This study demonstrates that a novel angiotensin I analog, angiotensinogen 3-11(Lys(11)), possesses a high affinity for angiotensin-converting enzyme (ACE), which is substantially greater than the endogenous substrates. This assessment is based on data derived from a variety of techniques. First, the binding characteristics of (125)I-angiotensinogen 3-11(Lys(11)) were examined. Equilibrium saturation isotherms utilizing guinea pig lung membranes revealed that (125)I-angiotensinogen 3-11(Lys(11)) bound a single high-affinity site in the presence of EDTA exhibiting a K(d) of 0.15 +/- 0.02 nM with a B(max) = 4295 +/- 535 fmol/mg of protein. Competition studies revealed the following rank order of binding affinity: (125)I-angiotensinogen 3-11(Lys(11)) >> bradykinin >> angiotensin I. Next, SDS-polyacrylamide gel electrophoresis analysis revealed that chemically cross-linked (125)I-angiotensinogen 3-11(Lys(11)) specifically bound a protein of M(r) 173,000 that had the same molecular weight as ACE. Utilizing in vitro autoradiography, the binding distributions of (125)I-angiotensinogen 3-11(Lys(11)) and the ACE inhibitor, (125)I-351A, were also compared. These experiments demonstrated that the binding distributions of (125)I-angiotensinogen 3-11(Lys(11)) and (125)I-351A are identical in the guinea pig lung and testes. Finally, the purification of ACE from guinea pig serum was monitored with (125)I-angiotensinogen 3-11(Lys(11)) and (125)I-351A binding. These results demonstrated that the binding site for (125)I-angiotensinogen 3-11(Lys(11)) and (125)I-351A copurified. These experiments indicate that the novel angiotensin I analog, (125)I-angiotensinogen 3-11(Lys(11)) binds to ACE and suggest that there are critical binding sites outside the catalytic domains of ACE that determine binding specificity and affinity.