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  • Title: Retinoic acid inhibition of thyroxine binding to human transthyretin.
    Author: Smith TJ, Davis FB, Deziel MR, Davis PJ, Ramsden DB, Schoenl M.
    Journal: Biochim Biophys Acta; 1994 Jan 05; 1199(1):76-80. PubMed ID: 8280758.
    Abstract:
    All-trans retinoic acid is a potent inhibitor of [125I]-thyroxine (T4) binding to human erythrocyte membranes and can block the activation by thyroid hormone of erythrocyte Ca(2+)-ATPase [J. Biol. Chem. (1989) 264, 687-689]. In the present studies, retinoic acid was examined for its ability to displace thyroxine from binding sites on human transthyretin (TTR). Scatchard analysis of [125I]T4 binding to purified TTR, determined by equilibrium dialysis, revealed two classes of binding sites with association constants of 3.2 x 10(9) M-1 and 8.1 x 10(6) M-1. All-trans retinoic acid also displaced [125I]T4; 40% of the specifically bound [125I]T4 was displaced at a retinoic acid concentration of 2 x 10(-5) M. Analysis of the high affinity T4 binding site suggests that the Ka for retinoic acid to that site is approx. 10(7) M-1. 8-Anilinonaphthalene-1-sulfonate (ANS), a strongly fluorescing dye, binds to the thyroxine binding sites on TTR. T4 and 3,5,3'-L-triiodothyronine (T3) shifted the fluorescence emission maximum and intensity of an ANS-TTR solution toward the spectrum obtained from uncomplexed ANS. All-trans retinoic acid caused a similar shift in the emission spectrum of ANS, but was less potent than T4. Retinol failed to quench the emission intensity of the ANS-TTR complex, while 13-cis-retinoic acid was less effective than all-trans retinoic acid.
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