Superoxide dismutase (SOD) catalyses the dismutation of superoxide radical. Which statement correctly distinguishes the isoforms of SOD in human cells?

• A SOD1 (Cu/Zn-SOD) is cytosolic and in the intermembrane space; SOD2 (Mn-SOD) is mitochondrial matrix; SOD3 is extracellular ✓
• B SOD1 (Cu/Zn-SOD) is mitochondrial; SOD2 (Mn-SOD) is cytosolic; both require copper and zinc
• C SOD2 deficiency causes ALS (amyotrophic lateral sclerosis) due to mutation in the cytosolic form
• D SOD catalyses 2O2•⁻ + 2H⁺ → H2O + O2, producing water as the end product without further enzymatic processing

Explanation

SOD1 (Cu/Zn-SOD) is located in the cytosol and mitochondrial intermembrane space; SOD2 (Mn-SOD) is in the mitochondrial matrix where the electron transport chain generates superoxide; SOD3 (extracellular SOD) is secreted. SOD converts superoxide to hydrogen peroxide (not water): 2O2•⁻ + 2H⁺ → H2O2 + O2. H2O2 is then further detoxified by catalase or glutathione peroxidase. Mutations in SOD1 (not SOD2) cause familial ALS.

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

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