Biotin

Biotin is a water-soluble vitamin and serves as a coenzyme for five carboxylases, which catalyze key steps in the metabolism of fatty acids, glucose, and amino acids in humans. Biotin also regulates gene expression, mediated by biotinylation of lysine residues in histones H2A, H3, and H4, and by various transcription factors. Holocarboxylase synthetase, biotinidase, sodium-dependent multivitamin transporter, and the biotin transporters SMVT and MCT1 play crucial roles in biotin homeostasis in mammals. Human biotin requirements are unknown, and recommendations for dietary intake are based on estimates of biotin intake in apparently healthy populations ("adequate intake"). Individuals carrying mutations in genes coding for holocarboxylase synthetase and biotinidase require lifelong supplementation with pharmacological doses of biotin. Reliable markers for biotin status include the activity of propionyl-CoA carboxylase in lymphocytes, and the urinary excretion of biotin and the metabolite 3-hydroxyisovaleric acid. Anticonvulsants and lipoic acid may interfere with biotin metabolism, thereby increasing biotin requirements. Severe biotin deficiency has been linked to birth defects and impaired immune function in animal studies. Whether these effects of marginal biotin deficiency occur spontaneously in humans remains unclear.