Refetoff S, Fang V S, Marshall J S
J Clin Invest. 1975 Jul;56(1):177-87. doi: 10.1172/JCI108066.
Thyroxine-binding globulin (TBG) and partially desialylated or slow TBG (STBG) were purified from human serum by affinity chromatography. Purified TBG was identical to TBG present in serum by the criteria of electrophoretic mobility, affinity for thyroxine (T4), and heat-inactivation response. Purified STBG had slower electrophoretic mobility and lower affinity for T4. Both bound T4 in an equimolar ratio, were immunoprecipitable, and had similar inactivation t1/2 at 61 degrees C. TBG and STBG were iodinated by the chloramine-T-catalyzed reaction. An average of from 0.02 to 6 atoms I could be incorporated per molecule of the protein by adjusting the conditions of the reaction (time, protein and iodide concentrations). 125-I, 131-I, and 127-I were used. Iodination increased the anodal mobility of TBG but did not affect the reversible T4-binding, precipitation by antiserum, or the heat-inactivation properties. "Heavily" and "lightly" iodinated TBG had identical disappearance half-times from serum in the rabbit. 15 min after the intravenous administration of [131-I]-STBG and [125-I]TBG mixture to rats, more than 90% of the injected 131-I dose was in the liver, and the liver 131-I/125-I ratio was 32-fold that of serum. Selective uptake of STBG by the liver was also observed in the rabbit and in man. The serum [125-I]STBG/[131-I]TBG ratio declined from 1 to 0.2 in 10 min in the intact rabbit but remained unchanged for 1 h in the acutely hepatectomized animal. In the rabbit, t 1/2 was approximately 3 min for STBG and 0.8-3.4 days for TBG. The radioiodine derived from the iodinated proteins is partly excreted in bile but the bulk was precipitable with specific antibodies. Some isotope in the form of iodide appeared in blood and was excreted in the urine. Since radioiodinated TBG and STBG preserve their biologic and immunologic properties they are useful as tracer materials for metabolic studies. In rat, rabbit, and man STBG is rapidly cleared from serum by the liver. Conversion of TBG to STBG may be the limiting step in the regulation of TBG metabolism.
通过亲和层析从人血清中纯化甲状腺素结合球蛋白（TBG）和部分去唾液酸化或慢迁移甲状腺素结合球蛋白（STBG）。纯化后的TBG在电泳迁移率、对甲状腺素（T4）的亲和力以及热灭活反应等方面与血清中的TBG相同。纯化后的STBG电泳迁移率较慢，对T4的亲和力较低。二者均以等摩尔比结合T4，均可被免疫沉淀，且在61℃时具有相似的灭活半衰期。通过氯胺-T催化反应对TBG和STBG进行碘化。通过调整反应条件（时间、蛋白质和碘化物浓度），每分子蛋白质平均可掺入0.02至6个碘原子。使用了125-I、131-I和127-I。碘化增加了TBG的阳极迁移率，但不影响其可逆性T4结合、抗血清沉淀或热灭活特性。“重度”和“轻度”碘化的TBG在兔血清中的消失半衰期相同。给大鼠静脉注射[131-I]-STBG和[125-I]TBG混合物15分钟后，超过90%的注射剂量的131-I在肝脏中，肝脏中131-I/125-I的比值是血清中的32倍。在兔和人中也观察到肝脏对STBG的选择性摄取。在完整兔中，血清中[125-I]STBG/[131-I]TBG的比值在10分钟内从1降至0.2，但在急性肝切除动物中1小时内保持不变。在兔中，STBG的半衰期约为3分钟，TBG的半衰期为0.8 - 3.4天。碘化蛋白质释放的放射性碘部分经胆汁排泄，但大部分可被特异性抗体沉淀。一些碘化物形式的同位素出现在血液中并经尿液排泄。由于放射性碘化的TBG和STBG保留了它们的生物学和免疫学特性，它们可用作代谢研究的示踪材料。在大鼠、兔和人中，STBG可被肝脏迅速从血清中清除。TBG向STBG的转化可能是TBG代谢调节的限速步骤。
