Biotin is a member of the B vitamin complex and is structurally a ureido group within a five-membered ring, fused with a tetrahydrothiophene ring with a valeric acid side chain. Biotin is generally found in plant foods such as cereals, with animal tissue, excluding liver and eggs, being poor sources. Biotin is also synthesised in the colon by microorganisms. Biotin is found bound to protein and so requires hydrolysis prior to absorption. Digestion of the protein yields free biotin, which is thought to be absorbed in the duodenum of the small intestine by an unknown mechanism. A protein in eggs white, avidin (literally, hungry protein) binds with biotin and prevents absorption because the proteolytic enzymes are not able to free biotin. Cooking the eggs denatures the avidin, this allowing biotin absorption. Biotin deficiency causes fatigue, neuropathy, anaemia, hypocholestrolaemia and dry skin. No known biotin toxicity exists, but biotin has no pharmacological actions.
Once absorbed biotin is taken up by cells where it reacts with magnesium ions and ATP to form biotinyl 5’-adenylate, the activated form of the vitamin. Biotin functions as a co-factor in the acetyl CoA carboxylase, propionyl CoA carboxylase and pyruvate carboxylase systems, and is involved in carbon dioxide fixation. Biotin is useful in this regard because once attached to the enzyme biotin can interact with the donor substrate to receive the carbon dioxide, with subsequent transfer of the carbon dioxide bound biotin to an acceptor substrate. Acetyl CoA carboxylase catalyses the formation of malonyl CoA, the fist step in the synthesis of fatty acid. Pyruvate carboxylase converts pyruvate to oxaloacetate to replenish citric acid cycle intermediates to allow gluconeogenesis to proceed. Propionyl CoA carboxylase converts propionate to succinate to allow catabolisation of certain amino acids in the citric acid cycle. Biotin is optically active with activity in humans resulting from the dextroisomer.
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