Binding of β-endorphin and its fragments to the nonopiod receptor of murine peritoneal macrophages

The tritium-labeled selective agonist of the nonopioid β-endorphin receptor the decapeptide immunorphin (³H]SLTCLVKGFY) with a specific activity of 24 Ci/mmol was prepared. It was shown that ³H]immunorphin binds with a high affinity to the non-opioid β-endorphin receptor of mouse peritoneal macrophages (K _d 2.4 ± 0.1 nM). The specific binding of ³H]immunorphin to macrophages was inhibited by unlabeled β-endorphin (K _i of the ³H]immunorphin-receptor complex 2.9 ± 0.2 nM) and was not inhibited by unlabeled naloxone, α-endorphin, γ-endorphin, and Met⁵]enkephalin (K _i > 10 μM). Thirty fragments of β-endorphin were synthesized, and their ability to inhibit the specific binding of ³H]immunorphin to macrophages was studied. It was found that the shortest peptide having practically the same inhibitory activity as β-endorphin is its fragment 12–19 (K _i 3.1 ± 0.3 nM).