Mechanism of oxidative DNA damage induced by metabolites of carcinogenic naphthalene

Naphthalene is a carcinogenic polycyclic aromatic hydrocarbon, to which humans are exposed as an air pollutant. Naphthalene is metabolized in humans to reactive intermediates such as 1,2-hydroxynaphthalene (1,2-NQH2), 1,4-NQH2, 1,2-naphthoquinone (1,2-NQ), and 1,4-NQ. We examined oxidative DNA damage by these naphthalene metabolites using 32P-labeled DNA fragments from human cancer-relevant genes. 1,2-NQH2 and 1,4-NQH2 induced DNA damage in the presence of Cu(II). The DNA-damaging activity of 1,2-NQH2 was significantly increased in the presence of the reduced form of nicotinamide adenine dinucleotide (NADH), whereas that of 1,4-NQH2 was not. In the presence of NADH, 1,2-NQ induced Cu(II)-dependent DNA damage, whereas 1,4-NQ did not. The calculated energy of the lowest unoccupied molecular orbital (LUMO), which corresponds to the reduction potential, was estimated to be −0.67 eV for 1,2-NQ and −0.75 eV for 1,4-NQ. These results suggest that 1,2-NQ was reduced more easily than 1,4-NQ. Furthermore, 1,2-NQH2, 1,4-NQH2, and 1,2-NQ plus NADH formed 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodG) as an oxidative DNA marker. Catalase and bathocuproine inhibited DNA damage, suggesting that H2O2 and Cu(I) were involved. These results indicate that NQH2s are oxidized to the corresponding NQs via semiquinone radicals, and that H2O2 and Cu(I) are generated during oxidation. 1,2-NQ is reduced by NADH to form the redox cycle, resulting in enhanced DNA damage. The formation of the corresponding semiquinone radicals was supported by an electron paramagnetic resonance (EPR) study. In conclusion, the redox cycle of 1,2-NQ/1,2-NQH2 may play a more important role in the carcinogenicity of naphthalene than that of 1,4-NQ/1,4-NQH2.