Identification of endogenous opioid receptor components in rat brain using a monoclonal antibody

A monoclonal antibody generated against the tertiary structure of a partially purified opioid binding protein was used to probe the structure of the dynorphin and β-endorphin receptors. The Fab fragment 3B4F11 inhibited completely the binding of ¹²⁵I-β-endorphin and [³H]dynorphin to rat brain P₂ membranes with IC₅₀ values of 26 ng/ml and 40 ng/ml, respectively. To explore further the interaction of 3B4F11 with the β-endorphin receptor, the effect of the Fab fragment on ¹²⁵I-β-endorphin cross-linking to rat brain membranes was examined. ¹²⁵I-β-endorphin was covalently bound to three major species of approximate molecular weights 108,000, 73,000 and 49,000. The δ-selective ligand D-Pen², D-pen⁵enkephalin was least effective at inhibiting the cross-linking of β-endorphin, whereas the μ-selective ligand Tyr-D-Ala-Gly-NMe-Phe-Gly-ol and κ-selective ligand U50488 inhibited β-endorphin cross-linking to the 108,000 and 73,000 Da species. Both 3B4F11 and β-endorphin prevented the covalent binding of ¹²⁵I-β-endorphin to all three labeled species. These findings suggest that μ and κ receptor types might have some structural similarities, whereas the δ receptor type might differ in molecular size. In addition, the μ, κ, and δ ligands might have different primary sequences, whereas their tertiary structures might share regions of molecular homology with all three receptor constituents labeled by ¹²⁵I-β-endorphin. 3B4F11 will be a valuable tool for the purification and isolation of the several components of the β-endorphin receptor complex.