Antioxidant and prooxidant activities of α-lipoic acid and dihydrolipoic acid

Reactive oxygen (ROS) and nitrogen oxide (RNOS) species are produced as by-products of oxidative metabolism. A major function for ROS and RNOS is immunological host defense. Recent evidence indicate that ROS and RNOS may also function as signaling molecules. However, high levels of ROS and RNOS have been considered to potentially damage cellular macromolecules and have been implicated in the pathogenesis and progression of various chronic diseases. α-Lipoic acid and dihydrolipoic acid exhibit direct free radical scavenging properties and as a redox couple, with a low redox potential of -0.32 V, is a strong reductant. Several studies provided evidence that α-lipoic acid supplementation decreases oxidative stress and restores reduced levels of other antioxidants in vivo. However, there is also evidence indicating that α-lipoic acid and dihydrolipoic acid may exert prooxidant properties in vitro. α-Lipoic acid and dihydrolipoic acid were shown to promote the mitochondrial permeability transition in permeabilized hepatocytes and isolated rat liver mitochondria. Dihydrolipoic acid also stimulated superoxide anion production in rat liver mitochondria and submitochondrial particles. α-Lipoic acid was recently shown to stimulate glucose uptake into 3T3-L1 adipocytes by increasing intracellular oxidant levels and/or facilitating insulin receptor autophosphorylation presumably by oxidation of critical thiol groups present in the insulin receptor β-subunit. Whether α-lipoic acid and/or dihydrolipoic acid-induced oxidative protein modifications contribute to their versatile effects observed in vivo warrants further investigation.