NOVEL, DELTA-OPIOID RECEPTOR-SELECTIVE PEPTIDE ANTAGONISTS - DEMONSTRATION OF A POSSIBLE OPIOID INTERACTION IN THE ABSENCE OF A PROTONATED NITROGEN AND ATTEMPTS TO LOCATE THE PROTONATED NITROGEN SITE

We have recently (Ronai et al., 1992) introduced a family of novel del ta-opioid receptor-selective peptide antagonists, based on the Tyr-Pro -Gly-Phe-Leu-Thr structure, where the nitrogen accepts substituents th at make protonation possible (e.g. diallyl) as well as substituents (e .g. t-Boc) where protonation cannot occur. In this paper, we present t he details of a design strategy where the structurally closely related biologically active and inactive compounds are suggestive of conforma tional requirements of action. Furthermore, since even those derivativ es of the antagonist peptides where the N-terminus was free were eithe r devoid of opioid agonist activity or were extremely weak agonists, i t is suggested that these antagonists do not interact with the convent ional ''opioid nitrogen site''. To find this ''conventional'' site, a number of N-substituted (phenylglycyl-, alpha-Boc-lysyl-, alpha-Phe-be ta-alanyl-) derivatives of Tyr-Pro-Gly-Phe-Leu-Thr hexapeptide were sy nthesized and their biological activities were determined in the mouse vas deferens bioassay.