Aspirin irreversibly inhibits cyclooxygenase (COX) by acetylating serine 529 of COX-1 and serine 516 of COX-2. In platelets, this is therapeutically beneficial for preventing thrombosis. Why does aspirin inhibit thromboxane A2 (TXA2) production more selectively than prostacyclin (PGI2) synthesis at low doses?
- A Aspirin selectively inhibits COX-1 in platelets while endothelial COX-2 is naturally resistant to aspirin
- B Thromboxane synthase is more sensitive to aspirin than prostacyclin synthase
- C Platelets lack COX-2 and cannot regenerate COX-1 (anucleate cells), so their TXA2 synthesis is permanently blocked for the platelet's lifespan ✓
- D Vascular endothelium expresses platelet-activating factor acetylhydrolase that regenerates COX from aspirin inhibition
Explanation
Platelets are anucleate and cannot synthesize new COX-1 protein. Once aspirin irreversibly acetylates and inactivates COX-1 in platelets, TXA2 production is blocked for the platelet's entire 7-10 day lifespan. Endothelial cells are nucleated and can regenerate COX-2 (and some COX-1) within hours, restoring PGI2 synthesis. Low-dose aspirin regimens (81 mg) preferentially target platelet COX-1 in the portal circulation before first-pass hepatic metabolism reduces systemic levels, minimizing endothelial COX inhibition. This differential recovery explains aspirin's net antithrombotic effect.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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