Binding of mouse and rabbit iron-59 transferrins to lactating mouse mammary epithelial cells

The binding of mouse and rabbit transferrins to lactating mouse mammary epithelial cells was tested in a 59Fe-protein-binding assay. The homologous and heterologous binding was slow during the first 30 min, after which the uptake steadily increased. In ligand concentration-dependent saturation studies, the heterologous rabbit protein showed a high degree of binding and required approximately 9.7 ng of ligand to saturate approximately 2 x 10(6) cells. The homologous mouse protein demonstrated a low degree of binding and failed to demonstrate saturation at the above ligand concentration. Scatchard plot for homologous binding data was nonlinear and implied a low (1.08 x 10(-10) M) and a high (1.82 x 10(-9) M) affinity interaction mechanism. However, the plot for heterologous binding was linear and characterized by one high affinity (1.0 x 10(-9) M) binding interaction. A total of 11,000 and 19,600 binding sites per cell were estimated for mouse and rabbit proteins, respectively. These data suggest a binding crossreactivity between mouse and rabbit transferrins. A high affinity binding mechanism seems to be conserved in proteins from both species; however, an additional low affinity binding was present only in the homologous system.