Isolation of an angiotensin II-binding protein from liver

A protein that specifically binds angiotensin II has been isolated in nearly homogeneous form by two independent approaches after solubilization from rabbit liver particles by treatment with digitonin. The protein purified by either of these methods resembles in size the single radioactive macromolecular component made by using disuccinimidyl suberate to crosslink radioiodinated angiotensin II with its receptor in the solubilized extract. In the first technique, angiotensin II as an affinity ligand specifically extracted the protein from a preparation that had been freed of angiotensin-degrading activity. In the second approach, the angiotensin II-protein complex was specifically precipitated by anti-angiotensin II antibodies and staphylococcal protein A-Sepharose. The protein could be eluted from the affinity column with angiotensin II or 4 M MgCl2. The angiotensin II-protein complex dissociated in the presence of sulfhydryl-containing reagents, and these could therefore be used to elute it from either the chemical or the immunoaffinity-based matrix. This effect of sulfhydryl-containing reagents and the paradoxical observation that the isolated protein after denaturation exhibited a slower electrophoretic mobility in its reduced form that in its unreduced form suggest that the binding configuration of this protein may be sensitive to reduction.