Solubilization and Characterization of σ‐Receptors from Guinea Pig Brain Membranes

Abstract: The σ‐receptor, a distinct binding site in brain tissue that may mediate some of the psychotomimetic properties of benzomorphan opiates and phencyclidine, has been sol‐ubilized using the ionic detergent sodium cholate. Binding assays were performed with the solubilized receptor using vacuum filtration over polyethyleneimine‐treated glass fiber filters. The pharmacological specificity of the solubilized binding site for σ‐receptor ligands is nearly identical to the membrane‐bound form of the receptor, with the order of potencies for displacement of the selective σ‐ligand [³H]di‐o‐tolylguanidine ([³H]DTG) closely correlated. The stereoselectivity for (+)‐benzomorphan opiate enantiomers was retained by the solubilized receptor. The soluble receptor retained high affinity for binding of [³H]DTG (K_D= 28 ± 0.5 nM) and (+)‐[³H]3‐(3‐hydroxyphenyl)‐N‐(l‐propyl)piperi‐dine {(+)‐[³H]3‐PPP} (K_D= 36 ± 2 nM). Photoaffinity labeling of the solubilized receptor by [³H]p‐azido‐DTG, a σ‐selective photoaffinity label, resulted in labeling of a 29‐kilodalton polypeptide identical in size to that labeled in intact membranes. Estimation of the Stokes radius of the [³H]DTG binding site was obtained by Sepharose CL‐6B chromatography in the presence of 20 mM cholate and calculated to be 8.7 nm. This value was identical to the molecular size found for the binding sites of the σ‐selective ligands (+)‐[³H]3‐PPP and (+)‐[³H]SKF‐10,047, supporting the hypothesis that all three ligands bind to the same macro‐molecular complex.