Regarding purine metabolism and the pathogenesis of gout, all of the following statements are correct EXCEPT:

Explanation

## Purine Metabolism Pathway and Gout Pathogenesis ### Correct Statements (Options 0, 1, 2) **Key Point:** Xanthine oxidase is the critical enzyme in the final steps of purine catabolism, converting hypoxanthine → xanthine → uric acid. This is a two-step oxidation catalyzed by the same enzyme complex. **High-Yield:** Allopurinol is a structural hypoxanthine analogue that competitively (not non-competitively) inhibits xanthine oxidase, reducing uric acid production. It is the gold-standard urate-lowering therapy for chronic gout management. **Clinical Pearl:** Uric acid is the end product of purine metabolism in humans (unlike other mammals that possess uricase). Approximately 60–70% of uric acid is eliminated via renal filtration and secretion; the remainder undergoes extra-renal degradation and intestinal excretion. ### The Incorrect Statement (Option 3) — THE ANSWER **Key Point:** Humans **lack functional uricase enzyme** (also called urate oxidase). This is a key evolutionary loss in primates and humans. Uricase is present in most other mammals (dogs, rats, bacteria) and converts uric acid to the more soluble allantoin, but **this enzyme is absent or non-functional in human hepatocytes**. **Warning:** This is a high-yield trap. Students often confuse humans with other species. The absence of uricase in humans is why uric acid accumulates and precipitates in tissues, leading to gout — a disease that does not occur in most other mammals. ### Summary Table: Purine Catabolism in Humans | Step | Enzyme | Substrate → Product | Notes | | --- | --- | --- | --- | | 1 | Adenosine deaminase | Adenosine → Inosine | Also acts on deoxyadenosine | | 2 | Purine nucleoside phosphorylase | Inosine → Hypoxanthine | Also acts on guanosine | | 3 | Xanthine oxidase | Hypoxanthine → Xanthine | Rate-limiting; inhibited by allopurinol | | 4 | Xanthine oxidase | Xanthine → Uric acid | Final step; uric acid is end product | | 5 | ~~Uricase~~ | ~~Uric acid → Allantoin~~ | **ABSENT in humans** | [cite:Lehninger Principles of Biochemistry Ch 22]