Analysis of selective binding epitopes for the κ-opioid receptor antagonist nor-binaltorphimine

The structural determinants for the selective binding of the nonpeptide opioid receptor antagonist nor-binaltorphimine (nor-BNI) to the ≃-opioid receptor were characterized using a systematic series of chimeras between the ≃ receptor and the homologous μ-opioid receptor. All 10 chimeric constructs bound the nonselective antagonists (-)-naloxone and diprenorphine with similar affinities, as did the two wild-type receptors. Introduction of amino-terminal segments of increasing length, extending to and including transmembrane segment VI, from the μ receptor into the ≃ receptor did not impair the high affinity binding of nor-BNI, and neither did introduction of the intracellular carboxyl-terminal extension of the μ receptor. In contrast, nor-BNI binding was impaired ≥600-fold in constructs in which extracellular loop 3 and transmembrane segment VII originated from the μ receptor. The exchange of a single residue within this region, Glu²⁹⁷, for lysine, the corresponding residue from the μ receptor, reduced the binding affinity of nor-BNI 142-fold, without affecting the binding of the nonselective compounds (-)-naloxone and diprenorphine. It is concluded that the selective binding of nor-BNI to the ≃-opioid receptor is determined by nonconserved residues located in extracellular loop 3 and transmembrane segment VII and that Glu²⁹⁷, located just outside transmembrane segment VI, plays a major role in the ≃-selective binding characteristics of nor- BNI.