Aspirin irreversibly inhibits the following enzyme in the formation of eicosanoids:
## **Core Concept**
Aspirin acts by irreversibly inhibiting an enzyme crucial in the synthesis of eicosanoids, which are key mediators in inflammation, pain, and fever. The eicosanoids include prostaglandins, thromboxanes, leukotrienes, and others, derived from arachidonic acid. The enzyme targeted by aspirin is involved in the cyclooxygenase (COX) pathway.
## **Why the Correct Answer is Right**
The correct answer, **Cyclooxygenase (COX)**, is the enzyme irreversibly inhibited by aspirin. Aspirin acetylates a serine residue in the active site of COX enzymes, specifically COX-1 and COX-2 isoforms, thereby blocking the conversion of arachidonic acid into prostaglandin H2, a precursor for various prostaglandins and thromboxanes. This inhibition affects the production of prostanoids, leading to aspirin's anti-inflammatory, analgesic, and antiplatelet effects.
## **Why Each Wrong Option is Incorrect**
- **Option A (Lipoxygenase):** Lipoxygenase is involved in the leukotriene pathway of eicosanoid synthesis, which is distinct from the COX pathway. Inhibitors of lipoxygenase do not have the same effects as aspirin.
- **Option B (Phospholipase A2):** Phospholipase A2 is an enzyme that precedes COX in the eicosanoid synthesis pathway, releasing arachidonic acid from membrane phospholipids. While crucial for eicosanoid production, it is not directly inhibited by aspirin.
- **Option D (Thromboxane Synthase):** Thromboxane synthase is involved in converting prostaglandin H2 into thromboxane A2, a step downstream of COX. Aspirin's action on COX indirectly affects thromboxane A2 production but does not directly inhibit thromboxane synthase.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that aspirin's effect on COX enzymes is irreversible and lasts for the lifespan of the platelet (about 7-10 days), which is why its antiplatelet effect persists even after the drug is no longer present in the system. This also underlies the risk of bleeding associated with aspirin use.
## **Correct Answer:** . Cyclooxygenase (COX)