Question

Synthetic/phase-II reactions involve

• A. Oxidation
• B. Reduction
• C. Hydrolysis
• D. Glucuronide conjugation

Hint:

To easily remember the difference: Phase I reactions (Oxidation, Reduction, Hydrolysis) break down or modify molecules, while Phase II reactions "add" or "synthesize" a larger polar group (conjugation).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Biotransformation (metabolism) is the chemical alteration of drug molecules within the body, primarily occurring in the liver.
The main goal of metabolism is to convert lipophilic (lipid-soluble) drugs into hydrophilic (water-soluble) compounds that can be easily excreted by the kidneys.
Biotransformation is divided into Phase I (non-synthetic) and Phase II (synthetic/conjugation) reactions.

Step 2: Detailed Explanation:
Let us distinguish between the phases of drug metabolism:
1. Phase I (Non-synthetic) Reactions:
These reactions introduce or unmask a functional polar group (such as $-OH$, $-NH_2$, $-SH$, or $-COOH$) on the drug molecule.
These reactions include:
- Oxidation (A): The addition of oxygen or removal of hydrogen, catalyzed primarily by cytochrome P450 enzymes.
- Reduction (B): The addition of hydrogen or removal of oxygen.
- Hydrolysis (C): The cleavage of chemical bonds by the addition of water (common for esters and amides).
These reactions do not synthesize a new complex compound but rather modify the existing molecule.
2. Phase II (Synthetic) Reactions:
These are conjugation reactions where an endogenous hydrophilic substrate is covalently attached to the functional group of the drug (or its Phase I metabolite).
These reactions require energy and specific transferase enzymes.
The most common Phase II reactions are:
- Glucuronide conjugation (D): Catalyzed by UDP-glucuronosyltransferases (UGT), where glucuronic acid is attached to the drug.
- Sulfate conjugation.
- Acetylation.
- Methylation.
- Glutathione conjugation.
- Amino acid conjugation.
Because these reactions synthesize a new conjugated compound, they are termed synthetic reactions.
Glucuronide conjugation is the most dominant and clinically important Phase II pathway in most mammalian species (with the notable exception of cats, which are deficient in certain glucuronosyltransferase enzymes).

Step 3: Final Answer:
Among the options, Glucuronide conjugation is a Phase II synthetic reaction, while the others are Phase I non-synthetic reactions.
Therefore, the correct option is (D).