Biotin was originally discovered as part of the complex called bios, which promoted the growth of yeast, and separately, as vitamin H, the protective or curative factor in egg white injury – the disease caused by diets containing large amounts of uncooked egg white. The glycoprotein avidin in egg white binds biotin with high affinity. This has been exploited to provide a variety of extremely sensitive assay systems.
Dietary deficiency of biotin sufficient to cause clinical signs is extremely rare in human beings, although it may be a problem in intensively reared poultry. However, there is increasing evidence that suboptimal biotin status may be relatively common, despite the fact that the vitamin is widely distributed in many foods, is synthesized by intestinal flora, and there is an efficient mechanism for conserving biotin after the catabolism of biotin-containing enzymes.
Metabolically, biotin is of central importance in lipogenesis, gluconeogenesis, and the catabolism of branched-chain (and other) amino acids. There are two well-characterized biotin-responsive inborn errors of metabolism, which are fatal if untreated: holocarboxylase synthetase deficiency and biotinidase deficiency. In addition, biotin induces a number of enzymes, including glucokinase and other key enzymes of glycolysis. Biotinylation of histones may be important in regulation of the cell cycle.
METABOLISM OF BIOTIN
As shown in Figure 11.1, biotin is a bicyclic compound with fused ureido (imidazolidone) and thiophene rings, and an aliphatic carboxylate side chain. It is bound covalently to enzymes by the formation of a peptide bond between the carboxyl groupof the side chain and the ε-amino groupof a lysine residue forming biocytin (biotinyl-lysine).