TFR-competition binding assay of rhTF and hTF in Caco-2 cells. Different concentrations of rhTF or hTF mixed with 1 μg/ml of 125I-hTF were added to confluent Caco-2 cells and incubated at 4°C for 1 hr. Cells were then washed with cold PBS, solubilized with 1 N NaOH, and the amount of radiolabelled hTF in cell lysate was determined. The total cellular protein in the cell lysate was measured, and used to normalize the data to ng of hTF per mg cell protein (“ng/mg cell protein”). Data is represented as average with error bars indicating standard deviation, n = 3.