Nonionic detergents increase the stoichiometry of ligand binding to the rat hepatic galactosyl receptor

When digitonin is used to expose intracellular galactosyl (Gal) receptors in isolated rat hepatocytes, only about half of the binding activity for 125I-asialoorosomucoid (ASOR) is found as compared to cells solubilized with Triton X-100. The increased ligand binding in the presence of detergent is not due to a decrease in Kd but could be due either to an increase in the number of ASORs bound per receptor or to exposure of additional receptors. Several experiments support the former explanation. No additional activity is exposed even when 80% of the total cell protein is solubilized with 0.4% digitonin. It is, therefore, unlikely that receptors are in intracellular compartments not permeabilized by digitonin and inaccessible to 125I-ASOR. Digitonin-treated cells are not solubilized by Triton X-100 if they are first treated with glutaraldehyde under conditions that retain specific binding activity. 125I-ASOR binding to these permeabilized/fixed cells increases about 2-fold in the presence of Triton X-100 and a variety of other detergents (e.g., Triton X-114, Nonidet P-40, Brij-58, and octyl glucoside) but not with the Tween series, saponin, or other detergents. When these fixed cells are washed to remove detergent, 125I-ASOR binding decreases almost to the initial level. Affinity-purified Gal receptor linked to Sepharose 4B binds approximately twice as much 125I-ASOR in the presence of Triton X-100 as in its absence. The results suggest that the increase in Gal receptor activity in the presence of nonionic detergents is due to an increase in the valency of the receptor rather than to exposure of additional receptors.