The goal of this review is to place the exciting advances that have occurred in our understanding of the molecular biology of the types 1, 2, and 3 (D1, D2, and D3, respectively) iodothyronine deiodinases into a biochemical and physiological context. We review new data regarding the mechanism of selenoprotein synthesis, the molecular and cellular biological properties of the individual deiodinases, including gene structure, mRNA and protein characteristics, tissue distribution, subcellular localization and topology, enzymatic properties, structure-activity relationships, and regulation of synthesis, inactivation, and degradation. These provide the background for a discussion of their role in thyroid physiology in humans and other vertebrates, including evidence that D2 plays a significant role in human plasma T3 production. We discuss the pathological role of D3 overexpression causing “consumptive hypothyroidism” as well as our current understanding of the pathophysiology of iodothyronine deiodination during illness and amiodarone therapy. Finally, we review the new insights from analysis of mice with targeted disruption of the Dio2 gene and overexpression of D2 in the myocardium. (Endocrine Reviews 23: 38–89, 2002)

Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases.

The requirement of the trace element selenium for life and its beneficial role in human health has been known for several decades. This is attributed to low molecular weight selenium compounds, as well as to its presence within at least 25 proteins, named selenoproteins, in the form of the amino acid selenocysteine (Sec). Incorporation of Sec into selenoproteins employs a unique mechanism that involves decoding of the UGA codon. This process requires multiple features such as the selenocysteine insertion sequence (SECIS) element and several protein factors including a specific elongation factor EFSec and the SECIS binding protein 2, SBP2. The function of most selenoproteins is currently unknown; however, thioredoxin reductases (TrxR), glutathione peroxidases (GPx) and thyroid hormone deiodinases (DIO) are well characterised selenoproteins involved in redox regulation of intracellular signalling, redox homeostasis and thyroid hormone metabolism. Recent evidence points to a role for selenium compounds as well as selenoproteins in the prevention of some forms of cancer. A number of clinical trials are either underway or being planned to examine the effects of selenium on cancer incidence. In this review we describe some of the recent progress in our understanding of the mechanism of selenoprotein synthesis, the role of selenoproteins in human health and disease and the therapeutic potential of some of these proteins.

https://www.liebertpub.com/doi/pdf/10.1089/ars.2007.1528

From selenium to selenoproteins: Synthesis, identity, and their role in human health

In mammalian tissues, at least two isozymes of 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD) catalyze the interconversion of hormonally active C11-hydroxylated corticosteroids (cortisol, corticosterone) and their inactive C11-keto metabolites (cortisone, 11-dehydrocorticosterone). The type 1 and type 2 11 beta-HSD isozymes share only 14% homology and are separate gene products with different physiological roles, regulation, and tissue distribution. 11 beta-HSD2 is a high affinity NAD-dependent dehydrogenase that protects the mineralocorticoid receptor from glucocorticoid excess; mutations in the HSD11B2 gene explain an inherited form of hypertension, the syndrome of apparent mineralocorticoid excess in which cortisol acts as a potent mineralocorticoid. By contrast, 11 beta-HSD1 acts predominantly as a reductase in vivo, facilitating glucocorticoid hormone action in key target tissues such as liver and adipose tissue. Over the 10 years, 11 beta-HSD has progressed from an enzyme merely involved in the peripheral metabolism of cortisol to a crucial pre-receptor signaling pathway in the analysis of corticosteroid hormone action. This review details the enzymology, molecular biology, distribution, regulation, and function of the 11 beta-HSD isozymes and highlights the clinical consequences of altered enzyme expression.

11 beta-Hydroxysteroid dehydrogenase.

The iodothyronine deiodinases initiate or terminate thyroid hormone action and therefore are critical for the biological effects mediated by thyroid hormone. Over the years, research has focused on their role in preserving serum levels of the biologically active molecule T3 during iodine deficiency. More recently, a fascinating new role of these enzymes has been unveiled. The activating deiodinase (D2) and the inactivating deiodinase (D3) can locally increase or decrease thyroid hormone signaling in a tissue- and temporal-specific fashion, independent of changes in thyroid hormone serum concentrations. This mechanism is particularly relevant because deiodinase expression can be modulated by a wide variety of endogenous signaling molecules such as sonic hedgehog, nuclear factor-κB, growth factors, bile acids, hypoxia-inducible factor-1α, as well as a growing number of xenobiotic substances. In light of these findings, it seems clear that deiodinases play a much broader role than once thought, with great ramifications for the control of thyroid hormone signaling during vertebrate development and metamorphosis, as well as injury response, tissue repair, hypothalamic function, and energy homeostasis in adults.

Cellular and Molecular Basis of Deiodinase-Regulated Thyroid Hormone Signaling

THYROXINE (T4), measured as protein-bound iodine, was the first hormone to be quantitated in human serum. As a consequence, it was possible to demonstrate clear relationships between circulating T4 concentrations and many of the physiological manifestations of thyroid hormone excess or deficiency. In 1952, 3,5,3′-triiodothyronine (T3) was identified in human plasma and metabolic studies in man soon led to the realization that this hormone was 3–4 times as potent metabolically as T4 (1–4). At that time, the question was first raised as to whether T4 or T3 was the “active” thyroid hormone (3). This question became even more pertinent when it was suggested first in 1955 (5) and shown conclusively in 1970 that T4 could be converted to T3 by peripheral human tissues (6, 7). These studies were made possible by the development of methods for quantitating T3 in human serum, initially by competitive protein binding techniques, followed soon thereafter by specific and sensitive RIAs.

Relationships between circulating and intracellular thyroid hormones: physiological and clinical implications.

Renal membranes (crude microsomal fraction) which catalyze the thiol-dependent outer ring deiodination of LT4 with the formation of L-T3 are also capable of the 5′-deiodination of rT3 with the production of equivalent amounts of I and 3,3′-T2. rT3 deiodination was followed by measuring the amount of 125I released from 125I-labeled (3′ or 5′-) L-rT3; iodide and iodothyronines were separated by a rapid method using Dowex 50W cation exchange columns. Iodide release was proportional to tissue concentration, was heat labile, and was unaffected by methylmercaptoimidazole. The reaction had a rapid phase (∼ 10–15 sec) which was unaffected by thiols and a slower thiol-dependent phase which was linear with time. Double reciprocal plots of product formation vs. rT3 concentration at fixed thiol concentrations yielded a family of parallel lines. Parallel lines were also obtained when the thiol concentration was variable and the rT3 concentration was fixed. These patterns of initial velocity kinetics resemble those obt...

Iodothyronine 5′-Deiodinase from Rat Kidney: Substrate Specificity and the 5′-Deiodination of Reverse Triiodothyr onine

Enzymatic 5′-deiodination of T4 in various tissues is known to be stimulated by thiols and inhibited by propylthiouracil (PTU). Dithiothreitol (DTT) stimulation of rat kidney T4 5′-deiodinase followed saturation kinetics. Inhibition of the enzyme by PTU (10-5 M) was overcome in a concentration-dependent manner by DTT, methimazole (MMI), and thiourea. Pretreatment of catalytically active kidney microsomal preparations with PTU caused persistent inhibition of enzyme activity, assayed in the absence of PTU. Similarly, injection of PTU in rats in a dosage of 5 μg/100 g BW or greater significantly impaired T4 5′-deiodination in subsequently isolated kidney microsomal preparations. Reagents that cleave disulfide bonds (DTT and KCN), as well as the reductant, Na2S2O4, and the thioureylene, MMI, partially restored activity to PTU-inactivated enzyme in a time- and concentration-dependent way. These observations suggest that PTU inhibition of T4 5′-deiodinase results from an interaction of PTU with the enzyme, poss...

Thyroxine 5′-Deiodinase Activity of Rat Kidney: Observations on Activation by Thiols and Inhibition by Propylthiouracil

Preincubation of canine thyroid slices in KRP for 1–2 hr increased the organic binding of iodine by such slices and their response to TSH and other stimulators of iodination. TSH, dibutyryl cyclic 3′,5 ′-AMP (dbcAMP), NaF and prostaglandin Ei (PGE1) each stimulated organic binding of iodine and formation of thyroxine in preincubated slices and their effects were not additive. Anti-TSH rabbit serum blocked the effect of TSH on organisation, but not the effect of dbcAMP. Stimulation of organic binding by TSH, dbcAMP, NaF and PGE1 was still apparent in the presence of perchlorate. The presence of hydrogen peroxide in the medium enhanced organic binding of iodide and thyroxine formation in thyroid slices and this effect was not additive with the effects of TSH, dbcAMP, NaF or PGE1. Under conditions where organic binding by slices was blocked, T/M radioiodine concentration ratios after 30 min and 6 hr incubation periods were lower than controls in the presence of TSH, dbcAMP, NaF and PGE1. Adenyl cyclase activ...

Iodine metabolism in thyroid slices: effects of TSH, dibutyryl cyclic 3',5'-AMP, NaF and prostaglandin E-1.

Rat kidney homogenates, in phosphate-EDTA buffer, consistently catalyzed the formation of T3 from added L-thyroxine (T4). The formation of T3 was assessed by both paper chromatography and RIA of T3. Conversion of T4 to T3 appeared to be enzymatic, showing pH and temperature optima (pH 7.0 and 37 C, respectively) and tissue and time dependence. Formation of T3 was unaffected by azide, cyanide, or catalase, nor was it dependent upon oxygen; indeed, under anaerobic conditions conversion of T4 to T3 was enhanced. Dialyzed homogenate retained full activity, and no cofactor requirement was demonstrated. A role of iron and thiol groups in the enzymatic formation of T3 from T4 was suggested by the inhibitory action of iron chelators and thiol-blocking reagents. The capacity of kidney for T3 formation was considerable and increased with increasing T4 concentrations, being approximately 2 nmol/g tissue/h at very high T4 levels. The apparent Km was estimated to be 3 x 10(-6) M. The conversion of T4 to T3 was inhibited by propylthiouracil at micromolar concentrations whereas methimazole, iodide, and lithium salts were without effect. The enzymatic activity of the homogenates was associated with its particulate components, the readily sedimenting fractions corresponding to plasma membranes and mitochondria being most active, and was absent from nuclei and cytosol.

Conversion of L-thyroxine to triiodothyronine in rat kidney homogenate.

T4 5′-deiodinase, found in cell membranes of kidney and liver, is a thiol-dependent enzyme inhibited by propylthiouracil (PTU). Pretreatment of enzymatically active kidney membrane preparations (Mx) with PTU (10 μm) and increasing concentrations of L-T4 under N2 resulted in increasing degrees of persistent inhibition, as assayed in the absence of pretreatment reagents. Aerobic pretreatment with PTU in the presence or absence of L-T4 resulted in 80% inhibition of T4 5′-deiodination. Sulfhydryl modification of kidney Mx with iodoacetate or Nethylmaleimide irreversibly inactivated T4 5′-deiodinase. The formation of a reversible PTU-enzyme complex was found to protect T4 5′-deiodination from irreversible inactivation by iodoacetate or N-ethylmaleimide. A dose-dependent increase in the 35S content of kidney Mx was found after injection of [35S]thiouracil (TU) into rats, and the Mx 35S content was reduced 4-fold by simultaneous injection of methimazole. Treatment of kidney Mx, containing bound ;ISS, with 10 mm ...

Isadore N. Rosenberg

Papers

Iodothyronine 5′-Deiodinase from Rat Kidney: Substrate Specificity and the 5′-Deiodination of Reverse Triiodothyr onine

Thyroxine 5′-Deiodinase Activity of Rat Kidney: Observations on Activation by Thiols and Inhibition by Propylthiouracil

Iodine metabolism in thyroid slices: effects of TSH, dibutyryl cyclic 3',5'-AMP, NaF and prostaglandin E-1.

Conversion of L-thyroxine to triiodothyronine in rat kidney homogenate.

Characterization of essential enzyme sulfhydryl groups of thyroxine 5'-deiodinase from rat kidney.