Define Iodothyronine Deiodinases - functions of selenium in humans?

Another group of selenoproteins are the iodothyronine deiodinases essential for the conversion of thyroxine or tetraiodothyronine (T₄) to its physiologically active form tri-iodothyronine (T₃). Three types of iodothyronine deiodinases have been identified, all of them being selenoproteins. When one iodine is removed from T₄, it is converted T₃. T₃ is more active than T₄. Thus, one of the deiodinase enzymes is involved in activating T+ When one or more iodine is removed from T₃, the resulting molecules do not have enzyme activity. Therefore, another selenium-dependent deiodinase inactivates T.

Type I iodothyronine deiodinase (a selenoprotein) is found in liver, kidney and thyroid tissue. The major role of this enzyme is to provide T₃ to peripheral tissues by deiodinating T₄ secreted by the thyroid gland. Selenium deficiency causes a decline in the type I deiodinase enzyme activity, but it may not result in hypothyroidism as there 'is a compensatory increase in plasma T₄ levels. Type II iodothyronine deiodinase, also a selenoprotein, is present in the brain, pituitary and placenta. The major function of this enzyme is to regulate T₃ levels in these tissues, and control the secretion of thyroid stimulating hormone. Type II enzyme activity is also reduced in selenium deficiency. Type III iodothyroizine deiodinase, another seleiloprotein, is involved mainly in degradation of the T₄ and T₃. How this enzyme is affected in selenium deficiency is not fully investigated. Thus, these selenoprotein enzymes regulate and maintain thyroid levels. Animal studies have shown that a combined deficiency of selenium and iodine produces much more severe hypothyroidism compared to iodine deficiency alone. Further, maternal deficiency of selenium and iodine is implicated in cretinism in newborn- the most severe outcome of thyroid hormone deficiency during pregnancy.