Alternative Medicine Review 2007 (Mar); 12 (1): 73–78 ~ FULL TEXT
From the FULL TEXT Article:
Biotin is a water-soluble B vitamin that is an essential cofactor for four carboxylase enzymes, each of which catalyzes an essential step in intermediary metabolism. Because humans and other mammals cannot synthesize biotin, it must be derived from dietary sources and de novo synthesis by intestinal bacteria. Biotin was originally recognized when rats fed protein derived from egg whites developed severe dermatitis, hair loss, and neuromuscular dysfunction. A growth factor found in liver, then called “Protective Factor X,” cured the condition; this growth factor is now known as biotin. It was later discovered that uncooked egg whites contain a glycoprotein, avidin, that binds to biotin and prevents its absorption, whether biotin is from the diet or from intestinal bacterial synthesis.  Besides genetic inborn errors of metabolism, biotin deficiency can occur during extended parenteral nutrition, pregnancy, or long-term anticonvulsant therapy. Conditions that may benefit from biotin supplementation include dyslipidemia, brittle nails, diabetes, dermatitis, and candidiasis.
The chemical structure of biotin, first elucidated in the early 1940s, is a bicyclic compound; one ring contains a
ureido group (-N-CO-N-) and the other sulfur (a tetrahydrothiophene ring). Only one of eight possible stereoisomers
of biotin is found in nature and is enzymatically active – d-(+)-biotin (or simply “D-biotin”).
Oral biotin is completely absorbed, even at high, pharmacological doses. Urinary excretion of biotin and its
metabolites is similar for intravenous dosing and oral supplementation at high doses, suggesting 100-percent bioavailability
of orally administered biotin.  Percutaneous absorption of biotin from a biotin-containing ointment has been
demonstrated in healthy subjects and patients with atopic dermatitis. 
Biotin is absorbed via a sodium-dependent, carrier-mediated system.  After transport from the intestines to
the peripheral circulation, biotin is taken up by the liver and eventually crosses the blood-brain barrier into the central
nervous system via a saturable system.  In healthy adults and children not receiving biotin supplementation, the kidneys clear biotin and creatinine in a ratio of approximately 0.4.1 Specific systems for transport of biotin from mother to fetus [6-8] and from mother to infant via breastmilk, [9, 10] have been described.
Mechanisms of Action
In humans, biotin is required as a prosthetic group for four major carboxylase enzymes involved in several
critical metabolic pathways, including gluconeogenesis, fatty acid synthesis, and amino acid catabolism. All four carboxylase
enzymes catalyze the incorporation of bicarbonate into a substrate as a carboxyl group. Three of these carboxylase
enzymes are located in mitochondria; the fourth (acetyl-CoA carboxylase; ACC) is found in both cytosol and mitochondria.
ACC catalyzes incorporation of bicarbonate into acetyl-CoA, and finally into malonyl CoA. Malonyl CoA subsequently acts as a substrate for fatty acid synthesis, with the effect of elongating the fatty acid chain. Other
carboxylases, decarboxylases, and a transcarboxylase are
also dependent on biotin as an enzyme cofactor.
Recent research has illuminated several other
mechanisms of action. Biotin at pharmacological doses
(3.1 µM/day) to healthy adults resulted in decreased
synthesis of cytokines (interleukin-1β and interleukin-
2) and decreased proliferation of peripheral blood
mononuclear cells (PBMC) – a combination of T-cells,
B-cells, and granulocytes. 
Biotin appears to exert an effect on gene transcription,  although research is in its infancy. Researchers
have identified more than 2,000 biotin-dependent
genes.  Biotin has been found to attach to histones,
a process catalyzed by the enzymes holocarboxylase
synthetase  and biotinidase.  This recent discovery
provides one mechanism whereby biotin might regulate
chromatin structures, gene expression, and DNA
Biotin deficiency in humans is rare and generally associated with extended parenteral nutrition, consumption of large quantities of raw egg whites, severe malnutrition, or inborn errors of metabolism (e.g., biotinidase deficiency, multiple carboxylase deficiency).
Studies of biotin status during pregnancy suggest marginal biotin deficiency, occurring in a significant number of otherwise normal pregnancies, may be teratogenic.  One study found 50 percent of pregnant women had increased urinary excretion of 3-hydroxyisovaleric acid (a reflection of biotin deficiency) that was reversed by supplementation of 300 mcg biotin for 14 days.  Individuals on long-term anticonvulsant therapy are also at high risk for a biotin deficiency (see Drug-Nutrient Interactions below). Signs and symptoms of severe biotin deficiency include erythematous skin lesions, vomiting, seizures, developmental delay, hypotonia, and ataxia.  One study found 3-hydroxyisovaleric acid to be the most reliable urinary indicator of biotin deficiency; urinary 3-hydroxypropionic acid and methylcitric acid
were not as reliable.  The most sensitive indicator of a biotin deficiency, reliably detecting even a marginal deficiency, appears to be measurement of lymphocyte propionyl-CoA carboxylase activity. 
Read the rest of this Full Text article now!