A simple, rapid, highly sensitive and reproducible fluorimetric assay for angiotensin-converting enzyme in untreated serum using the natural substrate based on a similar assay with hippuryl-L-histidine-L-leucine is described. Angiotensin I (0.2 mM in 0.1 M potassium phosphate-30 mM NaCl, pH 7.5; 37 C) is converted to angiotensin II and L-histidyl-L-leucine, which is quantified fluorimetrically (excitation = 360 nm; fluorescence, 500 nm) by formation of a fluorescent adduct with O-phthaldialdehyde. L-Histidyl-L-leucine peptidase was also monitored in order to correct for significant activity, which was observed only once, in less than 1% sera. The mean value of serum angiotensin-converting enzyme in sera from 45 normal subjects was 4.69+/-0.194 (SEM)+/-1.30 (SD) nmol/min/ml serum, compared with 31.7+/-1.53 (SEM)+/-10.3(SD) with the substrate analog hippuryl-L-histidine-L-leucine. There was a high degree of correlation between the velocity of cleavage of angiotensin I and hippuryl-L-histidyl-L-leucine (r - 0.903 to 0.993). The assay of serum angiotensin-converting enzyme is of use in the diagnosis and possible management of sarcoidosis and Gaucher's disease, and may have other applications.

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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-L-leucine, which is quantified spectrofluorimetrically (lamdba excitation = 360 nm; lamdba 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 mM. 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.

Angiotensin converting enzyme (ACE) (dipeptidyl carboxypeptidase; peptidyldipeptide hydrolase, EC 3.4.15.1) was localized in epithelioid and giant cells within ACE-rich sarcoidosis granulomas, but not control granulomas, by immunofluorescence by using a rabbit anti-human ACE immunoglobulin for localization and fluorescein-labeled goat anti-rabbit immunoglobulin for detection. ACE was frequently not detected in the very central epithelioid cells, perhaps due to a paucity of cellular or molecular stimulatory interaction in this region. The results suggest that marked elevation of ACE in the granulomas and often in the serum in sarcoidosis may be due to intense induction of ACE in most epithelioid and giant cells of the sarcoidosis granulomas.