The vitamin biotin is actually a group of stereo isomers but in humans only the D-biotin isomer is active. Biotin is a water soluble vitamin needed as a co-factor for a number of carboxylase enzymes. These carboxylases are involved in the gluconeogenic pathway, fatty acid synthesis and amino acid catabolism. Recent research also suggests a role for biotin in immune regulation, and cell proliferation as well as the stimulation of gene expression for the insulin receptor, the enzyme glucokinase and thiamine transporter-2, and in the suppression of gene expression of the enzyme phosphoenolpyruvate carboxykinase. Deficiency of biotin can lead to retarded growth, neurological disorders and disease of the skin. Bacteria and plants can synthesise biotin, and because of this some biotin is absorbed following production by microflora in the colon. Food sources include eggs, meat, milk, and organ meat.
The bioavailability of biotin from food depends on the source, with absorption ranging from close to zero to close to 100%. Biotin can be present in food bound to protein in which case the biotin is digested by peptidase enzyme to form biocytin (biotin bound to lysine). Biocytin is then hydrolysed from lysine by the action of the enzyme biotinidase to form the free form before absorption can occur. The absorption of free biotin is via an active Na+ dependent carrier, and the protein bound biotin is not recognised by the carrier. This transport is inhibited by anti-convusant drugs such as carbamazepine and primidone. Once absorbed to the epithelial cells biotin enters the circulation by a carrier mediated Na+ independent protein on the basolateral membrane. Microflora in the large intestine produce free biotin that is also absorbed by a Na+ dependent carrier mediated system.
RdB
