A Sensitive Fluorimetric Assay for Serum Angiotensin-con venrting Enzyme

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Abstract
A simple, rapid, highly sensitive and reproducible assay for angiotensin- converting enzyme in untreated serum is described. It is based on the conversion of the substrate analog, hippuryl-L-histidyl-L-leucine (5 mM in 0.1 M K phosphate, /pH 8.3-0.3 M NaCl) to hippurate and L-histidyl-Lleucine, which is quantified spectrofluorimetrically (λ excitation = 360 nm; λ fluorescence = 500 nm) by formation of a fluorescent adduct with ophthaldialdehyde. The chloride requirement and inhibition and activation patterns correspond to those for angiotensin-converting enzyme. The Km, for hippuryl-L-histidyl-L-leucine was 1.33 niM. The mean value of serum angiotensin-converting enzyme for 58 normal human subjects (mean age, 32 years; range 19-57) was 32.2± 1.30 (SE), with a standard deviation of 9.87 nmol/min/ml serum. The assay is useful for the diagnosis and possible management of sarcoidosis and may have other applications in the future.

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