Biotin

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BIOTIN

Biotin is also known as Vitamin H and Coenzyme R (Hexahydro-2-oxo-1H-thienol[3,4-d]- imidazole-4-pentatonic acid). It is found primarily in liver, kidney and muscle. Biotin functions as an essential cofactor for four carboxylases that catalyze the incorporation of cellular bicarbonate into the carbon backbone of organic compounds. Acetyl-CoA carboxylase (ACC) is located in the cytosol where it catalyzes the formation of malonyl-CoA which then serves as a substrate for fatty acid elongation. The other three enzymes are located in the mitochondria. Pyruvate carboxylase (PC) catalyzes the incorporation of bicarbonate into pyruvate to form oxaloacetate, an intermediate in the tricarboxylic acid cycle. In gluconeogenic tissues such as the liver and kidney, oxaloacetate can be converted to glucose. Methylcrotonyl-CoA carboxylase (MCC) catalyzes an essential step in the degradation of leucine, a branch-chained amino acid. Propionyl-CoA carboxylate (PCC) catalyzes the incorporation of bicarbonate into propionyl-CoA to form methylmalonyl-CoA which, in turn, is metabolized to other compounds that eventually enter the tricarboxylic acid cycle.

Deficiencies: Severe deficiency causes 1) thinning of hair, loss of hair color, and eventual complete loss of hair; 2) a scaly, red rash distributed around the openings of the eyes, nose, mouth, and perineal area; and 3) central nervous system abnormalities such as depression, lethargy, hallucinations, and paresthesias. In infants, the neurologic findings include hypotonia, lethargy, and developmental delay as well as a peculiar withdrawn behavior. In patients on total or near total intravenous feeding, an unusual distribution of facial fat coupled with the rash and hair loss produces a distinct appearance called "biotin deficiency facies."

Clinical uses: Biotin is routinely provided to individuals receiving total intravenous feeding and is incorporated into almost all nutritionally complete dietary supplements and infant formulas. In larger doses, biotin is also used to treat inborn errors of metabolism such as biotinidase deficiency, holocarboxylase synthetase deficiency, and the isolated deficiencies of PC, PCC and MCC. Marginal states of biotin deficiency may develop during normal pregnancy and in patients taking medications such as anticonvulsants that accelerate the rate of biotin breakdown.

Diet recommendations:

The Estimated Safe and Adequate Daily Dietary Intakes are:

Age (years)	ug/day
0 - 0.5	10
0.5 - 1	15
1 - 3	20
4 - 6	25
7-10	30
11+	30-100

Food sources: Biotin is widely distributed in food stuffs, but the amounts are small relative to other vitamins. Biotin deficiency is rare in the absence of total intravenous feedings without added biotin or chronic ingestion of raw egg white.

Toxicity: Oral and intravenous doses of biotin up to 20 mg have not produced frank toxicity in human subjects. In animal studies, even higher doses per kilogram body weight have not produced toxicity.

Recent research: It has been demonstrated that there are specific intestinal, renal, and hepatic transporters for this vitamin. Transport across the blood brain barrier is probably also carrier mediated but is not as specific as the intestinal absorption, hepatic uptake, or renal reabsorption of biotin. There might be a storage form of biotin as an inactive form of ACC.

For further information:

Mock, D.M. (1996) Biotin. In: Present Knowledge in Nutrition. (Filer, L.J. & Ziegler, E., eds.), International Life Sciences Institute - Nutrition Foundation, Washington, D. C.

Bonjour, J.-P. (1991) Biotin. In: Handbook of Vitamins (Machlin, L.J., ed.) pp. 393-427. Marcel Dekker, Inc. New York, NY.

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