BTD

The BTD gene provides instructions for making an enzyme called biotinidase. This enzyme helps the body reuse and recycle biotin, a B vitamin found in foods such as liver, egg yolks, and milk. Biotinidase removes biotin that is bound to proteins in food, leaving the vitamin in its free (unbound) state. The body needs free biotin to activate enzymes called biotin-dependent carboxylases. These carboxylases are involved in many critical cellular functions, including the production and breakdown of proteins, fats, and carbohydrates.

In addition to processing biotin obtained from the diet, biotinidase recycles biotin within the body. As biotin-dependent carboxylases are broken down, they release a molecule called biocytin. Biocytin is a complex made of up biotin and a protein building block (amino acid) called lysine. Biotinidase splits this complex, making free biotin available for reuse by other carboxylase enzymes.

Researchers suspect that biotinidase may have several additional functions. This enzyme may transport free biotin through the bloodstream. It might also have the ability to attach biotin to certain proteins through a process called biotinylation. Within the nucleus, biotinylation of DNA-associated proteins called histones may help determine whether certain genes are turned on or off. It is unclear, however, whether biotinidase plays a role in regulating gene activity.

biotinidase deficiency - caused by mutations in the BTD gene

More than 100 mutations in the BTD gene have been identified in people with biotinidase deficiency. Most of these mutations change single amino acids used to build biotinidase. These changes occur in critical regions of the enzyme, and reduce or eliminate the enzyme's activity.

Most BTD mutations cause profound biotinidase deficiency. This severe form of the disorder results when the activity of biotinidase is reduced to less than 10 percent of normal. Other mutations cause a milder form of the condition called partial biotinidase deficiency. These mutations reduce biotinidase activity to between 10 percent and 30 percent of normal. Without enough of this enzyme, biotin cannot be separated from proteins or recycled normally. As a result, not enough free biotin is available to activate biotin-dependent carboxylases and the body is less able to process important nutrients. These defects underlie the potentially serious medical problems associated with biotinidase deficiency.