Importance of the N-terminal domain of the type II angiotensin antagonist sarmesin for receptor blockade

Analogues of the competitive angiotensin antagonist [Sar1,Tyr(ME)4]angiotensin II (sarmesin) with modifications at the N-terminus have been prepared by the solid-phase method and purified by reversed-phase HPLC. Substitution of the Sar1 residue of sarmesin with N,N-dimethyl-Gly, N-ethyl-Gly, aminoisobutyric, (methylamino)isobutyric, aminocaproic, and oxamic acids gave analogues that had the following respective antagonist activities (pA2) in the rat isolated uterus assay: less than 6, 6.9, 5.5, 6.0, less than 6, and 5.3. The additional substitution of Ile for Phe at the C-terminus of the latter four peptides gave pA2 values of 7.1, 5.1, less than 5, and 5. Substitution of the Arg2 residue of sarmesin with Nle or Sar abolished antagonist activity. These data emphasize the stringent and discriminating structural requirements in the N-terminal domain of sarmesin that endow this analogue with its antagonist properties and suggest the presence of defined steric constraints in this region of the molecule during receptor blockade.