Transferrin Receptor on Chick Fibroblast Cell Surface and the Binding Affinity in Relevance to the Growth Promoting Activity of Transferrin: (transferrin/receptor/cultured fibroblast/molecular recognition/class dependent specificity).

We examined the transferrin (Tf) receptor of chick skin fibroblasts using chick 125 I-Tf. When the cells were incubated with 125 I-Tf on ice, most of the cell-associated 125 I-Tf was found on the cell surface; on the other hand, a large part of it was located inside the cells when incubated at 37°C. By equilibrium binding assay, the number of Tf receptors per cell was determined as 6.7 × 103 . Dissociation constant was estimated to be 2.6 × 10-8 M. The binding of 125 I-Tf was competitively inhibited by the addition of chick unlabeled Tf. Weaker inhibition was observed when bovine Tf was used as a competitor. Horse Tf had no effect on the binding of chick Tf. This agrees well qualitatively with chick cell growth-promoting activites of these Tfs. Removal of Fe from Tf affected the affinity for its receptors. About 5- to 10-fold higher concentrations of chick apo-Tf was needed to achieve the same degree of inhibition of 125 I-Tf binding as that made by chick Fe-Tf.