Partial purification of the mu opioid receptor irreversibly labeled with [3H]b-funaltrexamine

The mu opioid receptor in bovine striatal membranes was specifically and irreversibly labeled by incubation with 5 nM [3H]β-funaltrexamine (approx.-FNA) at 370C for 90 min in the presence of 100 mM NaCl. The specific and irreversible binding of [3H]β-FNA as defined by that blocked by 1 +M naloxone was about 60% of total irreversible binding. The specific irreversible binding was saturable, stereospecific, time-, temperature, and tissue-dependent. Mu opioid ligands were much more potent than delta or kappa ligands in inhibiting the specific irreversible labeling. SDS polyacrylamide gel electrophoresis of solubilized membranes in the presence of 2-mercaptoethanol yielded a major radiolabeled broad band of MW 68-97K daltons, characteristic of a glycoprotein band. This band was not observed in membranes labeled in the presence of excess unlabeled naloxone. The glycoprotein nature of the [3H]β-FNA-labeled opioid receptor was confirmed by its binding to a wheat germ agglutinin-Sepharose column and its elution with N-acetylglucosamine