| Abstract: | Rat hepatocytes, freshly isolated by a collagenase perfusion technique, bound 3H]asialo-orosomucoid in a sugar-specific and calcium-dependent manner as expected for the hepatic asialoglycoprotein receptor. At least 90% of the total cell surface-bound 3H]asialo-orosomucoid represented specific binding and could be removed by washing with EDTA. Freshly isolated cells had about 7 x 10(4) surface receptors per cell. However, when cells were incubated at 37 degrees C, the number of surface receptors per cell rapidly increased 2- to 3-fold to about 2.2 x 10(5). This increase in receptor number occurred in the absence of serum and began within minutes, depending on the particular conditions used to keep the cells in suspension. (The maximal rate of appearance of new receptors at 37 degrees C was about 70 receptors per cell per s.) When cells were first exposed to a brief EDTA treatment at 4 degrees C, before measuring the binding of 3H]asialo-orosomucoid, the number of surface receptors per cell was found to increase by about 45%. Therefore, about 30% of the surface receptors on freshly isolated cells have already bound endogenous asialoglycoproteins or are present in the membrane in a cryptic form. At 4 degrees C the binding of 3H]asialo-orosomucoid was rapid (kon greater than or equal to 1.8 x 10(4) M-1s-1), whereas the dissociation of bound 3H]asialo-orosomucoid, measured in the presence of excess nonradioactive glycoprotein, was extremely slow (koff less than or equal to 0.9 x 10(-5) s-1). The association constant calculated from these data (Ka = 2.0 x 10(9) M-1) agreed well with that obtained from equilibrium binding experiments (Ka = 2.4 x 10(9) M-1) using untreated cells or cells which had first been treated with EDTA or incubated at 37 degrees C. In all cases, when the concentration of 3H]asialo-orosomucoid was higher than about 600 ng/ml, the Scatchard plots were curvilinear. The data are, however, consistent with the conclusion that there is a single high affinity receptor on the hepatocyte surface. The additional receptors that appear on the surface when cells are incubated at 37 degrees C or exposed to EDTA are identical with those on untreated cells, |
