Angiotensin-1-converting enzyme (ACE) activity in human serum following plasma exchange.

Although angiotensin-1-converting enzyme (ACE), which catalyzes both the conversion of angiotensin 1 to angiotensin 2 and the hydrolysis of bradykinin, is known to be present in serum, little information is available about its source or turnover characteristics. We determined the removal and recovery characteristics of serum ACE for two normal subjects and 16 patients, with various diseases, who were undergoing partial plasma exchange. Cholesterol was removed as predicted, but the average actual removal of ACE, 70% of that originally present, was 28% greater than predicted. Recovery of serum levels of ACE after plasma exchange was 34% on Day 1, 62% on Day 3, 94% on Day 5, and 104% on Day 8, and the level remained relatively constant thereafter up to 28 days of study. The pattern of recovery of serum ACE is more compatible with a system of synthesis and release of ACE than with a simple exchange mechanism from sites on the plasma membrane of the endothelial cell or a shift of the enzyme from one space to another.