior to excretion in humans (Pritchard et al, 2005). Interestingly, other solute carrier families have been implicated in human BaP toxicity, such as SLC38A5 which is thought to indirectly favor cis DNA adduct formation by increasing the chloride ion concentration thus catalyzing S N 2 attack of the BPDE carbocation metabolite (Wolfe et al, 1994). However, literature concerning the role of solute family 22 in human BaP toxicity is limited and warrants further investigation given the potential role played by the orthologous gene in yeast BaP exposure and hydroquinone metabolite toxicity (Keum et al, 2006).…”

Synthesis and Characterization of Bay Region Halohydrins Derived from Benzo[a]pyrene Diol Epoxide and Their Role as Intermediates in Halide-Catalyzed Cis Adduct Formation

“…DNA (14,16,18,19). Initially, we observed that halide ions catalyze the hydrolysis of (()-anti-BPDE (14).…”

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

“…Trans addition of guanine to the C-10 position of (+) and (−)- anti -BPDE produces (+)- trans-N 2 -BPDE-dG and (−)- trans-N 2 -BPDE-dG, while cis addition results in (+) and (−)- cis-N 2 -BPDE-dG isomers ( Scheme 1 ). While trans-N 2 -BPDE-dG adducts are the most common diastereomers in vivo ( 10 ), the cis adducts are preferentially generated under high salt conditions ( 11 , 12 ). Cis and trans N 2 -BPDE-dG have distinct conformations in DNA, which influences their recognition by DNA repair proteins ( 13 , 14 ).…”