Characterization of the thyroid hormone transport system of isolated hepatocytes

Transport of 3,5-[3'-125I]triiodo-L-thyronine ([125I]T3) was studied in isolated rat liver hepatocytes. T3 transport was temperature-sensitive, the initial velocity of uptake, at low substrate concentration, was 60 times higher at 25 degrees C than at 0 degrees C. The activation energy of cellular uptake (26 kcal/mol) was different from that of binding to cytosolic proteins (6 kcal/mol), indicating that the latter was not the rate-limiting step. Uptake obeyed simple Michaelis-Menten kinetics, with an apparent Km of 0.34 microM and a Vmax of 0.15 fmol/min/cell at 25 degrees C. No simple diffusion occurred. Unlabeled T3, L-thyroxine (T4), 3,5,3'-triiodo-D-thyronine, and triiodothyroacetic acid inhibited T3 uptake with Kl values of 0.32, 1.4, 4.1, and 5.4 microM, respectively, indicating specificity of uptake which was different from specificity of intracellular binding sites. [125I]T4 was also taken up by a saturable process (Km = 0.65 microM and Vmax = 0.16 fmol/min/cell at 25 degrees C). T3 was a better competitor than T4 for the uptake of [125I]T4, indicating that both hormones were taken up by the same carrier system. Metabolic inhibitors had either no effect on T3 uptake or inhibitory effects unrelated to cellular ATP depletion. Uptake was not affected by modification of the membrane potential or the sodium ion gradient. T3 and T4 uptake was pH-dependent. It is suggested that the un-ionized 4'-OH form of the hormones was preferentially taken up. Inhibition of uptake by various compounds was compared to inhibition of thyroid hormone binding to transthyretin, nuclear receptor, and cytosolic-binding proteins. We conclude that, in freshly isolated hepatocytes, thyroid hormones are taken up by a saturable, stereospecific, Na+-independent carrier system.