HIGH-AFFINITY BINDING OF THYROID-HORMONES TO NEUROBLASTOMA PLASMA-MEMBRANES

The binding of thyroid hormones to isolated plasma membranes was studied in NB41A3 neuroblasts. Saturable binding of l-T3, d-T3 and l-T4 was observed. Binding was time-dependent, with eqilibrium reached in less than 60 min and maximal binding occurring between pH 7.4 and 7. Saturation experiments demonstrated two classes of sites for l-T3: a high-affinity site with Ka 8.4 · 109 M-1 and a low-affinity site with Ka 7.3 · 106 M-1. l-T3 and d-T3 inhibited each other's binding, l-T3 being several-times more potent. Affinity labeling of isolated membranes with bromoacetylated thyroid hormones disclosed stereospecific binding to SDS-PAGE bands with approximate molecular masses of 27 kDa (preferentially labeled by BrAc-l-T3), 32 kDa (preferentially labeled by BrAc-d-T3), and 48 and 87 kDa (preferentially labeled by BrAc-l-T4). Binding of BrAc-l-T3 to the 27 kDa band accounted for 3.4% of total binding, was selectively inhibited by excess l-T3, and may be involved in intracellular transport of l-T3. © 1990.