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Iodothyronine deiodinases ( and ) are a subfamily of deiodinase enzymes important in the activation and deactivation of thyroid hormones. Thyroxine (T4), the precursor of 3,5,3’-triiodothyronine (T3) is transformed into T3 by deiodinase activity. T3, through binding a nuclear thyroid hormone receptor, influences the expression of genes in practically every vertebrate cell. Iodothyronine deiodinases are unusual in that these enzymes contain selenium, in the form of an otherwise rare amino acid selenocysteine. These enzymes are not to be confused with the iodotyrosine deiodinases that are also deiodinases, but not members of the iodothyronine family. The iodotyrosine deiodinases (unlike the iodothyronine deiodinases) do not use selenocysteine or selenium. The iodotyrosine enzymes work on iodinated single tyrosine residue molecules to scavenge iodine, and do not use as substrates the double-tyrosine residue molecules of the various iodothyronines. == Activation and inactivation == In tissues, deiodinases can either activate or inactivate thyroid hormones: * ''Activation'' occurs by conversion of the prohormone thyroxine (T4) to the active hormone triiodothyronine (T3) through the removal of an iodine atom on the outer ring. * ''Inactivation'' of thyroid hormones occurs by removal of an iodine atom on the inner ring, which converts thyroxine to the inactive reverse triiodothyronine (rT3), or which converts the active triiodothyronine to diiodothyronine (T2). The major part of thyroxine deiodination occurs within the cells. Deionidase 2 activity can be regulated by ubiquitination: * The covalent attachment of ubiquitin inactivates D2 by disrupting dimerization and targets it to degradation in the proteosome.〔 * Deubiquitination removing ubiquitin from D2 restores its activity and prevents proteosomal degradation. * The Hedgehog cascade acts to increase D2 ubiquitination through WSB1 activity, decreasing D2 activity.〔 D-propranolol inhibits thyroxine deiodinase, thereby blocking the conversion of T4 to T3, providing some though minimal therapeutic effect. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Iodothyronine deiodinase」の詳細全文を読む スポンサード リンク
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