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 (CrO₈^3-) 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 CrO₈^3-show that decomposition of CrO₈^3- in water or in H₂O₂ does not generate significant amounts of .OH radicals. Addition of NADH to a solution containing CrO₈^3- 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: {A figure is presented}{A figure is presented}{A figure is presented}.