BIOTIN
AI (adults age 19 and older) = 30 g
Basic Chemical Structure: Consists of two rings – a ureido ring joined to a
thiophene ring – with an additional valeric acid side chain.
Nomenclature/Bioactive Forms: Coenzyme forms of biotin = holoenzyme
carboxylases, AKA holocarboxylases
Dietary Sources: Includes biotin in dietary foods as well as biotin made by
intestinal bacteria living within the large intestine. Good sources
include:
Liver
Soybeans
Egg yolk
Cereals
Legumes
Nuts
Within many foods, biotin is found either bound covalently to protein
or as biocytin, which consists of biotin bound to the amino acid lysine
and is sometimes called biotinyllysine.
Digestion/Absorption: Avidin, a glycoprotein found in raw egg whites, has
been found to bind biotin and prevent absorption and utilization by the
body. Avidin is heat labile and therefore not found in cooked egg
whites. Alcohol has been shown to inhibit intestinal biotin absorption.
Protein-bound biotin requires digestion by proteolytic enzymes prior to
absorption. Proteolysis yields free biotin, biocytin, or biotinyl peptides.
Biotinyl peptides can be further hydrolyzed by other proteases or
peptidases within the small intestine. Biocytin can be further
hydrolyzed by biotinidase to release free biotin and lysine. Some
undigested biocytin may be absorbed intact by peptide cariers and
subsequently hydrolyzed by biotinidase present in plasma or in most
other body tissues.
Free biotin is absorbed primarily in the jejunum followed by the ileum
due to differences in carrier concentrations. Dietary biotin is thought
to be nearly completely absorbed.
Biotin that is synthesized by colonic bacteria is absorbed in the
proximal and midtransverse colon. Bacterially made biotin cannot
totally meet the biotin needs of humans.
Mechanism of absorption varies with intake. Occurs by passive
diffusion with consumption of pharmacologic doses. With physiologic
intakes, absorption across the brush border membrane of the small
intestine and across the colonocytes is carrier mediated and sodium
dependent. The small intestines carrier also transports pantothenic
acid and lipoic acid.
Transport: Found in plasma mostly in a free, unbound state, with lesser
amounts bound to protein (albumin, - and -globulins, plasma
biotinidase).
Metabolic Functions: Biotin functions in cells covalently bound to enzymes
and thus is referred to as a coenzyme. In addition, biotin functions in
non-coenzyme roles including possible roles in gene expression and
the cell cycle. It functions in the following ways:
COENZYME ROLES (4 biotin-dependent carboxylases)
o Acetyl CoA carboxylase
Acetyl CoA malonyl CoA
Initiation of fatty acid synthesis
Requires ATP, Mg2+
o Pyruvate carboxylase
Pyruvate oxaloacetate
Requires acetyl CoA, ATP, Mg2+
o Propionyl CoA carboxylase
Ile, Thr OR Met propiony CoA
Requires ATP, Mg2+
o -methrlcrotonyl CoA carboxylase
Important in leucine catabolism
Requires ATP, Mg2+
NON-COENZYME ROLES
o Effects on DNA, gene expression, and cell development
and growth
Biological Basis of Requirements: ?
Excretion: Any absorbed biocytin not metabolized by biotinidase is excreted
in the urine. Catabolism of the biotin holocarboxylases by proteases
yields biotin oligopeptides and ultimately biocytin. Biocytin is then
degraded by biotinidase to yield lysine and free biotin. Some of this
biotin is excreted intact in the urine, while some may be reused or
degraded. Biotin that has been synthesized by intestinal bacteria but
not absorbed is excreted in the feces.
Interactions with Other Nutrients: ?
Deficiency: Characterized by lethargy, depression, hallucinations, muscle
pain, paresthesia in extremities, anorexia, nausea, alopecia, and scaly,
red dermatitis. Occurs due to excess ingestion of raw eggs, GI
disorders such as IBD, achlorhydria, anticonvulsant drug therapy or
chronic consumers of excessive amounts of alcohol.
Toxicity: Has not been reported.
Assessment of Nutriture: Evaluation of biotin in blood as well as in urine.
Urinary excretion.