Binding and endocytosis of monoterbium transferrin by K562 cells

Using isotopic labeling of human serum apotransferrin, the binding and the endocytosis of monoterbium transferrin (Tb_c-apotransferrin, Tb_c-apotransferrin-Fe_N) 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 Tb_c-apotransferrin binding is 4.1 × 10 mol⁻¹· L, for Tb_c-apotransferrin-Fe_N 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 Tb_C-apotransferrin-Fe_N, respectively.