Studies on the mechanism of transferrin iron uptake by rat reticulocytes

Mechanism of transferrin iron uptake by rat reticulocytes was studied using ⁵⁹Fe- and ¹²⁵I-labelled rat transferrin. Whereas more than 80% of the reticulocyte-bound ⁵⁹Fe was located in the cytoplasmic fraction, only 25–30% of ¹²⁵I-labelled transferrin was found inside the cells. As shown by the presence of acetylcholine esterase, 10–15% of the cytoplasmic ¹²⁵I-labelled transferrin might have been derived from the contamination of this fraction by the plasma membrane fragments. Electron microscopic autoradiography indicated 26% of the cell-bound ¹²⁵I-labelled transferrin to be inside the reticulocytes. Both the electron microscopic and biochemical studies showed that the rat reticulocytes endocytosed their plasma membrane independently of transferrin. Sepharose-linked transferrin was found to be capable of delivering ⁵⁹Fe to the reticulocytes. Our results suggest that penetration of the cell membrane by transferrin is not necessary for the delivery of iron and that, although it might make a contribution to the cellular iron uptake, internalization of transferrin reflects endocytotic activity of the reticulocyte cell membrane.