Takahashi K, Tavassoli M
Biochim Biophys Acta. 1982 Feb 8;685(1):6-12. doi: 10.1016/0005-2736(82)90027-x.
The kinetics of the cellular uptake of iron-transferrin complex was studied in L1210 murine leukemia cells and rat reticulocytes using 125I-transferrin. Saturation of transferrin with iron was necessary for optimal uptake. Following the incubation of cells with the radiolabeled complex a biphasic pattern of uptake was observed. The initial phase was rapid and relatively temperature-independent and was not altered by ethylamine, an inhibitor of transglutaminase activity which is necessary for receptor-mediated endocytosis. This phase was considered to result from receptor-ligand interaction which could be reversed to a great degree by replacement with unlabeled transferrin. A plateau was then reached, indicating a saturation of receptors. After 30 min a second phase of uptake was indicated by the second rise in the curve. This phase was slow, relatively temperature-dependent and could be abolished by ethylamine. It was interpreted as evidence of internalization of the ligand. Analysis of the data from competition studies with unlabeled transferrin indicated that the first phase might itself comprise a reversible and an irreversible step with a ratio of 5 to 1.4 for bound transferrin. Thus, the cellular uptake of iron-transferrin complex may consist of a reversible ligand-receptor interaction. Conformational changes may render this interaction irreversible and the internalization of the ligand may then follow.
利用¹²⁵I-转铁蛋白，在L1210小鼠白血病细胞和大鼠网织红细胞中研究了铁-转铁蛋白复合物的细胞摄取动力学。转铁蛋白用铁饱和对于最佳摄取是必要的。在用放射性标记的复合物孵育细胞后，观察到摄取呈双相模式。初始阶段迅速且相对不依赖温度，并且不受转谷氨酰胺酶活性抑制剂乙胺的影响，而转谷氨酰胺酶活性是受体介导的内吞作用所必需的。该阶段被认为是由受体-配体相互作用导致的，这种相互作用在很大程度上可以通过用未标记的转铁蛋白替代来逆转。然后达到一个平台期，表明受体饱和。30分钟后，曲线的第二次上升表明摄取进入第二阶段。该阶段缓慢，相对依赖温度，并且可以被乙胺消除。它被解释为配体内化的证据。对来自与未标记转铁蛋白竞争研究的数据的分析表明，第一阶段本身可能包括一个可逆步骤和一个不可逆步骤，结合的转铁蛋白的比例为5比1.4。因此，铁-转铁蛋白复合物的细胞摄取可能包括可逆的配体-受体相互作用。构象变化可能使这种相互作用不可逆，然后可能接着发生配体的内化。
