Thioctic Acid and Dihydrolipoic Acid are Novel Antioxidants Which Interact With Reactive Oxygen Species

Thioctic acid (TA) and its reduced form dihydrolipoic acid (DHLA) have recently gained somc recognition as useful biological antioxidants. In particular, the ability of DHLA to inhibit lipid peroxidation has been reported. In the present study, the effects of TA and DHLA on reactive oxygen species (ROS) generated in the aqueous phase have been investigated. Xanthine plus xanthine oxidase-generated superoxide radicals (O₂), detected by electron spin resonance spectroscopy (ESR) using DMPO as a spin trap. were eliminated by DHLA but not by TA. The sulhydryl content of DHLA, measured using Ellman's reagent decreased subsequent to the incubation with xanthine plus xanthine oxidase confirming the interaction between DHLA and O₂^-. An increase of hydrogen peroxide concentration accompanied the reaction between DHLA and O₂x, suggesting the reduction of O₂^- by DHLA. Competition of O₂^- with epinephrine allowed us to estimate a second order kinetic constant of the reaction between O₂^- and DHLA, which was found to be a 3.3 × 10⁵ M^-1 s^-1. On the other hand, the DMPO signal of hydroxyl radicals (HO ·) generated by Fenton's reagent were eliminated by both TA and DHLA. Inhibition of the Fenton reaction by TA was confirmed by a chemiluminescence measurement using luminol as a probe for HO ·. There was no electron transfer from Fe²⁺ to TA or from DHLA to Fe^{3 +} detected by measuring the Fe²⁺ -phenanthroline complex. DHLA did not potentiate the DMPO signal of HO · indicating no prooxidant activity of DHLA. These results suggest that both TA and DHLA possess antioxidant properties. In particular. DHLA is very effective as shown by its dual capability by eliminating both O₂^-; and HO ·.