Avidin is a protein found in the eggs of birds, amphibians, and reptiles. It is produced in the oviducts of these animals. An oviduct is the route leading from the ovaries to the outside of the body in non-mammalian vertebrates.
In eggs, avidin accounts for only 0.05 percent of their total protein. Avidin is tetramic, which means that it is composed of four identical subunits. The most important action of this protein is that it binds very easily to the vitamin biotin.
Biotin is a water-soluble vitamin that is necessary for several body functions. It is required for cell growth, fatty acid production, and the metabolism of both amino acids and fats. This vitamin also assists in the transfer of carbon dioxide, and promotes hair and nail growth. Natural sources of this vitamin include bananas, salmon, liver, and egg yolks. Bacteria present in the human intestinal tract also produce biotin.
With the abundance of biotin sources available, combined with the fact that the human body needs very little of it to function, a deficiency in this vitamin is rare. A diet high in uncooked egg whites or other forms of raw eggs can lead to a deficiency of this vitamin. The avidin present in the eggs will bind to any biotin present in the body, rendering it useless. When an egg is cooked, the avidin inside the egg white is deactivated, but biotin present in the egg yolk is unaffected.
A biotin deficiency causes hair loss, a scaly rash around the eyes, and high cholesterol in the blood. It also causes neurological symptoms, such as depression, hallucinations, and numbness in the hands and feet. A person lacking in biotin is often said to have a biotin-deficient face characterized by an eye rash and an odd distribution of fat in the facial area.
There are also genetic disorders that cause an increased need for biotin, and certain procedures, such as a stomach removal, can have the same effect. Excessive consumption of alcohol also leads to a greater need for biotin. Pregnancy is associated with a greater need for this vitamin, but only limited research has been done to determine the reason for this need.
Another form of avidin is streptavidin. It is produced by the bacterium streptomyces avidinii, and has the same affinity for biotin as avidin. Streptavidin also has a tetrmaic structure. This protein is used in laboratory applications, such as cell and tissue staining, and is also used as a biotin detector.