Mechanism of Oxidative DNA Damage Induced by a Heterocyclic Amine, 2-Amino-3,8-dimethylimidazo[4,5-f]quinoxaline

Abstract

Adduct formation has been considered to be a major causal factor of DNA damage by carcinogenic heterocyclic amines. By means of experiments with 32P‐labeled DNA fragments and an electrochemical detector coupled to a high‐pressure liquid chromatograph, we investigated whether the N‐hydroxy metabolite of 2‐amino‐3,8‐dimethylimidazo[4,5‐f]quinoxaline (MeIQx) can cause oxidative DNA damage or not. This metabolite [MeIQx(NHOH)] was found to cause Cu(II)‐mediated DNA damage, including 8‐oxo‐7,8‐dihydro‐2′‐deoxyguanosine formation. When an endogenous reductant, β‐nicotinamide adenine dinucleotide (NADH), was added, the DNA damage was greatly enhanced. Catalase and bathocuproine, a Cu(I)‐specific chelator, inhibited the DNA damage, suggesting the involvement of H2O2 and Cu(I). MeIQx(NHOH) frequently induced DNA cleavage at thymine and cytosine residues in the presence of NADH and Cu(II). A UV‐visible spectroscopic study showed that little decomposition of MeIQx(NHOH) occurred in the absence of Cu(II), whilst rapid spectral change was observed in the presence of Cu(II), suggesting that Cu(II) catalyzes the autoxidation. The addition of NADH reduced the oxidized product back to MeIQx(NHOH). These results suggest that a copper‐peroxo intermediate, derived from the reaction of Cu(I) with H2O2, participates in Cu(II)‐dependent DNA damage by MeIQx(NHOH), and NADH enhances the DNA damage via a redox cycle. We conclude that in addition to DNA adduct formation, oxidative DNA damage plays an important role in the carcinogenic process of MeIQx.