Formation of depurinating N3adenine and N7guanine adducts after reaction of 1,2-naphthoquinone or enzyme-activated 1,2-dihydroxynaphthalene with DNA. Implications for the mechanism of tumor initiation by naphthalene

Naphthalene is considered by the US Environmental Protection Agency to be a carcinogenic compound based on inhalation studies in rats. The primary metabolite of naphthalene is naphthalene 1,2-arene oxide. This unstable intermediate can lead to formation of 1-naphthol and naphthalene-1,2-dihydrodiol. Secondary metabolites include 1,2-dihydroxynaphthalene (1,2-DHN), which can be further oxidized to 1,2-naphthoquinone (1,2-NQ). Based on the metabolism of naphthalene and its similarity to the metabolic activation of carcinogenic natural estrogens, synthetic estrogens and benzene, we hypothesize that naphthalene is activated to initiate cancer by reaction of 1,2-NQ with DNA to form the depurinating adducts 1,2-DHN-4-N3Ade and 1,2-DHN-4-N7Gua. These adducts were synthesized by reaction of 1,2-NQ with Ade or dG in acetic acid/water/DMF (1:1:1). 1,2-NQ was reacted with DNA, and the depurinating 1,2-DHN-4-N3Ade and 1,2-DHN-4-N7Gua adducts were analyzed by ultraperformance liquid chromatography/tandem mass spectrometry and HPLC with electrochemical detection. After the reaction of 1,2-NQ with DNA, the N3Ade and N7Gua adducts were found. Similarly, when 1,2-DHN was activated by tyrosinase in the presence of DNA, higher amounts of the N3Ade and N7Gua adducts were detected. These same adducts were also formed when 1,2-DHN was activated by prostaglandin H synthase or 3-methylcholanthrene-induced rat liver microsomes in the presence of DNA. These depurinating adducts are analogous to those obtained from the ortho-quinones of natural estrogens, synthetic estrogens and benzene. These results suggest that reaction of ortho-quinones with DNA by 1,4-Michael addition is a general mechanism of weak carcinogenesis that occurs with naphthalene and a number of other aromatic compounds.     

Slow loss of deoxyribose from the N7deoxyguanosine adducts of estradiol-3,4-quinone and hexestrol-3',4'-quinone. Implications for mutagenic activity

A variety of evidence has been obtained that estrogens are weak tumor initiators. A major step in the multi-stage process leading to tumor initiation involves metabolic formation of 4-catechol estrogens from estradiol (E2) and/or estrone and further oxidation of the catechol estrogens to the corresponding catechol estrogen quinones. The electrophilic catechol quinones react with DNA mostly at the N-3 of adenine (Ade) and N-7 of guanine (Gua) by 1,4-Michael addition to form depurinating adducts. The N3Ade adducts depurinate instantaneously, whereas the N7Gua adducts depurinate with a half-life of several hours. Only the apurinic sites generated in the DNA by the rapidly depurinating N3Ade adducts appear to produce mutations by error-prone repair. Analogously to the catechol estrogen-3,4-quinones, the synthetic nonsteroidal estrogen hexestrol-3',4'-quinone (HES-3',4'-Q) reacts with DNA at the N-3 of Ade and N-7 of Gua to form depurinating adducts. We report here an additional similarity between the natural estrogen E2 and the synthetic estrogen HES, namely, the slow loss of deoxyribose from the N7deoxyguanosine (N7dG) adducts formed by reaction of E2-3,4-Q or HES-3',4'-Q with dG. The half-life of the loss of deoxyribose from the N7dG adducts to form the corresponding 4-OHE2-1-N7Gua and 3'-OH-HES-6'-N7Gua is 6 or 8 h, respectively. The slow cleavage of this glycosyl bond in DNA seems to limit the ability of these adducts to induce mutations.