Following restoration of blood flow after a prolonged period of intestinal ischaemia, tissue injury worsens rather than improves. Which enzyme, accumulating in ischaemic tissue through ATP degradation, generates the burst of superoxide responsible for this reperfusion injury?

• A Cytochrome P450 reductase
• B NADPH oxidase
• C Xanthine oxidase ✓
• D Monoamine oxidase

Explanation

During ischaemia, ATP is degraded to hypoxanthine and xanthine dehydrogenase is converted to xanthine oxidase. On reperfusion, oxygen returns and xanthine oxidase converts hypoxanthine to xanthine and then uric acid while generating large amounts of superoxide. NADPH oxidase produces superoxide in phagocytes and vascular cells but does not accumulate as hypoxanthine substrate does, so it cannot explain the timing of the injury peak seen here.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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