Selective agonist of nonopioid beta-endorphin receptor decapeptide immunorphin (SLTCLVKGFY) was labeled with tritium (the specific activity of 24 Ci/mmol). [3H]Immunorphin was found to bind to nonopioid beta-endorphin receptor of mouse peritoneal macrophages (Kd = 2.0 +/- 0.1 nM). The [3H]immunorphin specific binding with macrophages was inhibited by unlabeled beta-endorphin (Ki = 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 have been synthesized and their ability to inhibit the [3H]immunorphin specific binding to macrophages was studied. Unlabeled fragment 12-19 (TPLVTLFK, the author's name of the peptide octarphin) was found to be the shortest peptide possessing practically the same inhibitory activity as beta-endorphin (Ki = 3.1 +/- 0.3 nM). The peptide octarphin was labeled with tritium (the specific activity of 28 Ci/mmol). [3H]Octarphin was found to bind to macrophages with high affinity (Kd = 2.3 +/- 0.2 nM). The specific binding of [3H]octarphin was inhibited by unlabeled immunorphin and beta-endorphin (Ki = 2.4 +/- 0.2 and 2.7 +/- 0.2 nM, respectively).

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Synthetic peptide, corresponding to the amino acid sequence 11-24 of human adrenocorticotropic hormone (ACTH), was labeled with tritium (specific activity of 22 Ci/mmol). [(3)H]ACTH (11-24) was found to bind to rat adrenal cortex membranes with high affinity and specificity (K(d) = 1.8 +/- 0.1 nM). Twenty nine fragments of ACTH (11-24) have been synthesized and their ability to inhibit the specific binding of [(3)H]ACTH (11-24) to adrenocortical membranes has been investigated. Unlabeled fragment ACTH 15-18 (KKRR) was found to replace in a concentration-dependent manner [(3)H]ACTH (11-24) in the receptor-ligand complex (K(i) = 2.3 +/- 0.2 nM). ACTH (15-18) was labeled with tritium (specific activity of 20 Ci/mmol). [(3)H]ACTH (15-18) was found to bind to rat adrenal cortex membranes with high affinity (K(d) = 2.1 +/- 0.1 nM). The specific binding of [(3)H]ACTH (15-18) was inhibited by unlabeled ACTH (11-24) (K(i) = 2.2 +/- 0.1 nM). ACTH (15-18) at the concentration range of 1-1000 nM did not affect the adenylate cyclase activity in adrenocortical membranes.

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