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.
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.
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Zhou
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Samson
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Osty
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Francon
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Blondeau
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