The Phe3 and/or Tyr5 residues in dermorphin (H-Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2) and its N-terminal hexapeptide-amide were replaced by delta-Phe or by Phe5 in order to examine the effect on opioid activity. On GPI preparation, the substitution of Phe5 for Tyr5 was well tolerated, whereas the hexa and heptapeptides containing delta Phe in position 3 and/or 5 displayed low potency. The unsaturation at position 3 alone or at positions 3 and 5 was particularly detrimental to mu activity. In the tail flick test, the influence of unsaturation or substitution at positions 3 and 5 generally matched the results of the in vitro assay. Dehydropeptides showed comparatively low antinociceptive effects and [Phe5] analogues displayed about 50% of the analgesic potency of the original peptides.
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