Angiotensin I-converting enzyme (ACE, EC 3.4.15.1), an exopeptidase, is a circulating enzyme that participates in the body's renin-angiotensin system (RAS), which mediates extracellular volume (i.e. that of the blood plasma, lymph and interstitial fluid), and arterial vasoconstriction. It is secreted by pulmonary and renal endothelial cells and catalyzes the conversion of decapeptide angiotensin I to octapeptide angiotensin II.

ACE Ubiquitously expressed, with highest levels in lung, kidney, heart, gastrointestinal system and prostate. The testis-specific isoforn is expressed in spermatocytes, adult testis. ACE can Convert angiotensin I to angiotensin II by release of the terminal His-Leu, this results in an increase of the vasoconstrictor activity of angiotensin.