Inhibitor Binding Assay for Angiotensin-Converting Enzyme

We describe a new principle for the determination of en-zymes, here applied to angiotensin-converting enzyme (ACE, EC 3.4.15.1) in human serum. The enzyme inhibitor binding assay isbased on specific binding of labeled inhibitor to the active center of the enzyme. Serum (10-15 L) is incubated with 125I-labeled ACE inhibitor (“351A, ” a p-hydroxybenzamidine derivative of N-(1-carboxy-3-phenylpro-pyl)-L-lysyl-L-proline) at pH 7.0 at 37 #{176}Cfor 2 h in a non-equilibrated system. Inhibitor bound to ACE is separated by adsorption to coated charcoal, the radioactivity remaining in the supernate is counted, and the ACE value is calculated from a standard curve. Sensitivity for ACE in serum is 200 U/L, corresponding to 5.0 ng of ACE purified from human lung. The coefficient of variation was 3.9 % within assay, and 6.4 % between assays for normal ACE activities. Correlatio