DNA Strand Scission by Polycyclic Aromatic Hydrocarbon o-Quinones: Role of Reactive Oxygen Species, Cu(II)/Cu(I) Redox Cycling, and o-Semiquinone Anion Radicals,

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Abstract
In previous studies, benzo[a]pyrene-7,8-dione (BPQ), a polycyclic aromatic hydrocarbon (PAH) o-quinone, was found to be 200-fold more potent as a nuclease than (±)-anti-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, a suspect human carcinogen. The mechanism of strand scission mediated by naphthalene-1,2-dione (NPQ) and BPQ was further characterized using either φX174 DNA or poly(dG)·poly(dC) as the target DNA. Strand scission was extensive, dependent on the concentration of o-quinone (0−10 μM), and required the presence of NADPH (1 mM) and CuCl2 (10 μM). The production of reactive species, i.e., superoxide anion radical, o-semiquinone anion (SQ) radical, hydrogen peroxide (H2O2), hydroxyl radical (OH•), and Cu(I), was measured in the incubation mixtures. The formation of SQ radicals was measured by EPR spectroscopy under anaerobic conditions in the presence of NADPH. A Cu(II)/Cu(I) redox cycle was found to be critical for DNA cleavage. No strand scission occurred in the absence of Cu(II) or when Cu(I) was substituted, yet Cu(I) was required for OH• production. Both DNA strand scisson and OH• formation were decreased to an equal extent, albeit not completely, by the inclusion of OH• scavengers (mannitol, soduim benzoate, and formic acid) or Cu(I) chelators (bathocuproine and neocuproine). In contrast, although the SQ radical signals of NPQ and BPQ were quenched by DNA, no strand scission was observed. When calf thymus DNA was treated with PAH o-quinones, malondialdehyde (MDA) was released by acid hydrolysis. The formation of MDA was inhibited by OH• scavengers suggesting that OH• cleaved the 2‘-deoxyribose moiety in the DNA to produce base propenals. These studies indicate that for PAH o-quinones to act as nucleases, NADPH, Cu(II), Cu(I), H2O2, and OH•, were necessary and that the primary species responsible for DNA fragmentation was OH•, generated by a Cu(I)-catalyzed Fenton reaction. The genotoxicity of PAH o-quinones may play a role in the carcinogenicity and mutagenicity of the parent hydrocarbons.

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