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Rainbow Trout Liver Expresses Two Iodothyronine Phenolic Ring Deiodinase Pathways with the Characteristics of Mammalian Types I and II 5'-Deiodinases¹

Centro de Neurobiología (A.O., C.V-R.), Universidad Nacional Autonoma de Mexico, Apartado Postal 70–228, Ciudad Universitaria, México Distrito Federal 04510; and Department of Medicine (J.E.S.), Division of Endocrinology, McGill University, Montreal, Quebec H3T 1E2, Canada

Address all correspondence and requests for reprints to: Aurea Orozco, Centro de Neurobiología, Universidad Nacional Autonoma de Mexico, Apartado Postal 70 228, Ciudad Universitaria 04510, D.F., México. E-mail: aureao{at}servidor.unam.mx

Deiodinases are major determinants of thyroid hormone tissue availability and disposal. The knowledge of the expression of these enzymes in lower species is important to understand evolutionary and ontogenetic aspects of thyroid hormone action and metabolism. Here we have studied outer ring deiodination in the trout liver using both reverse T₃ (rT₃) and T₄ as substrates. The use of rT₃ disclosed two enzymatic components with the characteristics of mammalian types I and II 5'-deiodinases. The high rT₃-K_m type I 5'-deiodinase activity (180 nM) has a low cofactor requirement (5 mM dithiothreitol) and is relatively sensitive to propylthiouracil inhibition, whereas the low rT₃-K_m activity was akin to the outer ring deiodination of T₄ in these regards. The use of T₄ exhibited only a single type of activity with a low K_m (0.63 nM), a relatively high cofactor requirement (25 mM dithiothreitol), and propylthiouracil-resistance. Teleosts constitute a unique example of type II activity expression in the liver of an adult vertebrate. Furthermore, the V_max of this enzyme is as high as that found in comparable homogenates from hypothyroid mammalian tissues, whereas the V_max of the type I activity is lower than that of mammalian liver. These findings are in consonance with the peculiar kinetics of T₃ in trout liver, kinetics remarkably similar to those of the mammalian pituitary, cerebral cortex, and brown adipose tissue, which also preferentially express type II deiodinase.

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