Biotin, also known as vitamin H or coenzyme R, is a water-soluble B-vitamin (vitamin B7).
It is composed of a 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 for carboxylase enzymes, involved in the synthesis of fatty acids, isoleucine, and valine, and in gluconeogenesis.
|Jmol-3D images||Image 1
|Molar mass||244.31 g mol−1|
|Appearance||White crystalline needles|
|Melting point||232-233 °C|
|Solubility in water||22 mg/100 mL|
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. 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 this usage are weak. Nevertheless, biotin is found in many cosmetics and health products for the hair and skin.
Biotin deficiency is rare because, in general, intestinal bacteria produce biotin in excess of the body’s daily requirements. For that reason, statutory agencies in many countries, for example the USA and Australia, do not prescribe a recommended daily intake of biotin. However, a number of 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 has an unusual structure (see above figure), with two rings fused together via one of their sides. The two rings are ureido and thiophene 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. 7,8Diaminopelargonic acid (DAPA) aminotransferase is unusual in using S-adenosyl methionine (SAM) as the NH2 donor. Dethiobiotin synthethase catalyzes the formation of the ureido ring via a DAPA carbamate activated with ATP. Biotin synthase reductively cleaves SAM into a deoxyadenosyl radical—a first radical formed on dethiobiotin is trapped by the sulfur donor, which was found to be the iron-sulfur (Fe-S) center contained in the enzyme.
- Acetyl-CoA carboxylase alpha
- Acetyl-CoA carboxylase beta
- Methylcrotonyl-CoA carboxylase
- Propionyl-CoA carboxylase
- Pyruvate carboxylase
Biotin is important in fatty acid synthesis, branched-chain amino acid catabolism, and gluconeogenesis. It 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. In bacteria, biotin is attached to biotin carboxyl carrier protein (BCCP) by biotin protein ligase (BirA in E. coli). The attachment of biotin to various chemical sites, biotinylation, is 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 histone proteins, 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 on the order of 10−15 M, which is one of the strongest known protein-ligand interactions. This is often used in different biotechnological applications. Until 2005, very harsh conditions were thought to be required to break the biotin-streptavidin bond.
Sources of biotin
Biotin is consumed from a wide range of food sources in the diet, but few are particularly rich sources. Foods with a relatively high biotin content include Swiss chard, raw egg yolk (however, the consumption of avidin-containing egg whites with egg yolks minimizes the effectiveness of egg yolk’s biotin in one’s body), liver, Saskatoon berries, leafy green vegetables, and peanuts. The dietary biotin intake in Western populations has been estimated to be 35 to 70 μg/d (143–287 nmol/d).
Biotin is also available in supplement form and can be found in most pharmacies. The synthetic process developed by Leo Sternbach and Moses Wolf Goldberg in the 1940s uses fumaric acid as a starting material.
Biotin is also called vitamin H (the H represents Haar und Haut, German words for “hair and skin”) or vitamin B7. Studies on its bioavailability have been conducted in rats and in chicks. Based on these studies, 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 grains have about a 20-40% bioavailability of biotin.
The wide variability in biotin bioavailability may be due to the ability of an organism to break various biotin-protein bonds from food. Whether an organism has an enzyme with that ability will determine the bioavailability of biotin from the foodstuff.
Factors that affect biotin requirements
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 a partial gastrectomy or 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 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 rare and mild, and can be addressed with supplementation. It is caused by the consumption of raw egg whites (two or more daily for several months) due the avidin they contain, a protein which binds extremely strongly with biotin, making it unavailable. Such regimens have produced the only examples of biotin deficiency serious enough to produce symptoms.
The first demonstration of biotin deficiency in animals was observed in animals fed raw egg white. Rats fed egg white protein were found to develop dermatitis, alopecia, and neuromuscular dysfunction. This syndrome, called egg white injury, was discovered to be caused by a glycoprotein found in egg white, avidin. Avidin denatures upon heating (cooking), while the biotin remains intact.
Symptoms of biotin deficiency include:
- Hair loss (alopecia)
- Dermatitis in the form of a scaly, red rash around the eyes, nose, mouth, and genital area.
- Neurological symptoms in adults, such as depression, lethargy, hallucination, and numbness and tingling of the extremities
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 a high risk of biotin deficiency. Nearly half of pregnant women have abnormal increases of 3-hydroxyisovaleric acid, which reflects reduced status of biotin. Several studies have reported 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 the mother’s biotin deficiency that does not reach the appearance of physiological deficiency signs may cause a serious consequence in the infants.
Inherited metabolic disorders characterized by deficient activities of biotin-dependent carboxylases are termed multiple carboxylase deficiency. These include deficiencies in the enzymes 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 manifestations include: ketolactic acidosis, organic aciduria, hyperammonemia, skin rash, feeding problems, hypotonia, seizures, developmental delay, alopecia, and coma.
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, 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 and hepatic steatosis also can occur.
Health and diet
Diabetics may benefit from biotin supplementation. In both insulin-dependent and insulin-independent diabetics deficient in biotin, supplementation with biotin can improve blood sugar control and help lower fasting blood glucose levels; in one study, the reduction in fasting glucose approached 50%. 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.
Hair and nail problems
The signs and symptoms of biotin deficiency include hair loss, which progresses in severity to include loss of eyelashes and eyebrows in severely deficient subjects, as well as nails that break, chip, or flake easily. The recommended dose to treat deficiency for adults varies from 3,000 mcg per day for brittle fingernails up to 7,000 to 15,000 mcg per day for diabetics.
Patients with palmoplantar pustulosis had metabolic derangements of glucose and fatty acids, as well as immune dysfunction derived from biotin deficiency, which led to abnormal manifestations of skin, bone and other tissues and organs. All of the clinical, metabolic and immune disorders were improved by biotin administration. These findings indicate biotin deficiency was implicated in the outbreak and exacerbation of the disease and its complications. Supplementary addition of a probiotic agent to the biotin treatment intensified the therapeutic effect of the vitamin. Additionally, patients with psoriasis vulgaris, systemic lupus erythematosus, atopic dermatitis or rheumatoid arthritis also had biotin deficiency with the subsequent metabolic abnormalities and immune dysfunction, so the biotin treatment provided beneficial effects in the therapy of the diseases, as in the case of palmoplantar pustulosis.
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.
Biotin is widely used throughout the biotechnology industry to conjugate proteins for biochemical assays. Biotin’s small size means the biological activity of the protein will most likely be unaffected. This process is called biotinylation. Because both streptavidin and avidin bind biotin with high affinity (Kd of 10−14 mol/l to 10−15 mol/l) and specificity, biotinylated proteins of interest can be isolated from a sample by exploiting this highly stable interaction. The sample is incubated with streptavidin/avidin beads, allowing capture of the biotinylated protein of interest. Any other proteins binding to the biotinylated molecule will also stay with the bead and all other unbound proteins can be washed away. However, due to the extremely strong streptavidin-biotin interaction, very harsh conditions are needed to elute the biotinylated protein from the beads (typically 6M guanidine HCl at pH 1.5), which often will denature the protein of interest. To circumvent this problem, beads conjugated to monomeric avidin can be used, which has a decreased biotin-binding affinity of ~10−8 mol/l, allowing the biotinylated protein of interest to be eluted with excess free biotin.
ELISAs often make use of biotinylated secondary antibodies against the antigen of interest, followed by a detection step using streptavidin conjugated to a reporter molecule, such as horseradish peroxidase or alkaline phosphatase.
Animal studies have indicated few, if any, effects due to high level 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, in particular, when used in the treatment of metabolic disorders causing sebhorrheic dermatitis in infants.