No abstract

Catechol estrogens and catecholamines are metabolized to quinones, and the metabolite catechol (1,2-dihydroxybenzene) of the leukemogenic benzene can also be oxidized to its quinone. We report here that quinones obtained by enzymatic oxidation of catechol and dopamine with horseradish peroxidase, tyrosinase or phenobarbital-induced rat liver microsomes react with DNA by 1,4-Michael addition to form predominantly depurinating adducts at the N-7 of guanine and the N-3 of adenine. These adducts are analogous to the ones formed with DNA by enzymatically oxidized 4-catechol estrogens (Cavalieri,E.L., et al. (1997) PROC: Natl Acad. Sci., 94, 10937). The adducts were identified by comparison with standard adducts synthesized by reaction of catechol quinone or dopamine quinone with deoxyguanosine or adenine. We hypothesize that mutations induced by apurinic sites, generated by the depurinating adducts, may initiate cancer by benzene and estrogens, and some neurodegenerative diseases (e.g. Parkinson's disease) by dopamine. These data suggest that there is a unifying molecular mechanism, namely, formation of specific depurinating DNA adducts at the N-7 of guanine and N-3 of adenine, that could initiate many cancers and neurodegenerative diseases.

Benzo[a]pyrene (B[a]P) is an archetypal member of the family of polycyclic aromatic hydrocarbons (PAHs) and is a widely distributed environmental pollutant. B[a]P is known to induce cancer in animals, and B[a]P-containing complex mixtures are human carcinogens. B[a]P exerts its genotoxic and carcinogenic effects through metabolic activation forming reactive intermediates that damage DNA. DNA adduction by B[a]P is a complex phenomenon that involves the formation of both stable and unstable (depurinating) adducts. One pathway by which B[a]P can mediate genotoxicity is through the enzymatic formation of B[a]P-7,8-quinone (BPQ) from B[a]P-7,8-diol by members of the aldo-keto-reductase (AKR) family. Once formed, BPQ can act as a reactive Michael acceptor that can alkylate cellular nucleophiles including DNA and peptides. Earlier studies have reported on the formation of stable and depurinating adducts from the reaction of BPQ with DNA and nucleosides, respectively. However, the syntheses and characterization of the stable adducts from these interactions have not been addressed. In this study, the reactivity of BPQ toward 2'-deoxyguanosine (dG) and 2'-deoxyadenosine (dA) nucleosides under physiological pH conditions is examined. The identification and characterization of six novel BPQ-nucleoside adducts obtained from the reaction of BPQ and dG or dA in a mixture of phosphate buffer and dimethylformamide are reported. The structures of these adducts were determined by ultraviolet spectroscopy, electrospray mass spectrometry, and NMR experiments including (1)H, (13)C, two-dimensional COSY, one-dimensional NOE, ROESY, HMQC, HSQC, and HMBC. The reaction of BPQ with dG afforded four unique Michael addition products: two diastereomers of 8-N(1),9-N(2)-deoxyguanosyl-8,10-dihydroxy-9,10-dihydrobenzo[a]pyren-7(8H)-one (BPQ-dG(1,2)) and two diastereomers of 10-(N(2)-deoxyguanosyl)-9,10-dihydro-9-hydroxybenzo[a]pyrene-7,8-dione (BPQ-dG(3,4)). The BPQ-dG(1,2)( )()adducts suggest a 1,6-Michael addition reaction of dG, an oxidation of the hydroquinone to the quinone, a 1,4-Michael addition of water, and an internal cyclization. The BPQ-dG(3,4)( )()adducts suggest a 1,4-Michael addition reaction of dG, an oxidation of the hydroquinone to the quinone, and a 1,6-Michael addition of water. Under similar but extended reaction conditions, the reaction of BPQ with dA produced only one diastereomeric pair of adducts identified as 8-N(6),10-N(1)-deoxyadenosyl-8,9-dihydroxy-9,10-dihydrobenzo[a]pyren-7(8H)-one (BPQ-dA(1,2)). The BPQ-dA(1,2)( )()adducts suggest a 1,4-Michael addition reaction of dA, an oxidation of the hydroquinone to the quinone, a 1,6-Michael addition of water, and an internal cyclization. As considerable efforts have been placed in documenting the genotoxic effects of BPQ, this first report of the identification and characterization of these stable adducts of BPQ formed under physiological pH conditions is expected to contribute significantly to the area of BPQ-mediated genotoxicity and carcinogenesis.

No abstract