Thyroxine-protein interactions. Interaction of thyroxine and triiodothyronine with human thyroxine-binding globulin.

Korcek, Ladislav; Tabachnick, Milton

doi:10.1016/s0021-9258(17)33380-x

Cited by 65 publications

(7 citation statements)

References 15 publications

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“…In the present study an increase in the H+ ion concentration leads to an increase in the uptake of L-tri-iodothyronine by system I. Because of the pK values of the amino acid and of phenolic groups, 10% of the molecule exists in an ionized state at pH 7.4 (Korcek & Tabachnik, 1976). L-Tri-iodothyronine possesses partial-hydrophilic properties, and decreasing the pH leads to a decrease in the net charge of the molecule.…”

Section: Discussionmentioning

confidence: 47%

Uptake of L-tri-iodothyronine by isolated rat liver cells. A process partially inhibited by metabolic inhibitors; attempts to distinguish between uptake and binding to intracellular proteins

Eckel¹,

Rao²,

Rao³

et al. 1979

Biochemical Journal

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1. Rat liver cells obtained by dispersion with collagenase were used to investigate the mode of entry of L-tri-iodothyronine into the cell. 2. The hormone was taken up very rapidly at 23 degrees C; the linear phase of uptake lasted for up to approx. 20 s. 3. A plot of the initial rates of uptake against different concentrations of L-tri-iodothyronine yielded a sigmoidal curve. The Eadie--Hofstee plot (v/[S]2 versus v) yielded two straight lines. The uptake component with an apparent Kt value of 86 +/- 15 pM was designated as system I, and the second uptake component with an apparent Kt of 726 +/- 11 pM as system II. The Hill plot for system I was not linear; the apparent Hill coefficient for system II was calculated to be 2.1.4. Uptake of L-tri-iodothyronine by system I was higher at pH 6.4 than at pH 7.4; system II was relatively insensitive to changes in the pH of the external medium. 5. Both systems exhibited a transition temperature at about 16 degrees C in the Arrhenius plot. The activation energies of the two systems below and above 16 degrees C were 72.8 and 47.7 and 54.4 and 33.1 J/mol respectively. 6. Inhibitors of cellular energy reduced the uptake by system I to a larger extent than that by system II. 7. Replacement of Na+ in the external medium by either K+ or choline led to uptake that followed normal Michaelis--Menten kinetics. 8. Thiol-group-blocking agents reduced the uptake of the hormone by both systems. 9. Treatment of liver cells with beta-glucosidase, Pronase and neuraminidase led to a decrease in the uptake of L-tri-iodothyronine by system I, whereas uptake by system II was decreased after treatment with phospholipase A2, beta-galactosidase. Pronase and neuraminidase. 10. The stereoisomer D-tri-iodothyronine (100--3000 pM) did not affect system I, but uptake by system II decreased with increasing concentration of D-tri-iodothyronine. Reverse L-tri-iodothyronine (2--100 pM) and L-thyroxine (100--3000 pM) did not influence uptake by either system. 11. Under identical conditions of incubation, the uptake of L-tri-iodothyronine was 3.7 times higher than binding to cytosol proteins. The binding was insensitive to metabolic inhibitors. The results suggest that cytosol proteins are not directly involved in the uptake of L-tri-iodothyronine. 12. Plasma-membrane vesicles also take up the hormone rapidly at 23 degrees C. Increasing the osmolarity of the external medium led to a decrease in the uptake of L-tri-iodothyronine by vesicles. 13. Uptake as a function of L-tri-iodothyronine concentration exhibited a sigmoidal curve. The Eadie--Hofstee plot showed two uptake components with apparent Kt values of 96.8 and 1581 pM. 14. The results of our study are consistent with a carrier-mediated translocation of the hormone into the cell.

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Section: Discussionmentioning

confidence: 47%

Uptake of L-tri-iodothyronine by isolated rat liver cells. A process partially inhibited by metabolic inhibitors; attempts to distinguish between uptake and binding to intracellular proteins

Eckel¹,

Rao²,

Rao³

et al. 1979

Biochemical Journal

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“…The thyroxine-fluorophore was used to determine the proportional change in binding affinity, Kd/Kd 378C that takes place over a range of temperatures in human TBG and its engineered variants (table 1b). The use of the fluorophore-thyroxine Kd/Kd 378C ratios to calculate the proportional changes that will occur to the binding affinity of thyroxine to TBG in plasma is validated in table 1a and figure 2 by the agreement with values independently determined with thyroxine [18] and with the direct assays by others of free-thyroxine concentrations in the blood at room and body temperature [19,20]. The plot of Kd/Kd 378C ratios versus temperature in figure 1d and of consequent blood free-thyroxine concentrations (table 1b and figure 2) demonstrate how changes in the binding affinity of TBG provide an inherent adjustment of thyroxine levels to match metabolic needs.…”

Section: Resultsmentioning

confidence: 94%

“…Thyroxine indices: free thyroxine and % saturation of TBG and its variants over a range of temperatures were calculated using a free-thyroxine concentration at 378C of 20 pM based, with recent updating [16], on a plasma range of 12 -26 pM. The binding constant Kd of thyroxine with TBG at 378C and pH 7.4 has been variously reported (bracketed, inverse Ka Â10(10) M-1): in 1972 [17] as 60 pM (Ka 1.68), more definitively with isolated TBG by Korcek & Tabachnik [18] in 1976 as 110 pM (Ka 0.90), in plasma by Ross & Benraad [19] 1992 as 67 pM (Ka 1.5) and from our own competitive-assay with recombinant TBG [13] as 75 pM (Ka 1.33). We have adopted here the mean of these results, a Kd of 80 pM (Ka 1.25), but note that whatever affinity is chosen will affect only the magnitude of derived values and not the proportional changes central to this paper.…”

Section: Methodsmentioning

confidence: 96%

“…Variation in free thyroxine with temperature from Kd/Kd 378C ratios (hatched bars), 20 pM at 378C rising to 25 pM at 398C with wt-TBG, but dampened to 22 pM in the 191/283 TBG (Aus) variant (upper normal limit, dashed line). The open (non-hatched) bars show values from independent determinations by others using plasma-derived thyroxine[18] and direct assays of plasma-free thyroxine[19,20]. The comparative bars are based on a defined Kd of 80 pM, free thyroxine of 20 pM and a TBG saturation of 20%, at 378C.…”

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confidence: 99%

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Temperature-responsive release of thyroxine and its environmental adaptation in Australians

Chan

Read

et al. 2014

Proc. R. Soc. B.

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The hormone thyroxine that regulates mammalian metabolism is carried and stored in the blood by thyroxine-binding globulin (TBG). We demonstrate here that the release of thyroxine from TBG occurs by a temperature-sensitive mechanism and show how this will provide a homoeostatic adjustment of the concentration of thyroxine to match metabolic needs, as with the hypothermia and torpor of small animals. In humans, a rise in temperature, as in infections, will trigger an accelerated release of thyroxine, resulting in a predictable 23% increase in the concentration of free thyroxine at 39°C. The in vivo relevance of this fever-response is affirmed in an environmental adaptation in aboriginal Australians. We show how two mutations incorporated in their TBG interact in a way that will halve the surge in thyroxine release, and hence the boost in metabolic rate that would otherwise occur as body temperatures exceed 37°C. The overall findings open insights into physiological changes that accompany variations in body temperature, as notably in fevers.

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“…The mathematical model formulated is applicable to free-hormone assay systems involving any number of hormones and binding sites. This technique made it possible to simulate an assay system for free triiodothyronine in which four ligands react with up to 12 binding sites. The mass law equations determining the composition of the assay mixture at equilibrium were solved using a personal computer equipped with commercial software.…”

Section: Introductionmentioning

confidence: 99%

The mathematical theory of complex ligand-binding systems applied to free triiodothyronine immunoassays

Blomberg¹,

Engblom²

1991

Anal. Chem.

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A theoretical basis for direct immunoassays of free hormones in serum is presented. The multiple-ligand/multiple-site binding theory employed makes it possible to predict the distribution of hormones among exogenous and endogenous binding proteins in the assay mixture. The model allows simulation of assay systems involving any number of ligands and binding sites. The simulation of an assay of free triiodothyronine illustrates the way in which assay parameters such as antibody concentration, antibody affinity, serum dilution, and labeled hormone interactions with serum binding proteins affect the validity of free hormone assays. The simultaneous equations describing these complex binding systems at equilibrium were solved on a personal computer, with the use of commercial mathematics software. This general method for solving and modelling free hormone assay systems provides a tool for predicting the behavior of free-hormone assays.

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Thyroxine-protein interactions. Interaction of thyroxine and triiodothyronine with human thyroxine-binding globulin.

Cited by 65 publications

References 15 publications

Uptake of L-tri-iodothyronine by isolated rat liver cells. A process partially inhibited by metabolic inhibitors; attempts to distinguish between uptake and binding to intracellular proteins

Uptake of L-tri-iodothyronine by isolated rat liver cells. A process partially inhibited by metabolic inhibitors; attempts to distinguish between uptake and binding to intracellular proteins

Temperature-responsive release of thyroxine and its environmental adaptation in Australians

The mathematical theory of complex ligand-binding systems applied to free triiodothyronine immunoassays

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