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)

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Abstract
The transferrin (Tf) receptor of chick skin fibroblasts was examined using chick 125I-Tf. When the cells were incubated with 125I-Tf on ice, most of the cell-associated 125I-Tf was found on the cell surface; a large part of it was located inside the cells when incubated at 37.degree. C. By equilibrium binding assay, the number of Tf receptors/cell was 6.7 .times. 103. Kd was .apprx. 2.6 .times. 10-8 M. The binding of 125I-Tf was competitively inhibited by the addition of chick unlabeled Tf. Weaker inhibition was observed when bovine Tf was used as a competitor. Horses Tf had no effect on the binding of chick Tf. This agrees well qualitatively with chick cell growth-promoting activities of these Tf. 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 125I-Tf binding as that made by chick Fe-Tf.