Binding and endocytosis of monoterbium transferrin by K562 cells

Using isotopic labeling of human serumapotransferrin, the binding and the endocytosis of monoter-bium transferrin (TbC-apotransferrin, TbC-apotransferrin-FeN) by K562 cells, a human leukemic cell line, have beeninvestigated. There are about (8.58±2.41)×105 binding sites per cell surface at 0℃. The association constant for TbC-apotransferrin binding is 4.1×107 mol-1·L, for TbC-apotransferrin-FeN 2.7×107 mol-1·L at 0℃. At pH 7.4, upon warming cells to 37℃, endocytosis starts. The rate constantsfor 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.