H-1-NMR STUDIES OF SARAFOTOXIN SRTB, A NONSELECTIVE ENDOTHELIN RECEPTOR AGONIST, AND IRL-1620, AN ET(B) RECEPTOR-SPECIFIC AGONIST

H-1 NMR studies on the nonselective endothelin receptor agonist sarafo toxin SRTb have identified a helix between residues Asp 8 and His 16, and a beta-turn involving residues Cys 3 to Met 6; however, the biolog ically important C-terminal five residues were found to be conformatio nally variable. The average RMSD, measured for the final 43 refined st ructures to the average structure over residues 1-16, was 0.78 +/- 0.1 8 Angstrom for the backbone atoms and 1.39 +/- 0.22 Angstrom for all a toms. The torsion angles Cys 3 psi/Lys 4 phi, Thr 7 psi/Asp 8 phi, and Gln 17 phi were identified as sites of conformational variability. Di fferences were found between the structures in the bicyclic loop regio n for SRTb, and those published for ET1, another nonselective receptor agonist, which may explain the observed differences in potency of the se peptides. The conformation of an ET(B) receptor-specific agonist, I RL 1620, which lacks the N-terminal seven residues and the two intrach ain disulfides, was found by NMR and circular dichroism spectroscopy t o be predominantly random coil, despite the fact that its affinity for the ET(B) receptor almost equals that of ET1. However, close analysis of the NMR results indicated the presence of turn-like structures, or a nascent helix, in the part of the sequence corresponding to the hel ical region in the parent peptides. These results suggest that the hel ical conformation may be required for ligand binding to the ET(B) rece ptor as well as to the ET(A) receptor.