Superoxide dismutase, Catalase, Glutathione peroxidase and Glutathione MCQs With Answer

Superoxide dismutase, Catalase, Glutathione peroxidase and Glutathione MCQs With Answer

Superoxide dismutase, catalase, glutathione peroxidase and glutathione form the primary antioxidant defense against reactive oxygen species (ROS). For B. Pharm students, understanding enzyme mechanisms, cofactors (Cu/Zn, Mn, heme, selenium), subcellular localization (cytosol, mitochondria, peroxisome), kinetics, and regulation is vital for pharmacology, toxicology and drug development. These antioxidant systems influence drug metabolism, oxidative stress-related pathology and biomarker interpretation. Practical knowledge of assays, clinical implications, enzyme isoforms and therapeutic modulation will strengthen your biochemistry and pharmacotherapeutics foundation. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which chemical reaction is catalyzed by superoxide dismutase (SOD)?

• 2 H2O2 → 2 H2O + O2
• 2 O2•− + 2 H+ → H2O2 + O2
• H2O2 + 2 GSH → 2 H2O + GSSG
• Fe2+ + H2O2 → Fe3+ + OH• + OH−

Correct Answer: 2 O2•− + 2 H+ → H2O2 + O2

Q2. Which metal cofactors are characteristic of cytosolic and mitochondrial SOD isoforms respectively?

• Fe for cytosol, Cu/Zn for mitochondria
• Cu/Zn for cytosol, Mn for mitochondria
• Mn for cytosol, Cu/Zn for mitochondria
• Zn for both cytosol and mitochondria

Correct Answer: Cu/Zn for cytosol, Mn for mitochondria

Q3. Where is catalase primarily localized in mammalian cells?

• Mitochondrial matrix
• Endoplasmic reticulum lumen
• Peroxisomes
• Golgi apparatus

Correct Answer: Peroxisomes

Q4. Which statement best describes glutathione peroxidase (GPx)?

• It is a heme-containing enzyme that produces O2 from H2O2
• It reduces H2O2 and organic hydroperoxides using GSH and requires selenium
• It converts superoxide into hydrogen peroxide without cofactors
• It oxidizes GSSG to GSH using NAD+

Correct Answer: It reduces H2O2 and organic hydroperoxides using GSH and requires selenium

Q5. Which molecule regenerates reduced glutathione (GSH) from oxidized glutathione (GSSG)?

• Glutathione peroxidase using NADH
• Glutathione reductase using NADPH
• Catalase using FAD
• Superoxide dismutase using O2

Correct Answer: Glutathione reductase using NADPH

Q6. The selenocysteine residue in GPx participates by forming which oxidized intermediate during catalysis?

• Seleninic acid (R–SeO2H)
• Selenenic acid (R–SeOH)
• Selenium dioxide (SeO2)
• Selenate (SeO4 2−)

Correct Answer: Selenenic acid (R–SeOH)

Q7. Which assay principle is commonly used to measure SOD activity in vitro?

• Measurement of O2 evolution from H2O2 decomposition
• Inhibition of cytochrome c reduction by superoxide generated from xanthine oxidase
• Direct measurement of GSH oxidation to GSSG
• Measurement of hydroxyl radical formation via Fenton reaction

Correct Answer: Inhibition of cytochrome c reduction by superoxide generated from xanthine oxidase

Q8. Which antioxidant has the highest catalytic turnover (kcat) and rapidly decomposes H2O2 at high concentrations?

• Glutathione peroxidase
• Catalase
• Superoxide dismutase
• Glutathione reductase

Correct Answer: Catalase

Q9. Which form of GPx specifically reduces phospholipid hydroperoxides and is important for membrane protection?

• GPx1 (cytosolic GPx)
• GPx2 (gastrointestinal GPx)
• GPx3 (plasma GPx)
• GPx4 (phospholipid hydroperoxide GPx)

Correct Answer: GPx4 (phospholipid hydroperoxide GPx)

Q10. Selenium deficiency would most directly impair which antioxidant enzyme activity?

• Catalase
• Cu/Zn-SOD
• Glutathione peroxidase
• Glutathione reductase

Correct Answer: Glutathione peroxidase

Q11. Which reaction describes the Fenton chemistry that produces hydroxyl radical (OH•)?

• O2 + e− → O2•−
• Fe2+ + H2O2 → Fe3+ + OH• + OH−
• 2 H2O2 → 2 H2O + O2
• GSH + ROOH → GSSG + ROH + H2O

Correct Answer: Fe2+ + H2O2 → Fe3+ + OH• + OH−

Q12. Which therapeutic supplement directly provides a precursor to increase intracellular GSH levels?

• Vitamin C
• N-acetylcysteine (NAC)
• Iron sulfate
• Folic acid

Correct Answer: N-acetylcysteine (NAC)

Q13. A drug that inhibits glutathione reductase will most likely cause which cellular change?

• Increase in GSH:GSSG ratio
• Decrease in NADPH consumption
• Accumulation of GSSG and lower GSH:GSSG ratio
• Enhanced activity of catalase

Correct Answer: Accumulation of GSSG and lower GSH:GSSG ratio

Q14. Which antioxidant enzyme binds to extracellular matrix components and protects extracellular spaces from superoxide?

• Cu/Zn-SOD (SOD1)
• Mn-SOD (SOD2)
• Extracellular SOD (EC-SOD or SOD3)
• Glutathione peroxidase

Correct Answer: Extracellular SOD (EC-SOD or SOD3)

Q15. Between catalase and GPx, which enzyme has a higher affinity (lower Km) for hydrogen peroxide and is effective at low H2O2 concentrations?

• Catalase
• Glutathione peroxidase
• Both have identical Km
• Superoxide dismutase

Correct Answer: Glutathione peroxidase

Q16. Which clinical enzymatic deficiency impairs the cellular ability to regenerate GSH indirectly by lowering NADPH supply?

• Glucose-6-phosphate dehydrogenase (G6PD) deficiency
• Catalase deficiency
• SOD1 deficiency
• Glutathione peroxidase deficiency

Correct Answer: Glucose-6-phosphate dehydrogenase (G6PD) deficiency

Q17. Which laboratory method is appropriate to quantify catalase activity?

• Monitoring decrease in absorbance of NADPH at 340 nm
• Measuring O2 evolution or H2O2 decomposition spectrophotometrically
• Determining rate of cytochrome c reduction
• Measuring conjugation of GSH to CDNB (1-chloro-2,4-dinitrobenzene)

Correct Answer: Measuring O2 evolution or H2O2 decomposition spectrophotometrically

Q18. Which statement about SOD inhibition in cells is true?

• SOD inhibition decreases hydrogen peroxide levels
• SOD inhibition increases superoxide concentration and may enhance peroxynitrite formation
• SOD inhibition converts hydroxyl radical into water
• SOD inhibition directly elevates GSH synthesis

Correct Answer: SOD inhibition increases superoxide concentration and may enhance peroxynitrite formation

Q19. Which drug approach is used to mimic SOD activity therapeutically?

• Heme oxygenase inducers
• Manganese porphyrin complexes as SOD mimetics
• Selenium chelators
• Iron supplements

Correct Answer: Manganese porphyrin complexes as SOD mimetics

Q20. Which ratio is commonly used as an index of cellular oxidative stress?

• NAD+/NADH ratio
• ATP/ADP ratio
• GSH/GSSG ratio
• FAD/FADH2 ratio

Correct Answer: GSH/GSSG ratio

Q21. Which enzyme class catalyzes conjugation of glutathione to xenobiotics as part of phase II metabolism?

• CYP450 monooxygenases
• Glutathione S-transferases (GSTs)
• Acetyltransferases
• UDP-glucuronosyltransferases

Correct Answer: Glutathione S-transferases (GSTs)

Q22. The active site of GPx contains a selenocysteine encoded by which special mechanism during translation?

• Frameshift mutation incorporation
• Reinterpretation of UGA stop codon as selenocysteine using SECIS element
• Standard UGC cysteine codon usage
• Insertion via post-translational modification of serine

Correct Answer: Reinterpretation of UGA stop codon as selenocysteine using SECIS element

Q23. Which cellular compartment contains Mn-SOD (SOD2)?

• Cytosol
• Extracellular matrix
• Mitochondrial matrix
• Peroxisomes

Correct Answer: Mitochondrial matrix

Q24. Which antioxidant enzyme primarily prevents formation of hydroxyl radical by eliminating H2O2 at high concentrations?

• Glutathione peroxidase at high H2O2
• Catalase
• Superoxide dismutase
• Glutathione reductase

Correct Answer: Catalase

Q25. In oxidative drug toxicity, which change would you expect from excessive ROS generation?

• Increased GSH:GSSG ratio
• Decreased lipid peroxidation
• Oxidation of GSH to GSSG and decreased GSH:GSSG ratio
• Enhanced glutathione reductase activity leading to more GSH

Correct Answer: Oxidation of GSH to GSSG and decreased GSH:GSSG ratio

Q26. Which enzyme is most effective at reducing lipid peroxides in membranes preventing ferroptosis?

• Catalase
• GPx4 (phospholipid hydroperoxide glutathione peroxidase)
• Cu/Zn-SOD
• Glutathione reductase

Correct Answer: GPx4 (phospholipid hydroperoxide glutathione peroxidase)

Q27. A common oxidative stress biomarker is an altered ratio; which numeric range approximates a healthy cytosolic GSH:GSSG ratio?

• 1:1
• 10:1
• 100:1
• 0.1:1

Correct Answer: 100:1

Q28. Which intervention would most directly increase cellular GPx activity in selenium-deficient subjects?

• Oral iron supplementation
• Dietary selenium supplementation
• High-dose vitamin E alone
• Glucose infusion

Correct Answer: Dietary selenium supplementation

Q29. Which antioxidant enzyme reaction consumes reduced glutathione (GSH) directly during detoxification?

• Superoxide dismutase converting O2•− to H2O2
• Glutathione peroxidase reducing H2O2 to water
• Catalase converting H2O2 to O2 and water without GSH
• Glutathione reductase converting GSSG to GSH

Correct Answer: Glutathione peroxidase reducing H2O2 to water

Q30. Which molecular event links superoxide to nitric oxide to form a particularly damaging oxidant?

• Superoxide reacts with H2O2 to form water
• Superoxide reduces ferric iron to ferrous iron
• Superoxide reacts with nitric oxide to form peroxynitrite (ONOO−)
• Superoxide is converted to hydroxyl radical by catalase

Correct Answer: Superoxide reacts with nitric oxide to form peroxynitrite (ONOO−)

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