Inhibitor binding assay for angiotensin-converting enzyme

We describe a new principle for the determination of enzymes, here applied to angiotensin-converting enzyme (ACE, EC 3.4.15.1) in human serum. The enzyme inhibitor binding assay is based on specific binding of labeled inhibitor to the active center of the enzyme. Serum (10-15 microL) is incubated with 125I-labeled ACE inhibitor (" 351A ," a p- hydroxybenzamidine derivative of N-(1-carboxy-3-phenylpropyl)-L-lysyl-L-proline) at pH 7.0 at 37 degrees C for 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. Correlation with a comparison spectrophotometric method (Am J Med 59: 363-372, 1975) for ACE assay was excellent (r = 0.98) in 59 samples from healthy subjects and from patients with various diseases including active sarcoidosis. The novel assay principle presented here is simple and specific, and can be extended to use with various biological fluids and tissues, and to other enzymes as well.