An efficient route to N6 deoxyadenosine adducts of diol epoxides of carcinogenic polycyclic aromatic hydrocarbons

Studies of the Mechanisms of Adduction of 2‘-Deoxyadenosine with Styrene Oxide and Polycyclic Aromatic Hydrocarbon Dihydrodiol Epoxides

“…The structures of these standards were established by NMR and fast atom bombardment mass spectrometry (FAB-MS). Standards were also prepared independently by reaction of 6-chloro-or 6-fluoropurine 2′-deoxyribonucleoside or ribonucleoside (in the case of the styrene oxide reaction) with the appropriate amino alcohols (18,27 gave three major components, the structures of which were established by comparison with unlabeled standards. Two of them were identified as the N 6 R (R) and (S) adducts.…”

“…These were provisionally assigned as the cis adducts on the basis of comparison of UV spectra with those reported by Dipple (32). The structure of the trans adducts was confirmed by comparison with literature values (18,25), and the stereochemical assignments were confirmed by CD spectroscopy; the 1S isomer eluted before the 1R.…”

“…The two major products, eluting from a reverse-phase HPLC column at 9.5 and 9.8 min, were the diastereomeric N 6 (S) and -(R) trans adducts previously synthesized by Kim et al (18). Minor components at 8.7 and 8.9 min were obtained in quantities insufficient to permit identification, but were probably cis adducts.…”

“…UV absorbance was monitored at 258 and 333 nm. Structure (cis or trans) and stereochemistry were confirmed by comparison with literature data (18,23,24). 15 N 6 -labeled 2′-deoxyadenosine (10 mg, 0.04 mmol) was dissolved completely in 1 mL of 50 mM Tris-HCl (pH 7.2) at 42°C.…”

Hypothesis-based weight of evidence: A tool for evaluating and communicating uncertainties and inconsistencies in the large body of evidence in proposing a carcinogenic mode of action—naphthalene as an example

“…We propose that the balance of these activities differs between the mouse and rat nose, ultimately resulting in the observed difference in tumor formation� These questions are further addressed in the next section on genotoxicity� Key event 4-Genotoxicity Current data and hypothesized MoA An understanding of the potential genotoxicity of naphthalene and/or its metabolites is critical in developing an accurate MoA for naphthalene carcinogenesis� The Agency for Toxic Substances and Disease Registry (ATSDR, 2005) presented a review of over 40 naphthalene genotoxicity studies, 80% of which reported no evidence for genotoxicity for naphthalene� And the majority of positive studies are consistent with secondary effects produced by cytotoxicity� Several recent reviews (Schreiner, 2003;Brusick, 2008;Brusick et al�, 2008) have discussed the naphthalene genotoxicity data and that the evidence does not suggest that naphthalene or any of its metabolites drive naphthalene tumorigenicity� However, as outlined in proposed MoA (2) above, US EPA has proposed a genotoxic MoA for naphthalene (US EPA, 2004), based on data indicating genotoxicity of the naphthalene metabolite 1,2-naphthoquinone� The basis of US EPA's proposed MoA is derived, in part, from a 1,2-naphthoquinone mutagenesis study by Flowers-Geary et al� (1996), in which 1,2-naphthoquinone was found to cause a small increase in reversion mutations in Salmonella� In addition, 1,2-naphthoquinone and 1,4-naphthoquinone induced SCE in human MNLs (Wilson et al�, 1996)� Recent studies have shown that 1,2-naphthoquinone reacts with DNA in vitro (Saeed et al�, 2007) and in vivo in mouse skin (Saeed et al�, 2009) (2007) also observed stable DNA adducts in mouse skin that appeared to derive from the 1,2-naphthoquinone� Although depurinating adducts are known to cause mutations in DNA, at least in vitro, a recent review of the literature evaluating the biological significance for the involvement of N7-guanine adducts in mutagenesis found that there is insufficient evidence that N 7 -guanine adducts, or their apurinic sites, are the cause of mutations in cells and tissues, and that the stable DNA adducts may be more relevant (Boysen et al�, 2009)� Therefore, the relevance of naphthalene-induced stable DNA adducts needs further investigation� Studies by Kim et al� (1995) and Kim et al� (2000) suggest that naphthalene can induce stable N 6 adducts of deoxyadenosine from the tetrahydrodiol epoxide ("diol epoxide") of naphthalene� As shown in Figure 1, the 1,2-naphthoquinone and diol epoxide adducts are well downstream of the initial naphthalene epoxide and would not likely form until GSH has been depleted and cytotoxicity has already occurred� No in vivo studies have examined possible DNA adducts in naphthalene-exposed rat or mouse lung or nasal epithelial tissue� As we have mentioned, the balance of naphthalenemetabolizing, -detoxifying, and repair enzymes likely varies between species and tissues, and therefore the potential for formation of DNA adducts likely varies as well and should be examined in each relevant tissue (or cell type)� Th...…”