Characterization of a spectrophotometric assay for angiotensin converting enzyme

Although serum angiotensin converting enzyme (EC 3.4.15.1) activity has generally been shown to be increased in patients with sarcoidosis, considerable variation in the diagnostic usefulness of this test has been reported. We investigated the possibility that this variation may be the result of inhibition of the widely used spectrophotometric assay by various substances. We also prospectively examined the predictive value of measurements of this enzyme in serum from 100 patients being evaluated for sarcoidosis. The following did not significantly affect the results: storage at 4 or 25 degrees C for one week, hemoglobin, lipoproteins, or corticosteroid medications. Bilirubin, at concentrations of 20 mg/L of serum or move, significantly inhibited the assay. Sera with increased activity showed nonlinear reaction rates during the usual 60-min reaction interval, This was corrected by shortening the reaction time to 30 min. We found a predictive value of 88% when the serum angiotensin converting enzyme test was applied to the diagnosis of active sarcoidosis.