An angiotensin-converting enzyme (ACE) inhibitor in human serum. Increased sensitivity of the serum ACE assay for detecting active sarcoidosis.

Measurement of serum angiotensin-converting enzyme (ACE) is extremely useful as an aid in the diagnosis and longitudinal evaluation of patients with sarcoidosis. We have detected a human serum ACE-inhibitor which affects the ACE level obtained by activity measurements. The effect of the inhibitor can be eliminated by a mere eight-fold dilution of the serum sample with physiologic saline. We recommend that serum ACE be performed with 1:8 dilutions of serum to eliminate the effect of the ACE inhibitor. The inhibitor has a MW above 50,000 daltons, and reversibility of inhibition by dilution appears to be ion dependent. Dialysis of an inhibitor-containing serum sample against saline causes the inhibition to become irreversible, allowing the distinction between alinearity of the assay vs an inhibitor effect when a serum has ACE activity greater than 50 units/ml. The source of the serum ACE-inhibitor remains to be determined.