516 BIOTIN/Physiology Fidanza F (1991) Nutritional Status Assessment. London: side chain and by sulfur oxidation in the thiophane Chapman & Hall. ring. Biotin, bisnorbiotin, and biotin, d,l-sulfoxide Green NM (1970) Spectrophotometric determination of are the most abundant biotinyl compounds in urine avidin and biotin. Methods in Enzymology 18: 418–424. and plasma. Guilarte T (1985) Measurement of biotin levels in human plasma using a radiometric-microbiological assay. Nutrition Reports International 31: 1155–1163. Hentz NG and Bachas LG (1997) Fluorophore-linked Biochemical Function assays for high-performance liquid chromatography In mammals, holocarboxylase synthetase (EC 0002 postcolumn reaction detection of biotin and biocytin. 6.3.4.10) catalyzes the covalent binding of biotin Methods in Enzymology 279: 275–286. to the E-amino group of lysine in four different apo- Lizano S, Ramanathan S, Feltus A, Witkowski A and Dannert S (1997) Bioluminescence competitive binding carboxylases to form the active holocarboxylases assays for biotin based on photoprotein aequorin. (Figure 1). For acetyl-CoA carboxylase (EC 6.4.1.2), Methods in Enzymology 279: 296–303. a cytosolic (acetyl-CoA carboxylase a) and a mito- Schroeder HR, Vogelhut PO, Carrilco RJ, Boguslaski RC chondrial form (acetyl-CoA carboxylase b) have been and Buckler RT (1976) Competitive protein binding identified. Both the a and b forms catalyze the bind- assay for biotin monitored by chemoluminescence. ing of bicarbonate to acetyl-CoA to form malonyl- Analytical Chemistry 48: 1933–1937. CoA; the latter is a substrate for fatty acid synthesis. Although the a and b forms of acetyl-CoA carboxyl- ase catalyze the same reaction, they appear to have different roles in intermediary metabolism. Acetyl- Physiology CoA carboxylase a controls fatty acid synthesis in J Zempleni, University of Nebraska-Lincoln, Lincoln, the cytosol by providing the substrate malonyl-CoA; NE, USA acetyl-CoA carboxylase b controls fatty acid oxida- D M Mock, University of Arkansas for Medical tion inside the mitochondria via malonyl-CoA inhib- Sciences, Little Rock, AR, USA ition of fatty acid transport into mitochondria. (See Copyright 2003, Elsevier Science Ltd. All Rights Reserved. Fatty Acids: Metabolism.) The three other mammalian biotin-dependent car- 0003 boxylases are located exclusively in mitochondria: Introduction pyruvate carboxylase (EC 6.4.1.1), propionyl-CoA carboxylase (EC 6.4.1.3), and b-methylcrotonyl- 0001 Biotin is a water-soluble vitamin that serves as essen- tial coenzyme for four mammalian carboxylases. CoA carboxylase (EC 6.4.1.4). By synthesizing oxalo- Biotin may also play a role in the replication and acetate, pyruvate carboxylase provides a tricarboxylic transcription of DNA, possibly by an effect on bioti- acid cycle intermediate and catalyzes a step in gluco- nylation of histones. Frank biotin deficiency causes neogenesis. Propionyl-CoA carboxylase catalyzes an clinical findings such as hair loss and skin rash. Mar- essential step in the metabolism of isoleucine, valine, ginal biotin deficiency is frequently seen in pregnant the cholesterol side chain, and products of dietary women and theoretically could have teratogenic carbohydrate breakdown by intestinal microorgan- effects. Biotin status may be assessed using urinary isms. b-Methylcrotonyl-CoA carboxylase catalyzes excretion of biotin and its metabolites, and certain an essential step in leucine metabolism. organic acids that are byproducts of deficiency of Biotin deficiency causes reduced carboxylase activ- 0004 biotin-dependent carboxylases. Direct measurement ities; substrates are shunted to alternative pathways of biotin-dependent carboxylases and biotin in (Figure 1). For example, reduced activity of propio- lymphocytes and fibroblasts also show promise as nyl-CoA carboxylase results in increased formation indicators of biotin status. Inborn errors of biotin of 3-hydroxypropionic acid and 2-methylcitric acid; metabolism (e.g., biotinidase deficiency and holocar- reduced activity of b-methylcrotonyl-CoA carboxyl- boxylase synthetase deficiency to a variable degree) ase results in increased formation of 3-hydroxyisova- can produce symptoms similar to biotin deficiency. leric acid and 3-methylcrotonyl glycine. Increased For adults, the safe and adequate daily dietary intake urinary excretion of these organic acids has been of biotin is 30 mg. Available data suggest that pure used to diagnose biotin deficiency, as discussed below. biotin administered orally is absorbed completely. Biotin may also play a role in the regulation of 0005 Uptake of biotin into cells of the intestine and periph- DNA transcription and replication on the basis of eral tissues is mediated by a sodium-dependent trans- the following observations: porter that requires metabolic energy. Mammals 1. Biotinidase (EC 3.5.1.12) specifically biotinylates 0006 catabolize biotin by b-oxidation of the valeric acid histones. Histones are DNA-binding proteins that.