Question

Which of the following is Phase-II metabolism reaction:

• A. Reduction
• B. Acetylation
• C. Hydrolysis
• D. Oxidation

Hint:

Phase-II reactions like acetylation are conjugation reactions that increase the solubility of the drug, making it easier for the body to eliminate it.

Answer 1

The Correct Option is B

Acetylation is a Phase-II metabolism reaction, also known as conjugation. In this reaction, an acetyl group is added to the drug or its metabolite, increasing its water solubility and facilitating excretion. Phase-I reactions (like oxidation, reduction, and hydrolysis) modify the drug molecule, while Phase-II reactions involve conjugation with endogenous molecules like acetyl groups, sulfate, or glucuronic acid.

Answer 2

The question asks which of the listed biotransformation reactions belongs to Phase-II drug metabolism. Let's examine each one.

1. Reduction: Reduction adds electrons or hydrogen to a functional group, unmasking or introducing a reactive site on the molecule. This is a functionalisation reaction and belongs to Phase-I metabolism.
2. Acetylation: Acetylation attaches an acetyl group, donated by acetyl coenzyme A, onto an amine or hydroxyl group of the drug or its metabolite, catalysed by N-acetyltransferase enzymes. Because this reaction joins the substrate to a small endogenous molecule rather than simply modifying an existing group, it is a conjugation reaction, which is the defining feature of Phase-II metabolism.
3. Hydrolysis: Hydrolysis splits ester or amide bonds using water, exposing a new functional group on the molecule. Like oxidation and reduction, this modifies the parent structure directly and is classed as Phase-I metabolism.
4. Oxidation: Oxidation, most often carried out by cytochrome P450 enzymes, adds oxygen or removes hydrogen from the drug. This is the most common Phase-I reaction and, like the other options besides acetylation, precedes rather than follows conjugation.

Phase-I reactions, oxidation, reduction, and hydrolysis, unmask or introduce a functional group on the drug, while Phase-II reactions attach the drug to an endogenous molecule to boost its water solubility for excretion. Acetylation does exactly that.

Therefore, the correct answer is Acetylation.