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

Abstract

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 ( 3 ), is approximately 2 logs less potent than hU-II (pD 2=6.3 vs 8.4). A conformational analysis in solution of the active peptide 3 , 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 3 clearly shows that three out of four key residues (i.e., Phe 6, Lys 8 and Tyr 9) maintain the same side– chain orientation, while the fourth one, Trp 7, cannot be superimposed. This observation could explain the reduced biological activity of the synthetic analogue.