What is angiotensin I-converting enzyme (ACE)?

Angiotensin I-converting enzyme (ACE) is a central regulatory enzyme within the renin-angiotensin-aldosterone system (RAAS), a critical hormonal cascade that controls blood pressure, fluid balance, and electrolyte homeostasis. It is a zinc metallopeptidase primarily anchored to the plasma membranes of endothelial cells, especially in the lungs, but is also found in soluble form in blood plasma and various tissues. Its primary and eponymous biochemical function is to cleave the inactive decapeptide angiotensin I by removing a dipeptide from its carboxyl terminus, thereby producing the potent vasoconstrictor octapeptide angiotensin II. This conversion is the pivotal rate-limiting step that activates the pressor effects of the RAAS, as angiotensin II subsequently binds to receptors, causing vasoconstriction, aldosterone secretion, sodium retention, and increased blood pressure.

Beyond this canonical role, ACE exhibits a dual substrate specificity, functioning equivalently as kininase II. In this capacity, it inactivates bradykinin, a vasodilatory peptide that promotes nitric oxide release and lowers blood pressure. Therefore, ACE exerts a profound bidirectional influence on vascular tone: it simultaneously generates a potent vasoconstrictor and degrades a potent vasodilator. This dual action amplifies its net pressor effect, making it a uniquely powerful pharmacological target. The enzyme's structure, with two homologous catalytic domains (N and C) that have slightly different substrate affinities and activities, adds a layer of complexity to its physiological regulation and its interaction with drugs.

The clinical significance of ACE is overwhelmingly defined by the development and widespread use of ACE inhibitors, a cornerstone class of drugs for treating hypertension, heart failure, diabetic nephropathy, and post-myocardial infarction care. By competitively inhibiting the enzyme, these drugs reduce angiotensin II production and potentiate bradykinin levels, leading to vasodilation, reduced aldosterone, and decreased sodium reabsorption. The therapeutic benefits are attributed to both mechanisms, though the accumulation of bradykinin is also linked to the side effect of dry cough observed in some patients. Furthermore, a common insertion/deletion polymorphism in the ACE gene influences circulating enzyme levels, which has been extensively studied for potential associations with cardiovascular disease risk, athletic performance, and sarcoidosis, though with often conflicting and context-dependent results.

From a broader physiological and pathological perspective, ACE's role extends beyond cardiovascular regulation. Its presence in testis, where a distinct isozyme is encoded by the same gene but under a different promoter, is essential for male fertility. In macrophages and other tissues, it may modulate immune and inflammatory responses. The enzyme's involvement in bradykinin and substance P metabolism also implicates it in inflammatory pathways and pain perception. Thus, while ACE is fundamentally a protease executing a specific biochemical cleavage, its position at the intersection of two powerful peptide systems renders it a master regulator of vascular, renal, and inflammatory physiology, with its inhibition representing one of the most successful examples of rational drug design in modern medicine.