Solubilization and characterization of high-affinity [3H]serotonin binding sites from bovine cortical membranes

High-affinity [3H]serotonin binding activity has been solubilized from bovine cerebral cortical membranes by using Triton X-100, Tween-80, and octyl-beta-D-glucopyranoside. This mixture of detergents solubilizes the high-affinity [3H]serotonin binding activity present in crude membrane preparations with retention of 75-90% specific binding. The detergent mixture was chosen because it can easily be removed from the solubilized fraction by dialysis and polystyrene bead adsorption, thus permitting further purification and isolation of the binding sites. Saturation analysis reveals multiple components of high-affinity [3H]serotonin binding. In crude bovine cortical membranes, at least two binding components are present. A higher-affinity binding component, as defined from curvilinear Scatchard plots, has a Kd for [3H]serotonin of 1-3 nM, whereas a lower-affinity component has a Kd of 10-20 nM. In the solubilized preparation, only a single class of binding sites is apparent, with a Kd of 50-100 nM. Removal of detergents by dialysis and polystyrene bead adsorption results in restoration of the curvilinear Scatchard plot with apparent Kds similar to those observed in crude membrane preparations and with increased Bmax values for each component. [3H]Serotonin binding activity in the solubilized preparation is stable to Sephacryl S-300 column chromatography and to glycerol gradient sedimentation. Saturation analysis of the peak binding fractions from both these procedures once again yields curvilinear Scatchard plots, indicating that the multiple high-affinity binding components are preserved and migrate together. The molecular weight, Stokes radius, and frictional coefficient of the binding site(s) have been calculated. After detergent removal the solubilized material shows many of the characteristics usually attributed to S1 receptors, such as high affinity for [3H]serotonin and its analogs and low affinity for serotonin antagonists.