Friesema E C, Docter R, Moerings E P, Verrey F, Krenning E P, Hennemann G, Visser T J
Department of Internal Medicine, Erasmus University Medical Center, 3015 GE Rotterdam, The Netherlands.
Endocrinology. 2001 Oct;142(10):4339-48. doi: 10.1210/endo.142.10.8418.
Transport of thyroid hormone across the cell membrane is required for thyroid hormone action and metabolism. We have investigated the possible transport of iodothyronines by the human system L amino acid transporter, a protein consisting of the human 4F2 heavy chain and the human LAT1 light chain. Xenopus oocytes were injected with the cRNAs coding for human 4F2 heavy chain and/or human LAT1 light chain, and after 2 d were incubated at 25 C with 0.01-10 microM [(125)I]T(4), [(125)I]T(3), [(125)I]rT(3), or [(125)I]3,3'-diiodothyronine or with 10-100 microM [(3)H]arginine, [(3)H]leucine, [(3)H]phenylalanine, [(3)H]tyrosine, or [(3)H]tryptophan. Injection of human 4F2 heavy chain cRNA alone stimulated the uptake of leucine and arginine due to dimerization of human 4F2 heavy chain with an endogenous Xenopus light chain, but did not affect the uptake of other ligands. Injection of human LAT1 light chain cRNA alone did not stimulate the uptake of any ligand. Coinjection of cRNAs for human 4F2 heavy chain and human LAT1 light chain stimulated the uptake of phenylalanine > tyrosine > leucine > tryptophan (100 microM) and of 3,3'-diiodothyronine > rT(3) approximately T(3) > T(4) (10 nM), which in all cases was Na(+) independent. Saturation analysis provided apparent Michaelis constant (K(m)) values of 7.9 microM for T(4), 0.8 microM for T(3), 12.5 microM for rT(3), 7.9 microM for 3,3'-diiodothyronine, 46 microM for leucine, and 19 microM for tryptophan. Uptake of leucine, tyrosine, and tryptophan (10 microM) was inhibited by the different iodothyronines (10 microM), in particular T(3). Vice versa, uptake of 0.1 microM T(3) was almost completely blocked by coincubation with 100 microM leucine, tryptophan, tyrosine, or phenylalanine. Our results demonstrate stereospecific Na(+)-independent transport of iodothyronines by the human heterodimeric system L amino acid transporter.
甲状腺激素作用和代谢需要甲状腺激素穿过细胞膜。我们研究了人系统L氨基酸转运体对碘甲状腺原氨酸的可能转运，该转运体由人4F2重链和人LAT1轻链组成。将编码人4F2重链和/或人LAT1轻链的cRNA注射到非洲爪蟾卵母细胞中，2天后在25℃下用0.01 - 10μM的[(125)I]T(4)、[(125)I]T(3)、[(125)I]rT(3)或[(125)I]3,3'-二碘甲状腺原氨酸，或用10 - 100μM的[(3)H]精氨酸、[(3)H]亮氨酸、[(3)H]苯丙氨酸、[(3)H]酪氨酸或[(3)H]色氨酸进行孵育。单独注射人4F2重链cRNA会刺激亮氨酸和精氨酸的摄取，这是由于人4F2重链与内源性非洲爪蟾轻链二聚化所致，但不影响其他配体的摄取。单独注射人LAT1轻链cRNA不会刺激任何配体的摄取。同时注射人4F2重链和人LAT1轻链的cRNA会刺激苯丙氨酸＞酪氨酸＞亮氨酸＞色氨酸（100μM）以及3,3'-二碘甲状腺原氨酸＞rT(3)≈T(3)＞T(4)（10 nM）的摄取，在所有情况下均不依赖Na(+)。饱和分析得出T(4)的表观米氏常数（K(m)）值为7.9μM，T(3)为0.8μM，rT(3)为12.5μM，3,3'-二碘甲状腺原氨酸为7.9μM，亮氨酸为46μM，色氨酸为19μM。亮氨酸、酪氨酸和色氨酸（10μM）的摄取受到不同碘甲状腺原氨酸（10μM）的抑制，尤其是T(3)。反之，0.1μM T(3)的摄取在与100μM亮氨酸、色氨酸、酪氨酸或苯丙氨酸共同孵育时几乎完全被阻断。我们的结果表明，人异源二聚体系统L氨基酸转运体对碘甲状腺原氨酸具有立体特异性的不依赖Na(+)的转运作用。
