189 REVIEW Iodothyronine deiodinase structure and function: from ascidians to humans Veerle M Darras and Stijn L J Van Herck Animal Physiology and Neurobiology Section, Department of Biology, Laboratory of Comparative Endocrinology, KU Leuven, Naamsestraat 61, PO Box 2464, B-3000 Leuven, Belgium (Correspondence should be addressed to V M Darras; Email:
[email protected]) Abstract Iodothyronine deiodinases are important mediators of thyroid of each of them, however, varies amongst species, develop- hormone (TH) action. They are present in tissues throughout mental stages and tissues. This is especially true for 0 the body where they catalyse 3,5,3 -triiodothyronine (T3) amphibians, where the impact of D1 may be minimal. D2 production and degradation via, respectively, outer and inner and D3 expression and activity respond to thyroid status in ring deiodination. Three different types of iodothyronine an opposite and conserved way, while the response of D1 is deiodinases (D1, D2 and D3) have been identified in variable, especially in fish. Recently, a number of deiodinases vertebrates from fish to mammals. They share several have been cloned from lower chordates. Both urochordates common characteristics, including a selenocysteine residue and cephalochordates possess selenodeiodinases, although in their catalytic centre, but show also some type-specific they cannot be classified in one of the three vertebrate types. differences. These specific characteristics seem very well In addition, the cephalochordate amphioxus also expresses conserved for D2 and D3, while D1 shows more evolutionary a non-selenodeiodinase. Finally, deiodinase-like sequences diversity related to its Km, 6-n-propyl-2-thiouracil sensitivity have been identified in the genome of non-deuterostome and dependence on dithiothreitol as a cofactor in vitro.