Detoxification Pathways — MCQs

Detoxification Pathways Indian Medical PG Practice Questions and MCQs

Question 31: Trilene is degraded by:

A. Glutathione conjugation
B. Cytochrome P450 oxidation (Correct Answer)
C. Direct renal excretion
D. Acetylation

Explanation: ***Cytochrome P450 oxidation*** - **Trichloroethylene (Trilene)** was historically used as an inhalational anesthetic and industrial solvent - In humans, it undergoes **hepatic metabolism primarily through cytochrome P450 enzymes**, particularly **CYP2E1** - The oxidation pathway produces metabolites including **chloral hydrate, trichloroethanol, and trichloroacetic acid** - This is a classic example of **Phase I detoxification** involving oxidative biotransformation - The metabolites are then conjugated (Phase II) or excreted renally *Glutathione conjugation* - While some chlorinated compounds undergo glutathione conjugation as a Phase II reaction - For trichloroethylene, **oxidation by CYP450 is the primary metabolic pathway**, not direct glutathione conjugation - GSH conjugation may occur with some metabolites but is not the main degradation route *Direct renal excretion* - Trilene is **lipophilic** and requires hepatic metabolism before elimination - Direct renal excretion without biotransformation is **minimal** - Metabolites (after oxidation) are excreted via kidneys *Acetylation* - **Acetylation** is a Phase II conjugation reaction typically for compounds with **amino or sulfonamide groups** - Trichloroethylene lacks the appropriate functional groups for acetylation - This pathway is **not involved** in Trilene metabolism

Question 32: Biological role of metallothioneins is to sequester harmful metal ions. These bind which of the following ions?

A. Iron (Fe+++), Manganese (Mn++), and Chromium (Cr++)
B. Cadmium (Cd++), Copper (Cu++), and Zinc (Zn++) (Correct Answer)
C. Aluminum (Al+++), Mercury (Hg++), and Nickel (Ni++)
D. Platinum (Pt++), Arsenic (As+++), and Lead (Pb++)

Explanation: ***Cadmium (Cd²⁺), Copper (Cu²⁺), and Zinc (Zn²⁺)*** - Metallothioneins are **cysteine-rich proteins** that function as the primary binding proteins for **Group IB and IIB metals**. - They have high affinity for **essential metals** like **copper (Cu²⁺)** and **zinc (Zn²⁺)**, which are critical for normal cellular metabolism and enzyme function. - They also bind **toxic heavy metals** like **cadmium (Cd²⁺)**, providing cellular protection through sequestration and detoxification. - This combination represents the **classical triad** of metallothionein-binding metals most emphasized in medical biochemistry. *Aluminum (Al³⁺), Mercury (Hg²⁺), and Nickel (Ni²⁺)* - While metallothioneins do bind **mercury (Hg²⁺)** with high affinity, this option is incorrect because **aluminum (Al³⁺)** and **nickel (Ni²⁺)** are NOT primary metallothionein targets. - **Aluminum** is not typically sequestered by metallothioneins and its toxicity involves different mechanisms. - **Nickel** has lower affinity for metallothioneins compared to the classical binding metals. - The presence of two non-primary metals makes this option incorrect despite mercury being a valid binding target. *Iron (Fe³⁺), Manganese (Mn²⁺), and Chromium (Cr²⁺)* - **Iron (Fe³⁺)** metabolism is primarily regulated by dedicated proteins like **ferritin**, **transferrin**, and **hepcidin**, not metallothioneins. - **Manganese (Mn²⁺)** and **chromium (Cr²⁺)** are managed by other specific transport systems and binding proteins. - These metals do not have significant affinity for the cysteine-rich binding sites of metallothioneins. *Platinum (Pt²⁺), Arsenic (As³⁺), and Lead (Pb²⁺)* - While **lead (Pb²⁺)** can interact with metallothioneins to some extent, this is not their primary biological role. - **Platinum (Pt²⁺)** interactions are mainly relevant in the context of chemotherapy drug binding, not physiological metallothionein function. - **Arsenic (As³⁺)** toxicity involves different binding proteins and mechanisms. - None of these represents the classical, well-established metallothionein-binding metals emphasized in medical education.

Question 33: The cytochrome involved in monooxygenase-mediated detoxification of drugs is:

A. Cyt P 450 (Correct Answer)
B. Cytochrome b5
C. Cytochrome c
D. Cytochrome oxidase

Explanation: ***Cyt P 450*** - **Cytochrome P450 (CYP450)** enzymes are a superfamily of heme-containing monooxygenases primarily responsible for the **metabolism of xenobiotics**, including the detoxification of drugs. - They catalyze oxidation reactions, introducing a hydroxyl group to substrates, which typically increases their **hydrophilicity** and facilitates excretion. *Cytochrome c* - **Cytochrome c** is a component of the **electron transport chain** in mitochondria, primarily involved in cellular respiration and energy production. - It acts as an **electron carrier** between Complex III and Complex IV, not directly in drug detoxification. *Cytochrome b5* - **Cytochrome b5** participates in various metabolic reactions, including **fatty acid desaturation** and cholesterol biosynthesis, and can sometimes assist CYP450 enzymes. - However, it does not function as a primary monooxygenase for drug detoxification itself. *Cytochrome oxidase* - **Cytochrome oxidase** (Complex IV) is the terminal enzyme in the **electron transport chain**, responsible for the reduction of oxygen to water. - Its main role is in cellular respiration, and it is not directly involved in drug monooxygenation or detoxification.

Question 34: Which amino acid primarily conjugates with benzoic acid during its detoxification?

A. Alanine
B. Glycine (Correct Answer)
C. Tyrosine
D. Leucine

Explanation: ***Glycine*** - **Benzoic acid** is detoxified in the liver by conjugation with **glycine**, forming **hippuric acid**, which is then excreted in the urine. - This is a well-known Phase II detoxification reaction, enhancing the water solubility and elimination of the xenobiotic. *Alanine* - **Alanine** is involved in **gluconeogenesis** via the **glucose-alanine cycle** and protein synthesis. - It is not primarily involved in the conjugation of benzoic acid for detoxification. *Tyrosine* - **Tyrosine** is a precursor for **catecholamines**, thyroid hormones, and melanin. - It does not participate in the detoxification pathway of benzoic acid. *Leucine* - **Leucine** is a **branched-chain amino acid (BCAA)** essential for protein synthesis and muscle repair. - It has no known role in the conjugation or detoxification of benzoic acid.

Question 35: In liver, ethanol is converted to?

A. Acetaldehyde (Correct Answer)
B. Methanol
C. Lactate
D. Citric acid

Explanation: ***Acetaldehyde*** - In the liver, **ethanol** is primarily metabolized by **alcohol dehydrogenase (ADH)** into **acetaldehyde**. - **Acetaldehyde** is a highly toxic compound responsible for many of the adverse effects associated with alcohol consumption. *Methanol* - **Methanol** is a different type of alcohol (wood alcohol) and is not a product of ethanol metabolism. - Methanol is metabolized to **formaldehyde** and then **formic acid**, which are highly toxic. *Lactate* - **Lactate** is a product of anaerobic glycolysis and is not directly formed from ethanol metabolism. - While heavy alcohol consumption can lead to **lactic acidosis**, lactate itself is not the immediate, direct conversion product of ethanol. *Citric acid* - **Citric acid** is a key intermediate in the **Krebs cycle (citric acid cycle)**, involved in aerobic respiration. - It is not a direct product of ethanol metabolism; rather, **acetyl-CoA** (derived from acetaldehyde metabolism) can enter the Krebs cycle.

Practice by Chapter

Phase I Reactions: Cytochrome P450 System

Practice Questions

Phase II Conjugation Reactions

Practice Questions

Glutathione and Detoxification

Practice Questions

Drug Metabolism

Practice Questions

Metabolism of Xenobiotics

Practice Questions

Alcohol Metabolism

Practice Questions

Free Radical Generation and Antioxidant Defense

Practice Questions

Antioxidant Enzymes

Practice Questions

Detoxification of Heavy Metals

Practice Questions

Ammonia Detoxification Pathways

Practice Questions

Bilirubin Metabolism and Jaundice

Practice Questions

Biotransformation in Liver Disease

Practice Questions

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