RAINBOW-TROUT LIVER EXPRESSES 2 IODOTHYRONINE PHENOLIC RING DEIODINASE PATHWAYS WITH THE CHARACTERISTICS OF MAMMALIAN TYPE-I AND TYPE-II 5'-DEIODINASES

Deiodinases are major determinants of thyroid hormone tissue availabil ity 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 T-3 (rT (3)) and T-4 as substrates. The use of rT(3) disclosed two enzymatic c omponents with the characteristics of mammalian types I and II 5'-deio dinases. The high rT(3)-K-m type I 5'-deiodinase activity (180 nM) has a low cofactor requirement (5 mM dithiothreitol) and is relatively se nsitive to propylthiouracil inhibition, whereas the low rT(3)-K-m acti vity was akin to the outer ring deiodination of T-4 in these regards. The use of T-4 exhibited only a single type of activity with a low K-m (0.63 nM), a relatively high cofactor requirement (25 mM dithiothreit ol), and propylthiouracil-resistance. Teleosts constitute a unique exa mple of type II activity expression in the liver of an adult vertebrat e. Furthermore, the V-max of this enzyme is as high as that found in c omparable homogenates from hypothyroid mammalian tissues, whereas the V-max of the type I activity is lower than that of mammalian liver. Th ese findings are in consonance with the peculiar kinetics of T-3 in tr out liver, kinetics remarkably similar to those of the mammalian pitui tary, cerebral cortex, and brown adipose tissue, which also preferenti ally express type II deiodinase.