Biochemistry - The Chemical Reactions of Living Cells Volume 2 - D. Metzler 1980

Coenzymes - specialized natural reagents of a distinct type
Lipoic acid and oxidative decarboxylation of α-keto acids

The isolation of Lipoic Acid in 1951 occurred just a decade after the discovery that Protozoa such as *Tetrahymena geleii* require an unknown growth factor. Studies on Bacteria also pointed to the existence of a new coenzyme. It was observed that acetic acid promotes the rapid growth of *Lactobacillus casei*, but it could be replaced by an unknown "acetate-replacing factor." Another lactic acid bacterium, *Streptococcus faecalis*, is unable to oxidize Pyruvate in the absence of the "pyruvate oxidation factor." By 1949, these three unknown substances were recognized as identical. Through heroic efforts Processing roughly 10 tons of a Water-soluble Liver fraction, Reed of the University of Texas and his coworkers at the Eli Lilly and Co. laboratories isolated 30 mg of a fat-soluble acidic material which they named lipoic acid (this compound is also referred to as 6-thioctic acid).

While *Tetrahymena* must obtain lipoic acid from its diet, The Human Body appears capable of synthesizing it independently, and therefore it cannot be classified as a vitamin. Lipoic acid is present in Tissues in extremely minute amounts. Its only definitively known function is participation in The oxidative decarboxylation of α-keto acids [137, 137a]. Its Structure is simple, and the functional group is evidently a cyclic disulfide suspended at the end of a long chain. Lipoic acid does not exist in a free state; rather, much like biotin, it is covalently attached to Enzymes via an amide linkage formed with the terminal amino group of a Lysine residue:

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A specific enzyme catalyzes the attachment of lipoic acid to enzymes via S1C-type reactions (Table 7-2), which involve the Cleavage of ATP into AMP and pyrophosphate.