SELECTIVE PEPTIDE AND NONPEPTIDE LIGANDS DIFFERENTIALLY BIND TO ANGIOTENSIN-II AT(2) RECEPTOR AND A NON-ANGIOTENSIN-II CGP42112 BINDING-SITE

[I-125]CGP42112 [Nic-Tyr-(epsilon-CBZ (benzyloxycarbonyl)-Arg)Lys-His- Pro-IIe] does not only recognize angiotensin II AT(2) receptors, but h as also the capacity to label a high-affinity, non-angiotensin II bind ing site, selectively associated with macrophages and activated microg lia. We have searched for the structural requirements of the novel CGP 42112 binding site, and compared these with the requirements for bindi ng to the angiotensin II AT(2) site. We designed a series of CGP42112 analogs and evaluated the new compounds by using binding assays on rat spleen (CGP42112 site) and rat fetal (angiotensin II AT(2) site) memb ranes. The nonpeptidic analog Z-Arg(Pmc)OH(N(alpha)CBZ-N-G-2,2;5,7,8-p entamet ylchroman-6-sulphonyl-L-Arg), the side chain of CGP42112 subst ituted on the guanidinium group, was selective in recognizing the CGP4 2112 site, and did not displace binding from the angiotensin II A(2) s ite. This is a potential lead compound for development of CGP42112 sit e-selective analogs. Conversely, the CGP42112 analog lacking the CBZ-g roup (Nic-Tyr-(Ac-Arg)Lys-His-ProOH, III) and the peptide Nic-Tyr-Lys- His-Ala-HisOH (VI), were selective for the angiotensin II AT(2) site, and recognized the CGP42112 site poorly. Our results demonstrate that the structural requirements for the nonangiotensin II CGP42112 and the angiotensin II AT(2) binding sites are different. We propose that the CBZ group and the free carboxyl terminal group, together with their s patial orientation, are key components of the molecule for the interac tion in the non-angiotensin CGP42112 binding pocket.