Conjugation reactions Flashcards
General points of conjugation reactions?
- functional group required for conjugation
- conjugation molecules are endogenous compounds
- resulting conjugates are extremely polar and usually terminal metabolites
How are conjugates excreted?
Biliary and renal excretion, facilitated by efflux transporters
Approx pKa of resulting conjugates?
< 3
What are the two types of conjugation reactions?
- direct conjugation of the drug (often parallel to CYP metabolism, e.g. diclofenac, propofol
- conjugation of drug metabolites
What functional groups can glucuronidation occur at?
-OH, -COOH, -NH2
What types of conjugation can occur at -OH groups?
glucuronidation or sulphation
What types of conjugation can occur at -COOH groups?
glucuronidation or glycine conjugation
What types of conjugation can occur at -NH2 groups?
glucuronidation or acetylation
Pathways of midazolam metabolism?
hydroxylation (aliphatic or aromatic) - CYP mediated. can then be glucuronidated
alternatively - N-glucuronidation
Pathways of naloxone metabolism?
Glucuronidation of -OH
What are the two steps of conjugation reactions?
- Activation step
2. Synthetic step
What happens in the first step of conjugation?
activation of the conjugating agent (glucuronides, sulphation or acetyl conjugates) or the drug (amino acid conjugates)
complex and involves intermediary metabolism
What happens in the second step of conjugation?
simple - transferase enzymes involved
activated co-enzyme (or drug) combined with the drug (or co-substrate) to form the conjugate
What does UGT stand for?
UDP-glucuronosyltransferases
Where are in the cell are UGTs located?
luminal side of endoplasmic reticulum (different to CYP450s as not membrane bound)
Function of UGTs?
glycoproteins that catalyse the addition of glucuronic acid to a substrate
How many human UGTs are there?
22
Which UGT subfamilies are the most relevant?
1A and 2B
Where in the body are UGTs located?
predominantly liver, but also intestine and kidneys
Clinical importance of conjugation reactions?
approx 40% of top prescribed drugs are metabolised by UGTs
Example of drugs metabolised by glucuronidation?
Lamotrigine, mycophenolic acid, valproic acid
Example of endogenous compounds metabolised by glucuronidation?
bilirubin, steroid hormones, thyroxine, bile acids
Which glucuronide metabolites are pharmacologically active?
morphine-6 glucuronide, ezetimibe glucuronide
quite rare, most are terminal
How can glucuronides contribute to DDIs?
inhibit transporters or metabolic enzymes
How does gemfibrozil cause DDIs?
glucuronide metabolite is a potent inhibitor of CYP2C8 and OATP1B1
How does UGT expression vary between adults and children?
UGT1A1 activity and expression reach adult levels at 3-6 months old (newborns have far less glucuronidation activity)
cause of congenital jaundice - increased bilirubin
Which enzyme glucuronidates morphine?
UGT2B7
Neonates and UGT2B7?
have reduced activity so reduced metabolism of morphine
What is Gilbert’s syndrome?
UGT1A1 deficiency, which leads to unconjugated hyperbilirubinaemia
UGT1A1*28 - the polymorphism allelic variant
2-13% of caucasians
Risks of the UGT1A1*28 variant?
increased risk of neutropaeia when receiving irinotecan (lower starting dose recommended)
no glucuronidation so higher side effects
Mycophenolic acid inter-individual variability?
Different genotype groups have a significant impact on UGT activity and therefore PK/PD - significant variability found in paediatric transplant patients. can affect organ reception
Which glucuronidation enzymes are the most abundant in the kidneys?
UGT1A9, 2B7, 1B6
Glucuronidation in obese patients?
Reported increased glucuronidation in morbidly obese
GLucuronidation in CKD?
Evidence for uremic toxins inhibiting UGT1A9 and 2B7
What do SULT enzymes do?
Sulphation
Where are SULT enzymes located in the cell?
Cytosol
Relationship between UGTs and SULTs?
Similar, so often have the same substrates. SULTs far more readily saturated than UGTs
Where in the body are SULTs located?
Liver and small intestine (SULT1A1)
Examples of drugs metabolised by SULTs?
paracetamol, salbutamol, troglitazone
What SULT is expressed in human feotuses?
SULT1A3
SULTs and hepatotoxicity?
Some sulphate metabolites are linked to hepatotoxicity - e.g. troglitazone
What are the parallel pathways of paracetamol metabolism?
- CYP mediated N-hydroxylation and rearrangment
- Sulfation
- Glucuronidation
What happens in glycine conjugation?
Drug becomes activated with acyl coenzyme A intermediate
What happens in acetylation?
- limited to amines, formation of N-acetyl conjugates
- Acetyl CoA is coenzyme
Which enzyme catalyses acetylation reactions?
N-acetyl transferase (NAT)
Polymorphisms of NAT and the consequences?
get rapid and slow metabolisers
- clinically relevant in patients taking sulfasalazine
What are Phase III reactions?
Hydrolysis of conjugates
Which enzymes carry out phase III reactions?
glucuronidase (glucuronides)
Sulphatase (for sulphates)
Where to phase III reactions occur?
small intestine - metabolites excreted via biliary excretion of metabolites, which can then be enterohepatically recycled and prolong drug effect
