Fluorescence microphotometric studies of the transferrin receptor in human erythroid precursor cells

The receptor site for transferrin in normal human erythroid precursor cells was studied by fluorescence microscopy. F-transferrin saturated with iron was used as probe of the available receptor sites on reticulocytes and nucleated red cells. In a series of experiments specificity and certain structural details of the ligand site were evaluated. Hydrolytic cleavage of exposed carbohydrate moieties by purified glycosidases revealed increased fluorescence after treatment of fixed cells by neuramindase, no perceptible change after N-acetylhexosaminidase treatment, but a pronounced decrease after exposure to beta-galactosidase. Inhibitor studies with monosaccharides and tryptic glycopeptides of normal reticulocytes complemented and amplified the results obtained with enzymes. The data suggest that an oligosaccharide chain is essential for specific transferrin binding to erythroid precursors. N-acetyl-neuraminic acid, galactose, N-acetylgalactosamine, and fucose appear to be saccharides on the receptor. These studies also demonstrate the applicability of fluorescence microscopic methods to qualitative structural analysis of receptor biochemistry.