Inhibition and affinity chromatography of chicken lung angiotensin I-converting enzyme with captopril

1. Angiotensin I-converting enzyme (EC 3.4.15.1) has been purified to electrophoretic homogeneity from chicken lung by using a facile two-step protocol which included affinity chromatography on Sepharose-bound captopril. 2. Captopril was a potent inhibitor of chicken lung angiotensin I-converting enzyme with Ki values of 2.0 nmol/l and 1.6 nmol/l for detergent-extracted and trypsin-extracted angiotensin I-converting enzymes, respectively. 3. Molecular weight comparison of trypsin-extracted (M(r)270,000) and detergent-extracted (M(r)690,000) angiotensin I-converting enzyme indicated that membrane-binding sequence contributed to a large extent to the enzyme molecule. 4. Kinetic properties of both forms of the enzyme suggested that the membrane-bound sequence contributed to an increase of the enzyme-substrate affinity.