title: The site-specific basicity of thyroid hormones and their precursors as regulators of their biological functions.
abstract: 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.
authors.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.
authors.forename: Gergo
authors.initials: G
authors.lastname: Tóth
authors.forename: Sándor
authors.initials: S
authors.lastname: Hosztafi
authors.forename: Zsuzsanna
authors.initials: Z
authors.lastname: Kovács
authors.forename: Béla
authors.initials: B
authors.lastname: Noszál
mesh_terms: D000136:Acid-Base Equilibrium / Q000502:physiology*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D011485:Protein Binding / Q000502:physiology; D011498:Protein Precursors / Q000737:chemistry* / Q000502:physiology; D013056:Spectrophotometry, Ultraviolet / Q000379:methods; D013963:Thyroid Hormones / Q000737:chemistry* / Q000502:physiology; D013974:Thyroxine / Q000737:chemistry / Q000502:physiology; D014284:Triiodothyronine / Q000737:chemistry / Q000502:physiology
chemical_list: D011498:Protein Precursors; D013963:Thyroid Hormones; D014284:Triiodothyronine; D013974:Thyroxine