Triiodothyronine binding sites in the rat erythrocyte membrane: involvement in triiodothyronine transport and relation to the tryptophan transport System T.
The binding of L-triiodothyronine (T3) to rat erythrocyte membranes (ghosts and peripheral protein-depleted vesicles) was studied under equilibrium conditions. Ghosts contained high-affinity T3 binding sites whose dissociation constant (21 nM) was similar to the equilibrium-exchange Michaelis constant of T3 transport measured in ghosts. Each ghost contained about 8.10(3) high-affinity binding sites. The high-affinity T3 binding was stereospecific and was inhibited by L-tryptophan (Trp) but not by L-leucine. The iodothyronine and amino acid specificity of binding is therefore similar to that of System T, the erythrocyte T3/Trp transporter. These Trp-inhibitable high-affinity T3-binding sites were also present in peripheral protein-depleted membrane vesicles, indicating that they are integral part of the membrane. Ghosts prepared from human erythrocytes, which have very low System T transport activities, contained no detectable Trp-inhibitable high-affinity T3-binding sites. In rat erythrocyte ghosts, N-ethylmaleimide inactivated both the binding and the transport of T3. This inactivation was blocked by T3 and Trp with similar efficiencies. Phenylglyoxal, an arginine residue modifier, also inhibited both high-affinity T3 binding and System T transport activity. It is concluded that the Trp-inhibitable high-affinity T3-binding sites in the rat erythrocyte membrane are likely to be associated with System T.