Biotinidase and its roles in biotin metabolism.

Biotinidase is the enzyme responsible for the recycling of the vitamin biotin. Biotinidase acts as a hydrolase by cleaving biocytin and biotinyl-peptides, thereby liberating biotin for reutilization. Biotinidase is also important for making biotin bioavailable from bound dietary sources. The interest in this enzyme has been increased by the discovery of biotinidase deficiency, an inherited biotin-responsive disorder that can result in neurological and cutaneous abnormalities, but can be treated effectively with biotin supplementation. Biotinidase has recently been shown to be biotinylated in the presence of biocytin, but not biotin, at neutral and alkaline pH. This raises the possibility that biotinidase acts as a biotin-binding or biotin-carrier protein. Biotinidase has also been shown to have biotinyl-transferase activity resulting in the transfer of biotin from biocytin to nucleophilic acceptors, such as histones. Transferase activity occurs at physiological pH and at physiological concentrations of biocytin and, therefore, may be the main function of the enzyme in serum and other tissues. These novel functions of the enzyme may indicate that biotinidase plays a critical role in the metabolism of biotin in nuclei, particularly of neuronal cells. The role of biotinidase in biotin metabolism may be a paradigm for better understanding the metabolism of other vitamins.