Lipoic acid biological function

Rapid and reversible making and breaking of the sulfur-sulfur bond is essential to the biological function of a lipoic acid... 

Ascorbate is a major antioxidant, protecting cells and tissues from damage by free radicals, peroxides, and other metabolites of O2.P It is chemically suited to react with many biologically important radicals and is present in high enough concentrations to be effective. It probably functions in cooperation with glutathione (Box 11-B), a-tocopherol (Fig. 15-24), and lipoic acid. Ascorbate can react with radicals in one-election transfer reactions to give the monodehy-droascorbate radical ... 

Nowadays, the term vitamin is understood to mean an essential organic compound, which is not synthesised in sufficient quantities or at all in the human body (or in most animals), and has therefore to be supplied through the diet or other sources. Vitamins are defined by their biological function, but not as a chemically uniform group of substances. In contrast to energy-providing foodstuffs, they are needed in only very small amounts. By this definition, there are 13 compounds or compound families known, which constitute vitamins for humans (Tab. 7.1). [9] However, there are additional substances of value as dietary supplements and with vitamin-like properties fatty acids (Vitamin F), lipoic acid, ubiquinone, choline, myoinositol and S-adenosyhnethionine. 

The really unique reaction of the lipoate centre in ct-keto acid metabolism is the oxidative thioester formation from a thiamine-coordinated active aldehyde . Thiol transacetylase and dithiol-disulphide oxidation reduction roles are well-known attributes of other biological thiols. Unfortunately mechanistic studies on this reductive acylation of a cyclic disulphide have so far received little attention. Proposals that a lipoic acid-thiamine pyrophosphate compound was the functional entity in a-keto acid oxidation have been completely abandoned, but data supporting this concept remain unexplained. Investigations in this area might have some relevance for the reductive acylation process. 

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