Design, Synthesis, Conformational Analysis, and Biological Studies of Urotensin-II Lactam Analogues Original Research Article

Human urotensin II (hU-II; H-Glu-Thr-Pro-Asp-cyclo[Cys-Phe-Trp-Lys-Tyr-Cys]-Val-OH) is a disulfide bridged undecapeptide recently identified as the ligand of an orphan G protein-coupled receptor. hU-II has been described as the most potent vasoconstrictor compound identified to date. With the aim of replacing the disulfide bridge by a chemically more stable moiety, we have synthesized and tested a series of lactam analogues of hU-II minimum active fragment, that is hU-II(4–11). The contractile activity of the synthetic analogues on the rat isolated thoracic aorta was found to be dependent upon the dimension of the lactam bridge. The most active peptide, H-Asp-cyclo[Orn-Phe-Trp-Lys-Tyr-Asp]-Val-OH (), is approximately 2 logs less potent than hU-II (pD2=6.3 vs 8.4). A conformational analysis in solution of the active peptide , one of the inactive analogues, and hU-II was performed, using NMR and molecular modelling techniques. A superposition of the calculated structures of hU-II and clearly shows that three out of four key residues (i.e., Phe6, Lys8 and Tyr9) maintain the same side– chain orientation, while the fourth one, Trp7, cannot be superimposed. This observation could explain the reduced biological activity of the synthetic analogue.