Question

Which functional group in drug molecules is most likely to undergo phase I metabolic oxidation by cytochrome P450 enzymes:

• A. Ester group
• B. Amide group
• C. Hydroxyl group
• D. Methyl group

Hint:

In phase I metabolism, cytochrome P450 enzymes primarily oxidize methyl groups, introducing hydroxyl groups to enhance drug polarity for excretion.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
We need to identify which functional group cytochrome P450 enzymes are most likely to oxidize during phase I drug metabolism.

Step 2: Key Concept:
Phase I metabolism by cytochrome P450 mainly involves inserting oxygen into C-H bonds, a reaction that works well on alkyl carbons but does not apply to bonds that are already broken down by hydrolytic enzymes instead.

Step 3: Detailed Explanation:
Ester and amide groups are cleaved by hydrolysis, carried out by esterases and amidases respectively, so cytochrome P450 oxidation is not their main metabolic route.
A hydroxyl group is already an oxidized position on the molecule, so instead of being oxidized again, it usually gets conjugated with glucuronic acid or sulfate in phase II.
A methyl group, however, has C-H bonds that cytochrome P450 can directly hydroxylate, converting \( -\text{CH}_3 \) into \( -\text{CH}_2\text{OH} \), which can be oxidized further toward a carboxylic acid.

Step 4: Final Answer:
The methyl group is the functional group most likely to undergo cytochrome P450 oxidation in phase I metabolism.