CHARACTERIZATION OF A SPECIFIC BINDING-SITE FOR ANGIOTENSIN-II IN CHICKEN LIVER

We have characterized a specific binding site for angiotensin II (AngI I) in chicken liver membranes. Pseudo-equilibrium studies at 22 degree s C for 30 min have shown that this binding site recognizes AngII with a high affinity (pK(D) of 8.13 +/- 0.21). The binding sites are satur able and relatively abundant (maximal binding capacity varies from 0.3 18 to 0.88 pmol/mg of protein). Nonequilibrium kinetic analyses at 22 degrees C revealed a calculated kinetic pK(D) of 8.77 +/- 0.20. The bi nding site is pharmacologically distinct from the classic AngII recept ors AT(1) and AT(2). Competitive binding studies with chicken liver me mbranes demonstrated the following rank order of effectiveness: AngII (human; Asp-Arg-Val-Tyr-Ile-His-Pro-Phe) > AngI (Asp-Arg-Val-Tyr-Ile-H is-Pro-Phe-His-Leu) > AngIII (Arg-Val-Tyr-Ile-His-Pro-Phe) > AngIV (Va l-Tyr-Ile-His-Pro-Phe) > Ang(1-7) (Asp-Arg-Val-Tyr-Ile-His-Pro) > PD12 3319 -tetrahydro-1H-imidazo[4,5-c]pyridine-6-carboxylic acid) > DuP753 (2-n-butyl-4-chloro-5 '-1H-tetrazol-5-yl)biphenyl-4-yl)methyl]imidazo le. This atypical AngII binding site (chicken AT) was sensitive to inc reasing concentrations of DTT and Mn2+. The structure-activity relatio nship on position 1 of AngII showed that the primary N-terminal amine was essential for binding affinity ([Asp(1)]AngII > [Suc(1)]AngII grea ter than or equal to [Sar(1)]AngII), but modifications of the side cha in in position 1 had less influence on the affinity ([Gly(1)]AngII > [ Cys(1)]AngII approximate to [aminoisobutyryl(1)]AngII approximate to [ Ser(1)]AngII >>> [Sar(1)]AngII). The presence of substantial quantitie s of this binding site in chicken liver membranes suggests the possibi lity that the chicken AT may play an important, yet unrecognized, role in the renin-angiotensin system.