Metal-independent production of hydroxyl radicals by halogenated quinones and hydrogen peroxide: an ESR spin trapping study

The metal-independent production of hydroxyl radicals (√OH) from H2O2 and tetrachloro-1,4-benzoquinone (TCBQ), a carcinogenic metabolite of the widely used wood-preservative pentachlorophenol, was studied by electron spin resonance methods. When incubated with the spin trapping agent 5,5-dimethyl-1-pyrroline N-oxide (DMPO), TCBQ and H2O2 produced the DMPO/√OH adduct. The formation of DMPO/√OH was markedly inhibited by the √OH scavenging agents dimethyl sulfoxide (DMSO), ethanol, formate, and azide, with the concomitant formation of the characteristic DMPO spin trapping adducts with √CH3, √CH(CH3)OH, √COO−, and √N3, respectively. The formation of DMPO/√OH and DMPO/√CH3 from TCBQ and H2O2 in the absence and presence, respectively, of DMSO was inhibited by the trihydroxamate compound desferrioxamine, accompanied by the formation of the desferrioxamine-nitroxide radical. In contrast, DMPO/√OH and DMPO/√CH3 formation from TCBQ and H2O2 was not affected by the nonhydroxamate iron chelators bathophenanthroline disulfonate, ferrozine, and ferene, as well as the copper-specific chelator bathocuproine disulfonate. A comparative study with ferrous iron and H2O2, the classic Fenton system, strongly supports our conclusion that √OH is produced by TCBQ and H2O2 through a metal-independent mechanism. Metal-independent production of √OH from H2O2 was also observed with several other halogenated quinones.