Antioxidant Effect of α-Lipoic Acid on the Oxidation of Phosphatidylcholine Initiated by the Fenton Reaction

Mass spectrometry with negative-ion electrospray ionization proves the existence of not only monomers of α-lipoic acid (LA) but also its dimers and trimers in the aqueous phase. Upon incubation with ferrous ions (0.1 mM), new mass ions at m / z 261, 467, and 727 appear in the mass chromatogram in the positive ionization mode, and a mass ion at m / z 671 appears in the negative ionization mode. These mass ions are complexes of LA with ferrous ions, formed due to coordination bonds. The presence of complexes depends on the LA concentration. At concentrations of 0.25 to 4.00 mM, four complexes are observed. LA at a concentration of 4 mM (the maximum concentration that can be achieved by dissolving the acid in water) in the presence of 1 mM of hydrogen peroxide and 0.1 mM of Fe 2+ inhibits phospholipid oxidation by 25%. Such an effect was not observed at the LA concentrations below 4 mM. At a concentration of 0.06 mM, only one complex with iron ions is formed ( m / z 261). We found by IR spectroscopy that in the presence of iron ions and hydrogen peroxide, one or both sulfur atoms in the LA molecule are oxidized by the hydroxyl radical. The results suggest two mechanisms for the implementation of the antioxidant properties of LA: oxidation by a hydroxyl radical and chelation of ferrous ions.