INSULIN BINDING TO RAT-LIVER MEMBRANES PREDICTS A HOMOGENEOUS CLASS OF BINDING-SITES IN DIFFERENT AFFINITY STATES THAT MAY BE RELATED TO A REGULATOR OF INSULIN BINDING

Insulin binding to pericanicular, liver plasma membranes was measured at equilibrium as a function of temperature from 4-degrees-C to 37-degrees-C. Scatchard plots of the binding data obtained at temperatures from 4-degrees-C to 15-degrees-C were linear and the Hill plots were characterized by Hill coefficients equal to unity. Thus, insulin binding under these conditions was consistent with the presence of a single class of homogeneous, noninteracting binding sites. However, the Scatchard plots of binding data obtained above 15-degrees-C were curvilinear, and the Hill coefficients derived from these data were about 0.75. This apparent change in the complexity of the binding with increasing temperature was not due to gross ligand or receptor degradation and care was taken to ensure that all assumptions inherent in interpreting the equilibrium binding data were valid. Changes in membrane fluidity or the presence of a cryptic population of receptors which surface with increasing temperature also could not account for this apparent increase in the complexity of the binding above 15-degrees-C because identical observations were made using nonionic and ionic detergent-solubilized liver plasma membranes. Thus, we were able to rule out heterogeneity of binding sites as a model to explain the increased complexities of the binding above 15-degrees-C. We conclude that the temperature dependence of insulin binding in impure but intact receptor preparations is consistent with a two-state model of the insulin receptor. Using this model, we predict that one conformational state of the insulin receptor exists below 15-degrees-C but that two affinity states of the receptor exist at higher and physiological temperature. Our results are consistent with two states of the receptor resulting from the temperature-dependent interaction of the receptor with an affinity regulator of insulin binding.