ABSTRACT
Lipoic acid is a naturally occurring compound present in most kinds of pro-and eukaryotic cells. In our food, lipoic acid is present in several products, such as meat, liver, and heart. Lipoic acid gained attention in the late 1940s as a growth factor and was shown to be a requirement for pyruvate oxidation in certain micro-organisms. In 1951, 30 mg of lipoic acid was purified from 100 kg of liver residue by Reed et al. (1). In the following years the molecular structure was elucidated (3,4) and the compound was identified as 1,2-dithiolane-3-pentanoic acid. It has a chiral center at the C3 carbon atom, which is in the R configuration (Fig. 1). In humans, lipoic acid is part of several 2-oxo acid dehydrogenases, which are involved in energy formation. Linked to lysine residues of the 2-oxo acid dehydrogenase multienzyme complexes, lipoic acid acts as a cofactor (5,6). It binds acyl groups and transfers them from one part of the enzyme complex to another. During this process, lipoic acid is reduced to dihydrolipoic acid, which is subsequently reoxidized by lipoamide dehydrogenase under the formation of NADH. Lipoic acid and dihydrolipoic acid can thus act as a redox couple, carrying electrons from the substrate of the dehydrogenase to NAD+.
