Photoaffinity labeling of the angiotensin II receptor; pharmacology of the labeling peptides in the dark

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Abstract
The biological activities of photoaffinity labeling analogs of angiotensin II (ATII) and their precursors were measured in rabbit aorta strips in the dark. Most of the analogs behave as reversible, specific agonists, one as a competitive inhibitor. The activities are discussed in line with the current view of structural requirements. The modifications consisted of substitutions on the aromatic nuclei of Tyr4 and Phe8 in [Sar1]ATII with (4′-NO2)Phe, (4′-NH2)Phe, (4′-N3)Phe, (4′-N2+)Phe, and (4′-NH2-3′, 5′-I2)Phe. It is shown that the affinity of the ATII analogs modified in position 4 depends on the electronegativity and not on space-filling properties of the aromatic residue; rising electronegativity lowers the affinity, i.e., [Sar1, (4′-NO2)Phe4]ATII has no more measurable activity. Substituting the aromatic side chain in position 8 of [Sar1]ATII gives well-binding analogs with intrinsic activities from 0 to 100% and activity seems to depend only on stereochemical requirements. Agonists and partial agonists bear rather small groups like -NH2, -N3, -NO2, and -N2+. The only antagonist [Sar1, (4′-NH2-3′,5′-I2)Phe8]ATII resembles the antagonist [Sar1, Leu8]ATII in competitivity and binding.