Lipoic acid: a unique antioxidant in the detoxification of activated oxygen species

Lipoic acid (1,2-dithiolane-pentanoic acid) is a dithiol which is effective in affording protection against oxidative stress by virtue of its two sulphydryl moieties. It is present in all kinds of eukaryotic and prokaryotic cells. As lipoamide, it functions as a cofactor in the multienzyme complexes that catalyse the oxidative decarboxylation of α-keto acids such as pyruvate, α-ketoglutarate, and branched-chain α-keto acids. The complete enzyme pathway responsible for the de novo synthesis of lipoic acid has not yet been elucidated. Octanoic acid appears to be the precursor for the eight-carbon fatty acid chain, and cysteine the source of sulfur. Lipoic acid is unique, among antioxidants, because it retains powerful antioxidant properties in both its reduced (dihydrolipoic acid) and oxidised (lipoic acid) forms. Both lipoic and dihydrolipoic acids have metal-chelating ability and quench activated oxygen species either in the cytosol or in the hydrophobic domains. Dihydrolipoic acid has more antioxidant properties than lipoic acid, and it plays an important role in the recycling of other oxidised radical scavengers such as glutathione, ascorbate and tocopherol. However, dihydrolipoic acid can also exert pro-oxidant properties both by its iron-reducing ability and by its ability to generate sulfur-containing radicals that can damage proteins. There are few quantitative data on lipoic acid contents in vegetables. It has been found in asparagus, wheat and potatoes, and recently, the presence of both lipoic and dihydrolipoic acids in roots, leaves and in the stroma of wheat has been demonstrated.