Differences in backbone structure between angiotensin II agonists and type I antagonists

Type I angiotensin II antagonists with O-methyl-L-homoserine [HSer(gamma-OMe)] and delta-methoxy-L-norvaline [Nva(delta-OMe)] at position 8 have been prepared by the solid-phase method, purified by reverse-phase HPLC, and bioassayed in the rat uterus, and their backbone conformational properties were investigated by nuclear Overhauser effect (NOE) spectroscopy. [Sar1,HSer-(gamma-OMe)8]ANGII, [HSer(gamma-OMe)8]ANGII, [Des1,HSer(gamma-OMe)8]ANGII, [Sar1,Nva(delta-OMe)8]-ANGII, and [Des1,Nva(delta-OMe)8]ANGII had, respectively, the following antagonist activities, pA2: 7.6, 7.5, < 6.0, 7.1, and 6.9. Analogs of [Sar1]ANGII with delta-hydroxy-L-norvaline [Nva(delta-OH)], delta-methoxy-L-norvaline [Nva(delta-OMe)], 4'-carboxyphenylalanine [Phe(4'-COOH)], and 4'-(trifluoromethyl)phenylalanine [Phe(4'-CF3)] at position 4 were also prepared by solid phase and bioassayed in the rat uterus. [Sar1,Nva(delta-OH)4]ANGII, [Aib1,Nva(delta-OMe)4]ANGII, [Sar1,DL-Phe(4'-COOH)4]ANGII, and [Sar1,DL-Phe(4'-CF3)4]ANGII had, respectively, agonist activities as follows: 4%, 1.5%, 3%, < 0.1%, and < 0.1%. These data emphasize that replacement of Ile8 in Sarilesin with the higher homologs HSer(gamma-OMe) and Nva(delta-OMe) does not greatly alter the structural requirements necessary for expression of type I antagonist activity, while replacement of the tyrosine hydroxyl in [Sar1]ANGII by the carboxylate or the trifluoromethyl group abolishes activity, suggesting that the tyrosinate pharmacophore cannot be replaced by any negatively charged or electronegative group. Conformational investigation of the ANGII type I antagonists [HSer(gamma-OMe)8]ANGII and [Sar1Nva(delta-OMe)8]ANGII in DMSO by 1D-NOE spectroscopy revealed that the Tyr-Ile-His bend, a conformational property found in ANGII and [Sar1]ANGII (J. Biol. Chem. 1994, 269, 5303) is not present in type I antagonists, providing for the first time an important conformational difference between angiotensin II agonists and type I antagonists.