Dietary biotin occurs primarily in the form of protein-bound biotin, which is converted into biotin in the intestinal lumen. Enteric bacteria can also provide a source of protein-bound biotin; however, the importance of this source is not known. Biotin is absorbed from the intestine, more in the jejunum than the ileum, through the SMVT, which also transports

C5 of pentanoic acid ^ Figure 28.39 • Biotin.

pantothenic acid and lipoic acid.141'148 This explains the ability of these three substances to competitively inhibit absorption of each other and the ability of some drugs, such as carbamazepine and primidone, to produce biotin deficiency through competitive inhibition of absorption. The transporter is driven by the Na+ gradient that exists from the extracellular fluid to the intracellular fluid, transporting 2 Na+ for each monovalent anion of pantothenate, biotin, or lipoate.167 In addition, the inside-negative membrane potential may also act as a driving force because a net charge of +1 enters the cell. This same carrier is ubiquitous throughout bodily tissues and probably also accounts for cellular uptake from the blood. Converson to the active form most likely occurs inside the cell. Greater than 50% of a dose is excreted in the urine as intact biotin. The body appears unable to break the fused imidazolidone and tetrahydrothio-phene ring system. The metabolites bisnorbiotin, biotin-d, l-sulfoxide, bisnorbiotin methyl ketone, and biotin sulfone have been identified in the urine.141195

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