Which enzyme is irreversibly inhibited by aspirin?
**Core Concept**
Aspirin, a nonsteroidal anti-inflammatory drug (NSAID), exerts its therapeutic effects by inhibiting the enzyme responsible for the synthesis of prostaglandins, which play a crucial role in inflammation and pain.
**Why the Correct Answer is Right**
Aspirin irreversibly inhibits the enzyme cyclooxygenase (COX), specifically the COX-1 and COX-2 isoforms, by acetylating a serine residue at the active site, thereby preventing the enzyme from catalyzing the conversion of arachidonic acid to prostaglandin H2. This leads to a decrease in the production of prostaglandins, which are responsible for mediating pain, inflammation, and fever.
**Why Each Wrong Option is Incorrect**
**Option A:** While other NSAIDs, such as ibuprofen, may also inhibit COX enzymes, aspirin's unique mechanism involves irreversible inhibition.
**Option B:** Lipooxygenase is involved in the synthesis of leukotrienes, not prostaglandins, and is not the primary target of aspirin.
**Option C:** Phospholipase A2 is involved in the release of arachidonic acid from membrane phospholipids, but its inhibition is not directly responsible for aspirin's therapeutic effects.
**Clinical Pearl / High-Yield Fact**
Aspirin's irreversible inhibition of COX enzymes can lead to tachyphylaxis, a phenomenon where the therapeutic effects of aspirin are reduced with repeated dosing.
**Correct Answer: C. Cyclooxygenase (COX).**