L11: Phase II Metabolism

Question 1

What is the primary aim of Phase II metabolism?

Answer 1

To increase a compound’s water solubility and molecular weight, making it easier to excrete and less likely to cross cell membranes.

Question 2

What is the usual outcome of Phase II conjugation reactions?

Answer 2

Detoxification – the compound becomes pharmacologically inactive and more excretable.

Question 3

Do Phase I and Phase II reactions always occur in sequence?

Answer 3

Not necessarily. While many compounds undergo Phase I first, Phase II can occur directly if a functional group is already present.

Question 4

What are the main types of Phase II conjugation reactions?

Answer 4

Glucuronidation, Sulfation, Amino acid conjugation, Methylation, Acetylation, Glutathione conjugation.

Question 5

What enzyme catalyzes glucuronidation?

Answer 5

UDP-glucuronosyltransferases (UGTs)

Question 6

What cofactor is required for glucuronidation?

Answer 6

UDP-glucuronic acid

Question 7

What kind of compounds undergo glucuronidation?

Answer 7

Both xenobiotics and endogenous compounds like steroids, vitamins, bile acids, and bilirubin.

Question 8

Where are UGTs located?

Answer 8

On the luminal face of the endoplasmic reticulum.

Question 9

What are the structural features of UGTs?

Answer 9

Monomeric proteins (50–60 kDa), no prosthetic group, tissue-specific isoform distribution.

Question 10

What are the two gene families of human UGTs?

Answer 10

UGT1 and UGT2

Question 11

What does alternative splicing allow in UGT1 gene family?

Answer 11

It enables different isoforms with substrate-specific first exons but shared exons 2–5.

Question 12

What enzyme family is UGT2 similar to?

Answer 12

Cytochrome P450 enzymes

Question 13

What are examples of UGT1A isoforms and their substrates?

Answer 13

UGT1A1 → bilirubin, ethinyl estradiol; UGT1A4 → TCA antidepressants; UGT1A6 → paracetamol; UGT1A9 → valproate, naproxen.

Question 14

What is the main substrate of UGT2B7?

Answer 14

Morphine and ibuprofen (also metabolizes steroids)

Question 15

How are glucuronide conjugates excreted based on MW?

Answer 15

<400 Da → urine; ≥400 Da → bile.

Question 16

What is enterohepatic recirculation?

Answer 16

Gut bacteria cleave glucuronides in bile, regenerating the parent compound for reabsorption.

Question 17

What functional groups can UDP-glucuronic acid conjugate with?

Answer 17

Alcohols, carboxylic acids, amines, amides, and thiols.

Question 18

What are the effects of glucuronidation on drugs?

Answer 18

Promotes excretion, reduces biological activity, some glucuronides can be reactive and bind irreversibly to proteins (e.g. acyl glucuronides).

Question 19

What enzymes catalyze sulfation?

Answer 19

Sulfotransferases (SULTs)

Question 20

What is the cofactor for sulfation reactions?

Answer 20

PAPS (3’-phosphoadenosine 5’-phosphosulfate)

Question 21

What functional group does sulfation primarily target?

Answer 21

Hydroxyl (–OH) groups

Question 22

What is the advantage and disadvantage of sulfation?

Answer 22

✅ High water solubility; ⚠️ PAPS is expensive and limited, so sulfation is used at low substrate levels.

Question 23

When is glucuronidation favored over sulfation?

Answer 23

When substrate levels are high or PAPS is depleted.

Question 24

What harmful consequence can sulfation have?

Answer 24

Can activate procarcinogens into sulfoxy derivatives → spontaneous formation of nitrenium ions, which bind DNA.

Question 25

Where does amino acid conjugation take place?

Answer 25

In the mitochondria

Question 26

What are the key amino acids used in conjugation?

Answer 26

Glycine, taurine, and glutamine

Question 27

What is the activation step in amino acid conjugation?

Answer 27

Carboxyl group of xenobiotic is converted into an acyl-CoA thioester using ATP and CoA.

Question 28

What enzyme activates xenobiotics during amino acid conjugation?

Answer 28

MACS (medium-chain acyl-CoA synthetase)

Question 29

What enzyme conjugates the activated xenobiotic to glycine?

Answer 29

Glycine N-acyltransferase

Question 30

Why do few xenobiotics undergo amino acid conjugation?

Answer 30

Due to strict enzyme selectivity and the requirement for a carboxyl group.

Question 31

What is the pathway of benzoic acid conjugation?

Answer 31

Benzoic acid → benzoyl-CoA → hippuric acid (via glycine conjugation)

Question 32

What is the conjugation pathway of aspirin?

Answer 32

Aspirin → salicylic acid → salicyl-CoA → salicyluric acid (via glycine)

Question 33

What does PAPS stand for?

Answer 33

3'-Phosphoadenosine 5'-Phosphosulfate

Question 34

What is the significance of S-glucuronidation using disulfiram?

Answer 34

Breaks disulfide bond and glucuronidates each half separately.

Question 35

Which SULT isoforms are important for drug metabolism?

Answer 35

SULT1A1 → paracetamol, N-hydroxy PhIP; SULT1A3 → phenols; SULT1C2/4 → carcinogenic arylamines like N-hydroxy-2-acetylaminofluorene.

Question 36

Which of the following is a feature of glucuronidation? A) Increases lipophilicity B) Produces pharmacologically active metabolites only C) Uses PAPS as cofactor D) Facilitates drug excretion by increasing water solubility

Answer 36

D

Question 37

Which compound would most likely undergo amino acid conjugation? A) Paracetamol B) Benzoic acid C) Ethinyl estradiol D) Valproate

Answer 37

B

Question 38

Which cofactor is used in sulfation reactions? A) UDP-glucuronic acid B) Coenzyme A C) NADPH D) PAPS

Answer 38

D

Question 39

Where are sulfotransferase enzymes primarily located? A) Mitochondria B) Cytosol C) Endoplasmic reticulum D) Nucleus

Answer 39

B

Question 40

Which of the following explains why sulfation is less commonly used at high substrate levels? A) It activates the immune system B) It is energy-independent C) PAPS is expensive and limited D) SULT enzymes are only found in the liver

Answer 40

C

Question 41

What is the main reason enterohepatic recirculation can prolong drug activity? A) It produces new metabolites B) Bacterial enzymes regenerate the parent drug from glucuronides C) It enhances conjugation D) It increases CYP enzyme activity

Answer 41

B

Question 42

Which of the following is required to activate a xenobiotic for amino acid conjugation? A) PAPS B) Glycine N-acyltransferase C) MACS D) UDP-glucuronic acid

Answer 42

C

Question 43

Which UGT isoform is responsible for glucuronidating bilirubin? A) UGT1A1 B) UGT1A4 C) UGT1A9 D) UGT2B7

Answer 43

A