Exclusive Affinity-Labeling of [mu] Opioid Receptors by Morphiceptin Analogs Containing S-(3-Nitro-2-pyridylthio)cysteine

The S-(3-nitro-2-pyridylthio) (Npys) group only reacts with a free mercapto group to form a disulfide bond. A series of morphiceptin analogs containing SNpys-cysteine were designed and synthesized for specific affinity-labeling of μ opioid receptors. Affinity-labeling of opioid receptors was monitored and evaluated by radioligand receptor binding assays using rat brain membranes and specific tritium-labeled ligands of enkephalin analogs. It was found that analogs with Cys(Npys) at position 4 or 5 bind covalently to μ receptors, but not to δ receptors, resulting in a discriminative and exclusive affinity labeling of μ opioid receptor subtype.