Direct spectrophotometric assay for angiotensin-converting enzyme in serum.

The appropriate conditions for determining angiotensin-converting enzyme in human serum with use of 2-furanacryloyl-L-phenylalanylglycylglycine as substrate are described. The method, a modification of that reported by Holmquist et al. (Anal. Biochem. 95:540-548, 1979) for purified rabbit lung enzyme, is based on the blue shift of the absorption spectrum that occurs upon hydrolysis of the substrate into furanacryloyl-L-phenylalanine and glycylglycine. The Km value for the substrate is 0.31 mmol/L. Some kinetic properties determined by this method are similar to those previously reported for the purified serum enzyme. Results by the present assay and Cushman's modified method correlate closely (r = 0.995). The normal reference interval for 42 adult donors was 43-137 U/L (mean 90 U/L, SD 23 U/L). The within-run and between-run CVs ranged from 3.0 to 4.1%. Its rapidity, simplicity, sensitivity, and precision make the proposed method very suitable for routine work, clinical investigation, and kinetic and structural studies of the serum enzyme.