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.
Samson
M
M
Unité de Recherche sur la Glande Thyroïde et la Régulation Hormonale (U.96), Institut National de la Santé et de la Recherche Médicale, Le Kremlin-Bicêtre, France.
Osty
J
J
Francon
J
J
Blondeau
J P
JP
D000818:Animals; D001665:Binding Sites; D001692:Biological Transport; D004910:Erythrocyte Membrane / Q000378:metabolism*; D005033:Ethylmaleimide / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D010658:Phenylglyoxal / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D014284:Triiodothyronine / Q000378:metabolism*; D014364:Tryptophan / Q000378:metabolism*
D014284:Triiodothyronine; D014364:Tryptophan; D010658:Phenylglyoxal; D005033:Ethylmaleimide