Q.13 The two common antioxidants that also act as pro-oxidants in biological systems are
I. Ascorbate and Uric acid
2. Ascorbate and Polyphenols
3. Beta carotene and Uric acid
4. Ascorbate and Beta carotene
Ascorbate and beta-carotene are the two common antioxidants that can also act as pro-oxidants in biological systems, making option 4 correct.
Option Analysis
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Option 1: Ascorbate and Uric acid. Incorrect. Ascorbate (vitamin C) acts as both antioxidant (scavenging ROS) and pro-oxidant (generating H₂O₂ via metal-catalyzed reactions at high doses), but uric acid primarily functions as an antioxidant by neutralizing peroxyl radicals and does not typically exhibit pro-oxidant activity in physiological contexts.
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Option 2: Ascorbate and Polyphenols. Incorrect. Polyphenols (e.g., flavonoids) are mainly antioxidants via radical scavenging and metal chelation; while some can show pro-oxidant effects under specific conditions (high concentrations, metal presence), they are not commonly classified as dual antioxidants/pro-oxidants alongside ascorbate in standard biology texts.
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Option 3: Beta carotene and Uric acid. Incorrect. Beta-carotene (vitamin A precursor) can act as a pro-oxidant in smokers or high-oxygen environments by forming carotenoid radicals, but uric acid lacks significant pro-oxidant behavior, focusing instead on antioxidant protection against oxidative stress.
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Option 4: Ascorbate and Beta carotene. Correct. Both are classic examples of antioxidants that switch to pro-oxidants: ascorbate generates reactive oxygen species (ROS) like H₂O₂ in the presence of transition metals (e.g., Fe³⁺), while beta-carotene promotes oxidation under pro-oxidant conditions such as high partial pressure of oxygen or cigarette smoke exposure.
Introduction to Antioxidants as Pro-Oxidants
Antioxidants that also act as pro-oxidants in biological systems, such as ascorbate (vitamin C) and beta-carotene, play dual roles by neutralizing ROS at low doses but generating oxidative stress at high concentrations or under specific conditions. This redox duality is crucial for exams like GATE Life Sciences, highlighting cellular homeostasis mechanisms. Understanding ascorbate and beta-carotene’s switch helps explain therapeutic uses and risks in oxidative biology.
Dual Role Mechanisms
Ascorbate donates electrons to scavenge superoxide but, with Fe³⁺/Cu²⁺, reduces metals to catalyze Fenton reactions producing hydroxyl radicals (pro-oxidant). Beta-carotene quenches singlet oxygen as an antioxidant yet forms radicals in high-oxygen or smoke-exposed lungs, promoting lipid peroxidation.
| Antioxidant | Antioxidant Action | Pro-Oxidant Condition | Biological Impact |
|---|---|---|---|
| Ascorbate | ROS scavenging, enzyme cofactor | High dose + transition metals | H₂O₂ generation, potential ferroptosis |
| Beta-carotene | Singlet oxygen quencher | High O₂, smoking | Carotenoid radicals, lung oxidation |
Biological and Exam Relevance
In cells, this pro-oxidant shift aids cancer therapy (e.g., pharmacologic ascorbate induces apoptosis) but risks damage in deficiencies/excess. Polyphenols/uric acid lack this balanced dual classification, emphasizing ascorbate-beta carotene pairing for molecular biology questions.
