Uptake of transferrin by rat peritoneal macrophages

Activated rat peritoneal macrophages bind 125I-apotransferrin in a time- and temperature-dependent process, the amount of transferrin taken up at 4 degrees C amounting to only about 15% of that bound at physiological temperatures. Binding is reversible, saturable, and largely abolished by prior treatment of the cells with Pronase. A single class of high affinity binding sites is evidenced by Scatchard analysis, each cell binding about 110 000 apotransferrin molecules with an apparent affinity constant of 1.4 x 10(6) l mol-1. Macrophages are also capable of binding about one-third as much iron-saturated transferrin as iron-free transferrin. Since binding of neither form of the protein is influenced by the presence of the other, separate and independent binding sites for apotransferrin and iron transferrin are presumed to exist on the macrophage.