| Abstract: | Cells grown in the presence of ferric ammonium citrate or hemin exhibited a concentration and time-dependent decrease in 125I-transferrin (Trf) binding. In contrast, cells grown in the presence of protoporphyrin IX or picolinic acid (an iron chelator) exhibited a marked increase in Trf binding. The decrease or increase in binding activity observed under these different conditions of culture reflected, respectively, a reduction or increase in receptor number rather than an alteration in ligand receptor affinity. Growth of the cells in the presence of saturating concentrations of apotransferrin only induced a slight reduction in receptor number. Investigation of the Trf receptors' turnover and biosynthesis clearly showed that iron and hemin decreased the synthesis of Trf receptors without any modification of the receptor turnover; in contrast, protoporphyrin IX and picolinic acid markedly increased the synthesis of Trf receptors. Our results suggest that hemin, iron, and protoporphyrin IX may represent the main molecules involved in the regulation of Trf receptors. |
