|Molar mass||244.31 g mol−1|
|Appearance||White crystalline needles|
|Solubility in water||22 mg/100 mL|
(what is this?) |
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Biotin 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 a coenzyme in the metabolism of fatty acids and leucine, and it plays a role in gluconeogenesis.
Biotin is necessary for cell growth, the production of fatty acids, and 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 reactions, but also helps to transfer carbon dioxide. Biotin may also be helpful in maintaining a steady blood sugar level. Biotin is often recommended for strengthening hair and nails. Consequently, it is found in many cosmetics and health products for the hair and skin, though it cannot be absorbed through the hair or skin itself.
Biotin deficiency is rare, as intestinal bacteria generally produce an excess of the body's recommended daily requirement. For that reason, statutory agencies in many countries, for example the USA, and Australia, do not prescribe a recommended daily intake of biotin.
and so, is important in fatty acid synthesis, branched-chain amino acid catabolism, and gluconeogenesis. Biotin covalently attaches to the epsilon-amino group of specific lysine residues in these carboxylases. This biotinylation reaction requires ATP and is catalyzed by holocarboxylase synthetase. The attachment of biotin to various chemical sites can be used as an important laboratory technique to study various processes including protein localization, protein interactions, DNA transcription and replication. Biotinidase itself is known to be able to biotinylate histones, but little biotin is found naturally attached to chromatin.
Biotin binds very tightly to the tetrameric protein avidin (also streptavidin and neutravidin), with a dissociation constant Kd in the order of 10−15 which is one of the strongest known protein-ligand interactions, approaching the covalent bond in strength. This is often used in different biotechnological applications. Until 2005, very harsh conditions were required to break the biotin-streptavidin bond.
Biotin is consumed from a wide range of food sources in the diet, however there are few particularly rich sources. Foods with a relatively high biotin content include egg yolk, liver, and some vegetables. The dietary biotin intake in Western populations has been estimated to be 35 to 70 μg/d (143–287 nmol/d).
Biotin is also called vitamin H or vitamin B7. Studies on the bioavailability of biotin have been conducted in rats and in chicks. From these studies, it was concluded that biotin bioavailability may be low or variable depending on the type of food being consumed. In general, biotin exists in food as protein bound form or biocytin . Proteolysis by protease is required prior to absorption. This process assists free biotin release from biocytin and protein bound biotin.The biotin present in corn is readily available; however, most grain have about a 20-40% bioavailability of biotin.
A possible explanation for the wide variability in biotin bioavailability is that it is due to ability of an organism to break various biotin-protein bonds from food. Whether an organism has an enzyme with the ability to break that bond will determine the bioavailability of biotin from the foodstuff.
The frequency of marginal biotin status is not known, but the incidence of low circulating biotin levels in alcoholics has been found to be much greater than in the general population. Also, relatively low levels of biotin have been reported in the urine or plasma of patients who have had partial gastrectomy or who have other causes of achlorhydria, burn patients, epileptics, elderly individuals and athletes. Pregnancy and lactation may be associated with an increased demand for biotin. In pregnancy, this may be due to a possible acceleration of biotin catabolism, whereas in lactation, the higher demand has yet to be elucidated. Recent studies have shown that marginal biotin deficiency can be present in human gestation, as evidenced by increased urinary excretion of 3-hydroxyisovaleric acid, decreased urinary excretion of biotin and bisnorbiotin, and decreased plasma concentration of biotin. Additionally, smoking may further accelerate biotin catabolism in women.
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 (20 eggs/day would be required to induce it), which contain high levels of the protein avidin, which binds biotin strongly. Avidin is deactivated by cooking, while the biotin remains intact.
Symptoms of overt biotin deficiency include:
The characteristic facial rash, together with an unusual facial fat distribution, has been termed the "biotin-deficient face" by some experts. Individuals with hereditary disorders of biotin deficiency have evidence of impaired immune system function, including increased susceptibility to bacterial and fungal infections.
Pregnant women tend to have high risk of biotin deficiency. Research has shown that nearly half of pregnant women have an abnormal increase of 3-hydroxyisovaleric acid which reflects reduced status of biotin. Numbers of studies reported that this possible biotin deficiency during the pregnancy may cause infants' congenital malformations such as cleft palate. Mice fed with dried raw egg to induce biotin deficiency during the gestation resulted in up to 100% incidence of the infants' malnourishment. Infants and embryos are more sensitive to the biotin deficiency. Therefore even a mild level of mother's biotin deficiency which does not reach the appearance of physiological deficiency signs may cause a serious consequence in the infants.
Inherited metobolic disorders characterized by deficient activites of biotin-dependent carboxylases are termed multiple carboxylase deficiency. These include deficiencies in the enzyme holocarboxylase synthetase or biotinidase. Holocarboxylase synthetase deficiency prevents the body's cells from using biotin effectively, and thus interferes with multiple carboxylase reactions. Biochemical and clinical manifestation includes: ketolactic acidosis, organic aciduria, hyperammonemia, skin rash, feeding problems, hypotonie, seizures, development delay, alopecia, and coma. This disease is lethal, however, mentioned manifestation can be reversed by pharmacologic doses of biotin (10-100 mg per day). Biotinidase deficiency is not due to inadequate biotin, but rather to a deficiency in the enzymes that process it. Biotinidase catalyzes the cleavage of biotin from biocytin and biotinyl-peptides (the proteolytic degradation products of each holocarboxylase) and thereby recycles biotin. It is also important in freeing biotin from dietary protein-bound biotin. General symptoms include decreased appetite and growth. Dermatologic symptoms include dermatitis, alopecia (hair loss) and achromotrichia (absence or loss of pigment in the hair.) Perosis (a shortening and thickening of bones) is seen in the skeleton. Fatty liver and kidney syndrome (FLKS) and hepatic steatosis also can occur.
Biotin supplements are often recommended as a natural product to counteract the problem of hair loss in both children and adults. 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.
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.
Diabetics may also benefit from biotin supplementation. In both insulin-dependent and non-insulin-dependent diabetes, supplementation with biotin can improve blood sugar control and help lower fasting blood glucose levels, in some studies the reduction in fasting glucose exceeded 50 percent. Biotin can also play a role in preventing the neuropathy often associated with diabetes, reducing both the numbness and tingling associated with poor glucose control.
Animal studies have indicated few, if any, effects due to toxic doses of biotin. This may provide evidence that both animals and humans could tolerate doses of at least an order of magnitude greater than each of their nutritional requirements. There are no reported cases of adverse effects from receiving high doses of the vitamin, particularly when used in the treatment of metabolic disorders causing sebhorrheic dermatitis in infants.
In the biology laboratory, biotin is often chemically linked, or tagged, to a molecule or protein for biochemical assays. This process is called biotinylation. Because 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.