Characterization of solubilized opiate receptors from human placenta

A solubilized opiate receptor from human placental 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 Sepharose 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.