Tietze F, Kohn L D, Kohn A D, Bernardini I, Andersson H C, Adamson M D, Harper G S, Gahl W A
Laboratory of Molecular and Cell Biology, National Institute of Diabetes and Digestive and Kidney Disease, Bethesda, Maryland 20892.
J Biol Chem. 1989 Mar 25;264(9):4762-5.
Monoiodotyrosine (MIT) crosses the lysosomal membrane of rat FRTL-5 thyroid cells by a carrier-mediated process. In egress studies, MIT lost from inside lysosomes was quantitatively recovered outside lysosomes as MIT, indicating that the compound was transported intact across the lysosomal membrane. In uptake studies, [125I]MIT entry required intact lysosomes and exhibited saturation kinetics. The apparent Km for MIT was approximately 1.5 microM and the Vmax was approximately 0.24 pmol/unit hexosaminidase/min. Countertransport of MIT was demonstrated, with an initial velocity of [125I]MIT uptake which reached a maximum at high intralysosomal MIT loading. Nonradioactive MIT and diiodotyrosine competed to approximately equivalent extents with [125I]MIT for uptake in countertransport experiments. The existence of a lysosomal MIT carrier in thyroid cells may explain how this product of thyroglobulin catabolism is transported to the cytosol for iodine salvage and reutilization.
单碘酪氨酸（MIT）通过载体介导的过程穿过大鼠FRTL-5甲状腺细胞的溶酶体膜。在流出研究中，从溶酶体内部丢失的MIT在溶酶体外部作为MIT被定量回收，这表明该化合物完整地穿过了溶酶体膜。在摄取研究中，[125I]MIT的进入需要完整的溶酶体，并表现出饱和动力学。MIT的表观Km约为1.5微摩尔，Vmax约为0.24皮摩尔/单位己糖胺酶/分钟。证实了MIT的反向转运，[125I]MIT摄取的初始速度在高溶酶体内MIT负荷时达到最大值。在反向转运实验中，非放射性MIT和二碘酪氨酸与[125I]MIT竞争摄取的程度大致相当。甲状腺细胞中溶酶体MIT载体的存在可能解释了甲状腺球蛋白分解代谢产物如何被转运到细胞质中进行碘的 salvage 和再利用。
