Andersson H C, Kohn L D, Bernardini I, Blom H J, Tietze F, Gahl W A
Section on Human Biochemical Genetics, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892.
J Biol Chem. 1990 Jul 5;265(19):10950-4.
Lysosomal transport of monoiodotyrosine was characterized in countertransport experiments using rat FRTL-5 thyroid cell lysosomes. Monoiodotyrosine carrier activity was temperature-dependent (Ea = 11.65 kcal/mol) and had a pH optimum of 7.5. Carrier activity was minimally inhibited by KCl and NaCl, but unaffected by the presence of other ions or ATP. Monoiodotyrosine transport was unaffected by the presence of carbonyl cyanide m-chlorophenylhydrazone, nigericin, or ammonium chloride, indicating that a proton or K+ gradient is not necessary for monoiodotyrosine transport across the lysosomal membrane. Monoiodotyrosine countertransport showed a 6-fold increase in lysosomes from FRTL-5 cells grown in medium containing thyrotropin by comparison to cells grown without this hormone. Thyrotropin responsiveness raised the possibility that monoiodotyrosine was transported by system h, the only known lysosomal carrier whose activity is enhanced by thyrotropin. Consistent with this, monoiodotyrosine-loaded lysosomes exhibited countertransport of [3H]tyrosine, [3H]phenylalanine, and [3H]leucine, three system h ligands, but not [3H]cystine, a nonsystem h ligand. Unlabeled tyrosine, phenylalanine, and leucine, but not cystine or proline, inhibited [125I]monoiodotyrosine countertransport, and leucine inhibition of [3H]tyrosine countertransport and [125I]monoiodotyrosine countertransport yielded virtually identical KI values, 3.5 and 3.2 microM, respectively. Competition studies with monoiodotyrosine analogues showed that system h recognizes a broad range of ligands with an alpha-amino acid configuration at one end and a hydrophobic region at the other. Ring-substituted halogens, regardless of mass or ring position, but not amino, nitro, hydroxy, or methoxy groups, enhanced carrier recognition of system h analogues. It appears that a single system effects the transport of iodinated (e.g. monoiodotyrosine) and noniodinated (e.g. tyrosine) thyroglobulin catabolites into the cytosol for salvage and reutilization by FRTL-5 thyroid cells.
利用大鼠FRTL-5甲状腺细胞溶酶体进行的反向转运实验对单碘酪氨酸的溶酶体转运进行了表征。单碘酪氨酸载体活性具有温度依赖性（活化能 = 11.65千卡/摩尔），最适pH值为7.5。载体活性受到氯化钾和氯化钠的最小抑制，但不受其他离子或ATP存在的影响。单碘酪氨酸的转运不受羰基氰化物间氯苯腙、尼日利亚菌素或氯化铵的影响，这表明质子或钾离子梯度对于单碘酪氨酸跨溶酶体膜的转运不是必需的。与在不含这种激素的培养基中生长的细胞相比，在含有促甲状腺激素的培养基中生长的FRTL-5细胞的溶酶体中，单碘酪氨酸反向转运增加了6倍。促甲状腺激素反应性增加了单碘酪氨酸由系统h转运的可能性，系统h是唯一已知的其活性可被促甲状腺激素增强的溶酶体载体。与此一致的是，负载单碘酪氨酸的溶酶体表现出[3H]酪氨酸、[3H]苯丙氨酸和[3H]亮氨酸（三种系统h配体）的反向转运，但不表现出[3H]胱氨酸（一种非系统h配体）的反向转运。未标记的酪氨酸、苯丙氨酸和亮氨酸，但不是胱氨酸或脯氨酸，抑制[125I]单碘酪氨酸的反向转运，亮氨酸对[3H]酪氨酸反向转运和[125I]单碘酪氨酸反向转运的抑制产生了几乎相同的抑制常数（KI）值，分别为3.5和3.2微摩尔。与单碘酪氨酸类似物的竞争研究表明，系统h识别范围广泛的配体，一端具有α-氨基酸构型，另一端具有疏水区域。环取代的卤素，无论质量或环位置如何，但氨基、硝基、羟基或甲氧基不增强系统h类似物的载体识别。似乎单个系统影响碘化（如单碘酪氨酸）和非碘化（如酪氨酸）甲状腺球蛋白分解代谢产物向细胞质的转运，以供FRTL-5甲状腺细胞回收和再利用。
