Question

Which of the following is Phase-II metabolism reaction:

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

Answer 1

The Correct Option is B

The correct option is (B): Acetylation.

Answer 2

Drug metabolism is generally split into Phase I reactions, which introduce or expose a functional group, and Phase II reactions, which attach (conjugate) a small molecule onto the drug or its Phase I metabolite to make it more water soluble for excretion. The question asks which of the four listed reactions is a Phase II type.

1. Reduction: Reduction adds electrons or hydrogen to a functional group, such as converting a ketone to an alcohol, and is classified as a Phase I reaction because it modifies an existing functional group rather than attaching a new conjugating molecule.
2. Acetylation: Acetylation attaches an acetyl group, usually donated by acetyl coenzyme A, onto an amine or hydroxyl group on the drug molecule. This transfer of a small conjugating group from a cofactor is exactly what defines a Phase II (conjugation) reaction.
3. Hydrolysis: Hydrolysis breaks a bond such as an ester or amide using water, which exposes a functional group like a carboxylic acid or amine rather than attaching anything new, placing it firmly in Phase I.
4. Oxidation: Oxidation reactions, most often carried out by cytochrome P450 enzymes, add or expose oxygen-containing functional groups on the drug molecule. This functional-group modification is the hallmark of Phase I metabolism.

Since acetylation is the only one of the four that conjugates a small group onto the drug rather than modifying an existing functional group, it is the Phase II reaction on this list.

The correct answer is Acetylation.