Angiotensin I-converting enzyme (ACE, kininase II) is a single-chain protein containing two active site domains (named N- and C-domains according to position in the chain). ACE is bound to plasma membranes by its C-terminal hydrophobic transmembrane anchor. Ileal fluid, rich in ACE activity, obtained from patients after surgical colectomy was used as the source. Column chromatography, including modified affinity chromatography on lisinopril-Sepharose, yielded homogeneous ACE after only a 45-fold purification. N-terminal sequencing of ileal ACE and partial sequencing of CNBr fragments revealed the presence of an intact N terminus but only a single N-domain active site, ending between residues 443 and 559. Thus, ileal-fluid ACE is a unique enzyme differing from the widely distributed two-domain somatic enzyme or the single C-domain testicular (germinal) ACE. The molecular mass of ileal ACE is 108 kDa and when deglycosylated, the molecular mass is 68 kDa, indicating extensive glycosylation (37% by weight). In agreement with the results reported with recombinant variants of ACE, the ileal enzyme is less Cl(-)-dependent than somatic ACE; release of the C-terminal dipeptide from a peptide substrate was optimal in only 10 mM Cl-. In addition to hydrolyzing at the C-terminal end of peptides, ileal ACE efficiently cleaved the protected N-terminal tripeptide from the luteinizing hormone-releasing hormone and its congener 6-31 times faster, depending on the Cl- concentration, than the C-domain in recombinant testicular ACE. Thus we have isolated an active human ACE consisting of a single N-domain. We suggest that there is a bridge section of about 100 amino acids between the active N- and C-domains of somatic ACE where it may be proteolytically cleaved to liberate the active N-domain. These findings have potential relevance and importance in the therapeutic application of ACE inhibitors.