Structure-activity studies of endothelin leading to novel peptide ETA antagonists

The receptor binding affinity of monocyclic [Ala3,Xaa11]endothelin-1 analogs correlates with inducible helix length

Endothelin-1, a bicyclic 21-amino acid peptide with disulfide bridges between cysteines 1 and 15 as well as between cysteines 3 and 11, has been reported to be partially helical based on both CD and NMR data. However, this remains an area of controversy with some claims that CD data indicate no alpha-helical structure (Calas, B.; Harricane, M.-C.; Gulmard, L.; Heitz, F.; Mendre, C.; Chabrier, P.E.; Bennes, R. Peptide Res. 1992, 5, 97) and a recent X-ray crystal structure placing the helix at a different locus (Janes, R.W.; Peapus, D.H.; Wallace, B.A. Structural Biology 1994, 1, 311). The CD studies reported herein indicate that the helical structures reported in NMR studies (e.g. Andersen, N.H.; Chen, C.; Marschner, T.M.; Krystek, Jr. S.R.; Bassolino, D.A. Biochemistry 1992, 31, 1280) apply to pure aqueous media as well. The helix located from Lys9 to the Cys15/His16 juncture is ca 75% populated in pH 4 aqueous buffer. Titration difference CDs reveal that the helix extent increases by one to two residues and that the 'helical conformation' is more completely populated upon addition of TFE to 50+ volume-%. Comparison with a more helical analog suggests that the helix propagates towards (but not to the end of) the C-terminus upon fluoroalcohol addition. A variety of monocyclic derivatives of [Nle7] ET-1 lacking the 3,11-disulfide were evaluated for biological activity and examined by TFE titration difference CD. The series included an Aib11 and a Pro11 analog. The helix promoting Aib analog was the most active while the Pro analog exhibited significantly lower vasoconstrictor activity and binding affinity for the ETA receptor. All of the monocyclic analogs became significantly more helical upon addition of fluoroalcohols. The inclusion of a proline residue at position 11 does not preclude helix formation upon addition of fluoroalcohols. Rather, helix formation is relatively easily induced but limited to a 5 residue span. Apparently this is insufficient to orient required side chains optimally for interaction with the ETA receptor. For the 1,15-monocyclic analogs differing only at position 11, ETA binding affinity and vasoconstrictor potency correlate with the facility which a 7-8 residue long helix can be induced. This presumably includes the segment Glu10-->Cys15 in all cases and may represent the full sequence from Lys9-->His16. CD studies also reveal that the C-terminal fragment of endothelins is not a fully disordered 'random coil' either alone or attached to the endothelin core.

The development of potent peptide agonists and antagonists for the endothelin receptors

The endothelins (ETs), sarafotoxins (SRTXs), vasoactive intestinal contractor (VIC), and bibrotoxin are a family of potent vasoconstrictor peptides. All peptides in this family possess 21 amino acids arranged in a unique bicyclic motif formed between cystine bridges in the 1-15 and 3-11 positions. Since the discovery of endothelin-1 (ET-1) in 1988, significant effort has been focused on the understanding of its structure-activity relationships. The identification of endothelin receptor subtypes has led to the discovery/design of potent peptide agonists and antagonists, along with nonpeptide antagonists of endothelin with varying levels of potency and receptor subtype selectivity. In keeping with the theme of this journal, this review will focus only on the development of peptidic-based agonists and antagonists of endothelin in addition to their applications in understanding the physiological and/or pathophysiological role of endothelin and its isopeptides.