Design of a functional hexapeptide antagonist of endothelin

Endothelin-1, a bicyclic 21-amino acid peptide, is a potent constrictor of vascular smooth muscle. Since the isolation of ET-1 from the supernatant of cultured porcine endothelial aortic cells, human genomic analysis has identified two structurally and functionally related isopeptides (ET-2 and ET-3). Previous structure-activity analyses have shown the importance of the C-terminal L-tryptophan indole ring, its carboxylate, and the two cystine bridges (1-15 and 3-11) for vasoconstrictor activity in certain tissues. In addition, in vitro binding (IC sub(50) approximately equals 50-70 mu M) to endothelin receptors in rat cardiac and rabbit pulmonary tissue preparations has been demonstrated for the C-terminal hexapeptide (His-Leu-Asp-Ile-Ile-Trp and Ac-His-Leu-Asp-Ile-Ile-Trp (compounds 2 and 3, Table I)). Using D-amino acids to probe the importance of the individual residues, we observed that incorporation of D-histidine in the 16 position (compound 4) led to a 20-fold enhancement of the binding affinity in several tissue beds.