Deiodinase
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Deiodinase (eg. EC 1.97.1.10 (http://www.expasy.org/cgi-bin/nicezyme.pl?1.97.1.10)) is an enzyme important in the action of thyroid hormones. In the tissues, deiodinase converts the prohormone thyroxine to the active hormone triiodothyronine by the removal of an iodine atom. The major part of thyroxine deiodination occurs within the target cells.
There are three types of enzymes (in humans ?) that can deiodinate thyroxine. Iodothyronine deiodinase type I (IDI) is commonly found in the liver, kidney, muscle tissue and thyroid gland, and type II (IDII) in the testis and thyroid. Type III (IDIII) is found in the fetal tissue, brain matter and the placenta. IDII can only deiodinate the outer ring of the prohormone thyroxine or the metabolically inactive reverse triiodothyronine. IDIII can only deiodinate the inner ring of thyroxine or triiodothyronine. IDI can deiodinate both rings.
Deiodinases are unusual in that the enzyme contains selenium, in the form of an otherwise rare amino acid selenocysteine.