THYROID Volume 15, Number 8, 2005 © Mary Ann Liebert, Inc. Biochemical Mechanisms of Thyroid Hormone Deiodination George G.J.M. Kuiper, Monique H.A. Kester, Robin P. Peeters, and Theo J. Visser Deiodination is the foremost pathway of thyroid hormone metabolism not only in quantitative terms but also because thyroxine (T4) is activated by outer ring deiodination (ORD) to 3,3’,5-triiodothyronine (T3), whereas both T4 and T3 are inactivated by inner ring deiodination (IRD) to 3,3’,5-triiodothyronine and 3,3’- diiodothyronine, respectively. These reactions are catalyzed by three iodothyronine deiodinases, D1-3. Although they are homologous selenoproteins, they differ in important respects such as catalysis of ORD and/or IRD, deiodination of sulfated iodothyronines, inhibition by the thyrostatic drug propylthiouracil, and regulation during fetal and neonatal development, by thyroid state, and during illness. In this review we will briefly discuss recent developments in these different areas. These have resulted in the emerging view that the biological activity of thyroid hormone is regulated locally by tissue-specific regulation of the different deiodinases. HYROID HORMONE is essential for growth, development, thyrostatic drug 6-propyl-2-thiouracil (PTU). D1 activity is Tand regulation of energy metabolism (1–3). Amphibian positively regulated by T3, reflecting regulation of D1 ex- metamorphosis is an important example of thyroid hormone pression by T3 at the pretranslational level. actions on development (4). Equally well known is the crit- In humans, D2 activity is found in brain, anterior pitu- ical role of thyroid hormone in development and function itary, placenta, thyroid and skeletal muscle, and D2 mRNA of the human central nervous system (5,6). Thyroid hor- has also been detected in the human heart. In rodents D2 is mone is produced by the thyroid in the form of the biolog- also expressed in brown adipose tissue. D2 has only ORD ically inactive precursor thyroxine (T4). The prinicipal bioac- activity, preferring T4 over rT3 as the substrate, with appar- tive form of the hormone is triiodothyronine (T3). In ent Km values in the nanomolar range. In general, D2 activ- humans, only approximately 20% of T3 is secreted by the ity is increased in hypothyroidism and decreased in hyper- thyroid; most circulating T3 is derived from outer ring deio- thyroidism. Both pre- and posttranslational mechanisms are dination (ORD) of T4 in peripheral tissues. Both T4 and T3 involved in the regulation of D2 expression by thyroid state, undergo inner ring deiodination (IRD) to metabolites which with distinct roles for T3, and for T4 and rT3, respectively. do not interact with T3 receptors, reverse triiodothyronine Although perhaps D2 in skeletal muscle may contribute to Ј Ј (rT3) and 3,3 -diiodothyronine (3,3 -T2), respectively. Thus, circulating T3, the enzyme is particularly important for local ORD is regarded as an activating pathway and IRD as an T3 production in brain and anterior pituitary. inactivating pathway. ORD is also the main pathway for the In human and rodents, D3 is located in brain, placenta, metabolism of rT3, representing another route for the gen- pregnant uterus, and fetal tissues. D3 has only IRD activity, Ј eration of 3,3 -T2. Three iodothyronine deiodinases are in- and is thus important for the inactivation of thyroid hor- volved in the deiodination of iodothyronines, namely, mone. It shows preference for T3 over T4 as the substrate, D1–D3 (7–9). with apparent Km values in the nanomolar range. The high In humans and rodents, D1 is located primarily in liver, D3 activity in placenta, pregnant uterus and different fetal kidney, and thyroid. Lower D1 activities are expressed in tissues seems to serve the purpose of protecting the fetus other tissues, including rat anterior pituitary. Although D1 against undue exposure to active thyroid hormone that may has both ORD and IRD activities, it appears particularly im- be detrimental for the development of different tissues, in portant for the generation of plasma T3 and clearance of particular the brain. In brain, D3 activity is increased in hy- plasma rT3. ORD of rT3 is the most efficient reaction cat- perthyroidism and decreased in hypothyroidism but the alyzed by D1, while IRD of both T4 and T3 are strongly ac- mechanism of this regulation remains to be established. celerated by sulfation of these iodothyronines. Michaelis In this short review we will focus on recent insights in Menten constant (Km) values for substrates of D1 are in the deiodinase structure-function relationships and physiologi- micromolar range. The enzyme is potently inhibited by the cal roles of deiodinases. For a more in-depth discussion on Department of Internal Medicine, Erasmus Medical Center, Rotterdam, The Netherlands. 787 788 KUIPER ET AL. the concepts underlying much of the work to be presented Iodothyronine substrate interaction the reader is referred to earlier reviews (8–11). Cloning of D1 from various species (rat, mouse, cat, dog, human) and careful analysis of kinetic properties with a Structure-Function Relationship of range of substrates (T , T , rT , rT S, T , T S) has enabled the Iodothyronine Deiodinases 4 3 3 3 2 2 identification of a region that is involved in substrate inter- Catalytic center action. Comparative structural–functional analysis of human and dog D1 enzymes showed that the region between amino Iodothyronine deiodinases are selenoproteins, containing acid residue 30 and 70 accounts for the difference in K value a single selenocysteine residue (SeC) in the core catalytic cen- m for rT ORD between dog and human D1 (19). Dog D1 has ter. This core catalytic center consists of approximately 15 3 an approximately 30-fold higher K for rT ORD than hu- amino acid residues surrounding the SeC and is highly con- m 3 man D1. More detailed studies demonstrated that it is mainly served within and between the deiodinase subtypes. The SeC the Phe65Leu substitution that explains the slow ORD of rT is encoded by a UGA stop codon which in the presence of a 3 by dog D1 versus human and rat D1 (19). The same type of so-called SECIS (selenocysteine insertion sequence) element studies comparing cat and rat D1 enzyme also indicated that in the 3Ј untranslated region (UTR) is recoded from a stop the region between residue 40 and 70 is involved in substrate to a SeC codon (12). The SeC residue is essential for enzyme interaction. By site-directed mutagenesis it was found that a activity because replacement with Ala or Ser residues (es- combination of mutations was necessary to improve the sentially replacing the SeH group by H or OH) eliminates deiodination of rT by cat D1. For efficient rT deiodination, activity. As far as we know only replacement of SeC with 3 3 a Phe at position 65 and the insertion of the Thr-Gly-Met- Cys (substituting S for Se) in D1, D2, or D3 maintains enzy- Thr-Arg sequence (residue 48–52) as well as the amino acids matic activity, although with strongly reduced substrate Gly and Glu at position 45–46 are essential (20). An intrigu- turnover numbers and significantly increased K values for m ing property of cat D1 is the facilitated deiodination of rT S, the iodothyronine substrates (13–15). For some reason dur- 3 and the combination of the described changes did not affect ing evolution nature has decided that deiodinases should be this property (V /K rT ϭ 3 and V /K rT S ϭ 81). selenoenzymes, despite the fact that the synthesis of a se- max m 3 max m 3 The negatively charged sulfate group of rT S might interact lenoprotein is expensive for the cell. From an energetic point 3 with the positively charged side group of a basic amino acid of view the extraction of iodonium (Iϩ) from an aromatic ring (Lys, Arg), thereby stabilizing interaction with D1. Our stud- is a difficult step and most likely the more negatively charged ies indicated that this basic amino acid is probably situated selenol group (SeH ↔ SeϪ) is better capable of accomplish- outside the region between residue 40 and 70, and remains ing this than the sulfydryl group (SH). The selenol group to be identified. The fact that D1 catalyzes ORD (rT3, rT3S, might interact with side groups of other amino acid residues T ) and IRD (T , T S, T , T S) suggests different orientations facilitating its deprotonation. For D1 the formation of an es- 4 4 4 3 3 of substrate binding within a single site, so that either the sential imidazolium–selenolate ion pair was postulated on iodines of the inner ring or of the outer ring are in close prox- the basis of experiments with histidine-directed reagents and imity of the catalytic center. In other words, the D1 substrate side-directed mutagenesis studies (8,16,17). binding site is very flexible and therefore it will be difficult More detailed insights in deiodinase structure and cat- to gain more insight in the structure–activity relationship by alytic mechanism must come from the three-dimensional site-directed mutagenesis studies with the thus far cloned D1 structure when this is resolved by crystallographic studies. enzymes, unless D1 variants from other species turn up Unfortunately, these studies are greatly hampered by the dif- which for instance lack IRD activity or do not show facili- ficulties encountered with overexpressing these membrane- tated deiodination of sulfated iodothyronines. For similar integrated enzymes in a soluble and active form. An alter- reasons, that is no large variations in kinetic properties for native strategy consists of attempting to model deiodinase different substrates among D2 and D3 enzymes in various structure on the basis of structural resemblance to other pro- species, no progress has been made in the identification of teins for which experimental structure information exists.