Biotin

Vitamin H redirects here. In medical slang, Vitamin H may also refer to haloperidol.

Biotin, also known as vitamin H or B₇, has the chemical formula C₁₀H₁₆N₂O₃S (Biotin; Coenzyme R, Biopeiderm), is a water-soluble B-complex vitamin which is composed of an ureido (tetrahydroimidizalone) ring fused with a tetrahydrothiophene ring. A valeric acid substituent is attached to one of the carbon atoms of the tetrahydrothiophene ring. Biotin is important as a cofactor in the catalysis of essential metabolic reactions to synthesize fatty acids, in gluconeogenesis, and in metabolising leucine.

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General overview

Biotin is used in cell growth, the production of fatty acids and in the metabolism of fats and amino acids. It plays a role in the Citric acid cycle, which is the process by which biochemical energy is generated during aerobic respiration. Biotin not only assists in various metabolic chemical conversions, but also helps to transfer carbon dioxide. Biotin is also helpful in maintaining a steady blood sugar level. Biotin is often recommended for strengthening hair and nails. Consequently, it is found in many cosmetic and health products for the hair and skin.

Deficiency is extremely rare, as intestinal bacteria generally produce in excess of the body's daily requirement. For that reason, statutory agencies in many countries (e.g., the Australian Department of Health and Aging) do not prescribe a recommended daily intake.

Uses

Hair problems

Biotin supplements are often recommended as a natural product to counteract the problem of hair loss in both children and adults. There are, however, no studies that show any benefit in any case where the subject is not actually biotin deficient. The signs and symptoms of biotin deficiency include hair loss which progresses in severity to include loss of eye lashes and eye brows in severely deficient subjects. Some shampoos are available that contain biotin, but it is doubtful whether they would have any useful effect, as biotin is not absorbed well through the skin.

Cradle cap (seborrheic dermatitis)

Children with a rare inherited metabolic disorder called phenylketonuria (PKU; in which one is unable to break down the amino acid phenylalanine) often develop skin conditions such as eczema and seborrheic dermatitis in areas of the body other than the scalp. The scaly skin changes that occur in people with PKU may be related to poor ability to use biotin. Increasing dietary biotin has been known to improve seborrheic dermatitis in these cases.

Diabetes

People with type 2 diabetes often have low levels of biotin. Biotin may be involved in the synthesis and release of insulin. Preliminary studies in both animals and people suggest that biotin may help improve blood sugar control in those with diabetes, particularly type 2 diabetes.^[1]

Deficiency

Biotin deficiency is relatively rare and mild, and can be addressed with supplementation. Such deficiency can be caused by the excessive consumption of raw egg whites, which contain high levels of the protein avidin, which binds biotin strongly. Avidin is inactivated by cooking, while the biotin remains active.

Biotinidase deficiency is not due to inadequate biotin, but rather to a deficiency in the enzymes which process it.

Biochemistry

Biotin is a cofactor responsible for carbon dioxide transfer in several carboxylase enzymes:

• Acetyl-CoA carboxylase alpha
• Acetyl-CoA carboxylase beta
• Methylcrotonyl-CoA carboxylase
• Propionyl-CoA carboxylase
• Pyruvate carboxylase

The attachment of biotin to various chemical sites, called biotinylation, can be used as an important laboratory technique to study various processes including protein localization, protein interactions, DNA transcription and replication. Biotin itself is known to biotinylate histones, but is not found naturally on DNA. Holocarboxylase synthetase is involved in the binding of biotin.

Biotin binds very tightly to the tetrameric protein avidin (also streptavidin and neutravidin), with a dissociation constant K_d in the order of 10^-15 mol/L (Bonjour, 1977; Green 1975; and Roth, 1985). This is often used in different biotechnological applications. Until 2005, very harsh conditions were required to break the biotin-streptavidin bond.^[2]

Laboratory uses

In the biology laboratory, biotin is sometimes chemically linked, or tagged, to a molecule or protein for biochemical assays. This process is called biotinylation. Since avidins bind preferentially to biotin, biotin-tagged molecules can be extracted from a sample by mixing them with beads with covalently-attached avidin, and washing away anything unbound to the beads.

For example, biotin can be attached to a molecule of interest (e.g. a protein), and this modified molecule will be mixed with a complex mixture of proteins. Avidin or streptavidin beads are added to the mixture, and the biotinylated molecule will bind to the beads. Any other proteins binding to the biotinylated molecule will also stay with the beads. All other unbound proteins can be washed away, and the scientist can use a variety of methods to determine which proteins have bound to the biotinylated molecule.

Biotinylated antibodies are used to capture avidin or streptavidin in both the ELISPOT and ELISA techniques.

References

1. ^ Campbell, R. Keith (Nov 2006). "A Critical Review of Chromium Picolinate and Biotin". U.S. Pharmacist 31 (11).
2. ^ Holmberg A, Blomstergren A, Nord O et al. (2005). "The biotin-streptavidin interaction can be reversibly broken using water at elevated temperatures". Electrophoresis 26 (3): 501-10. doi:10.1002/elps.200410070. PMID 15690449.

See also

• Biotinylation
• Avidin
• Streptavidin
• NeutrAvidin