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 transthy retin (TTR). Scatchard analysis of [I-125]T-4 binding to purified TTR, determined by equilibrium dialysis, revealed two classes of binding s ites 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 sp ecifically bound [I-125]T-4 was displaced at a retinoic acid concentra tion of 2 x 10(-5) M. Analysis of the high affinity T-4 binding site s uggests that the K-a for retinoic acid to that site is approx. 10(7) M (-1), 8-Anilinonaphthalene-1-sulfonate (ANS), a strongly fluorescing d ye, binds to the thyroxine binding sites on TTR. T-4 and 3,5,3'-L-trii odothyronine (T-3) shifted the fluorescence emission maximum and inten sity of an ANS-TTR solution toward the spectrum obtained from uncomple xed ANS. All-trans retinoic acid caused a similar shift in the emissio n spectrum of ANS, but was less potent than T-4. Retinol failed to que nch the emission intensity of the ANS-TTR complex, while 13-cis-retino ic acid was less effective than all-trans retinoic acid.