Copper-mediated DNA damage by metabolites of p-dichlorobenzene

p-Dichlorobenzene (p-DCB) has been reported to be carcinogenic for rodents, although it does not seem to be mutagenic in bacterial test systems. In this study, the mechanism of DNA damage by metabolites of p-DCB in the presence of metals was investigated by a DNA sequencing technique using 32P-labeled DNA fragments and by an electrochemical detector coupled to an HPLC. 2,5-Dichlorohydroquinone (DCHQ), one of the major metabolites, caused DNA damage in the presence of Cu(II). 2,5-Dichloro-p-benzoquinone (DCBQ) slightly induced DNA damage in the presence of Cu(II), but addition of NADH induced DNA damage very efficiently. DCHQ plus Cu(II) induced piperidine-labile sites at thymine residues at high frequency. A similar DNA cleavage pattern was observed with DCBQ plus Cu(II) in the presence of NADH. Both DCHQ and DCBQ plus NADH increased 8-oxo-7,8-dihydro-2'-deoxyguanosine in calf thymus DNA in the presence of Cu(II). Typical hydroxyl radical scavengers showed no inhibitory effects on this Cu(II)-mediated DNA damage. Bathocuproine and catalase inhibited the DNA damage, indicating the participation of Cu(I) and hydrogen peroxide (H2O2) in the DNA damage. UV-visible and ESR spectroscopy has demonstrated that DCHQ is rapidly autoxidized to DCBQ via a semiquinone radical, even in the absence of metal ions, indicating that the semiquinone radical itself is not the main active species inducing DNA damage. These results suggest that a semiquinone radical produced by autoxidation of DCHQ and/or reduction of DCBQ by NADH reacts with O2 to form superoxide and subsequently H2O2. Consequently, it is considered that the active species derived from the reaction of H2O2 with Cu(I) participates in the DNA damage.