Endogenous antioxidants – enzymatic and non-enzymatic systems MCQs With Answer

Endogenous antioxidants – enzymatic and non-enzymatic systems MCQs With Answer

Endogenous antioxidants form an integrated defense network against oxidative stress and reactive oxygen species (ROS) in the body. This topic covers enzymatic antioxidants such as superoxide dismutase (SOD), catalase, glutathione peroxidase (GPx), glutathione reductase and thioredoxin systems, plus non-enzymatic antioxidants including glutathione (GSH), vitamins C and E, uric acid, bilirubin and coenzyme Q. B. Pharm students must understand mechanisms, cellular localization, cofactors (selenium, NADPH), regulation by Nrf2, biomarkers like MDA and clinical relevance in drug-induced oxidative injury. Mastery of these concepts aids pharmacotherapy, toxicology and antioxidant assay interpretation. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the primary role of endogenous antioxidant systems in cells?

• Neutralize reactive oxygen species and repair oxidative damage
• Generate reactive oxygen species for signaling only
• Synthesize free radicals to aid digestion
• Transport oxygen to tissues

Correct Answer: Neutralize reactive oxygen species and repair oxidative damage

Q2. Which enzyme catalyzes the dismutation of superoxide (O2•–) into hydrogen peroxide (H2O2)?

• Catalase
• Superoxide dismutase (SOD)
• Glutathione peroxidase (GPx)
• Glutathione reductase

Correct Answer: Superoxide dismutase (SOD)

Q3. Catalase is primarily localized in which subcellular organelle?

• Peroxisomes
• Mitochondria
• Cytosol
• Lysosomes

Correct Answer: Peroxisomes

Q4. Glutathione peroxidase (GPx) requires which molecule as an essential cofactor to reduce hydrogen peroxide?

• Reduced glutathione (GSH)
• Vitamin C
• Heme
• Iron

Correct Answer: Reduced glutathione (GSH)

Q5. Which enzyme uses NADPH to convert oxidized glutathione (GSSG) back to its reduced form (GSH)?

• Glutathione peroxidase
• Glutathione reductase
• Thioredoxin
• Catalase

Correct Answer: Glutathione reductase

Q6. Which non-enzymatic antioxidant is the most abundant intracellular thiol and critical for detoxification?

• Uric acid
• Glutathione (GSH)
• Vitamin E
• Bilirubin

Correct Answer: Glutathione (GSH)

Q7. Which antioxidant primarily protects membrane lipids by scavenging lipid peroxyl radicals?

• Vitamin C
• Vitamin E (tocopherol)
• Uric acid
• Glutathione

Correct Answer: Vitamin E (tocopherol)

Q8. Which property best describes vitamin C’s role in antioxidant networks?

• Lipid-soluble chain-breaking antioxidant in membranes
• Water-soluble antioxidant that regenerates vitamin E
• Protein-bound metal chelator only
• Primary mitochondrial enzyme

Correct Answer: Water-soluble antioxidant that regenerates vitamin E

Q9. Activation of which transcription factor upregulates genes encoding antioxidant enzymes via the antioxidant response element (ARE)?

• NF-κB
• Nrf2 (nuclear factor erythroid 2–related factor 2)
• AP-1
• p53

Correct Answer: Nrf2 (nuclear factor erythroid 2–related factor 2)

Q10. Which biomarker is commonly used to assess lipid peroxidation in biological samples?

• Glutathione
• Malondialdehyde (MDA)
• Superoxide
• Ferritin

Correct Answer: Malondialdehyde (MDA)

Q11. Peroxiredoxins reduce peroxides and are regenerated by which system?

• Glutathione peroxidase system
• Thioredoxin system
• Catalase only
• P450 monooxygenase system

Correct Answer: Thioredoxin system

Q12. Which antioxidant enzyme contains selenium as a critical component of its active site?

• Catalase
• Superoxide dismutase
• Glutathione peroxidase (GPx)
• Glutathione reductase

Correct Answer: Glutathione peroxidase (GPx)

Q13. Which of the following is NOT primarily considered an endogenous antioxidant enzyme?

• Superoxide dismutase (SOD)
• Catalase
• Cytochrome P450 (CYP450)
• Glutathione peroxidase

Correct Answer: Cytochrome P450 (CYP450)

Q14. Which protein sequesters free iron intracellularly, thereby limiting Fenton chemistry and oxidative damage?

• Transferrin
• Ferritin
• Albumin
• Hemoglobin

Correct Answer: Ferritin

Q15. A decreased cellular GSH/GSSG ratio indicates which condition?

• Enhanced antioxidant capacity
• Oxidative stress and a more oxidized intracellular environment
• Unchanged redox balance
• Reduced reactive oxygen species production

Correct Answer: Oxidative stress and a more oxidized intracellular environment

Q16. Which cellular process is the major physiological source of reactive oxygen species in aerobic cells?

• Mitochondrial electron transport chain leakage
• Glycolysis in cytosol
• Protein synthesis in ribosomes
• Lysosomal digestion

Correct Answer: Mitochondrial electron transport chain leakage

Q17. Which enzyme catalyzes the decomposition of hydrogen peroxide into water and oxygen rapidly?

• Glutathione peroxidase
• Catalase
• Superoxide dismutase
• Glutathione reductase

Correct Answer: Catalase

Q18. Glutathione peroxidase can reduce which of the following substrates besides hydrogen peroxide?

• Lipid hydroperoxides
• Superoxide radical directly
• Metal ions like Fe3+
• Nitric oxide

Correct Answer: Lipid hydroperoxides

Q19. Thioredoxin reductase maintains thioredoxin in its reduced form using which cofactor?

• NADH
• NADPH
• FADH2 only
• ATP

Correct Answer: NADPH

Q20. Which endogenous molecule acts as a significant antioxidant in plasma and can scavenge peroxyl radicals?

• Bilirubin
• Uric acid
• Ferritin
• Glutathione

Correct Answer: Uric acid

Q21. Moderate levels of which breakdown product of heme have antioxidant properties in plasma?

• Heme
• Bilirubin
• Biliverdin
• Haptoglobin

Correct Answer: Bilirubin

Q22. Which assay is commonly used to measure superoxide dismutase (SOD) activity in vitro?

• ELISA for catalase
• Nitroblue tetrazolium (NBT) reduction assay
• MDA thiobarbituric acid reactive substances (TBARS) assay
• Bradford protein assay

Correct Answer: Nitroblue tetrazolium (NBT) reduction assay

Q23. Chronic alcohol consumption commonly impairs antioxidant defense primarily by depleting which molecule?

• Vitamin E
• Glutathione (GSH)
• Uric acid
• Superoxide dismutase

Correct Answer: Glutathione (GSH)

Q24. An overdose of which common drug produces a reactive metabolite that depletes glutathione and causes oxidative liver injury?

• Ibuprofen
• Acetaminophen (paracetamol)
• Aspirin
• Amoxicillin

Correct Answer: Acetaminophen (paracetamol)

Q25. The rate-limiting enzyme in glutathione biosynthesis is:

• Glutathione peroxidase
• Gamma-glutamylcysteine synthetase (glutamate–cysteine ligase)
• Glutathione reductase
• Glutathione S-transferase

Correct Answer: Gamma-glutamylcysteine synthetase (glutamate–cysteine ligase)

Q26. Which antioxidant enzyme exists in Cu/Zn and Mn isoforms located in cytosol and mitochondria respectively?

• Catalase
• Superoxide dismutase (SOD)
• Glutathione peroxidase
• Peroxiredoxin

Correct Answer: Superoxide dismutase (SOD)

Q27. Which oxidative modification of proteins is often used as a marker of irreversible protein oxidation?

• Protein carbonylation
• Protein phosphorylation
• Protein glycosylation
• Protein acetylation

Correct Answer: Protein carbonylation

Q28. Which vitamin is directly involved in regenerating oxidized vitamin E to its reduced active form?

• Vitamin A
• Vitamin C (ascorbic acid)
• Vitamin K
• Vitamin D

Correct Answer: Vitamin C (ascorbic acid)

Q29. Binding of Nrf2 to antioxidant response elements (ARE) in gene promoters primarily results in:

• Downregulation of antioxidant enzymes
• Upregulation of phase II detoxifying and antioxidant enzymes
• Immediate apoptosis
• Inhibition of glutathione synthesis

Correct Answer: Upregulation of phase II detoxifying and antioxidant enzymes

Q30. Which laboratory assay estimates overall antioxidant capacity by measuring the ferric reducing ability of plasma?

• TBARS assay
• FRAP (Ferric Reducing Ability of Plasma) assay
• SOD zymography
• Western blot for catalase

Correct Answer: FRAP (Ferric Reducing Ability of Plasma) assay

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