Phase II

Question 1

Name all enzymes involved in phase 2 metabolism

Answer 1

UDP-glucuronyltransferases (UGTs)
Sulfotransferases (SULTs)
Glutathione S transferases (GSTs)
Amino acid conjugation enzymes
N-acetyltransferases (NATs)
Methytransferases

Question 2

Where are UGTs found

Answer 2

Collocated with CYPs and FMOs in liver microsomes

Question 3

Where are SULTs found

Answer 3

In the cytosolic - wide tissue distribution

Question 4

Where are amino acid conjugating enzymes found

Answer 4

In mitochondria

Question 5

Where are NATs found

Answer 5

Cytosol

Question 6

Where are GSTs found

Answer 6

Cytosol

Question 7

What is the most common route of phase 2 metabolism

Answer 7

Glucuronidation
Followed by sulfation

Question 8

Why is conjugation in phase 2 necessary

Answer 8

Gives greater molecular weight
More water soluble
Less able to pass through membranes
Easier to excrete

Question 9

What is the glucuronidation cofactor

Answer 9

UDP-glucuronic acid

Question 10

Substrates for UDP-GTs

Answer 10

Steroids
Fatty acids
Bile acids
Bilirubin

Question 11

Where are UGTs found

Answer 11

Endoplasmic reticulum

Question 12

Tissue distribution of UGTs

Answer 12

Highest in liver but also kidney lung small intestine

Question 13

How does glucuronidation promote excretion

Answer 13

Larger MW and more water soluble and loss of biological activity
However some glucuronides have biological activity (morphine glucuronide)

Question 14

Families of UGTs?

Answer 14

UGT1 and UGT2

Question 15

Describe elimination products of glucuronidated compounds

Answer 15

Conjugates MW>400 excreted in bile
Conjugates MW<400 excreted in urine

Question 16

What may happen to glucuronides excreted in bile

Answer 16

Enterihepatic recirculation - increase half life

Question 17

How do drugs undergo enterohepatic recirculation

Answer 17

Excreted to intestine via bile duct
Reabsorbed through portal vein to liver

Question 18

UDP-glucuronic acids reacts with

Answer 18

Alcohol
Carboxylic acids
Amines
Amides
Thiols

Question 19

Describe glucuronidation of paracetamol

Answer 19

UDP is lost
OH on paracetamol condenses with UDP-glucuronic acid

Question 20

Example of another glucuronidation reaction

Answer 20

Formation of disulphiram glucuronide from disulphiram

Question 21

Can UGTs be induced

Answer 21

Yes but not as much as CYPs
UGT1 induced by polycyclic aromatic hydrocarbons

Question 22

Important SULT enzymes and their locations

Answer 22

SULT1A1 - liver
SULT1A3 - intestine
SULT1C2 - stomach & foetal liver

Question 23

How do SULTs transfer sulfate

Answer 23

Using PAPS as a source of sulfate

Question 24

General alcohol sulfation reaction

Answer 24

PAPS + ROH —> PAP + ROSO3-

Question 25

Example substrate of SULTs

Answer 25

Oestrone & paracetamol —> paracetamol sulfate

Question 26

Name an amine conjugated by SULTs

Answer 26

Metoclopramide - dopamine receptor agonist Desipramine - tricyclic antidepressant

Question 27

Why isn’t sulfation as common as glucuronidation

Answer 27

Sulfate availability is limited

Question 28

Can sulfation activate carcinogens

Answer 28

Yes - sulfoxy derivatives of pro-carcinogens are unstable and spontaneously form reactive nitrenium compounds

Question 29

Example of SULT regulation

Answer 29

AhR ligands (dioxins) are SULT substrates They can have a negative effect on SULT activity in mice

Question 30

What was the first amino acid conjugation reaction to be discovered

Answer 30

Benzoic acid metabolised to hippuric acid

Question 31

What is required for amino acid conjugation

Answer 31

Carboxyl group

Question 32

What are the major conjugating amino acids

Answer 32

Glycine Taurine Glutamine

Question 33

General reaction for amino acid conjugation

Answer 33

COOH to is acyl-CoA derivative Then amide formation with amino group from amino acid

Question 34

What enzyme carries out the initial reaction in aa conjugation

Answer 34

CoA ligases (MACS)

Question 35

With what are MACS active with

Answer 35

One active with benzoic acid One active with salicylic acid

Question 36

Benzoic acid conjugation

Answer 36

Benzoic acid —> acyl adenylate —> benzyl CoA using MACS —> linkage to glycine forming hippuric acid

Question 37

Conjugation of salicylate

Answer 37

Salicylic acid —> salicyl CoA (MACS) —> salicyluric acid (glycine)

Question 38

Why are so few xenobiotics conjugated to amino acids

Answer 38

N-acyltransferase is selective Not all CoAs formed by MACS are conjugated Metabolite of hypoglycin is conjugated at a low rate Hypoglycin is in Jamaican Ackee fruit and causes Jamaican vomiting sickness

Question 39

What compounds are acetylated?

Answer 39

Compounds containing amino, hydroxyl and sulfhydryl groups

Question 40

What does acetyltransferase use

Answer 40

Acetyl CoA to donate acetyl group

Question 41

What are the acetylation enzymes

Answer 41

NAT1 and NAT2

Question 42

What are the important substrates for NAT enzymes

Answer 42

Sulphamethoxazole - NAT1 Isoniazid - NAT2 (slow acetylation phenotype leads to isoniazid toxicity)

Question 43

NATs location and tissue distribution

Answer 43

Cytosol NAT1 - most tissues NAT2 - liver, intestine

Question 44

Are NATs inducible

Answer 44

No - they are expressed at a constant level

Question 45

Which NAT is more polymorphic

Answer 45

NAT2 - 50% of population can’t acetylate certain compounds

Question 46

What’s the effect of acetylation

Answer 46

Masks an amine group with and acetyl group Decreased solubility

Question 47

What’s the link with NATs and cancer

Answer 47

NATs can activate arylamines in well-cooked food and produces reactive intermediates that initiate carcinogenesis

Question 48

What do GSTs do

Answer 48

Conjugate glutathione to electrophilic compounds using nucleophilic cysteine thiol group

Question 49

Subfamilies of GST

Answer 49

Alpha Mu Pi Theta (small molecules)

Question 50

Which GST classes are inducible

Answer 50

Alpha and mu - by barbiturates and hydrocarbons

Question 51

Why don’t GSTs have a general role in metabolism

Answer 51

They have a specialised role because most drugs aren’t electrophiles

Question 52

Example substrates for GSTs

Answer 52

Cyclophosphamide Paracetamol

Question 53

Explain toxicity linked to GSTs

Answer 53

NAPQI is a metabolism product of paracetamol It is further metabolised by GSTs NAPQI toxicity is caused by GST depletion

Question 54

3 families of methyltransferase

Answer 54

O-methyltransferase N-methyltransferase S-methyltransferase

Question 55

Methylation usually increases …

Answer 55

Lipophilicity

Question 56

What cofactor to methyltramsferases use

Answer 56

SAM

Question 57

Example of methylation reaction

Answer 57

L-Dopa —> O-methyl-L-dopa

Question 58

How does methylation aid arsenic toxicity

Answer 58

Methylation of arsenic makes it more able to bind to DNA