Oligosaccharide specific endocytosis by isolated rat hepatic reticuloendothelial cells

We have examined the specificity and kinetics of endocytosis of iodinated glycopeptides and glycoproteins by isolated rat reticuloendothelial cell preparations. All of the probes which were recognized by this system contain within their oligosaccharide moiety the following common structural unit which we hypothesize to be the minimum required structure for binding and endocytosis: GlcNAc beta 1,6 or Man alpha 1,6 Man alpha 1,6[Man alpha 1,3]Man beta 1,4GlcNAc beta 1,4GlcNAc beta Asn. Desialyzed tetra-antennary complex oligosaccharides contain this minimum structure but are not endocytosed until after removal of the beta 1,4-linked terminal Gal residues by Diplococcal beta-galactosidase. Further degradation by Diplococcal beta-hexosaminidase removes only the beta 1,2-linked GlcNAc residues resulting in a product which still contains the minimum structure and can be taken up by this system. Desialyzed triantenary and biantenary complex oligosaccharides are not endocytosed following treatment with Diplococcal beta-galactosidase and beta-hexosaminidase. High mannose type oligosaccharides which contain the minimum structure are endocytosed, whereas hybrid structures are not, indicating that the presence of a beta 1,4-linked GlcNAc on the beta-linked core mannose inhibits endocytosis. The Michaelis-Menten kinetic constants are similar for glycopeptides and glycoproteins, ranging from 1.1-6.4 X 10(-7) M for all ligands recognized by this system. In the range of 50-150 X 10(3) ligand binding sites per cell are estimated to be on the cell surface with a capacity to internalize from 13-80 X 10(3) molecules per min per cell at 37 degrees C. All of the structural information required for binding and internalization resides in the oligosaccharide moiety itself.