Isolation of angiotensin converting enzyme (ACE) binding protein from human serum with an ACE affinity column

Immobilized angiotensin-converting enzyme (ACE) was utilized as an affinity ligand to isolate a naturally occurring ACE binding protein from normal hu man serum. The enzyme was isolated from solubilized bovine lung membrane pr eparations by lisinopril affinity chromatography. It had an estimated molec ular weight of 180000 and was recognized by the anti-ACE antibody for the r abbit testicular ACE in immunoblots. ACE was immobilized onto epoxy Sepharo se as well as Affi-Gel 15. Immobilized ACE on Affi-Gel 15 had higher cataly tic activity (0.176 U/mL) compared with the enzyme immobilized on epoxy Sep harose (0.00005 U/mL). Immobilized ACE served as the affinity ligand for th e identification of the ACE binding protein in human serum with an estimate d molecular weight of 14000 as observed by SDS polyacrylamide gel electroph oresis. The identification and further characterization of ACE binding prot eins in serum and tissues may facilitate the greater understanding of the e ndogenous regulation of this key enzyme, which is involved in blood pressur e homeostasis.