Yan Z, Hinkle P M
Department of Pharmacology, University of Rochester School of Medicine and Dentistry, New York 14642.
J Biol Chem. 1993 Sep 25;268(27):20179-84.
The mechanism of uptake of the thyroid hormones, 3,5,3'-triiodo-L-thyronine (L-T3) and L-thyroxine (L-T4), was studied in rat pituitary GH4C1 cells. The major portion (approximately 65%) of L-T3 transport was stereospecific and saturable. Transport of L-T3 was 8-10 times more rapid than transport of D-T3. [125I]L-T3 transport was saturable at microM concentrations; a Lineweaver-Burk plot was linear with Km = 0.4 microM and Vmax = 4 pmol/min/10(6) cells. Unlabeled analogs competed with [125I]L-T3 uptake in the order L-T3 > or = L-T4 >> 3,3',5'-triiodo-L-thyronine (reverse-T3), D-T3, D-T4, and L-thyronine. L-T3 and L-T4 also both effectively inhibited [125I]L-T4 transport. Uptake of [125I]L-T3 was inhibited 40-55% by large neutral amino acids and 77% by 80 microM beta-2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid, an inhibitor selective for the L system of amino acid uptake. Conversely, L-T3 inhibited the transport of [3H]leucine by pituitary cells (IC50 = 2 microM), but D-T3 and 3,5,3'-triiodothyroacetic acid (Triac) did not. L-Leucine was transported much more efficiently (Vmax = 0.65 mumol/min/10(6) cells) than L-T3 by GH4C1 cells. The results show that L-T3 and L-T4 share the same stereospecific transport pathway in pituitary cells, that the transport mechanism is saturable at supraphysiological thyroid hormone concentrations, and that the L system is partially responsible for L-T3 transport.
在大鼠垂体GH4C1细胞中研究了甲状腺激素3,5,3'-三碘-L-甲状腺原氨酸（L-T3）和L-甲状腺素（L-T4）的摄取机制。L-T3转运的主要部分（约65%）具有立体特异性且可饱和。L-T3的转运速度比D-T3快8至10倍。[125I]L-T3转运在微摩尔浓度下可饱和；Lineweaver-Burk图呈线性，Km = 0.4微摩尔，Vmax = 4皮摩尔/分钟/10(6)个细胞。未标记的类似物与[125I]L-T3摄取竞争的顺序为L-T3≥L-T4>>3,3',5'-三碘-L-甲状腺原氨酸（反式-T3）、D-T3、D-T4和L-甲状腺原氨酸。L-T3和L-T4也都能有效抑制[125I]L-T4的转运。大中性氨基酸使[125I]L-T3摄取受到40%至55%的抑制，80微摩尔β-2-氨基双环-(2,2,1)-庚烷-2-羧酸（一种对氨基酸摄取L系统具有选择性的抑制剂）使其受到77%的抑制。相反，L-T3抑制垂体细胞对[3H]亮氨酸的转运（IC50 = 2微摩尔），但D-T3和3,5,3'-三碘甲状腺乙酸（Triac）则无此作用。GH4C1细胞对L-亮氨酸的转运效率（Vmax = 0.65微摩尔/分钟/10(6)个细胞）远高于L-T3。结果表明，L-T3和L-T4在垂体细胞中共享相同的立体特异性转运途径，转运机制在超生理甲状腺激素浓度下可饱和，且L系统部分参与L-T3的转运。
