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) x 10(5) binding sites per cell surface at 0degreesC. The association constant for Tbc-apotransferrin binding is 4.1 x 1107 mol(-1) (.) L, for Tbc-apo-transferrin-Fe-N 2.7 x 10(7) mol(-1) (.) L at 0degreesC. At pH 7.4, upon warming cells to 37degreesC, 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.