One-electron reduction of Chromium(VI) by alpha-lipoic acid and related hydroxyl radical generation, dG hydroxylation and nuclear transcription factor-kappa B activation

Reaction of chromium(VI) with cu-lipoic acid (reduced form, also called 1,2-dithiolane-3-pentanoic acid) generated Cr(V) and hydroxyl radical ((OH)-O-.) as measured by electron spin resonance and ESR spin trapping. 5,5-Dimethyl-1-pyrroline was used as a spin trapping agent. Catalase inhibited the (OH)-O-. generation and enhanced the Cr(V) formation. Superoxide dismutase had an opposite effect. H2O2 enhanced the (OH)-O-. generation and decreased the Cr(V) formation in a dose-dependent manner. Metal chelators, EDTA, diethylenetriaminepentaacetic acid, deferoxamine, and 1,10-phenanthroline inhibited (OH)-O-. radical generation in the order of EDTA > 1,10-phenanthroline > DTPA > deferoxamine. Oxygen consumption measurements indicated that molecular oxygen was used to generate (OH)-O-. radical in the mixture of Cr(VI) and alpha-lipoic acid. H2O2 and superoxide radical (O-2(-)) were involved as reactive intermediates. The (OH)-O-. radical was generated via Cr(V)-mediated Fenton-like reaction (Cr(V) + H2O2 --> Cr(VI) + OH- + (OH)-O-.). HPLC measurements show that the (OH)-O-. radical generated by this reaction is capable of generating 8-hydroxyl-2'-deoxyguanosine from 2-deoxyguanosine. Incubation of Cr(VI) with cultured Jurkat cells resulted in an activation of DNA binding activity of the nuclear factor (NF)-kappa B. Addition of alpha-lipoic acid enhanced the NF-kappa B activation, while the (OH)-O-. radical scavenger, sodium formate, inhibited it, showing that alpha-lipoic acid enhanced Cr(VI)-induced NF-kappa B activation via free radical reactions. The results indicate that while alpha-lipoic acid is considered to be an antioxidant, it may be a cellular one-electron Cr(VI) reductant and could be involved in the mechanism of Cr(VI)-induced carcinogenesis. (C) 1997 Academic Press.