An electron microscope autoradiographic study of the carbohydrate recognition systems in rat liver. I. Distribution of 125I-ligands among the liver cell types.

Abstract

EM autoradiography was used to study the cellular localization of 7 glycoproteins rapidly cleared from the circulating plasma of rats and taken up by the liver. One and 15 min after i.v. administration of the 125I-glycoproteins, livers were fixed in situ by perfusion and processed for autoradiography. Autoradiographic grains in the developed sections represented the intact 125I-ligand. A quantitative analysis of the distribution and concentration (density) of autoradiographic grains over the 3 major cell types of the liver was performed. Three molecules, asialo-fetuin, asialo-orosomucoid and lactosaminated RNase A dimer, the oligosaccharide chains of which terminate in galactose residues, were bound and internalized almost exclusively (> 90%) by hepatocytes. Four molecules, the oligosaccharide chains of which terminate in either N-acetyl-glucosamine (agalacto-orosomucoid) or mannose (ahexosamino-orosomucoid, preputial .beta.-glucuronidase and mannobiosaminated RNase A dimer), were specifically bound and internalized by cells lining the blood sinusoids, i.e., by Kupffer cells and endothelial cells. Endothelial cells were 26 times more active (on a cell volume basis) than were Kupffer cells in the internalization of these 4 125I-ligands. Mannose and N-acetylglucosamine-terminated glycoproteins competed with each other for uptake into either endothelial cells or Kupffer cells, indicating that a single system recognized mannose or N-acetyl-glucosamine residues. Agalacto-orosomucoid and ahexosaminoorosomucoid were also associated with hepatocytes, but competition experiments utilizing excess asialo-orosomucoid demonstrated that residual galactosyl residues were responsible for this association.