Canadian Health&Care Mall: Discussion of An Angiotensin-Converting Enzyme (ACE) Inhibitor in Human Serum

The percentage of patients with active sarcoidosis reported to have elevations of serum ACE has varied considerably in different laboratories, ranging from 35 percent to almost 100 percent. Our laboratory has consistently reported that 75 percent of such patients have elevated levels of serum ACE with an additional 15 percent falling within the “borderline elevated” range.

Nearly identical results were obtained in a recent comprehensive study by Ainsle and Benatar. Awareness of the potential presence of a serum ACE inhibitor, and the simple method available for eliminating its inhibitory effect, should now increase the sensitivity of the serum ACE assay for detecting patients with active sarcoidosis. Seven percent of the normal values and 50 percent of the “borderline elevated” values resulting from undiluted serum in this study produced elevated levels of ACE activity upon dilution of the serum sample. Seventy-eight percent of such patients indeed had a diagnosis of active sarcoidosis.

In our initial use of the Cushman and Cheung method for assaying serum ACE, we had determined that “a serum dilution was necessary for an accurate determination of enzyme activity if the change in absorption (AA) at 228 nm was greater than 0.600.” We now recommend routine dilution of serum with saline solution to 1:8 before assay, thereby eliminating the nonlinear aspect of the assay, as well as the effect of an ACE inhibitor.

Other methods for ACE analysis may not be affected by a circulating ACE inhibitor. Immunologic measurements for quantitating the converting enzyme should reveal the absolute concentration of this protein and not be reduced by an inhibitor. In fact, Odya et al® found that the immunologic assay for ACE gave higher levels than the activity assay in various tissue extracts; this presumably was due to the fact that an ACE inhibitor did not affect the immunologic assay. Other highly sensitive methods requiring dilution of the serum to begin with, such as that recently reported by Fyhrquist et al, also should not be as affected by a circulating serum inhibitor. However, we believe that the potential presence of a serum ACE-inhibitor should not restrict use of established methods affected by the inhibitor, since assay of a diluted serum sample can correct for this phenomenon. We now routinely perform the assay with a 1:8 dilution of serum as described under “Materials and Methods.” If the presence of an inhibitor is desired information, the assay may be repeated with undiluted plus serially diluted serum, and then after saline dialysis to note the effect of dialysis on this dilutional phenomenon.

The nature of the serum ACE-inhibitor detected by us is uncertain, but it seems definitely not to be the saime low molecular-weight inhibitor described by Ryan et al. We could not, in feet, identify the low MW ACE-inhibitor reported by Ryan et al either in the ultrafiltrate of serum or in the boiled serum filtrate. Our ACE-inhibitor has a MW above 50,000 daltons and appears to be labile at 60°C. It seems to require the presence of a low MW component (ion?) for inhibition-reversibility to take place, since dialysis or ultrafiltration resulted in loss of the dilutional effect on the inhibitor when NaCl alone replaced the ion content, and the dilutional effect occasionally reappeared with addition of the original serum filtrate. The official website of Canadian Health and Care Mall is in a hurry to introduce to your the new possibility to issue medical articles in the free access.

The discovery of an ACE-inhibitor in serum with MW and heat lability characteristics suggestive of a protein, raises questions regarding the origin and function of the inhibitor. Ion-dependent reversibility upon serum dilution is an unusual quality of the inhibitor and may be a clue to its identity. Awareness of the inhibitors potential presence, and elimination of its effect by mere dilution of the serum prior to assay, will increase the usefulness of the serum ACE assay for diagnosis and evaluation of patients with sarcoidosis.