Purification of human serum angiotensin I-converting enzyme by affinity chromatography

A relatively simple procedure is described for purifying human serum angiotensin-converting enzyme. The enzyme was purified 130,000-fold to electrophoretic homogeneity using affinity chromatography as the principal purification step. The ligand was an immobilized competitive inhibitor, d-cysteinyl-l-proline. A six-carbon spacer arm was satisfactory for trapping the enzyme; 80% of the bound enzyme was eluted with 3 m urea-1.0 m NaCl-0.1 m Tris, pH 8.3. The specific activity was 39 units/mg protein and the molecular weight (155,000), isoelectric point (4.7), kinetic properties, and the effect of various inhibitors are in agreement with published reports.