## Mechanism of ACE Inhibitors **Key Point:** ACE inhibitors (ACEIs) block angiotensin-converting enzyme, preventing the conversion of angiotensin I (inactive) to angiotensin II (active vasoconstrictor). ### Lisinopril — Correct Answer Lisinopril is a **competitive inhibitor of ACE**, a zinc-dependent metalloproteinase. By blocking this enzyme, ACEIs: 1. Reduce angiotensin II formation → decreased vasoconstriction 2. Reduce aldosterone secretion → decreased sodium and water retention 3. Reduce sympathetic nervous system activity 4. Reduce degradation of bradykinin → vasodilation **High-Yield:** ACEIs are first-line agents for hypertension, especially in patients with diabetes, chronic kidney disease, or heart failure. ### Mechanism Comparison Table | Drug Class | Mechanism | Site of Action | | --- | --- | --- | | ACE inhibitor (Lisinopril) | Blocks ACE enzyme | Converts Ang I → Ang II | | ARB (Losartan) | Blocks AT1 receptor | Angiotensin II receptor | | CCB (Amlodipine) | Blocks L-type Ca²⁺ channels | Vascular smooth muscle | | Beta-blocker (Labetalol) | Blocks β-adrenergic receptors | Sympathetic nervous system | **Clinical Pearl:** ACEIs are contraindicated in pregnancy (teratogenic in 2nd and 3rd trimesters) and in patients with a history of angioedema from prior ACEIs. [cite:KD Tripathi 8e Ch 21]
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