The site-specific basicity of thyroid hormones and their precursors as regulators of their biological functions.
Summary of "The site-specific basicity of thyroid hormones and their precursors as regulators of their biological functions."
The complete macro- and microequilibrium analyses of thyroxine, liothyronine, reverse liothyronine and their biological precursors - diiodotyrosine, monoiodotyrosine and tyrosine are presented. Their biosyntheses, receptor- and transport protein-binding are shown to be distinctively dependent on the phenolate basicity. The protonation macroconstants were determined by (1)H NMR-pH and/or UV-pH titrations. Microconstants of the minor microspecies were determined by deductive methods, in which O-methylated and carboxymethylated derivatives were synthesized, and the combination of their NMR-pH and UV-pH titration provided the experimental base to evaluate all the microconstants. NMR-pH profiles, macro-, and microscopic protonation schemes, and species-specific diagrams are included. Biosyntheses of the thyroid hormones take place by oxidative coupling of two iodotyrosine residues catalyzed by thyreoperoxidase in thyreoglobulin. On the grounds of our phenolate microconstants of precursors the thyroxine over liothyronine ratio needs to be 9:1 after their biosynthesis in thyroid gland, which is in good agreement with biochemical data. The microconstants show that the phenolates are in proton donor (-OH) form in liothyronine whereas they occur in proton acceptor (-O(-)) form in thyroxine at the pH of blood. These facts explain several facts that have previously been empirically known: the affinity of liothyronine for the receptor is higher than that of thyroxine, the affinity of thyroxine for the transport proteins is higher than that of liothyronine and the selectivity of thyroxine for the OATP1C1 organic anion transporter is higher than that of liothyronine.
Affiliation
Department of Pharmaceutical Chemistry, Semmelweis University, Research Group of Drugs of Abuse and Doping Agents, Hungarian Academy of Sciences, Budapest H-1092, Hőgyes Endre u. 9, Hungary.
Journal Details
This article was published in the following journal.
Name: Journal of pharmaceutical and biomedical analysis
ISSN: 1873-264X
Pages: 156-64
Links
- PubMed Source: http://www.ncbi.nlm.nih.gov/pubmed/22177412
- DOI: http://dx.doi.org/10.1016/j.jpba.2011.11.020
Medical and Biotech [MESH] Definitions
Hyperthyroidism
Hypersecretion of THYROID HORMONES from the THYROID GLAND. Elevated levels of thyroid hormones increase BASAL METABOLIC RATE.
Receptors, Thyroid Hormone
Specific high affinity binding proteins for THYROID HORMONES in target cells. They are usually found in the nucleus and regulate DNA transcription. These receptors are activated by hormones that leads to transcription, cell differentiation, and growth suppression. Thyroid hormone receptors are encoded by two genes (GENES, ERBA): erbA-alpha and erbA-beta for alpha and beta thyroid hormone receptors, respectively.
Thyroid Hormones
Natural hormones secreted by the THYROID GLAND, such as THYROXINE, and their synthetic analogs.
Immunoglobulins, Thyroid-stimulating
Autoantibodies that bind to the thyroid-stimulating hormone (TSH) receptor (RECEPTORS, THYROTROPIN) on thyroid epithelial cells. The autoantibodies mimic TSH causing an unregulated production of thyroid hormones characteristic of GRAVES DISEASE.
Diiodotyrosine
A product from the iodination of MONOIODOTYROSINE. In the biosynthesis of thyroid hormones, diiodotyrosine residues are coupled with other monoiodotyrosine or diiodotyrosine residues to form T4 or T3 thyroid hormones (THYROXINE and TRIIODOTHYRONINE).
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