Biochemical and pharmacological characterization of three opioid-nociceptin hybrid peptide ligands reveals substantially differing modes of their actions

In an attempt to design opioid-nociceptin hybrid peptides, three novel bivalent ligands, H-YGGFGGGRYYRIK-NH2, H-YGGFRYYRIK-NH2 and Ac-RYYRIKGGGYGGFL-OH were synthesized and studied by biochemical, pharmacological, biophysical and molecular modelling tools. These chimeric molecules consist of YGGF sequence, a crucial motif in the N-terminus of natural opioid peptides, and Ac-RYYRIK-NH2, which was isolated from a combinatorial peptide library as an antagonist or partial agonist that inhibits the biological activity of the endogenously occurring heptadecapeptide nociceptin. Solution structures for the peptides were studied by analysing their circular dichroism spectra. Receptor binding affinities were measured by equilibrium competition experiments using four highly selective radioligands. G-protein activating properties of the multitarget peptides were estimated in [35S]GTPγS binding tests. The three compounds were also measured in electrically stimulated mouse vas deferens (MVD) bioassay. H-YGGFGGGRYYRIK-NH2 (BA55), carrying N-terminal opioid and C-terminal nociceptin-like sequences interconnected with GGG tripeptide spacer displayed a tendency of having either unordered or β-sheet structures, was moderately potent in MVD and possessed a NOP/KOP receptor preference. A similar peptide without spacer H-YGGFRYYRIK-NH2 (BA62) exhibited the weakest effect in MVD, more α-helical periodicity was present in its structure and it exhibited the most efficacious agonist actions in the G-protein stimulation assays. The third hybrid peptide Ac-RYYRIKGGGYGGFL-OH (BA61) unexpectedly displayed opioid receptor affinities, because the opioid message motif is hidden within the C-terminus. The designed chimeric peptide ligands presented in this study accommodate well into a group of multitarget opioid compounds that include opioid-non-opioid peptide dimer analogues, dual non-peptide dimers and mixed peptide- non-peptide bifunctional ligands.