Evaluation of a test kit for the rapid and simple colorimetric measurement of angiotensin I-converting enzyme in serum.

We have evaluated a recently introduced colour test kit for the determination of serum angiotensin I-converting enzyme catalytic activity. p-Hydroxyhippuric acid, liberated from p-hydroxyhippuryl-L-histidyl-L-leucine by angiotensin I-converting enzyme, is converted into a quinoneimine dye with an absorption maximum at 505 nm. The procedure shows excellent linearity over the whole range of catalytic activities found in serum. Intra- and inter-assay coefficients of variation are 2-5 and 7-10% respectively. Correlation with a modified Cushman-Cheung ((1971) Biochem. Pharmacol. 20, 1637-1648) method currently used in our laboratory is good, with r = 0.985 and a regression equation of y (colour kit) = 0.423 x (Cushman-Cheung) + 0.765. Haemoglobin, lipids, bilirubin and prednisone do not interfere but uric acid in concentrations higher than 600 mumol/l does. No extraction step is required. The assay is very rapid, and more than twenty samples can be determined in an hour.