A liquid chromatography-assisted assay for angiotensin-converting enzyme (peptidyl dipeptidase) in serum

I describe modification of the spectrophotometric assay described by Cushman and Cheung [biochem. Pharmacol. 20, 1637 (1971)] for serum angiotensin-converting enzyme, with use of "high-pressure" liquid chromatography to measure the hippuric acid end product. After reaction of 10 microL of untreated serum with the angiotensin-converting enzyme substrate analog hippuryl-L-histidyl-L-leucine, the hippuric acid produced is extracted into ethyl acetate and quantitated, relative to an added internal standard, by liquid chromatography. Total chromatographic running time is 3 min per sample, with a within-run CV of 4.3% and a day-to-day CV of 6.6%, for aliquots of plasma supplemented with 1 mmol of hippuric acid per liter. The measurements are linear for hippuric acid in amounts up to 20 nmol per assay. The presence of large quantities of lipid in the serum did not affect the accuracy of the determination.