Studies on a mammalian hepatic binding protein specific for asialoglycoproteins. Evidence for receptor recycling in isolated rat hepatocytes

Abstract

Freshly isolated rat hepatocytes attained maximal ability to bind, internalize, and degrade 125I-asialo-orosomucoid after 5 h in suspension culture at 37 degrees C. Comparison of the number and distribution of the asialoglycoprotein binding sites of these cells revealed that 5% (6.7 x 10(4) receptors/cell) were on the external cell membrane with an average residency time of slightly less than 3 min. The remaining 95% were located intracellularly, as determined with detergent-solubilized hepatocytes. Binding of ligand (asialo-orosomucoid) was time-dependent, saturable, and dissociable. The dissociation constant for the single high affinity binding site was calculated to be 3.4 x 10(-8) M. The amount of asialo-orosomucoid metabolized by these cells over a period of 3 h at 37 degrees C was reduced 50% by the inclusion of 1 mM cycloheximide in the incubation medium. However, even in the absence of protein synthesis, 34 times more asialo-orosomucoid was metabolized than could be bound by the cell surface receptors, or twice the total capacity of the intact hepatocyte. These results provide clear evidence for the stability of the binding receptor under conditions where the ligand is being continually destroyed and support the previously proposed recycling hypothesis (Tanabe, T., Pricer, W.E., Jr., and Ashwell, G. (1979) J. Biol. Chem. 254, 1038-1043).