Binding and endocytosis of monoterbium transferrin by K562 cells

Using isotopic labeling of human serum apotransferrin, the binding and the endocytosis of monoterbium transferrin (Tbc-apotransferrin, Tbc-apotransferrin-FeN) by K562 cells, a human leukemic cell line, have been investigated. There are about (8.58 ± 2.41) × 105 binding sites per cell surface at 0°C. The association constant for Tbc-apotransferrin binding is 4.1 × 10 mol 1· L, for Tbc-apotransferrin-FeN 2.7×107 mol 1 · L at 0°C. At pH 7.4, upon warming cells to 37°C, endocytosis starts. The rate constants for the endocytosis are about 0.97 min 1 and 0.31 min 1 and the endocytosis ratio reaches 56% and 80% for Tbc-apotransferrin and TbC-apotransferrin-FeN, respectively.