Role of phenoxyl radicals in DNA adduction by chlorophenol xenobiotics following peroxidase activation.

Chlorophenol (CP) toxins are classified as probable human carcinogens and are known to undergo bioactivation to generate benzoquinone (BQ) electrophiles that react covalently with biopolymers. Recently, we characterized the ability of pentachlorophenol (PCP) to react covalently with deoxyguanosine (dG) following treatment with horseradish peroxidase (HRP)/H2O2 or myeloperoxidase to yield a C8-dG oxygen (O)-adduct that suggested the intermediacy of the pentachlorophenoxyl radical in covalent bond formation [Dai, J., Wright, M. W., and Manderville, R. A. (2003) Chem. Res. Toxicol. 16, 817-821]. Investigations currently focus on a wider range of CP substrates (PCP, 2,4,6-trichlorophenol (2,4,6-TCP), 2,4,5-TCP, and 2,4-dichlorophenol (2,4-DCP)) to establish their reactivity toward dG and duplex DNA (calf thymus (CT)) following activation by HRP/H2O2, as a representative peroxidase system. Our data show that chlorophenoxyl radicals may either react directly with dG and CT-DNA to form C8-dG O-adducts in an irreversible process or couple to yield 1,4-BQ electrophiles that react with dG to afford adducts of the benzetheno variety. These results are the first to establish the in vitro relevance of C8-dG O-adducts of phenolic toxins. The 1H NMR chemical shifts and reactivity of the benzetheno adducts favor 4' '-hydroxy-1,N2-benzetheno-dG adduct assignment, which is in contrast to other literature which has assigned the 1,4-BQ-dG adduct as 3' '-hydroxy-1,N2-benzetheno-dG. Overall, the results from this current study have provided new insights into peroxidase-mediated activation of CP substrates and have strengthened the hypothesis that direct reactions of phenoxyl radicals with DNA contribute to peroxidase-driven toxic effects of phenolic xenobiotics.