Biotin

Biotin is a water-soluble B vitamin, also called vitamin B7 and formerly known as vitamin H or coenzyme R. It is involved in a wide range of metabolic processes, both in humans and in other organisms, primarily related to the utilization of fats, carbohydrates, and amino acids. Biotin is a water-soluble B vitamin, also called vitamin B7 and formerly known as vitamin H or coenzyme R. It is involved in a wide range of metabolic processes, both in humans and in other organisms, primarily related to the utilization of fats, carbohydrates, and amino acids. Biotin deficiency can be caused by inadequate dietary intake or inheritance of one or more inborn genetic disorders that affect biotin metabolism. Subclinical deficiency can cause mild symptoms, such as hair thinning or skin rash typically on the face. Neonatal screening for biotinidase deficiency began in the United States in 1984, with many countries testing for this disorder at birth. Individuals born prior to 1984 are unlikely to have been screened, obscuring the true prevalence of the disorder. Biotin is an important component of enzymes involved in metabolizing fats and carbohydrates, influencing cell growth, and affecting amino acids involved in protein synthesis. Biotin assists in various metabolic reactions involving the transfer of carbon dioxide. It may also be helpful in maintaining a steady blood sugar level. Biotin is often recommended as a dietary supplement for strengthening hair and nails, though scientific data supporting these outcomes are weak. Nevertheless, biotin is found in many cosmetics and health products for the hair and skin. Biotin deficiency is rare. The amounts needed are small, a wide range of foods contain biotin, and intestinal bacteria synthesize biotin, which is then absorbed by the host animal. For that reason, statutory agencies in various countries, for example the USA and Australia, have not formally established a recommended daily intake of biotin. Instead, an adequate intake (AI) is identified based on the theory that average intake meets needs. Future research could result in biotin AIs with EARs and RDAs (see Dietary Reference Intake section). A number of rare metabolic disorders exist in which an individual's metabolism of biotin is abnormal, such as deficiency in the holocarboxylase synthetase enzyme which covalently links biotin onto the carboxylase, where the biotin acts as a cofactor. Biotin is composed of a ureido ring fused with a tetrahydrothiophene ring. The ureido ring acts as the carbon dioxide carrier in carboxylation reactions. A valeric acid substituent is attached to one of the carbon atoms of the tetrahydrothiophene ring. Biotin is a coenzyme for multiple carboxylase enzymes, which are involved in the digestion of carbohydrates, synthesis of fatty acids, and gluconeogenesis. Biotin is also required for the catabolism and utilization of the three branched-chain amino acids: leucine, isoleucine, and valine. In 1916, W.G. Bateman observed that a diet high in raw egg whites caused toxic symptoms in dogs, cats, rabbits, and humans. This study was followed in 1927 by Margaret Averil Boas, who found that a diet of only egg whites caused dermatitis, alopecia, and loss of muscular coordination in rats. She called this syndrome 'egg white injury'. In 1939, six years after he began investigating the cause of egg white injury, Hungarian scientist Paul Gyorgy confirmed the existence of a protecting factor, which he called vitamin H. By this point, many independent groups had isolated biotin. In 1936, Kögl and Tönnis isolated a growth factor from egg yolk they called bios. After experiments performed with yeast and Rhizobium R, West and Wilson isolated a compound called co-enzyme R. In 1940, Gyorgy proved that vitamin H, bios and coenzyme R were the same substance; he called it biotin. Biotin has an unusual structure (above figure), with two rings fused together via one of their sides. The two rings are ureido and tetrahydrothiophene moieties. Biotin is a heterocyclic, S-containing monocarboxylic acid. It is made from two precursors, alanine and pimeloyl-CoA via three enzymes. 8-Amino-7-oxopelargonic acid synthase is a pyridoxal 5'-phosphate enzyme. The pimeloyl-CoA, could be produced by a modified fatty acid pathway involving a malonyl thioester as the starter. 7,8-Diaminopelargonic acid (DAPA) aminotransferase is unusual in using S-adenosyl methionine (SAM) as the NH2 donor. Dethiobiotin synthetase catalyzes the formation of the ureido ring via a DAPA carbamate activated with ATP. Biotin synthase reductively cleaves SAM into a deoxyadenosyl radical, which abstracts an H atom from dethiobiotin to give an intermediate that is trapped by the sulfur donor. This sulfur donor is an iron-sulfur cluster. D-(+)-Biotin is a cofactor responsible for carbon dioxide transfer in several carboxylase enzymes: