| Abstract: | The tritium-labeled selective agonist of the nonopioid beta-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 beta-endorphin receptor of mouse peritoneal macrophages (Kd 2.4 +/- 0.1 nM). The specific binding of 3H]immunorphin to macrophages was inhibited by unlabeled beta-endorphin (Ki of the 3H]immunorphin-receptor complex 2.9 +/- 0.2 nM) and was not inhibited by unlabeled naloxone, alpha-endorphin, gamma-endorphin, and Met5]enkephalin (Ki > 10 microM). Thirty fragments of beta-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 beta-endorphin is its fragment 12-19 (Ki 3.1 +/- 0.3 nM). |
