Answering your questions: ACE test.

Q Our physicians are interested in adding the angiotensin-converting enzyme (ACE) to our test panel. Could you give me some information as to how the test is performed?

A Angiotensin-converting enzyme is an enzyme present in vascular endothelial cells, primarily in the lung. The enzyme converts angiotensin I to angiotensin II. ACE-inhibitors are drugs that are widely used in the management of hypertension. Measurement of ACE levels is helpful in the diagnosis and management of sarcoidosis. Sarcoidosis is a chronic granulomatous disease of unclear etiology. Individuals with sarcoidosis have consistently elevated ACE levels, and levels are somewhat correlated with disease burden and activity. (1), (2) Elevated ACE levels are non-specific, however, and may be present in a wide variety of diseases including histoplasmosis, cirrhosis, and tuberculosis. ACE levels can also be decreased with drug therapy--particularly steroids.

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Fluorometric and spectrophotometric methods have been described to measure ACE levels. With the spectrophotometric method, plasma is combined with an artificial substrate molecule (N-[3-(2-furyl)acryloyl]-L-phenylalanylglycylglycine [FAPGG]). (3) The combination is incubated together, and the substrate molecule undergoes hydrolysis to (N-[3-(2-furyl)acryloyl]-L-phenylalanine [FAP]). The change in absorbance is correlated with the level of ACE in the plasma. In the less commonly used fluorometric method, a common substrate is hippuryl-histidyl-leucine. Additional methods for measuring ACE have been described and include radioassays, inhibitor-binding assays, and ELISA methods. A variety of commercially available ACE methods are on the market. Kits are available that utilize almost any of the methods described above.

--Nicholas Radovich, MD

Department of Pathology

Oregon Health and Science University

Portland, OR

References

(1.) Muthuswamy PP, Lopez-Majano V, Ranginwala M, Trainor WD. Serum angiotensin-converting enzyme (SACE) activity as an indicator of total body granuloma load and prognosis in sarcoidosis. Sarcoidosis. September 1987;4(2):142-148.

(2.) Mimori Y. Sarcoidosis: correlation of HRCT findings with results of pulmonary function tests and serum angiotensin-converting enzyme assay. Kurume Med J. 1998;45(3):247-256.

(3.) Johansen KB, Marstein S, Aas P. Automated method for the determination of angiotensin-converting enzyme in serum. Scand J Clin Lab Invest. June 1987;47(4):411-414.

Edited by Daniel M. Baer, MD

Daniel M. Baer, MD, is professor emeritus of laboratory medicine at Oregon Health and Science University in Portland, OR, and a member of MLO's editorial advisory board.

MLO's "Tips from the Clinical Experts" provides practical, up-to-date solutions to readers' technical and clinical issues from a panel of experts in various fields. Readers may send questions to Dan Baer by e-mail at [e-mail].