RECEPTOR-BINDING PROPERTIES OF THE PEPTIDES CORRESPONDING TO THE BETA-ENDORPHIN-LIKE SEQUENCE OF HUMAN-IMMUNOGLOBULIN-G

The decapeptide H2N-Ser-Leu-Thr-Cys-Leu-Val-Lys-Gly-Phe-Tyr-COOH (term ed immunorphin) corresponding to the sequence 364-373 of the CH3 domai n of the human immunoglobulin G1 Eu heavy chain and displaying a 43% i dentity with the antigenic determinant of beta-endorphin was synthesiz ed. Immunorphin was found to compete with I-125-beta-endorphin for hig h-affinity receptors on murine peritoneal macrophages (K = 2.5 +/- 0.9 x 10(-9) M) and with H-3-morphin for receptors on murine thymocytes ( K-i = 2.7 +/- 0.6 x 10(-9) M) and murine macrophages (K-i = 5.9 +/- 0. 7 x 10(-9)M). In particular two types of receptors to I-125-beta-endor phin with K-d1 = 6.1 +/- 0.6 x 10(-9) M and K-d2 = 3.1 +/- 0.2 x 10(-8 ) M were revealed on macrophages. The second type of receptors interac ted with I-125-beta-endorphin, H-3-Met-enkephalin, H-3-Leu-enkephalin and H-3-morphin; the first displayed reactivity with I-125-beta-endorp hin, H-3-morphin and immunorphin. The first type receptors are not pre sent on murine brain cells nor are inhibited by naloxone. A minimum fr agment of immunorphin practically completely retaining its inhibitory activity in the competition tests with I-125-beta-endorphin for common receptors on thymocytes was found to correspond to the tetrapeptide H 2N-Lys-Gly-Phe-Tyr-COOH (K-i = 5.6 +/- 0.5 x 10(-9) M).