Superoxide dismutase (SOD) converts superoxide radical (O2•−) to hydrogen peroxide (H2O2). What enzyme then converts H2O2 to water in the liver cytoplasm, and what cofactor does it require?

• A Catalase — requires heme (iron-porphyrin)
• B Glutathione peroxidase — requires selenium (selenocysteine at active site)
• C Both A and B are correct for different subcellular compartments ✓
• D Thioredoxin reductase — requires FAD

Explanation

Both catalase and glutathione peroxidase convert H2O2 to water, but in different compartments. Catalase (heme-containing, Fe3+/Fe4+) is located primarily in peroxisomes and degrades H2O2 generated by peroxisomal oxidases (2H2O2 → 2H2O + O2). Glutathione peroxidase (GPx, selenoprotein with selenocysteine) is cytoplasmic and mitochondrial; it reduces H2O2 using GSH (2GSH + H2O2 → GSSG + 2H2O). GPx is more important at low H2O2 concentrations; catalase at high concentrations. Both are operative in hepatocytes.

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

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