Local levels of active thyroid hormone (3,3′,5-triiodothyronine) are controlled by the action of activating and inactivating iodothyronine deiodinase enzymes. Deiodinases are selenocysteine-dependent membrane proteins catalyzing the reductive elimination of iodide from iodothyronines through a poorly understood mechanism. We solved the crystal structure of the catalytic domain of mouse deiodinase 3 (Dio3), which reveals a close structural similarity to atypical 2-Cys peroxiredoxin(s) (Prx). The structure suggests a route for proton transfer to the substrate during deiodination and a Prx-related mechanism for subsequent recycling of the transiently oxidized enzyme. The proposed mechanism is supported by biochemical experiments and is consistent with the effects of mutations of conserved amino acids on Dio3 activity. Thioredoxin and glutaredoxin reduce the oxidized Dio3 at physiological concentrations, and dimerization appears to activate the enzyme by displacing an autoinhibitory loop from the iodothyronine binding site. Deiodinases apparently evolved from the ubiquitous Prx scaffold, and their structure and catalytic mechanism reconcile a plethora of partly conflicting data reported for these enzymes.hyroid hormones regulate mammalian development as well as energy expenditure and metabolism in the adult (1). The active, nuclear receptor-binding form of thyroid hormone is 3,3′,5-triiodothyronine (T 3 ). The thyroid gland mainly releases thyroxine [3,3′,5,5′-tetraiodothyronine (T 4 ); Fig. 1A], and T 3 is formed and degraded through elimination of iodine atoms from the 5′-and 5-positions, respectively (2) (Fig. S1). Three types of deiodinase enzymes are involved in activation and inactivation of thyroid hormones (Fig. S1). They form a family of transmembrane enzymes with homologous catalytic domains (2). Type II deiodinase (Dio2) catalyzes the activating (outer ring) 5′-deiodination of the prohormone T 4 to T 3 , whereas type III deiodinase (Dio3) catalyzes inactivating (inner ring) 5-deiodination (2). Type I deiodinase (Dio1) is capable of both types of reactions (3). Cells targeted by the hormone can thereby fine-tune intracellular T 3 levels through deiodinase expression according to their needs during development or upon metabolic challenges. Dio1, Dio2, and Dio3 share a conserved amino acid sequence and a selenocysteine residue essential for efficient deiodination, although nonmammalian cysteine-deiodinases have been found (4, 5). Although most of the characterized selenoproteins act as peroxidases or protein reductases, deiodinases are the only selenoenzymes known to catalyze halogen eliminations from aromatic rings. Despite a large body of experimental data, there is no coherent model for deiodinase catalysis that incorporates the bulk of available data. Results and DiscussionTo explore the unique deiodinase catalytic mechanism, we determined the crystal structure of the cytoplasmic catalytic domain of Mus musculus Dio3 (Dio3 cat ) with Sec 170 replaced by cysteine ( Fig. 1 B and C and Table 1). The structu...

Cellular thyroid hormone uptake and efflux are mediated by transmembrane transport proteins. One of these, monocarboxylate transporter 8 (MCT8) is mutated in Allan-Herndon-Dudley syndrome, a severe mental retardation associated with abnormal thyroid hormone constellations. Since mice deficient in Mct8 exhibit a milder neurological phenotype than patients, we hypothesized that alternative thyroid hormone transporters may compensate in murine brain cells for the lack of Mct8. Using qPCR, Western Blot, and immunocytochemistry, we investigated the expression of three different thyroid hormone transporters, i.e., Mct8 and L-type amino acid transporters Lat1 and Lat2, in mouse brain. All three thyroid hormone transporters are expressed from corticogenesis and peak around birth. Primary cultures of neurons and astrocytes express Mct8, Lat1, and Lat2. Microglia specifically expresses Mct10 and Slco4a1 in addition to high levels of Lat2 mRNA and protein. As in vivo, a brain microvascular endothelial cell line expressed Mct8 and Lat1. 158N, an oligodendroglial cell line expressed Mct8 protein, consistent with delayed myelination in MCT8-deficient patients. Functional T(3)- and T(4)-transport assays into primary astrocytes showed K(M) values of 4.2 and 3.7 μM for T(3) and T(4). Pharmacological inhibition of L-type amino acid transporters by BCH and genetic inactivation of Lat2 reduced astrocytic T(3) uptake to the same extent. BSP, a broad spectrum inhibitor, including Mct8, reduced T(3) uptake further suggesting the cooperative activity of several T(3) transporters in astrocytes.

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In hepatocyte-specific conditional mouse models, Secisbp2 gene inactivation is less detrimental than tRNA[Ser]Sec inactivation. A role of Secisbp2 in stabilizing selenoprotein mRNAs in vivo was uncovered.