Biotin (vitamin B7, also called vitamin H) is the B vitamin that is the essential cofactor of all carboxylase enzymes — the enzymes that catalyse the biotin-dependent carboxylation reactions that are essential for the synthesis of fatty acids, for the catabolism of fatty acids, for gluconeogenesis, and for the catabolism of the branched-chain amino acids. Biotin is covalently attached to the lysine residue in the biotinidase enzyme (which recycles biotin from the breakdown of the biotin-dependent carboxylases) and to the apocarboxylase enzymes (which are the inactive precursors of the active carboxylase holoenzymes), forming the biotin-carboxyl carrier protein (BCCP) domain that is present in all biotin-dependent enzymes. The five biotin-dependent enzymes in the human body are pyruvate carboxylase (which fixes bicarbonate onto pyruvate to form oxaloacetate, the first step of gluconeogenesis), acetyl-CoA carboxylase 1 and 2 (which catalyse the carboxylation of acetyl-CoA to form malonyl-CoA, the first step of fatty acid synthesis), propionyl-CoA carboxylase (which catalyses the carboxylation of propionyl-CoA to form methylmalonyl-CoA, a step in the catabolism of the odd-chain fatty acids and of the branched-chain amino acids isoleucine, valine, and methionine), and 3-methylcrotonyl-CoA carboxylase (which catalyses the carboxylation of 3-methylcrotonyl-CoA in the leucine catabolism pathway). Without adequate biotin, these carboxylase enzymes cannot function, and the metabolic pathways that depend on them — including gluconeogenesis, fatty acid synthesis, and the catabolism of fatty acids and of the branched-chain amino acids — are severely impaired.
Biotinidase and Biotin Recycling
Biotinidase is the enzyme that is essential for the recycling of biotin from the breakdown of the biotin-dependent carboxylase enzymes and from the dietary biotin that is bound to proteins (which must be released by proteolysis in the gut and then covalently attached to the apocarboxylases by holocarboxylase synthetase). Biotinidase is also essential for the release of free biotin from the biotinylated proteins that are present in the diet, and it is this release that allows the absorption of dietary biotin in the distal small intestine, where biotin is absorbed by the sodium-dependent multivitamin transporter (SMVT). The importance of biotinidase for biotin homeostasis is demonstrated by the genetic deficiency of biotinidase — an autosomal recessive disorder that is characterised by the inability to recycle biotin and that produces a clinical syndrome that includes seizures, developmental delay, hypotonia, ataxia, dermatitis, and in severe cases, coma and death if untreated. The treatment of biotinidase deficiency involves biotin supplementation at 5-20mg daily, which bypasses the defective recycling enzyme and restores the biotin cofactor for the carboxylase enzymes. The dramatic response to biotin in biotinidase deficiency is one of the most effective examples of targeted nutritional therapy in all of medicine.
Biotinidase deficiency is detected by newborn screening in many countries, and the early initiation of biotin supplementation prevents the development of the neurological symptoms that would otherwise occur. The recommended screening test is the measurement of biotinidase activity in dried blood spots — an activity of less than 10% of normal indicates profound biotinidase deficiency, while an activity of 10-30% of normal indicates partial biotinidase deficiency. Both profound and partial biotinidase deficiency are treated with biotin supplementation, with the dose adjusted according to the severity of the deficiency.
Biotin and Skin Health
Biotin is essential for the health of the skin, hair, and nails — it is required for the synthesis of the fatty acids that are incorporated into the skin lipids, for the metabolism of the amino acids that are the building blocks of keratin (the protein that is the primary structural component of hair and nails), and for the function of the enzymes that regulate the turnover of skin cells. Biotin deficiency (which is rare but which can develop in people who consume large amounts of raw egg whites, which contain avidin, a protein that binds biotin and prevents its absorption) produces a characteristic syndrome that includes dermatitis (particularly a scaly, red rash around the eyes, nose, mouth, and genitals, called the biotin-deficient rash), hair loss (alopecia), and brittle nails. The treatment of biotin deficiency involves biotin supplementation at 2-5mg daily, which rapidly reverses the skin and hair manifestations of deficiency.
Practical Application
For general biotin supplementation, the evidence-based dose is 30-100mcg of biotin daily (which is approximately the AI of 30mcg daily for adults). Most people achieve adequate biotin from a varied diet — biotin is found in egg yolks, liver, nuts, seeds, salmon, avocado, and sweet potatoes. Biotin supplementation at high doses (2.5-10mg daily) is used for the treatment of brittle nails, hair loss, and skin conditions, with some evidence supporting its use for these indications. For comprehensive metabolic and skin health support, biotin pairs well with the other B-complex vitamins (which are required for the function of the carboxylase enzymes and for the metabolism of the amino acids that are the building blocks of skin and hair), with zinc (which is required for the synthesis of keratin and for the function of the enzymes that regulate skin cell turnover), with vitamin D (which has immunomodulatory effects that support skin health), and with the omega-3 fatty acids (which have anti-inflammatory effects in the skin and which support the integrity of the skin lipid barrier).
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