Purification and properties of an angiotensin-binding protein from rabbit liver particles

An angiotensin II-binding protein was purified more than 8000-fold after solubilization from rabbit liver particles with digitonin. The procedure comprised fractionation with ammonium sulfate, chromatography on DEAE-cellulose and Affi-Gel 501, gel filtration through Sephacryl S-200, and chromatography with hydroxylapatite. The purified preparation exhibited Kd and Bmax values of 6.7 nM and 8.4 nmol of angiotensin II bound/mg protein. The latter figure represents more than 60% of the theoretical value calculated for a protein of Mr 75,000 as estimated for the major protein component by gel electrophoresis. The purified preparation displayed comparable or slightly higher affinities for various angiotensin antagonists and angiotensin III than that for angiotensin II, whereas angiotensin I as well as the hexapeptide and smaller carboxy-terminal fragments were less tightly bound. Binding of angiotensin II by the isolated protein was highly dependent upon the presence of p-chloromercuriphenylsulfonic acid and also required ethylene diaminetetraacetic acid which could be almost completely replaced by ethylene glycol bis(beta-aminoethyl ether) N,N'-tetraacetic acid but not by o-phenanthroline.