During ischemia-reperfusion injury, which enzyme system is responsible for generating the burst of reactive oxygen species (ROS) that occurs specifically at reperfusion rather than during ischemia?

• A Xanthine oxidase, generated by proteolytic conversion of xanthine dehydrogenase during ischemia and acting on accumulated hypoxanthine upon O2 reintroduction ✓
• B NADPH oxidase in neutrophils recruited to the ischemic zone upon reperfusion
• C Mitochondrial Complex I reverse electron transport driven by succinate accumulation during ischemia
• D Nitric oxide synthase uncoupling due to tetrahydrobiopterin (BH4) depletion during ischemia

Explanation

During ischemia, ATP is degraded to hypoxanthine (an accumulating substrate) and xanthine dehydrogenase is irreversibly converted to xanthine oxidase by calcium-activated proteases. Upon reperfusion with oxygen, xanthine oxidase rapidly oxidizes hypoxanthine to xanthine and uric acid, generating superoxide (O2•−) and hydrogen peroxide (H2O2) in a burst — this is the classic mechanism of reperfusion ROS injury. While NADPH oxidase in recruited neutrophils and mitochondrial ROS also contribute, xanthine oxidase is the primary enzymatic source at the moment of reperfusion. Mitochondrial reverse electron transport at Complex I from succinate is a more recently characterized mechanism that supplements xanthine oxidase-mediated injury. eNOS uncoupling contributes to later endothelial dysfunction.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

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