iodothyronine deiodinase

Iodothyronine deiodinase ( 1.97.1.10  ) (DI)  is the vertebrate enzyme responsible for the deiodination of the prohormone thyroxine (T4 or 3,5,3',5'-tetraiodothyronine) into the biologically active hormone T3 (3,5,3'-triiodothyronine) and of T3 into the inactive metabolite T2 (3,3'-diiodothyronine). All known DI are proteins of about 250 residues that contain a selenocysteine at their active site. Three types of DI selenoenzymes are known, type II is essential for providing the brain with the appropriate levels of T3 during the critical period of development, and type III is essential for the regulation of thyroid hormone inactivation during embryological development.