Elucidation of antioxidant activity of α-lipoic acid toward hydroxyl radical

The photosensitive organic hydroperoxidz, NP-III, which produces hydroxyl radicals on illumination by UVA light, was used to examine the antioxidant activity of α-lipoic acid and its derivatives toward hydroxyl radical. Apolipoprotein (apo-B) of human low density lipoprotein (LDL) and bovine serum alubumin (BSA) were irradiated with UVA in the presence of NP-III and α-lipoic acid. The oxidation of BSA and the apo-B protein of LDL by NP-III was completely suppressed by α-lipoic acid. ESR studies using dimethylpyrroline oxide (DMPO) as a spin trapping reagent also revealed that in the presence of α-lipoic acid, the DMPO-OH adduct produced from the irradiation of NP-III and DMPO completely disappeared. DMPO-OH quenching experiments were performed in the presence or absence of desferoxamine but no change in the signal intensity was found. Hence, the quenching activity of α-lipoic acid is not due to its chelating activity toward transition metals (ferrous ions). The results lead us to conclude that α-lipoic acid is an efficient hydroxyl radical quencher owing to the disulfide bond in the dithiolane ring.