A sulphur-containing coenzyme that is synthesized in mitochondria from octanoic acid. It is involved in the decarboxylation of pyruvate by the enzyme pyruvate dehydrogenase. This reaction has to take place before carbohydrates can enter the Krebs cycle during aerobic respiration. It is also a coenzyme for another Krebs cycle enzyme, alpha-ketoglutarate dehydrogenase, which catalyses the conversion of alpha-ketoglutarate to succinyl-CoA. Various properties of lipoic acid, including antioxidant activity, have been identified using cultured cells in vitro, and deficiency in humans has not been described. The value of dietary supplementation remains to be established.