Intracellular segregation of asialo-transferrin and asialo-fetuin following uptake by the same receptor system in suspended hepatocytes.

Asialo-transferrin and asialo-fetuin are both taken up into suspended hepatocytes by the asialo-glycoprotein receptor and with similar kinetics (Tolleshaug, H., Chindemi, P. and Regoeczi, E. (1981) J. Biol. Chem. 265, 6526-6528). However, the intracellular fate of the two ligands differ. Asialo-fetuin is carried to the lysosomes and degraded. Internalized asialo-transferrin is recycled with the receptors back to the cell surface, from which it may be released by calcium chelators. In the current studies, we fractionated cell homogenates in sucrose density gradients in order to trace the pathways taken by asialo-transferrin and asialo-fetuin within the cells. More than one-half of the intracellular asialo-transferrin sedimented within a novel kind of 'light' endosomes which were recovered at 1.11 g/ml in sucrose gradients. When cells were fractionated 6 min after the addition of trace concentrations of 125I-asialo-fetuin and 131I-asialo-transferrin, their intracellular distributions were found to be roughly similar. After 24 min their distributions were clearly disparate, relatively more asialo-fetuin being recovered in a peak of heavy endosomes at 1.15 g/ml. The ligand molecules in this part of the gradient (e.g., asialo-fetuin) were delivered to the lysosomes to be degraded, while the material in the lighter peak was degraded much more slowly. The data indicate that asialo-fetuin and asialo-transferrin enter a light endosome fraction immediately after receptor-mediated endocytosis. Subsequently, they are separated; the asialo-transferrin remains receptor-bound and is returned to the cell surface, while the asialo-fetuin is transferred to endosomes of density 1.15 g/ml and is eventually degraded in the lysosomes.