conjugation reactions Flashcards
what are the phases of metabolism?
some drugs needs conjugation reactions after oxidation, these can then be eliminated.
They can be eliminated after phase I reactions but many will form conjugates and get excreted.
there is also a chance of direct conjugation of the parent molecule
what are phase II reactions?
- Require functional group as a site for conjugation- often introduced by the phase I metabolite
- Drug is transported into cells where it can be metabolised
- Conjugating molecules are endogenous- glucose or amino acid derivatives often come from other metabolic reactions,
- Resulting conjugates are extremely polar (pKa<3) – TERMINAL metabolites- these are the ones that are eliminated
- Biliary and renal excretion of conjugates – especially for glucuronides facilitated via efflux transporters (e.g., MRP2, MRP4)
- Some gluconides get into blood: other transporters, basolateral influx into systemic circulation (MRP3 and MRP4) then these will get excreted via the kidneys
what are the general principles for phase II reactions?
drug with a certain functional group + endogenous compound ==> conjugate
what are the two typws of conjugation reactions?
A) Direct conjugation of the drug- as they already have the particular groups in the structure
Functional group Phase II reaction
-OH Glucuronidation or Sulphation
-COOH Glucuronidation or Glycine conjugation
-NH2 Glucuronidation or Acetylation
Often parallel reaction to CYP metabolism - diclofenac, propofol
B) Conjugation of the phase I metabolite
what are the functional groups that can take part in phase II reactions?
OH = glucuronidation or sulphation COOH = glucuronidation or glycine conjugation NH2 = glucuronidation of acetylation
what are examples of conjugation reaction?
midazolma CYP3A4 via oxidation reaction produces 1 and 4 hydroxy midazolam
what are the common mechanisms of conjugation?
A. ACTIVATION STEP- 2 different things can be activated
B. SYNTHETIC STEP
what is the activation step?
- Most common: Conjugating agent (gets activated) as a coenzyme (for glucuronides, sulphates and acetyl conjugates)- this is a multi step process that we don’t need to know or
- Drug (for amino acid conjugates)
what is the synthetic step?
simple, transferase enzymes that transfer the conjugate to the molecule and forms the metabolite
where are the enzymes located for glucuronidation via uDP?
Enzymes located on the luminal side of the endoplasmic reticulum- membrane bound but face the luminal side- this affects the molecules access to the active site
what are the common functional groups that are likely to glucuronosyltransferase?
OH, -COOH, -NH2, -SH
what are the super families of enzymes?
UGT1A and UGT2B subfamilies the most relevant - UGT1A1, UGT1A9, UGT2B7: clinically relevant- metabolise a lot of drugs on the market
where are the enzymes mainly present for UGT?
mainly in liver but also in the intestine and kidney
what are the importance of conjugation metabolic pathways in drug development?
- glucuronidation is the most common mainly for lamotrigin, mycophenolic acid, valproic
- conjugates are often pharmacologically inactive
- renal toxicity
- contribute to drug-drug interactions
what happens to bilirubin?
extensively Glucuronidated, steroid hormones, thyroxine and bile acids
what is the ontogeny of UGT?
UGT1A1 activity and protein expression reach adult levels at 3-6 months of age (vary between different enzymes)
Congenital jaundice - Increase in bilirubin levels (UGT1A1) that doesn’t get conjugated due to reduced activity- does pass as enzymes develop
Morphine glucuronidation (UGT2B7) deficient in young infants
what is gilberts syndrome?
unconjugated hyperbilirubinemia, deficiency in UGT1A1 in adults (UGT1A1*28)- don’t have this enzyme which causes higher levels of bilirubin
Occurs in 2-13 % of Caucasian
what is irinotecan disposition/
Does undergo CYP metabolism
UGT1A1 is needed for metabolism (right)
If deficient, this increases neutropenia as the SN-38 metabolite is directly related to neutropenia
Could use the genotype to see if dose adjustments are needed
what are the most abundant glucuronidation enzymes in kideny and tumour tissue?
UGT1A9, UGT2B7, and UGT1A6
what are the enzymes present in liver and small intestine?
SULT1A1
Metabolism of paracetamol, salbutamol, troglitazone
Sulphates linked to hepatoxicity of some drugs (troglitazone)- not always the case with all drugs
SULT1A3 expressed in the human foetal liver- differs to UGTs
what is the parallel metabolism of paracetamol?
- Blue is the dominant metabolism compared to CYPs direct glucuronidation and direct sulfation
- Red is the CYP mediated mechanism
- Purple is linked to toxicity/ overdose linked to paracetamol.
what is glycine conjugation?
Limited to acids
Drug becomes activated as acyl coenzyme A intermediate
what is acetylation?
Limited to amines – formation of N-acetyl conjugates
Acetyl CoA is the co-enzyme
Genetic polymorphisms in N-acetyl transferase (NAT)- rapid or slow (CYP450 lecture)
rapid and slow acetylators
what is enterohepatic recirculation after drug admin?
via the gluronide step, to include a metabolite.
Give the durg and it is absorbed by the small intestine, reaches circulation and enters liver (standard oral absorption process).
In the liver you will get metabolism via UGT, to produce glucuronide, this will get taken into the bile by efflux transporters (MRP2), which goes to the small intestine. Parts of that will be converted by beta glucuronidase back to the drug not all of it has to go this way, there are some elements that will be excreted via bile to the faeces but portion of it can be converted back into drug and will be reabsorbed in the lumen of the small intestine
