Synthetic β-endorphin-like peptide immunorphin binds to non-opioid receptors for β-endorphin on T lymphocytes

The synthetic decapeptide H-SLTCLVKGFY-OH (termed immunorphin) corresponding to the sequence 364-373 of the CH3 domain of human immunoglobulin G heavy chain was found to compete with [125I]β-endorphin for high-affinity receptors on T lymphocytes from the blood of healthy donors (Ki = 0.6 nM). Besides immunorphin, its synthetic fragments H-Val-Lys-Gly-Phe-Tyr-OH (Ki = 15 nM), H-Leu-Val-Lys-Gly-Phe-Tyr-OH (Ki = 8.0 nM), H-Cys-Leu-Val-Lys-Gly-Phe-Tyr-OH (Ki = 3.4 nM), H-Thr-Cys-Leu-Val-Lys-Gly-Phe-Tyr-OH (Ki = 2.2 nM), H-Leu-Thr-Cys-Leu-Val-Lys-Gly-Phe-Tyr-OH (Ki = 1.0 nM) possessed the ability to inhibit specific binding of [125I]β-endorphin to T lymphocytes. Tests of the specificity of the receptors revealed that they are not sensitive to naloxone and Met-enkephalin, i.e. they are not opioid receptors. Kd values characterizing the specific binding of 125I- labeled immunorphin and its fragment H-Val-Lys-Gly-Phe-Tyr-OH to the receptors have been determined to be 7.4 nM and 36.3 nM, respectively.