One-electron reduction of chromium(VI) by α-lipoic acid and related hydroxyl radical generation, dG hydroxylation and nuclear transcription factor-κb activation

Abstract

Reaction of chromium(VI) with α-lipoic acid (reduced form, also called 1,2-dithiolane-3-pentanoic acid) generated Cr(V) and hydroxyl radical (·OH) 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 generation and enhanced the Cr(V) formation. Superoxide dismutase had an opposite effect. H2O2 enhanced the ·OH generation and decreased the Cr(V) formation in a dose-dependent manner. Metal chelators, EDTA, diethylenetriaminepentaacetic acid, deferoxamine, and 1,10-phenanthroline inhibited ·OH radical generation in the order of EDTA > 1,10-phenanthroline > DTPA > deferoxamine. Oxygen consumption measurements indicated that molecular oxygen was used to generate ·OH radical in the mixture of Cr(VI) and α-lipoic acid. H2O2 and superoxide radical (O2) were involved as reactive intermediates. The ·OH radical was generated via Cr(V)-mediated Fenton-like reaction (Cr(V) + H2O2 →Cr(VI) + OH + ·OH). HPLC measurements show that the ·OH 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)-κB. Addition of α-lipoic acid enhanced the NF-κB activation, while the ·OH radical scavenger, sodium formate, inhibited it, showing that α-lipoic acid enhanced Cr(VI)-induced NF-κB activation via free radical reactions. The results indicate that while α- 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.