Allopurinol lowers serum urate in gout. Its hypouricemic action is best classified as:

• A Reversible competitive inhibition of urate oxidase
• B Feedback inhibition of purine biosynthesis by direct binding to PRPP amidotransferase
• C Allosteric activation of hypoxanthine-guanine phosphoribosyltransferase
• D Mechanism-based (suicide) inhibition of xanthine oxidase ✓

Explanation

Xanthine oxidase itself converts allopurinol to oxypurinol (alloxanthine), which remains tightly bound to the reduced molybdenum-sulfide center of the enzyme and inactivates it irreversibly. Because the target enzyme generates its own inhibitor, this is classic mechanism-based (suicide) inhibition. Urate oxidase is absent in humans, and rasburicase is the recombinant version used for tumor lysis. Febuxostat, by contrast, is a reversible non-purine inhibitor of the same enzyme.

Reference: Katzung Basic and Clinical Pharmacology, 16th ed.

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