Characterization of solubilized opiate receptors from human placenta

A solubilized opiate receptor from human pla-cental membranes binds [³H]etorphine saturably, reversibly, and stereospecifically. The binding activity is shown to be truly soluble because it migrates between two soluble proteins on Se-pharose CL-6B. The solubilized macromolecular complex behaves as a molecule with an apparent Stokes radius of 70 A and contains a protein as an essential constituent. The solubilized receptors bind [³H]ethylketocyclazocine with high affinity in a saturable and stereospecific manner. Naloxone inhibits [³H]etorphine and [³H]ethylketocyclazocine binding to the solubilized receptor. Sodium inhibits [³H]etorphine but not [³H]naloxone binding. These observations indicate that the solubilized receptor from human placental membranes is active and retains properties of membrane-bound receptors.