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

Abstract

The metal-independent production of hydroxyl radicals ( OH) from H 2O 2 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 H 2O 2 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 CH 3, CH(CH 3)OH, COO −, and N 3, respectively. The formation of DMPO/ OH and DMPO/ CH 3 from TCBQ and H 2O 2 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/ CH 3 formation from TCBQ and H 2O 2 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 H 2O 2, the classic Fenton system, strongly supports our conclusion that OH is produced by TCBQ and H 2O 2 through a metal-independent mechanism. Metal-independent production of OH from H 2O 2 was also observed with several other halogenated quinones.