Oxidative DNA damage by an N-hydroxy metabolite of the mutagenic compound formed from norharman and aniline

Abstract

Norharman (9 H-pyrido[3,4- b]indole), which is a heterocyclic amine included in cigarette smoke or cooked foodstuffs, is not mutagenic itself. However, norharman reacts with non-mutagenic aniline to form mutagenic aminophenylnorharman (APNH), of which DNA adducts formation and hepatocarcinogenic potential are pointed out. We investigated whether N-OH-APNH, an N-hydroxy metabolite of APNH, can cause oxidative DNA damage or not, using 32 P -labeled DNA fragments. N-OH-APNH caused Cu(II)-mediated DNA damage. When an endogenous reductant, β-nicotinamide adenine dinucleotide (NADH) was added, the DNA damage was greatly enhanced. Catalase and a Cu(I)-specific chelator inhibited DNA damage, suggesting the involvement of H 2O 2 and Cu(I). Typical OH scavenger did not inhibit DNA damage. These results suggest that the main reactive species are probably copper-hydroperoxo complexes with DNA. We also measured 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodG) formation by N-OH-APNH in the presence of Cu(II), using an electrochemical detector coupled to a high-pressure liquid chromatograph. Addition of NADH greatly enhanced 8-oxodG formation. UV–VIS spectra and mass spectra suggested that N-OH-APNH was autoxidized to nitrosophenylnorharman (NO-PNH). We speculated that NO-PNH was reduced by NADH. Cu(II) facilitated the redox cycle. In the presence of NADH and Cu(II), very low concentrations of N-OH-APNH could induce DNA damage via redox reactions. We conclude that oxidative DNA damage, in addition to DNA adduct formation, may play an important role in the expression of genotoxicity of APNH.