WHAT IS BIOTINIDASE DEFICIENCY?

Biotinidase deficiency is a treatable, inherited metabolic disorder in which the body cannot process the vitamin biotin in a normal manner.

Metabolic processes occur within the body.
The human body is very complex. Inside of our bodies, thousands of chemical reactions occur involving the foods we eat, the air we breathe, how we sense the environment, and how we develop and grow. Some of these chemical reactions allow substances like foods to be processed or metabolized so that they can be used by the body.

Enzymes carry out metabolic processes.
Enzymes are special proteins made by the body that allow chemical reactions to occur. Enzymes that work properly are said to be active or to have activity. Some enzymes need vitamins in order to be active. Often, the activity of a single enzyme may be required to make several different substances that the body needs.

Sometimes the body does not make enough of a particular enzyme or the enzyme that is made does not work properly. This causes the enzyme to have decreased activity and it cannot carry out its normal work load. Other chemical reactions that rely on that enzyme may also be slowed down or interrupted.

If the enzyme activity is less than 10% of normal activity, it is usually considered to be deficient. Enzyme deficiencies often lead to medical problems. Medical conditions resulting from enzyme deficiencies are called metabolic disorders because the metabolic or chemical processes within the body are not occurring normally.

Biotinidase is an enzyme that sometimes has decreased activity.
Biotinidase is an important enzyme in the body. People with biotinidase deficiency do not have enough biotinidase activity. An individual with biotinidase deficiency usually has enzyme activity that is less than 10% of normal and sometimes there is almost no activity at all. Individuals with less than 10% of normal biotinidase activity are considered to have profound biotinidase deficiency. The parents of children with biotinidase deficiency have approximately 50% of normal biotinidase activity which is usually enough activity to prevent medical problems. People classified as having partial biotinidase deficiency have intermediate levels of biotinidase activity, 10-30% of normal. Figure 1 shows these ranges of biotinidase activity in serum (blood) and compares them with the normal range of activity.

Active biotinidase is needed for the body to process dietary biotin.
Biotinidase is an important enzyme in the biotin cycle, the chain of chemical reactions involved in the use and reuse of the vitamin biotin, as shown in Figure 2. One important role of biotinidase is to separate or free biotin from proteins to which it is bound in foods. Biotin in its free form can then be used by the body. Additionally, biotinidase lets the body recycle or reuse the biotin over and over again so that we do not need to consume large amounts of this vitamin in our diets.

Biotin is an important vitamin.
Free biotin is required to allow a group of enzymes called carboxylases to function properly. Carboxylases are important in the production of certain fats and carbohydrates and for the breakdown of proteins. In order for inactive carboxylases to become active, free biotin must be attached to the amino acid, lysine, at a special site on the inactive carboxylases. Eventually, the carboxylases are broken down, but the biotin stays attached to lysine. This biotin-lysine complex is known as biocytin. Biocytin is normally broken down by biotinidase into free biotin and lysine. In this way, biotin is recycled and can be reused by the carboxylases.

Biotinidase deficiency interrupts the biotin cycle.
When biotinidase activity is deficient, protein-bound biotin in the diet cannot be used by carboxylases. The carboxylases are unable to perform their normal functions, altering fat, carbohydrate and protein metabolism. This causes harmful by-products to collect in the body. Without adequate biotinidase activity, the body needs large amounts of free biotin, more than is supplied by the normal diet. When this is not available, health problems usually result. These are discussed in the next section.

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