Academic literature on the topic 'Thyroid Hormone Synthesis'

Thyroid hormones are important modulators of lipid metabolism because the liver is a primary hormonal target. The hypolipidaemic effects of thyroid hormones result from the balance between direct and indirect actions resulting in stimulation of lipid synthesis and lipid oxidation, which favours degradation pathways. Originally, it was believed that thyroid hormone activity was only transduced by alteration of gene transcription mediated by the nuclear receptor thyroid hormone receptors, comprising the classic action of thyroid hormone. However, the discovery of other effects independent of this classic mechanism characterised a new model of thyroid hormone action, the non-classic mechanism that involves other signalling pathways. To date, this mechanism and its relevance have been intensively described. Considering the increasing evidence for non-classic signalling of thyroid hormones and the major influence of these hormones in the regulation of lipid metabolism, we reviewed the role of thyroid hormone in cytosolic signalling cascades, focusing on the regulation of second messengers, and the activity of effector proteins and the implication of these mechanisms on the control of hepatic lipid metabolism.

Iodine is an essential micronutrient for producing thyroid hormone (TH); however, iodide excess can lead to adverse thyroidal effects. Unfortunately, the lack of a proper in vitro model system hampered the studies of the effect of iodide excess on thyroid physiology and pathology. Here, we demonstrated that excessive iodide intake downregulated the genes related to TH synthesis in the thyroids of mice. Since sodium iodide has no effect on these genes in cultured cell lines, we developed a three-dimensional (3D) culture system to enable the murine thyrocytes to form organoids in vitro with thyroid follicle-like structures and function and found that the in vivo effect of iodide excess could be mimicked in these thyroid organoids. Our data indicate that iodide excess mainly activated the XBP1-mediated unfolded protein response in both murine thyroid and thyroid organoids, while activation of XBP1 was able to mimic the sodium iodide effect on genes for the synthesis of TH in murine thyroid organoids. Lastly, our results suggest that XBP1 might transcriptionally repress the genes involved in the synthesis of TH. Based on these findings, we propose that iodide excess inhibits the transcription of genes related to TH synthesis through a mechanism involving XBP1-mediated action.

The adequate availability and metabolism of three essential trace elements, iodine, selenium and iron, provide the basic requirements for the function and action of the thyroid hormone system in humans, vertebrate animals and their evolutionary precursors. Selenocysteine-containing proteins convey both cellular protection along with H2O2-dependent biosynthesis and the deiodinase-mediated (in-)activation of thyroid hormones, which is critical for their receptor-mediated mechanism of cellular action. Disbalances between the thyroidal content of these elements challenge the negative feedback regulation of the hypothalamus–pituitary–thyroid periphery axis, causing or facilitating common diseases related to disturbed thyroid hormone status such as autoimmune thyroid disease and metabolic disorders. Iodide is accumulated by the sodium-iodide-symporter NIS, and oxidized and incorporated into thyroglobulin by the hemoprotein thyroperoxidase, which requires local H2O2 as cofactor. The latter is generated by the dual oxidase system organized as ‘thyroxisome’ at the surface of the apical membrane facing the colloidal lumen of the thyroid follicles. Various selenoproteins expressed in thyrocytes defend the follicular structure and function against life-long exposure to H2O2 and reactive oxygen species derived therefrom. The pituitary hormone thyrotropin (TSH) stimulates all processes required for thyroid hormone synthesis and secretion and regulates thyrocyte growth, differentiation and function. Worldwide deficiencies of nutritional iodine, selenium and iron supply and the resulting endemic diseases are preventable with educational, societal and political measures.

A paper published in this issue of the Journal of Endocrinology has revisited the hypothesis that thyroid hormones may be generated by tissues outside the thyroid gland in higher organisms including mammals. This commentary appraises the strengths and weaknesses of the study, the alternative explanations for the findings and possible future measures to investigate further. The concept of extrathyroidal thyroxine and triiodothyronine synthesis has previously been proposed; by assuming that Nagao et al. and earlier authors are correct, the plausibility and possible mechanisms underlying the hypothesis are discussed.

Thyroid hormones circulate in blood mainly bound to plasma proteins. Transthyretin is the major thyroxine plasma carrier in mice. Studies in transthyretin-null mice revealed that the absence of transthyretin results in euthyroid hypothyroxinemia and normal thyroid hormone tissue distribution, with the exception of the choroid plexus in the brain. Therefore, transthyretin does not influence normal thyroid hormone homeostasis under standard laboratory conditions. To investigate if transthyretin has a buffer/storage role we challenged transthyretin-null and wild-type mice with conditions of increased hormone demand: (i) exposure to cold, which elicits thermogenesis, a process that requires thyroid hormones; and (ii) thyroidectomy, which abolishes thyroid hormone synthesis and secretion and induces severe hypothyroidism. Transthyretin-null mice responded as the wild-type both to changes induced by stressful events, namely in body weight, food intake and thyroid hormone tissue content, and in the mRNA levels of genes whose expression is altered in such conditions. These results clearly exclude a role for transthyretin in thyroid hormone homeostasis even under conditions of increased hormone demand.

Abstract. Thyroid hormones play an important role in body homeostasis by facilitating metabolism of lipids and glucose, regulating metabolic adaptations, responding to changes in energy intake, and controlling thermogenesis. Proper metabolism and action of these hormones requires the participation of various nutrients. Among them is zinc, whose interaction with thyroid hormones is complex. It is known to regulate both the synthesis and mechanism of action of these hormones. In the present review, we aim to shed light on the regulatory effects of zinc on thyroid hormones. Scientific evidence shows that zinc plays a key role in the metabolism of thyroid hormones, specifically by regulating deiodinases enzymes activity, thyrotropin releasing hormone (TRH) and thyroid stimulating hormone (TSH) synthesis, as well as by modulating the structures of essential transcription factors involved in the synthesis of thyroid hormones. Serum concentrations of zinc also appear to influence the levels of serum T3, T4 and TSH. In addition, studies have shown that Zinc transporters (ZnTs) are present in the hypothalamus, pituitary and thyroid, but their functions remain unknown. Therefore, it is important to further investigate the roles of zinc in regulation of thyroid hormones metabolism, and their importance in the treatment of several diseases associated with thyroid gland dysfunction.

Deuterium, a stable isotope of hydrogen, is abundant in organisms. It is known to produce various biological effects. However, its impact in thyroid hormone synthesis and secretion is poorly studied. The aim of this investigation was to evaluate the dynamics of thyroid hormones and pituitary thyroid-stimulating hormone secretion during bilateral shifts in deuterium supply and assess a possible role of the Na+/I− symporter (NIS), the main iodide transporter, in altered thyroid function. The experiment was performed on adult male Wistar rats, which consumed deuterium-depleted ([D] = 10 ppm) and deuterium-enriched ([D] = 500,000 ppm) water for 21 days. The assessment of total thyroxine and triiodothyronine and their free fractions, as well as thyroid-stimulating hormone in blood serum, revealed the rapid response of the thyroid gland to shifts in the deuterium/protium balance. The present investigation shows that the bilateral changes in the deuterium body content similarly modulate thyroid hormone production and functional activity of the pituitary gland, but the responses of the thyroid and pituitary glands differ. The response of the thyroid cells was to increase the synthesis of the hormones and the pituitary thyrotropes, in order to reduce the production of the thyroid-stimulating hormone. The evaluation of NIS serum levels found a gradual increase in the rats that consumed deuterium-enriched water and no differences in the group exposed to deuterium depletion. NIS levels in both groups did not correlate with thyroid hormones and pituitary thyroid-stimulating hormone production. The data obtained show that thyroid gland has a higher sensitivity to shifts in the deuterium body content than the hypothalamic–pituitary complex, which responded later but similarly in the case of deuteration or deuterium depletion. It indicates a different sensitivity of the endocrine glands to alterations in deuterium content. It suggests that thyroid hormone production rate may depend on deuterium blood/tissue and cytosol/organelle gradients, which possibly disturb the secretory process independently of the NIS.

The prevalence of thyroid diseases including autoimmune hyperthyroidism (Graves’ disease), autoimmune thyroiditis, and diff erent forms of thyroid cancer is increasing every year, while the eff ectiveness of their treatment remains low and is limited, mainly, to replacement therapy with thyroid hormones and surgical and radioisotope methods. This review presents the current state of the problem of pharmacological correction in thyroid diseases including new approaches to the regulation of the functional activity of the components of the thyroid hormone synthesis system in thyroid follicular cells, in particular, its initial, sensory component, the thyroid-stimulating hormone receptor. Among the drugs that are currently being developed, it is necessary to focus on allosteric regulators of the thyroid-stimulating hormone receptor, specifi c antibodies to it, as well as selective agonists of the β-isoform of thyroid hormone receptors.

ABSTRACT Growth hormone (GH) and the thyroid hormones interact in the hypothalamus, pituitary and peripheral tissues. Thyroid hormone exerts a permissive effect upon the anabolic and metabolic effects of GH, and increases pituitary synthesis of this protein hormone. GH depresses the secretion of thyrotropin and the thyroid hormones and increases the peripheral conversion of thyroxine to triiodothyronine. In the adult male rat experimental hypothyroidism produced by ingestion of propylthiouracil depresses the GH secretory response to GH-releasing hormone in vivo and in vitro, reflecting the lowered pituitary stores of GH in the hypothyroid state. Short term administration of large amounts of thyroxine with induction of the hyperthyroid state does not affect the in vivo GH secretory response to GH-releasing hormone in this animal.

Abstract Many xenobiotics are identified as potential thyroid disruptors due to their action to reduce circulating levels of thyroid hormone, most notably thyroxine (T4). Developmental neurotoxicity is a primary concern for thyroid disrupting chemicals yet correlating the impact of chemically induced changes in serum T4 to perturbed brain development remains elusive. A number of thyroid-specific neurodevelopmental assays have been proposed, based largely on the model thyroid hormone synthesis inhibitor propylthiouracil (PTU). This study examined whether thyroid disrupting chemicals acting distinct from synthesis inhibition would result in the same alterations in brain as expected with PTU. The perfluoroalkyl substance perfluorohexane sulfonate (50 mg/kg/day) and the antimicrobial Triclosan (300 mg/kg/day) were administered to pregnant rats from gestational day 6 to postnatal day (PN) 21, and a number of PTU-defined assays for neurotoxicity evaluated. Both chemicals reduced serum T4 but did not increase thyroid stimulating hormone. Both chemicals increased expression of hepatic metabolism genes, while thyroid hormone-responsive genes in the liver, thyroid gland, and brain were largely unchanged. Brain tissue T4 was reduced in newborns, but despite persistent T4 reductions in serum, had recovered in the PN6 pup brain. Neither treatment resulted in a low dose PTU-like phenotype in either brain morphology or neurobehavior, raising questions for the interpretation of serum biomarkers in regulatory toxicology. They further suggest that reliance on serum hormones as prescriptive of specific neurodevelopmental outcomes may be too simplistic and to understand thyroid-mediated neurotoxicity we must expand our thinking beyond that which follows thyroid hormone synthesis inhibition.

Les espèces réactives de l’oxygène ont initialement été identifiées comme des produits délétères dérivés du métabolisme aérobie. Il est maintenant accepté que ces espèces sont produites de manière régulée par des enzymes et interviennent dans des fonctions cellulaires telles que la défense immunitaire, la signalisation intracellulaire, la biosynthèse des hormones et la modification de matrice extracellulaire. Les NADPH (Nicotinamide Adénine Dinucléotide Phosphate) oxydases (NOX) forment une famille d’enzymes transmembranaires capables de former de l’anion superoxyde (O2.-) par transfert d’électrons du NADPH à l’oxygène moléculaire (O2). DUOX1 et DUOX2 sont deux des sept membres composant cette famille qui génèrent directement de l’H2O2 comme produit de réduction de l’O2.

Initialement clonés à partir de la thyroïde dans notre laboratoire, les ADNc codant pour les protéines DUOX ont été identifiées dans d’autres tissus, comme par exemple la prostate ou l’épithélium respiratoire où DUOX1 est majoritaire. DUOX2 se retrouve également dans les glandes salivaires, dans la muqueuse rectale et tout le long du tractus digestif. D’autre part, un orthologue de DUOX, appelé Udx1, a été identifié en 2004 au niveau de la membrane ovocytaire chez l’oursin. Dans chacun de ces tissus, l’H2O2 produit par les protéines joue un rôle clef.

Le mécanisme d’activation de DUOX dans tous ces tissus n’a été identifié que récemment. En effet, pour être exprimé sous forme active à la surface cellulaire, les protéines DUOX nécessitent un facteur de maturation spécifique. Ces facteurs, appelés DUOXA1 et 2 pour « DUOX activator », suivent l’expression tissulaire de leur DUOX respectif. Nous avons montré que la région COOH-terminale de DUOXA1 est responsable de l’activité génératrice d’H2O2 de DUOX1. DUOX2 peut produire de l’H2O2 ou de l’O2.-. L’extrémité NH2-terminale de DUOXA2 est critique dans cette activité et détermine le type de dérivé oxygéné produit. Dans notre système, DUOXA2 n’est pas détecté à la surface cellulaire, sauf en cas de modification de son extrémité amino-terminale par l’addition d’un épitope. DUOXA1 peut être exprimé à la membrane plasmique mais sa présence n’est pas nécessaire au sein du complexe formé avec DUOX pour que ce dernier soit actif. Les facteurs de maturation jouent donc un rôle de protéine chaperonne, induisant la maturation et la translocation d’une protéine DUOX active à la surface cellulaire.

Dans la thyroïde, l’H2O2 produit par les protéines DUOX constitue le cofacteur de la thyroperoxydase catalysant l’oxydation de l’iode et le couplage de sa forme oxydée sur des résidus tyrosines de la thyroglobuline menant in fine à la synthèse des hormones thyroïdiennes T3 et T4 et leur relarguage dans la circulation sanguine. Plusieurs mutations dans le gène DUOX2 ont déjà été décrites chez des patients atteints de dyshormonogenèse transitoire ou permanente. Nous avons mis en évidence qu’une inactivation totale de la protéine DUOX2 était compatible avec un état hypothyroïdien peu sévère et transitoire, indiquant l’intervention probable de DUOX1 dans la synthèse des hormones thyroïdiennes. Le défaut génétique identifié est composé d’une délétion génomique partielle d’un allèle associée à une mutation faux-sens (G1518S) sur l’autre allèle du patient hypothyroïdien. Cette mutation, située dans le site catalytique de l’enzyme, mène à une abolition de l’activité de l’enzyme qui est néanmoins exprimée partiellement à la surface cellulaire.

Les messagers des DUOX ont été identifiés récemment dans les tractus digestif et respiratoire. Le rôle joué par l’H2O2 dans ces tissus semble avant tout être un rôle de défense contre les micro-organismes en mettant en jeu la lactoperoxydase oxydant le thiocyanate en composé bactéricide actif. Nous avons montré que l’H2O2 produit par DUOX exerce un effet répulsif sur les bactéries. En effet, l’invasion de cellules CHO exprimant de manière stable DUOX2 et DUOXA2 par Salmonella Typhimurium est diminuée lorsque la production d’H2O2 de ces cellules est stimulée. Cet effet répulsif constituerait un rôle primordial pour DUOX au niveau des muqueuses respiratoire et digestive.

Lors de la fertilisation, une explosion respiratoire a lieu et de l’H2O2 est produit. Cet H2O2 fourni à l’ovoperoxydase permettrait la formation d’une enveloppe rigide autour de l’ovocyte, bloquant ainsi l’entrée de spermatozoïdes surnuméraires. Ce phénomène a été largement étudié dans l’ovocyte d’oursin, dans lequel la NADPH oxydase responsable de la production d’H2O2 a été caractérisée: il s’agit de Udx1, l’orthologue de DUOX. Chez les mammifères, le phénomène existe mais le mécanisme est en grande partie inconnu. Nous avons montré que les ARNm des DUOX sont exprimés dans l’ovocyte humain ;ceci nous permet d’émettre l’hypothèse que l’inhibition de la polyspermie chez l’homme pourrait être similaire à celle de l’oursin.

Doctorat en Sciences biomédicales et pharmaceutiques
info:eu-repo/semantics/nonPublished

A series of twenty two 2-phenoxybenzoic acids were prepared in various yields using two different methods. It was found that the method involving a modified Ullmann coupling of a phenol with 2-chlorobenzoic acid gave the best yields. Reaction between diphenyliodo-2-carboxylate and a phenoxide gave lower yields and xanthone as byproduct. Using the modified Ullmann reaction 3-phenoxybenzoic acid and two thiophenoxybenzoic acids were also prepared in good yield. Transthyretin (TTR) was isolated from human plasma by column chromatography and the prepared phenoxybenzoic acids were tested for binding to the isolated protein. The most potent compounds were found to be 3-phenoxybenzoic acid (26), 2-(2,4- dimethylphenoxy)benzoic acid (10), 2-(2,3-dimethylphenoxy)benzoic acid (9) and 2- (3-iodophenoxy)benzoic acid (25). All of the compounds tested were up to two orders of magnitude less active than the corresponding nitrogen bridged anthranilic acids, indicating that replacing the nitrogen atom with either an oxygen or sulfur lowers activity. Interestingly, 3-phenoxybenzoic acid binds more strongly to TTR than 2-phenoxybenzoic acid and is almost as active as 2-phenoxyanthranilic acid, indicating that the preparation of analogues with the carboxylic acid in alternative positions on the ring may lead to even more potent anti-thyroid drugs.

Thyroxine (T4), the main secretory hormone of the thyroid gland, is produced on thyroglobulin by thyroid peroxidase (TPO)/hydrogen peroxide/iodide system. The synthesis of T4 by TPO involves two independent steps: iodination of tyrosine and phenolic coupling of the resulting iodotyrosine residues. The prohormone T4 is then converted to its biologically active form T3 by a selenocysteine-containing iodothyronine deiodinase (ID-I), which is present in highest amounts in liver, kidney, thyroid and pituitary. The 5'-deiodination catalyzed by ID-I is a ping-pong, bisubstrate reaction in which the selenol (or selenolate) group of the enzyme (E-SeH or E-Se-) first reacts with thyroxine (T4) to form a selenenyl iodide (E-SeI) intermediate. Subsequent reaction of the selenenyl iodide with an as yet unidentified intracellular cofactor completes the catalytic cycle and regenerates the selenol. Although the deiodination reactions are essential for the function of thyroid gland, the activation of thyroid stimulating hormone (TSH) receptor by auto-antibodies leads to an overproduction of thyroid hormones. In addition, these antibodies stimulate ID-I and probably other deiodinases to produce relatively more amount of T3. Figure 1. Synthesis of thyroid hormones by heme-containing Thyroid Peroxidase(TPO)(Refer PDF File) As these antibodies are not under pituitary feedback control system, there is no negative influence on the thyroid activity and, therefore, the uncontrolled production of thyroid hormones leads to a condition called “hyperthyroidism”. Under these conditions, the overproduction of T4 and T3 can be controlled by specific inhibitors, which either block the thyroid hormone biosynthesis or reduce the conversion of T4 to T3. A unique class of such inhibitors is the thiourea drugs, methimazole (1, MMI), 6-n-propyl-2-thiouracil (3, PTU), and 6-methyl-2-thiouracil (5, MTU). Although these compounds are the most commonly employed drugs in the treatment of hyperthyroidism, the detailed mechanism of their action is still not clear. According to the initially proposed mechanism, these drugs may divert oxidized iodides away from thyroglobulin by forming stable electron donor-acceptor complexes with diiodine, which can effectively reduce the thyroid hormone biosynthesis. It has also been proposed that these drugs may block the thyroid hormone synthesis by coordinating to the metal center of thyroid peroxidase (TPO). After the discovery that the ID-I is responsible for the activation of thyroxine, it has been reported that PTU, but not MMI, reacts with the selenenyl iodide intermediate (E-SeI) of ID-I to form a selenenyl sulfide as a dead end product, thereby blocking the conversion of T4 to T3 during the monodeiodination reaction. The mechanism of anti-thyroid activity is further complicated by the fact that the gold-containing drugs such as gold thioglucose (GTG) inhibit the deiodinase activity by reacting with the selenol group of the native enzyme. Recently, the selenium analogues 2 (MSeI), 4 (PSeU) and 6 (MSeU) attracted considerable attention because these compounds are expected to be more nucleophilic than their sulfur analogues and the formation of an –Se–Se– bond may occur more readily than the formation of an –Se–S– bond with the ID-I enzyme. However, the data derived from the inhibition of TPO by selenium compounds show that these compounds may inhibit the TPO activity by a different mechanism. Therefore, further studies are required to understand the mechanism by which the selenium compounds exert their inhibitory action. Our initial attempts to isolate 2 were unsuccessful and the final stable compound in the synthesis was characterized to be the diselenide (8). In view of the current interest in anti-thyroid drugs and their mechanism, we extended our approach to the synthesis and biological activities of a number of sulfur and selenium derivatives bearing the methimazole pharmacophore. The thesis consists of five chapters. The first chapter gives a general introduction to thyroid hormone synthesis and anti-thyroid drugs. In this chapter, the biosynthesis of thyroid hormones, structure and function of heme peroxidases, activation of thyroid hormones by iodothyronine deiodinases are discussed. This chapter also gives a brief introduction to some common problems associated with the thyroid gland, with a particular emphasis on hyperthyroidism. The structure and activity of some commonly used anti-thyroid drugs and the role of selenium in thyroid are discussed. The literature references related to this work are provided at the end of the chapter. The second chapter deals with the synthesis and characterization of the selenium analogue (MSeI) of anti-thyroid drug methimazole and a series of organoselenium compounds bearing N-methylimidazole pharmacophore are described. The clinically employed anti-thyroid drug, methimazole (MMI), exists predominantly in its thione form, which is responsible for its anti-thyroidal activity. The selenium analogue MSeI, on the other hand, is not stable in air and spontaneously oxidizes to the corresponding diselenide (MSeIox). Experimental and theoretical studies on MSeI suggest that this compound exists in a zwitterionic form in which the selenium atom carries a large negative charge. The structure of MSeI was studied in solution by NMR spectroscopy and the 77Se NMR chemical shift shows a large upfield shift (-5 ppm) in the signal as compared to the true selones for which the signals normally appear in the downfield range (500-2500 ppm). This confirms that MSeI exists predominantly in its zwitterionic form in solution. Our theoretical studies show that the formation of the diselenide (MSeIox) from selenol tautomer is energetically more favored than the formation of the disulfide (MMIox) from the thiol tautomer of MMI. This study also shows that the replacement of the N−H group in MSeI by an N-methyl or N-benzyl substituent does not affect the nature of C−Se bond. In the third chapter, the inhibition of lactoperoxidase-catalyzed oxidation of ABTS by anti-thyroid drugs and related derivatives is described. The commonly used anti-thyroid agent methemazole (MMI) inhibits the lactoperoxidase (LPO) with an IC50 value of 7.0 µM which is much lower than that of the other two anti-thyroid drugs, PTU and MTU. The selenium analogue of methimazole (MSeI) also inhibits LPO with an IC50 value of 16.4 µM, which is about 4-5 times lower than that of PTU and MTU. In contrast to thiones and selones, the S- and Se-protected compounds do not show any noticeable inhibition under identical experimental conditions. While the inhibition of LPO by MMI cannot be reversed by increasing the hydrogen peroxide concentration, the inhibition by MSeI can be completely reversed by increasing the peroxide concentration. Some of the selenium compounds in the present study show interesting anti-oxidant activity in addition to their inhibition propertities. In the presence of glutathione (GSH), MSeI constitutes a redox cycle involving a catalytic reduction of H2O2 and thereby mimics the glutathione peroxidase (GPx) activity in vitro. These studies reveal that the degradation of the intracellular H2O2 by the selenium analogues of anti-thyroid drugs may be beneficial to the thyroid gland as these compounds may act as antioxidants and protect thyroid cells from oxidative damage. Because the drugs with an action essentially on H2O2 can reversibly inhibit thyroid peroxidase, such drugs with a more controlled action could be of great importance in the treatment of hyperthyroidism. Figure 2. (A) Concentration-inhibition curves for the inhibition of LPO-catalyzed oxidation of ABTS by MMI and MSeI at pH 7.0 and 30 °C. (B) Plot of initial rates (vo) for the LPO-catalyzed oxidation of ABTS vs concentration of H2O2. (a) Control activity, (b) 40 µM of MSeI, (c) 40 µM of MSeIox, (d) 80 µM of PTU, (e) 80 µM of MTU, (f) 40 µM of MMI. The incubation mixture contained 6.5 nM LPO, 1.4 mM ABTS, 0.067 M phosphatebuffer(pH7).(Refer PDF File) The fourth chapter describes the inhibition of lactoperoxidase (LPO)-catalyzed iodination of L-tyrosine by anti-thyroid drug methimazole (MMI) and its selenium analogue (MSeI). These inhibition studies show that MSeI inhibits LPO with an IC50 value of 12.4 µM, which is higher than that of MMI (5.2 µM). The effect of hydrogen peroxide on the inhibition of LPO by MMI and MSeI is also discussed. These studies also reveal that the inhibition of LPO-catalyzed iodination by MSeI can be completely reversed by increasing the peroxide concentration. On the other hand, the inhibition by MMI cannot be reversed by increasing the concentration of the peroxide. To under stand the nature of compounds formed in the reactions between anti-thyroid drugs and iodine, the reactions of MSeI with molecular iodine is described. MSeI reacts with I2 to produce novel ionic diselenides, and the nature of the species formed in this reaction appears to be solvent dependent. The formation of ionic species (mono and dications) in the reaction is confirmed by UV-Vis, FT-IR and FT-Raman spectroscopic investigations and single crystal x-ray studies. The major conclusion drawn from this study is that MSeI reacts with iodine, even in its oxidized form, to form ionic diselenides containing iodide or polyiodide anions, which might be possible intermediates in the inhibition of thyroid hormones. Dication X-ray crystal structure of the monocation X-ray crystal structure of the dication In the fifth chapter, the synthesis and characterization of several thiones and selones having N,N-disubstituted imidazole moiety are described. Experimental and theoretical studies were performed on a number of selones, which suggest that these compounds exist as zwitterions in which the selenium atom carries a large negative charge. The structures of selones were studied in solution by NMR spectroscopy and the 77Se NMR chemical shifts for the selones show large upfield shifts in the signals, confirming the zwitterionic structure of the selones in solution. The thermal isomerization of some S- and Se-substituted methyl and benzyl imidazole derivatives to produce the thermodynamically more stable N-substituted derivatives is described. A structure–activity correlation was attempted on the inhibition of LPO-catalyzed oxidation and iodination reactions by several thiouracil compounds, which indicates that the presence of an n-propyl group in PTU is important for an efficient inhibition. In contrast to the S- and Se-substituted derivatives, the selones produced by thermal isomerization exhibited efficient inhibition, indicating the importance of reactive selone (zwitterionic) moiety in the inhibition. The inhibition data on another well-known anti-thyroid agent carbimazole (CBZ) support the assumption that CBZ acts as a prodrug, requiring a conversion to methimazole (MMI) for its inhibitory action on thyroid peroxidase. (Refer pdf file/original thesis)

Thyroxine (T4), the main secretory hormone of the thyroid gland, is produced on thyroglobulin by thyroid peroxidase (TPO)/hydrogen peroxide/iodide system. The synthesis of T4 by TPO involves two independent steps: iodination of tyrosine and phenolic coupling of the resulting iodotyrosine residues. The prohormone T4 is then converted to its biologically active form T3 by a selenocysteine-containing iodothyronine deiodinase (ID-I), which is present in highest amounts in liver, kidney, thyroid and pituitary. The 5'-deiodination catalyzed by ID-I is a ping-pong, bisubstrate reaction in which the selenol (or selenolate) group of the enzyme (E-SeH or E-Se-) first reacts with thyroxine (T4) to form a selenenyl iodide (E-SeI) intermediate. Subsequent reaction of the selenenyl iodide with an as yet unidentified intracellular cofactor completes the catalytic cycle and regenerates the selenol. Although the deiodination reactions are essential for the function of thyroid gland, the activation of thyroid stimulating hormone (TSH) receptor by auto-antibodies leads to an overproduction of thyroid hormones. In addition, these antibodies stimulate ID-I and probably other deiodinases to produce relatively more amount of T3. Figure 1. Synthesis of thyroid hormones by heme-containing Thyroid Peroxidase(TPO)(Refer PDF File) As these antibodies are not under pituitary feedback control system, there is no negative influence on the thyroid activity and, therefore, the uncontrolled production of thyroid hormones leads to a condition called “hyperthyroidism”. Under these conditions, the overproduction of T4 and T3 can be controlled by specific inhibitors, which either block the thyroid hormone biosynthesis or reduce the conversion of T4 to T3. A unique class of such inhibitors is the thiourea drugs, methimazole (1, MMI), 6-n-propyl-2-thiouracil (3, PTU), and 6-methyl-2-thiouracil (5, MTU). Although these compounds are the most commonly employed drugs in the treatment of hyperthyroidism, the detailed mechanism of their action is still not clear. According to the initially proposed mechanism, these drugs may divert oxidized iodides away from thyroglobulin by forming stable electron donor-acceptor complexes with diiodine, which can effectively reduce the thyroid hormone biosynthesis. It has also been proposed that these drugs may block the thyroid hormone synthesis by coordinating to the metal center of thyroid peroxidase (TPO). After the discovery that the ID-I is responsible for the activation of thyroxine, it has been reported that PTU, but not MMI, reacts with the selenenyl iodide intermediate (E-SeI) of ID-I to form a selenenyl sulfide as a dead end product, thereby blocking the conversion of T4 to T3 during the monodeiodination reaction. The mechanism of anti-thyroid activity is further complicated by the fact that the gold-containing drugs such as gold thioglucose (GTG) inhibit the deiodinase activity by reacting with the selenol group of the native enzyme. Recently, the selenium analogues 2 (MSeI), 4 (PSeU) and 6 (MSeU) attracted considerable attention because these compounds are expected to be more nucleophilic than their sulfur analogues and the formation of an –Se–Se– bond may occur more readily than the formation of an –Se–S– bond with the ID-I enzyme. However, the data derived from the inhibition of TPO by selenium compounds show that these compounds may inhibit the TPO activity by a different mechanism. Therefore, further studies are required to understand the mechanism by which the selenium compounds exert their inhibitory action. Our initial attempts to isolate 2 were unsuccessful and the final stable compound in the synthesis was characterized to be the diselenide (8). In view of the current interest in anti-thyroid drugs and their mechanism, we extended our approach to the synthesis and biological activities of a number of sulfur and selenium derivatives bearing the methimazole pharmacophore. The thesis consists of five chapters. The first chapter gives a general introduction to thyroid hormone synthesis and anti-thyroid drugs. In this chapter, the biosynthesis of thyroid hormones, structure and function of heme peroxidases, activation of thyroid hormones by iodothyronine deiodinases are discussed. This chapter also gives a brief introduction to some common problems associated with the thyroid gland, with a particular emphasis on hyperthyroidism. The structure and activity of some commonly used anti-thyroid drugs and the role of selenium in thyroid are discussed. The literature references related to this work are provided at the end of the chapter. The second chapter deals with the synthesis and characterization of the selenium analogue (MSeI) of anti-thyroid drug methimazole and a series of organoselenium compounds bearing N-methylimidazole pharmacophore are described. The clinically employed anti-thyroid drug, methimazole (MMI), exists predominantly in its thione form, which is responsible for its anti-thyroidal activity. The selenium analogue MSeI, on the other hand, is not stable in air and spontaneously oxidizes to the corresponding diselenide (MSeIox). Experimental and theoretical studies on MSeI suggest that this compound exists in a zwitterionic form in which the selenium atom carries a large negative charge. The structure of MSeI was studied in solution by NMR spectroscopy and the 77Se NMR chemical shift shows a large upfield shift (-5 ppm) in the signal as compared to the true selones for which the signals normally appear in the downfield range (500-2500 ppm). This confirms that MSeI exists predominantly in its zwitterionic form in solution. Our theoretical studies show that the formation of the diselenide (MSeIox) from selenol tautomer is energetically more favored than the formation of the disulfide (MMIox) from the thiol tautomer of MMI. This study also shows that the replacement of the N−H group in MSeI by an N-methyl or N-benzyl substituent does not affect the nature of C−Se bond. In the third chapter, the inhibition of lactoperoxidase-catalyzed oxidation of ABTS by anti-thyroid drugs and related derivatives is described. The commonly used anti-thyroid agent methemazole (MMI) inhibits the lactoperoxidase (LPO) with an IC50 value of 7.0 µM which is much lower than that of the other two anti-thyroid drugs, PTU and MTU. The selenium analogue of methimazole (MSeI) also inhibits LPO with an IC50 value of 16.4 µM, which is about 4-5 times lower than that of PTU and MTU. In contrast to thiones and selones, the S- and Se-protected compounds do not show any noticeable inhibition under identical experimental conditions. While the inhibition of LPO by MMI cannot be reversed by increasing the hydrogen peroxide concentration, the inhibition by MSeI can be completely reversed by increasing the peroxide concentration. Some of the selenium compounds in the present study show interesting anti-oxidant activity in addition to their inhibition propertities. In the presence of glutathione (GSH), MSeI constitutes a redox cycle involving a catalytic reduction of H2O2 and thereby mimics the glutathione peroxidase (GPx) activity in vitro. These studies reveal that the degradation of the intracellular H2O2 by the selenium analogues of anti-thyroid drugs may be beneficial to the thyroid gland as these compounds may act as antioxidants and protect thyroid cells from oxidative damage. Because the drugs with an action essentially on H2O2 can reversibly inhibit thyroid peroxidase, such drugs with a more controlled action could be of great importance in the treatment of hyperthyroidism. Figure 2. (A) Concentration-inhibition curves for the inhibition of LPO-catalyzed oxidation of ABTS by MMI and MSeI at pH 7.0 and 30 °C. (B) Plot of initial rates (vo) for the LPO-catalyzed oxidation of ABTS vs concentration of H2O2. (a) Control activity, (b) 40 µM of MSeI, (c) 40 µM of MSeIox, (d) 80 µM of PTU, (e) 80 µM of MTU, (f) 40 µM of MMI. The incubation mixture contained 6.5 nM LPO, 1.4 mM ABTS, 0.067 M phosphatebuffer(pH7).(Refer PDF File) The fourth chapter describes the inhibition of lactoperoxidase (LPO)-catalyzed iodination of L-tyrosine by anti-thyroid drug methimazole (MMI) and its selenium analogue (MSeI). These inhibition studies show that MSeI inhibits LPO with an IC50 value of 12.4 µM, which is higher than that of MMI (5.2 µM). The effect of hydrogen peroxide on the inhibition of LPO by MMI and MSeI is also discussed. These studies also reveal that the inhibition of LPO-catalyzed iodination by MSeI can be completely reversed by increasing the peroxide concentration. On the other hand, the inhibition by MMI cannot be reversed by increasing the concentration of the peroxide. To under stand the nature of compounds formed in the reactions between anti-thyroid drugs and iodine, the reactions of MSeI with molecular iodine is described. MSeI reacts with I2 to produce novel ionic diselenides, and the nature of the species formed in this reaction appears to be solvent dependent. The formation of ionic species (mono and dications) in the reaction is confirmed by UV-Vis, FT-IR and FT-Raman spectroscopic investigations and single crystal x-ray studies. The major conclusion drawn from this study is that MSeI reacts with iodine, even in its oxidized form, to form ionic diselenides containing iodide or polyiodide anions, which might be possible intermediates in the inhibition of thyroid hormones. Dication X-ray crystal structure of the monocation X-ray crystal structure of the dication In the fifth chapter, the synthesis and characterization of several thiones and selones having N,N-disubstituted imidazole moiety are described. Experimental and theoretical studies were performed on a number of selones, which suggest that these compounds exist as zwitterions in which the selenium atom carries a large negative charge. The structures of selones were studied in solution by NMR spectroscopy and the 77Se NMR chemical shifts for the selones show large upfield shifts in the signals, confirming the zwitterionic structure of the selones in solution. The thermal isomerization of some S- and Se-substituted methyl and benzyl imidazole derivatives to produce the thermodynamically more stable N-substituted derivatives is described. A structure–activity correlation was attempted on the inhibition of LPO-catalyzed oxidation and iodination reactions by several thiouracil compounds, which indicates that the presence of an n-propyl group in PTU is important for an efficient inhibition. In contrast to the S- and Se-substituted derivatives, the selones produced by thermal isomerization exhibited efficient inhibition, indicating the importance of reactive selone (zwitterionic) moiety in the inhibition. The inhibition data on another well-known anti-thyroid agent carbimazole (CBZ) support the assumption that CBZ acts as a prodrug, requiring a conversion to methimazole (MMI) for its inhibitory action on thyroid peroxidase. (Refer pdf file/original thesis)

碩士
國立嘉義大學
水生生物科學系研究所
99
There were two aims in this study: (1) to confirm which type of thyroid hormone of tilapia was gifted from maternal origin; (2) when did thyroid hormone start to synthesize in tilapia larvae. We identified that 100 % of the yolk disappeared at 9 dph (day post hatching). We also observed whole body of T3 content was gradually lower before 11dph, but T4 contents can’t be detected at the same stage. Besides, TU can not inhibit T3 synthesis in 0- 13 dph, but inhibited effects by TU was observed during 13- 26 dph. According to the results, we suggested that the female fish transfer T3 to its filial generation, but not T4. On the other hand, the first thyroid follicular cells were found at 3 dph, and the immuno-cytochemistry (ICC) signals of TSH were located at pituitary of 9 dph tilapia. The T4 content was detected at 13 dph stage, and the type I deiodinase (D1) mRNA expression was started to increase at 15 dph. Based on these data, we confirmed that the maternal orgin provid T3 not T4 to the eggs. The thyroid hormone synthesis of tilapia was started at 9- 13 dph, and it showed a secretion peak on T4 at 19 dph.

yes
Thyroid hormone (TH) is essential for adult brain function and its actions include several key roles in the hypothalamus. Although TH controls gene expression via specific TH receptors of the nuclear receptor class, surprisingly few genes have been demonstrated to be directly regulated by TH in the hypothalamus, or the adult brain as a whole. This study explored the rapid induction by TH of retinaldehyde dehydrogenase 1 (Raldh1), encoding a retinoic acid (RA)-synthesizing enzyme, as a gene specifically expressed in hypothalamic tanycytes, cells that mediate a number of actions of TH in the hypothalamus. The resulting increase in RA may then regulate gene expression via the RA receptors, also of the nuclear receptor class. In vivo exposure of the rat to TH led to a significant and rapid increase in hypothalamic Raldh1 within 4 hours. That this may lead to an in vivo increase in RA is suggested by the later induction by TH of the RA-responsive gene Cyp26b1. To explore the actions of RA in the hypothalamus as a potential mediator of TH control of gene regulation, an ex vivo hypothalamic rat slice culture method was developed in which the Raldh1-expressing tanycytes were maintained. These slice cultures confirmed that TH did not act on genes regulating energy balance but could induce Raldh1. RA has the potential to upregulate expression of genes involved in growth and appetite, Ghrh and Agrp. This regulation is acutely sensitive to epigenetic changes, as has been shown for TH action in vivo. These results indicate that sequential triggering of two nuclear receptor signalling systems has the capability to mediate some of the functions of TH in the hypothalamus.

“Female Sexual Biochemistry” reviews the key hormones and neurotransmitters that have a major role in female sexuality. Estrogens—estradiol, estrone, and estriol—as well as major androgens, such as testosterone and dihydrotestosterone (DHT), are presented with a discussion of their role in the support of the reproductive organs and genitals as well as their actions on the central nervous system to affect sexual desire, arousal, and responsiveness. The interaction and regulation of estrogen by progesterone and thyroid hormone is included. A review of the dual-control model of sexual responsiveness is presented, including excitatory and inhibitory factors, as well as a summary of major neurotransmitters that work to enhance sexual arousal or inhibit it. The sexual response cycle is reviewed and relevant changes in pregnancy, childbirth, perimenopause, and menopause are presented. Finally, there is mention of how synthetic hormones and environmental toxins with hormone activity may alter a woman’s sexual response.

There is a direct correlation between successful metamorphosis from larvae to post-larvae and the quality of the resultant juveniles or fry. Juvenile quality, in turn, is a major factor influencing fish production level and market price. However, following the profound morphological and physiological changes occurring during metamorphosis, the emerging juveniles in some species characteristically demonstrate heterotrophic growth, poor pigmentation, cannibalism and generally poor survival. The white grouper (Epinephelus aeneus) in Israel and the Pacific threadfin (Polydactylussexfilis) in Hawaii are two promising candidates for mariculture that have high market value but a natural fishery that has sharply declined in recent years. Unfortunately, their potential for culture is severely hampered by variable metamorphic success limiting their production. The main objective was to compare the efficacy and economic viability of dietary or environmental iodine on metamorphic success and juvenile quality in the white grouper and the pink snapper which would lead to improved commercial rearing protocols and increased production of these species both in Israel and the US. The Hawaii Institute of Marine Biology encountered problems with the availability of pink snapper brood stock and larvae and changed to Pacific threadfin or moi which is rapidly becoming a premier aquaculture species in Hawaii and throughout the Indo-Pacific. The white grouper brood stock at the National Center for Mariculture was lost as a result of a viral outbreak following the sudden breakdown of the ozone purification system. In addition, the NCM suffered a devastating fire in the fall of 2007 that completely destroyed the hatchery and laboratory facilities although the BARD project samples were saved. Nevertheless, by studying alternate species a number of valuable findings and conclusions that can contribute to improved metamorphosis in commercially valuable marine species resulted from this collaborative effort. The Israeli group found that exposing white grouper larvae to external TH levels synchronized and increased the rate of metamorphosis. This suggested that sub-optimal synthesis of TH may be a major factor causing size heterogeneity in the larval population and high mortality through cannibalism by their larger more metamorphosed cohorts. Two protocols were developed to enrich the larvae with higher levels of the TH precursor, iodine; feeding iodine enriched Artemia or increasing the level of seawater iodine the larvae are exposed to. Results of accumulated iodine in gilthead seabream larvae indicated that the absorption of iodine from the water is markedly more efficient than feeding iodine enriched Artemia nauplii. Samples for TH, which will be analyzed shortly, will be able to determine if another dietary factor is lacking to effectively utilize surplus tissue iodine for TH synthesis. Moreover, these samples will also clarify which approach to enriching larvae with iodine, through the live food or exposure to iodine enriched seawater is the most efficient and cost effective. The American group found that moi larvae reared in ocean water, which possessed substantially higher iodine levels than those found in seawater well water, grew significantly larger, and showed increased survival compared with well water reared larvae. Larvae reared in ocean water also progressed more rapidly through developmental stages than those in low-iodine well seawater. In collaboration with Israeli counterparts, a highly specific and precise radioimmunoassay procedure for thyroid hormones and cortisol was developed. Taken altogether, the combined Hawaiian and Israeli collaborative research suggests that for teleost species of commercial value, adequate levels of environmental iodine are more determinate in metamorphosis than iodine levels in the live zooplankton food provided to the larvae. Insuring sufficiently high enough iodine in the ambient seawater offers a much more economical solution to improved metamorphosis than enriching the live food with costly liposomes incorporating iodine rich oils.