Rasburicase lowers uric acid by converting it to which metabolite?

• A. Urate
• B. Urea
• C. Allantoin
• D. Xanthine

Answer: (C)

Rasburicase provides urate oxidase activity, converting uric acid into allantoin. Allantoin dissolves much more readily in water than uric acid, so it’s excreted in the urine more easily. Humans normally lack active urate oxidase, so uric acid tends to accumulate; introducing this enzyme with rasburicase accelerates the breakdown to a highly soluble metabolite, lowering uric acid levels and helping prevent uric acid–related kidney problems during rapid cell turnover. The other substances listed aren’t produced by this enzyme: uric acid is the substrate, urea comes from amino acid catabolism, and xanthine is an upstream intermediate in purine degradation.