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

Abstract

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 T 3 (rT 3 ) and T 4 as substrates. The use of rT 3 disclosed two enzymatic components with the characteristics of mammalian types I and II 5Ј-deiodinases. The high rT 3 -K m type I 5Ј-deiodinase activity (180 nM) has a low cofactor requirement (5 mM dithiothreitol) and is relatively sensitive to propylthiouracil inhibition, whereas the low rT 3 -K m activity 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 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 T 3 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. (Endocrinology 138: 254 -258, 1997) T HE THYROID gland secretes predominantly T 4 , which functions primarily as a prohormone because it is largely converted to T 3 in peripheral tissues. T 3 is about 10 times more active than T 4 and is produced in an organspecific manner by enzymatic outer ring deiodination (ORD) of its precursor. In mammalian species, ORD is catalyzed by two distinct enzymes, well characterized in operational terms and tissue distribution: the type I and type II 5Ј-deiodinases (1-3).The complement of deiodinases present in vertebrate liver varies with the species and developmental stage of the organism. In fetal rats, for example, the liver expresses both type II and type III (an inner ring deiodinase), whereas, after birth, type I activity predominates (Aceves, C., unpublished data from our laboratory). Chicken embryo liver exhibits types I and II (4), whereas, in developing tadpoles, only type III is expressed (5). However variable the expression of deiodinases could be during ontogeny, adult liver of vertebrate species expresses predominantly, if not exclusively, type I deiodinase activity. In fish liver, it seems that two ORD processes are present, but it is not clear whether they correspond to the types I and II mammalian deiodinase activities (6 -8). In trout, for example, enzyme kinetic analyses have demonstrated vastly differing K m values for hepatic T 4 -ORD. Whereas initial studies reported K m values of 190 nm (6) and 13 nm (7), a subsequent report by Mac Latchy and Eales (8) has suggested that tw...

In Vivo and In Vitro Debromination of Decabromodiphenyl Ether (BDE 209) by Juvenile Rainbow Trout and Common Carp

“…Fenton et al (27) have measured DI activity in rainbow trout tissues, including the skin. Other studies have also measured DI activity in rainbow trout brain, liver, and gonads (28)(29)(30)(31)(32). Sweeting and Eales (33) conducted several experiments with rainbow trout liver microsomes and determined that outer ring deiodination of thyroxine (T4) was the dominant thyroid hormone process in these cell fractions.…”

Characterizing the in vitro hepatic biotransformation of the flame retardant BDE 99 by common carp

“…DIs), and specifically to prevent disulfide bonds from forming between active sites in selenocysteine residues. DTT is generally regarded as an essential thiol co-factor for investigating DI activity in in vitro assay systems 30a, 34. Specifically, studies examining DI activity in fish hepatic microsomes have found that DTT is required to maintain DI activity and that increasing the amount of DTT can often increase the level of DI activity 30a, 34.…”

Iodothyronine deiodinases: a functional and evolutionary perspective

“…The difficulty in identifying amphibian D1 activity resulted from the biochemical properties of the enzyme. Indeed, a critically important characteristic of D1-catalyzed deiodination is its remarkable sensitivity to inhibition by 6-n-propyl-2-thiouracil (PTU), with the exception of fish (Orozco et al 1997, Sanders et al 1997) and amphibian D1 , which are less sensitive to this inhibitor. In fact, PTU sensitivity has been used to demonstrate the specificity of T 4 to T 3 conversion and to distinguish D1 from the other Ds.…”