Evidence for a Fenton-type mechanism for the generation of .OH radicals in the reduction of Cr(VI) in cellular media.

Electron spin resonance (ESR) spectroscopy has been employed to examine the role of tetraperoxochromate (V) ions (CrO3-8) and other Cr(V) species in the generation of hydroxyl (OH) radicals in the reaction of Cr(VI) with H2O2 in biological media. In contrast to earlier suggestions, the present ESR studies using crystalline K3CrO8 as a source of CrO3-8 show that decomposition of CrO3-8 in water or in H2O2 does not generate significant amounts of OH radicals. Addition of NADH to a solution containing CrO3-8 yields a Cr(V)-NADH complex, which readily reacts with H2O2 to generate OH radicals. Similar results obtained from several other biological reductants, including vitamin B2, indicate that a vacant coordination site on a Cr(V) complex facilitates its reaction with H2O2 to generate .OH radicals. We thus suggest that in biological media, reaction [3] instead of [1] or [2] is the major pathway for the .OH radical generation: [formula, see text]