β-Endorphin omission analogs: Dissociation of immunoreactivity from other biological activities

An analog of human β-endorphin with omission of four residues at positions 11, 14, 20, and 22 has been synthesized. This analog and other synthetic analogs with deletion of a single amino acid at position 2, 5, 6, 10, 11, 12, 13, 15, or 22 have been assayed for analgesic potency, ileal opiate activity, opiate receptor-binding activity, and immunoreactivity. Results show that deletion of a single amino acid of the β-endorphin molecule outside of the enkephalin segment to give (des-Gln11-, des-Thr:12-, des-Pro13-, des-Leu14-, des-Val15-, des-Asn20-, des- Ile22-β-endorphin) markedly reduced or abolished the immunoreactivity yet gave substantial retention of opiate potencies. Deletion of a single amino acid of β-endorphin within the enkephalin segment (des-Gly2- or des-Met5-β-endorphin) did not markedly affect the immunoactivity; however, the opiate activities were abolished or markedly reduced. The data indicate a clear dissociation of immunoactivity from analgesic, ileal-opiate, and opiate receptor-binding activities.