ACE Inhibitors and Peptide Biology

Natriuretic Peptides and Blood Pressure

40% mortality reduction

ACE inhibitors reduce all-cause mortality by 25-40% in heart failure patients, making them one of the most impactful drug classes ever developed from peptide biochemistry.

Ahmad et al., JRAAS, 2023

Ahmad et al., JRAAS, 2023

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The story of ACE inhibitors begins with a snake. In the 1960s, Brazilian pharmacologist Sergio Ferreira isolated peptides from the venom of the lancehead viper (Bothrops jararaca) that potentiated the effects of bradykinin, a vasodilatory peptide. These bradykinin-potentiating peptides (BPPs) worked by inhibiting angiotensin-converting enzyme (ACE), the zinc metalloprotease that both activates the vasoconstrictor angiotensin II and degrades the vasodilator bradykinin. Gouda et al. (2021) reviewed the history and therapeutic potential of snake venom-derived BPPs, documenting how this discovery created the template for one of the most prescribed drug classes in medicine.^[1]

ACE inhibitors are, at their core, peptide science applied to pharmacology. They exploit the specificity of a peptide-processing enzyme to shift the balance between two opposing peptide systems: the vasoconstrictive angiotensin pathway and the vasodilatory bradykinin pathway. This article examines how ACE works, why blocking it lowers blood pressure, and what the research shows about both synthetic ACE inhibitors and natural peptide alternatives. For the broader context of how the renin-angiotensin system regulates blood pressure, see the renin-angiotensin system: how peptides control blood pressure. For the pillar overview of peptide-based blood pressure regulation, see sacubitril/valsartan: the drug that boosts natriuretic peptides.

The Enzyme: What ACE Actually Does

Angiotensin-converting enzyme is a type I transmembrane zinc metalloprotease anchored to the surface of endothelial cells throughout the vasculature, with the highest concentrations in the pulmonary capillary bed. ACE has two catalytic domains (N-domain and C-domain), each containing a zinc ion essential for peptide bond cleavage. The enzyme acts as a dipeptidyl carboxypeptidase: it removes two amino acids from the C-terminus of its substrates.

ACE's two most important substrates define its dual role in blood pressure regulation:

Angiotensin I (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu) is a 10-amino-acid peptide with minimal biological activity. ACE cleaves the C-terminal His-Leu dipeptide to produce angiotensin II (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe), an 8-amino-acid peptide that is one of the most potent vasoconstrictors in human physiology. Angiotensin II raises blood pressure by constricting arterioles, stimulating aldosterone secretion (which causes sodium and water retention), activating the sympathetic nervous system, and promoting cardiac and vascular remodeling.

Bradykinin (Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg) is a 9-amino-acid vasodilatory peptide. ACE degrades bradykinin by removing the C-terminal Phe-Arg dipeptide, inactivating it. Bradykinin promotes vasodilation through endothelial nitric oxide and prostacyclin release. When ACE is inhibited, bradykinin accumulates, contributing to blood pressure reduction.

Ahmad et al. (2023) conducted a systematic analysis of ACE inhibitors and their mechanisms, detailing how ACE's zinc coordination site provides the target for all approved ACE inhibitors. Each drug contains a zinc-binding group (sulfhydryl in captopril, carboxyl in enalapril and lisinopril, phosphoryl in fosinopril) that coordinates with the catalytic zinc ion, blocking substrate access.^[2]

ACE also cleaves other bioactive peptides, including substance P (a pain-transmitting neuropeptide), enkephalins, and neurotensin. The degradation of substance P by ACE explains why ACE inhibitor therapy causes dry cough in 5-20% of patients: accumulated substance P and bradykinin stimulate airway sensory nerves. This side effect is a direct pharmacological consequence of blocking peptide degradation, not an idiosyncratic reaction.

From Snake Venom to the Pharmacy Shelf

The rational design of captopril is one of the most celebrated stories in drug development. In 1970, Ng and Vane demonstrated that bradykinin-potentiating peptides from Bothrops jararaca venom inhibited the conversion of angiotensin I to angiotensin II during pulmonary passage. The most potent of these venom peptides was a nonapeptide (teprotide) that lowered blood pressure in hypertensive patients when administered intravenously but was clinically impractical because it required injection.

Miguel Ondetti and David Cushman at Squibb recognized that the venom peptide's inhibitory mechanism depended on its interaction with ACE's zinc active site. Using the crystal structure of carboxypeptidase A (a related zinc metalloprotease) as a model, they designed small molecules that could mimic the critical peptide-enzyme interactions in an orally bioavailable form. On March 13, 1974, they conceived their molecular model, and within 18 months and approximately 60 synthetic modifications, they had created captopril: a small molecule with a sulfhydryl group that coordinates directly with ACE's catalytic zinc ion.

Captopril gained FDA approval on April 6, 1981. Its development earned Cushman and Ondetti the 1999 Lasker Award for Clinical Medical Research. The success prompted development of second-generation ACE inhibitors: enalapril (prodrug converted to enalaprilat), lisinopril (no hepatic conversion required), ramipril, perindopril, and others. Each improves on captopril's pharmacokinetics or side effect profile while preserving the core mechanism of zinc-site blockade.

Sheth et al. (2002) compared omapatrilat (a dual ACE/neprilysin inhibitor, a precursor concept to sacubitril/valsartan) with lisinopril in heart failure patients, finding that omapatrilat produced greater reductions in BNP and norepinephrine levels, suggesting that combined peptidase inhibition offered advantages over ACE inhibition alone.^[3] This finding foreshadowed the development of sacubitril/valsartan two decades later.

Clinical Impact: The Evidence Base

The clinical evidence for ACE inhibitors spans four decades and dozens of randomized controlled trials. The landmark studies established benefits across heart failure, post-myocardial infarction, hypertension, and diabetic nephropathy.

The SOLVD Treatment Trial (1991) demonstrated that enalapril reduced mortality by 16% and hospitalizations for heart failure by 26% in patients with symptomatic heart failure and reduced ejection fraction. The extended 12-year follow-up (X-SOLVD) showed cumulative all-cause mortality of 50.9% with enalapril versus 56.4% with placebo.

The HOPE Trial (2000) enrolled 9,297 high-risk patients (age 55+, with vascular disease or diabetes plus one other risk factor) without heart failure or reduced ejection fraction. Ramipril reduced the composite endpoint of cardiovascular death, myocardial infarction, or stroke by 22% (from 17.8% to 14.0%). The sub-study MICRO-HOPE showed a 24% reduction in overt nephropathy in diabetic patients.

The SAVE Trial (1992) demonstrated that captopril reduced mortality by 19% in post-MI patients with reduced ejection fraction. The AIRE Trial (1993) showed ramipril reduced all-cause mortality by 27% in post-MI patients with clinical heart failure.

These trials established ACE inhibitors as first-line therapy for heart failure with reduced ejection fraction, post-MI left ventricular dysfunction, diabetic nephropathy, and cardiovascular risk reduction in high-risk populations. Kalea et al. (2010) reviewed the emerging role of ACE2 and apelin in cardiovascular disease, noting that ACE inhibitors' benefits may extend beyond simple angiotensin II reduction through effects on the ACE2/Ang(1-7)/MAS axis, which promotes vasodilation and anti-fibrotic effects.^[4]

Food-Derived ACE Inhibitory Peptides

The discovery that ACE is a peptidase that can be inhibited by peptides naturally raised the question: can dietary peptides from food proteins also inhibit ACE and lower blood pressure? This question has generated an enormous body of research, with hundreds of ACE inhibitory peptides identified from dairy, egg, fish, plant, and fermented food sources.

Rao et al. (2012) isolated ACE inhibitory peptides from hen egg white digestion hydrolysate, identifying several di- and tripeptides with IC50 values in the micromolar range.^[5] Ma et al. (2023) conducted a comprehensive characterization of ACE inhibitory peptide sequence features, finding that hydrophobic amino acids at the C-terminus (particularly proline, phenylalanine, and tryptophan) and positively charged residues (arginine, lysine) at the N-terminus are the most common structural features of potent ACE inhibitory peptides.^[6]

Adams et al. (2020) characterized casein-derived peptides and found that different peptide fractions had differential effects on the two ACE catalytic domains (N-domain and C-domain), suggesting that food-derived peptides could be selected for domain-specific inhibition with potentially different physiological effects.^[7] Manoharan et al. (2023) asked directly whether discovering new ACE inhibitors from natural products remains relevant, concluding yes, but only with comprehensive approaches that address the critical gap between in vitro activity and in vivo efficacy.^[8]

You et al. (2023) demonstrated that mussel-derived ACE inhibitory peptides produced measurable blood pressure reductions in spontaneously hypertensive rats, with systolic blood pressure decreasing by 20-30 mmHg after 4 weeks of oral administration.^[9] Bhadkaria et al. (2023) purified novel ACE inhibitory peptides from plant sources and validated their activity through molecular docking and in vivo studies, showing dose-dependent antihypertensive effects.^[10]

The critical limitation remains bioavailability. Most food-derived ACE inhibitory peptides are degraded by gastrointestinal proteases before reaching the systemic circulation. Even those that survive digestion must cross the intestinal epithelium and resist plasma peptidases to reach ACE in the pulmonary vasculature at sufficient concentrations. IC50 values measured in a test tube do not translate directly to blood pressure reduction in a living organism. The most promising food-derived ACE inhibitory peptides are short (2-3 amino acids, particularly tripeptides like VPP and IPP from fermented milk) because they resist proteolysis better than longer peptides and can be absorbed intact through intestinal peptide transporters. For more on food-derived bioactive peptides, see food-derived peptides for blood pressure: dairy, fish, and plant sources.

ACE2: The Counter-Regulatory Enzyme

ACE2 is a homolog of ACE that performs the opposite function: it converts angiotensin II (vasoconstrictor) to angiotensin 1-7 (vasodilator and anti-inflammatory peptide). The balance between ACE and ACE2 activity determines the net effect of the renin-angiotensin system on blood pressure and tissue inflammation.

Majumder et al. (2015) made a striking discovery: the egg-derived tripeptide IRW (Ile-Arg-Trp) not only inhibited ACE but also increased ACE2 expression while decreasing proinflammatory gene expression in endothelial cells.^[11] This dual mechanism, simultaneously reducing angiotensin II production (via ACE inhibition) and increasing angiotensin II degradation (via ACE2 upregulation), would amplify the blood pressure-lowering and anti-inflammatory effects beyond what either mechanism alone could achieve.

However, Wang et al. (2023) tested whether ACE2 activation is a common feature of food-derived ACE inhibitory peptides and found that it is not: most peptides that inhibit ACE do not also upregulate ACE2.^[12] The IRW tripeptide appears to be an exception rather than the rule. This finding is important because it means the biological effects of food-derived ACE inhibitors cannot be assumed to mirror those of pharmaceutical ACE inhibitors, which also do not directly activate ACE2.

The ACE2 connection gained new relevance during the COVID-19 pandemic, when SARS-CoV-2 was found to use ACE2 as its cell entry receptor. Initial concerns that ACE inhibitors might increase COVID-19 susceptibility by upregulating ACE2 were not supported by clinical data; multiple large studies found no increased risk, and some suggested potential benefit through reduced inflammation.

What the Research Has Not Settled

Several questions remain open in ACE inhibitor pharmacology. The relative contribution of angiotensin II reduction versus bradykinin accumulation to the clinical benefits is still debated. ARBs (angiotensin receptor blockers) reduce angiotensin II signaling without affecting bradykinin, yet their clinical outcomes in heart failure are broadly similar to ACE inhibitors. If bradykinin accumulation were a major contributor to benefit, ACE inhibitors should consistently outperform ARBs, which has not been convincingly demonstrated.

The therapeutic potential of food-derived ACE inhibitory peptides remains unproven in rigorous human clinical trials. Most evidence comes from in vitro assays and animal models. The few human trials that exist (primarily with VPP/IPP lactotripeptides) show modest blood pressure reductions (2-4 mmHg systolic) that, while statistically significant, are clinically small compared to pharmaceutical ACE inhibitors (typically 10-15 mmHg).

Domain-specific ACE inhibition (targeting the N-domain or C-domain independently) remains an unexplored clinical frontier. The two domains have different substrate specificities and tissue distributions, raising the possibility that selective domain inhibition could provide cardiovascular benefits with fewer side effects. Food-derived peptides with domain selectivity, like those characterized by Adams et al. (2020), could serve as lead compounds for this approach, bridging the gap between nutrition science and cardiovascular pharmacology.

For the broader cardiovascular peptide landscape, see ANP: the atrial peptide that lowers blood pressure and GLP-1 drugs and heart disease.

The Bottom Line

ACE inhibitors are pharmaceutical descendants of snake venom peptides, designed to block the enzyme that produces the vasoconstrictor angiotensin II and degrades the vasodilator bradykinin. Their clinical evidence base includes mortality reductions of 25-40% in heart failure and 22% cardiovascular event reduction in high-risk patients. Food-derived ACE inhibitory peptides show in vitro promise but face bioavailability barriers that limit clinical translation. The dual ACE/ACE2 axis adds complexity, with rare peptides like IRW simultaneously inhibiting ACE and upregulating ACE2.

Frequently Asked Questions

How do ACE inhibitors lower blood pressure?

ACE inhibitors block angiotensin-converting enzyme from converting the inactive peptide angiotensin I into the vasoconstrictor angiotensin II. This reduces arteriolar constriction, aldosterone-mediated sodium retention, and sympathetic nervous system activation. Simultaneously, ACE inhibitors prevent the degradation of bradykinin, a vasodilatory peptide, allowing it to accumulate and promote nitric oxide-mediated vessel relaxation. Ahmad et al. (2023) documented that this dual mechanism, targeting both the Ang II and bradykinin pathways, is what distinguishes ACE inhibitors from ARBs.

Were ACE inhibitors developed from snake venom?

Yes. The first ACE inhibitor, captopril, was rationally designed from peptides found in the venom of the Brazilian lancehead viper (Bothrops jararaca). Gouda et al. (2021) reviewed how bradykinin-potentiating peptides in the venom led Cushman and Ondetti to create captopril in 1975. They used the venom peptide's interaction with ACE's zinc active site as a blueprint, designing a small molecule that could mimic those interactions orally.

Can food peptides act as natural ACE inhibitors?

Hundreds of peptides from dairy, egg, fish, and plant proteins inhibit ACE in laboratory assays. You et al. (2023) showed mussel-derived peptides reduced blood pressure by 20-30 mmHg in hypertensive rats. However, most food-derived peptides are degraded during digestion before reaching sufficient blood concentrations. Human trials with the best-studied examples (VPP/IPP from fermented milk) show only modest 2-4 mmHg reductions compared to 10-15 mmHg from pharmaceutical ACE inhibitors.

Why do ACE inhibitors cause a cough?

ACE degrades not only angiotensin I but also bradykinin and substance P. When ACE is inhibited, these peptides accumulate in the airways. Bradykinin stimulates C-fiber sensory nerve endings in the lungs and trachea, triggering the cough reflex. Substance P amplifies this effect. The cough occurs in 5-20% of patients and is more common in women and people of East Asian descent. It resolves within 1-4 weeks of stopping the medication.

What is the difference between ACE inhibitors and ARBs?

ACE inhibitors block the enzyme that produces angiotensin II, which also prevents bradykinin degradation. ARBs (angiotensin receptor blockers) allow angiotensin II to be produced but block it from binding its AT1 receptor. ARBs do not cause cough because they do not affect bradykinin metabolism. Clinical outcomes in heart failure are broadly similar, though ACE inhibitors have a longer evidence base from trials like SOLVD, HOPE, SAVE, and AIRE.

Do ACE inhibitors affect ACE2?

Most ACE inhibitors do not directly affect ACE2 activity. Majumder et al. (2015) found that the food-derived peptide IRW simultaneously inhibits ACE and upregulates ACE2, but Wang et al. (2023) demonstrated this is not a common feature of ACE inhibitory peptides. ACE2 converts angiotensin II to the protective angiotensin 1-7, so upregulating ACE2 would theoretically amplify the cardiovascular benefits of ACE inhibition, but this dual mechanism is the exception rather than the rule.