RETINOIC ACID INHIBITION OF THYROXINE-BINDING TO HUMAN TRANSTHYRETIN

All-trans retinoic acid is a potent inhibitor of [I-125]-thyroxine (T-4) binding to human erythrocyte membranes and can block the activation by thyroid hormone of erythrocyte Ca2+-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 [I-125]T-4 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 [I-125]T-4; 40% of the specifically bound [I-125]T-4 was displaced at a retinoic acid concentration of 2 x 10(-5) M. Analysis of the high affinity T-4 binding site suggests that the K-a 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. T-4 and 3,5,3'-L-triiodothyronine (T-3) 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 T-4. 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.