Sensitive colorimetric assay for angiotensin converting enzyme in serum.

A sensitive colorimetric procedure has been developed for the assay of angiotensin converting enzyme (EC 3.4.15.1) in serum. Serum (10 microL) is incubated for 30 min with hippuryl-glycyl-glycine as described earlier (Clin Chem 28: 1352-1355, 1982). After a Folin-Wu deproteinization, the liberated glycyl-glycine is derivatized with a borate-buffered (pH 9.3) trinitrobenzenesulfonate solution (60 mmol/L) to form trinitrophenyl-glycylglycine, the absorbance of which is read at 420 nm vs a serum blank. The linear range extends to an activity of more than 900 U/L of serum and the detection limit is less than 4 U/L. The mean activity for serum from 50 blood bank donors and 25 patients with active sarcoidosis was 281 (SD 77) and 693 (SD 81) U/L, respectively. The method demonstrates good precision (CVs less than 2.8%) and correlates well (r = .99) with results from a "high-pressure" liquid chromatographic procedure for determining hippuric acid. In addition, the proposed method is widely applicable, involving only commonly available apparatus.