Uptake of 3,5,3'-L-tri-iodothyronine in human erythrocytes

The mechanisms of 3,5,3'-L-tri-iodothyronine (T3) uptake into human erythrocytes were examined. Purified membranes of human erythrocytes were shown to have two classes of T3-binding sites with one being a high-affinity site (dissociation constant, 59.2 +/- 17.8 nmol/l; maximum binding capacity, 344.3 +/- 95.5 fmol/micrograms protein). Furthermore, it was shown that there were two pathways for T3 uptake in human erythrocytes; one was saturable, stereospecific (T3 much greater than thyroxine greater than 3,5,3'-D-tri-iodothyronine), energy-dependent and dominant at 15 degrees C; the other was not displaced by unlabelled T3 and was energy-independent but did not occur by passive diffusion. The former pathway which, it is suggested, is a receptor-mediated transport pathway, was inhibited by monodansylcadaverine, phloretin or oligomycin at 15 or 37 degrees C, but the latter pathway was not inhibited by these inhibitors. Our results strongly suggest that uptake of T3 by the energy-independent pathway became predominant over the energy-dependent pathway at 37 degrees C and accounted for 83% of total T3 uptake of human erythrocytes.