Thyroid Hormones and their Analogues: Synthesis, Reactivity and Cell Membrane Transport Studies

Abstract

The thyroid hormones produced by the thyroid gland are essential for regulating various physiological functions in the human body such as cardiovascular function, normal growth and development, overall metabolism of fat, protein, and carbohydrate. It predominantly produces the prohormone L-thyroxine (T4) and a smaller amount of the physiologically active form triiodo-L-thyronine (T3). After biosynthesis, they are transported to various organs by different transporter proteins like thyroxine-binding globulin (TBG), transthyretin (TTR), and human serum albumin (HSA). The cellular uptake of the hormones, however, is mediated by a specific transmembrane transporter, monocarboxylate transporter 8 (MCT8). After entering inside the cells, thyroid hormones undergo various metabolic processes. One of the major pathways is the deiodination by iodothyronine deiodinase enzymes (DIOs), which contain a selenocysteine residue in their active site. Deiodination from the phenolic ring of L-thyroxine produces the active form, T3, which binds to the nuclear receptors and regulates the biological functions. My thesis work focuses on the synthesis of various thyroid hormone analogues and their activity towards deiodinase mimics. It is known that halogen bond plays a crucial role in the deiodination of T4. Here, I will discuss how modulating the strength of halogen bonds can alter the regioselectivity of T4 deiodination. More specifically an electron withdrawing group substitution at the phenolic ring of T4 favours the phenolic ring deiodination by deiodinase mimics. Similarly, we have synthesized several other thyroid hormones analogues by substituting the iodine atom with other halogen atoms and checked their activity towards deiodinase mimic. Additionally, during this study, we observed an unusual activity of the mimic. In addition to its deiodination activity, it can reduce nitro group to amine group under mild conditions. This method can be applied to synthesize various amine group containing important biomolecules. In the next part of my work, I have presented about the development a novel fluorescent probe for monitoring thyroid hormone transport activity across the plasma membrane through MCT8. The specificity of these molecules towards MCT8 is characterized in different cells including MCT8 KO cell lines as well by using various inhibitors. Further study reveals that free carboxylate moiety is essential for the specificity of probe towards MCT8. The fluorescent probe can be used to monitor the changes in the MCT8 expression in mammalian cells. Our study further revealed that some of the antipsychotic drugs like chlorpromazine and clozapine alter the expression of MCT8, leading to changes in the thyroid hormone uptake. In order to understand the role halogen atom in MCT8 expression by these drugs, we synthesized various other halogenated promazine derivatives and found that halogen atom does play a key role in the altering the MCT8 expression in mammalian cells. In the final chapter, I will discuss about the halogen atom-mediated recognition of iodothyronines by MCT8 for their cellular uptake. Further studies based on the structure activity relationship we have identified the smallest molecular weight compound which can be recognised by MCT8.