Evidence for a close link between the thyroid hormone transport system and the aromatic amino acid transport system T in erythrocytes.
The transport of [125I]triiodothyronine ([125I]T3) and [3H]tryptophan ([3H]Trp) by washed rat erythrocytes was studied at 25 degrees C in the presence of leucine in order to block the neutral amino acid transport system L. Eadie-Hofstee plots of initial velocity data gave the following values of Km (micromolar) and Vmax (nanomole/min/10(8) cells): 0.122 +/- 0.007 and 0.140 +/- 0.021 for T3, and 558 +/- 28 and 17.4 +/- 2.3 for Trp (n = 5). The Trp transport system in rat erythrocytes is similar to the human erythrocyte aromatic amino acid-specific system T described by Rosenberg et al. (Rosenberg, R., Young, J. D., and Ellory, J. C. (1980) Biochim. Biophys. Acta 598, 375-384). Unlabeled aromatic amino acids (e.g. Trp, phenylalanine, tyrosine) competitively inhibited [125I]T3 uptake and unlabeled iodothyronine analogues (e.g. T3, D-T3, thyroxine, thyronine) competitively inhibited [3H]Trp uptake. The inhibition constants of these competitors measured with each labeled substrate were highly correlated. N-Ethylmaleimide irreversibly inhibited T3 and Trp transport and each substrate protected the transport system of the other from inactivation by N-ethylmaleimide. The Vmax of T3 and Trp transport by human erythrocytes were 500 and 120 times lower, respectively, than those of rat erythrocytes (0.30 and 126 pmol/min/10(8) cells, respectively). The T3 and Trp transport activities of sheep erythrocytes were undetectable. These results indicate that T3 and Trp either share a common multi-specific transport system or are transported by closely linked systems which interact in the erythrocyte membrane.
Zhou
Y
Y
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.
Samson
M
M
Osty
J
J
Francon
J
J
Blondeau
J P
JP
D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D004912:Erythrocytes / Q000378:metabolism*; D005033:Ethylmaleimide / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D010649:Phenylalanine / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D012756:Sheep; D013974:Thyroxine / Q000494:pharmacology; D014284:Triiodothyronine / Q000097:blood* / Q000494:pharmacology; D014364:Tryptophan / Q000097:blood* / Q000494:pharmacology; D014443:Tyrosine / Q000494:pharmacology
D014284:Triiodothyronine; D014443:Tyrosine; D010649:Phenylalanine; D014364:Tryptophan; D005033:Ethylmaleimide; D013974:Thyroxine