Phase II Flashcards
Name all enzymes involved in phase 2 metabolism
UDP-glucuronyltransferases (UGTs)
Sulfotransferases (SULTs)
Glutathione S transferases (GSTs)
Amino acid conjugation enzymes
N-acetyltransferases (NATs)
Methytransferases
Where are UGTs found
Collocated with CYPs and FMOs in liver microsomes
Where are SULTs found
In the cytosolic - wide tissue distribution
Where are amino acid conjugating enzymes found
In mitochondria
Where are NATs found
Cytosol
Where are GSTs found
Cytosol
What is the most common route of phase 2 metabolism
Glucuronidation
Followed by sulfation
Why is conjugation in phase 2 necessary
Gives greater molecular weight
More water soluble
Less able to pass through membranes
Easier to excrete
What is the glucuronidation cofactor
UDP-glucuronic acid
Substrates for UDP-GTs
Steroids
Fatty acids
Bile acids
Bilirubin
Where are UGTs found
Endoplasmic reticulum
Tissue distribution of UGTs
Highest in liver but also kidney lung small intestine
How does glucuronidation promote excretion
Larger MW and more water soluble and loss of biological activity
However some glucuronides have biological activity (morphine glucuronide)
Families of UGTs?
UGT1 and UGT2
Describe elimination products of glucuronidated compounds
Conjugates MW>400 excreted in bile
Conjugates MW<400 excreted in urine
What may happen to glucuronides excreted in bile
Enterihepatic recirculation - increase half life
How do drugs undergo enterohepatic recirculation
Excreted to intestine via bile duct
Reabsorbed through portal vein to liver
UDP-glucuronic acids reacts with
Alcohol
Carboxylic acids
Amines
Amides
Thiols
Describe glucuronidation of paracetamol
UDP is lost
OH on paracetamol condenses with UDP-glucuronic acid
Example of another glucuronidation reaction
Formation of disulphiram glucuronide from disulphiram
Can UGTs be induced
Yes but not as much as CYPs
UGT1 induced by polycyclic aromatic hydrocarbons
Important SULT enzymes and their locations
SULT1A1 - liver
SULT1A3 - intestine
SULT1C2 - stomach & foetal liver
How do SULTs transfer sulfate
Using PAPS as a source of sulfate
General alcohol sulfation reaction
PAPS + ROH —> PAP + ROSO3-
Example substrate of SULTs
Oestrone & paracetamol —> paracetamol sulfate
Name an amine conjugated by SULTs
Metoclopramide - dopamine receptor agonist
Desipramine - tricyclic antidepressant
Why isn’t sulfation as common as glucuronidation
Sulfate availability is limited
Can sulfation activate carcinogens
Yes - sulfoxy derivatives of pro-carcinogens are unstable and spontaneously form reactive nitrenium compounds
Example of SULT regulation
AhR ligands (dioxins) are SULT substrates
They can have a negative effect on SULT activity in mice
What was the first amino acid conjugation reaction to be discovered
Benzoic acid metabolised to hippuric acid
What is required for amino acid conjugation
Carboxyl group
What are the major conjugating amino acids
Glycine
Taurine
Glutamine
General reaction for amino acid conjugation
COOH to is acyl-CoA derivative
Then amide formation with amino group from amino acid
What enzyme carries out the initial reaction in aa conjugation
CoA ligases (MACS)
With what are MACS active with
One active with benzoic acid
One active with salicylic acid
Benzoic acid conjugation
Benzoic acid —> acyl adenylate
—> benzyl CoA using MACS
—> linkage to glycine forming hippuric acid
Conjugation of salicylate
Salicylic acid —> salicyl CoA (MACS)
—> salicyluric acid (glycine)
Why are so few xenobiotics conjugated to amino acids
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
What compounds are acetylated?
Compounds containing amino, hydroxyl and sulfhydryl groups
What does acetyltransferase use
Acetyl CoA to donate acetyl group
What are the acetylation enzymes
NAT1 and NAT2
What are the important substrates for NAT enzymes
Sulphamethoxazole - NAT1
Isoniazid - NAT2 (slow acetylation phenotype leads to isoniazid toxicity)
NATs location and tissue distribution
Cytosol
NAT1 - most tissues
NAT2 - liver, intestine
Are NATs inducible
No - they are expressed at a constant level
Which NAT is more polymorphic
NAT2 - 50% of population can’t acetylate certain compounds
What’s the effect of acetylation
Masks an amine group with and acetyl group
Decreased solubility
What’s the link with NATs and cancer
NATs can activate arylamines in well-cooked food and produces reactive intermediates that initiate carcinogenesis
What do GSTs do
Conjugate glutathione to electrophilic compounds using nucleophilic cysteine thiol group
Subfamilies of GST
Alpha
Mu
Pi
Theta (small molecules)
Which GST classes are inducible
Alpha and mu
- by barbiturates and hydrocarbons
Why don’t GSTs have a general role in metabolism
They have a specialised role because most drugs aren’t electrophiles
Example substrates for GSTs
Cyclophosphamide
Paracetamol
Explain toxicity linked to GSTs
NAPQI is a metabolism product of paracetamol
It is further metabolised by GSTs
NAPQI toxicity is caused by GST depletion
3 families of methyltransferase
O-methyltransferase
N-methyltransferase
S-methyltransferase
Methylation usually increases …
Lipophilicity
What cofactor to methyltramsferases use
SAM
Example of methylation reaction
L-Dopa —> O-methyl-L-dopa
How does methylation aid arsenic toxicity
Methylation of arsenic makes it more able to bind to DNA
