Detoxification by the liver Flashcards
summary
- Oxidation in Phase I reactions; Cytochrome P450 enzymes
- Conjugation in Phase II reactions (for renal excretion)
- Drug interactions
- Active metabolites of a drug
- Ethanol metabolism
Cytochrome P450 enzymes
They are present in the smooth Endoplasmic Reticulum (hence “microsomal” enzymes).
• They oxidise the substrate and reduce oxygen,
• They have a cytochrome reductase subunit which uses NADPH,
• They are inducible – enzyme activity is increased by certain drugs, some dietary components, and some environmental toxins eg smoking,
• They generate a reactive free radical compound.
Cytochrome P450 enzymes & drug interactions
Cytochrome P450 enzymes are inducible, which may accelerate the breakdown of some medications as well as endogenous steroids
Cytochrome P450 enzymes can be inhibited by various drugs and foodstuffs (usually takes effect quicker than induction),
• that can result in increased blood concentrations of certain medications (less breakdown).
Inactivation of a xenobiotic
- Phenobarbital is relatively lipophilic; drug distributes into fat tissue.
- The amount that remains in the plasma is mostly bound to plasma proteins.
- only a small fraction of the drug is found freely dissolved in the blood plasma,
- Elimination of the unmodified drug is thus very slow, and most of the drug is excreted after
Active drug to active metabolites (1st example - opiates
- An active drug may be converted to another active form, for example codeine is metabolised to morphine (a phase-1 reaction).
- Codeine is a morphine molecule with one hydroxyl group replaced by a methyl group which makes the compound less susceptible to first-pass metabolism (in gut mucosa and liver).
- Codeine is active, and is de-methylated in the liver to morphine which is also active.
paracetamol
Paracetamol is usually safely metabolised by glucuronidation or sulphation, but…
In paracetamol overdose the toxic metabolite NAPQI builds up and causes liver cell damage
Phase 1 reactions
Oxidation
Hydroxylation (addition of -OH group)
N- and O- Dealkylation (removal of -CH side chains)
Deamination (removal of -NH side chains)
Epoxidation (formation of epoxides)
Oxygen addition (sulfoxidation, N-oxidation)
Hydrogen removal
Reduction
Hydrogen addition (unsaturated bonds to saturated)
Donor molecules include GSH, FAD, NAD(P)H
Oxygen removal
Hydrolysis
Splitting of C-N-C (amide) and C-O-C (ester) bonds
phase 2 reactions
• Glycoside conjugation - glucuronidation
• Sulphate - sulphation
• Glutathione (GSH)
• Methylation
• Acylation
• Acetylation
• Amino acid conjugation
• Deacetylation
• Phosphate conjugation
Glucuronidation is the most common Phase II reaction, and the next commonest is sulphation (as we saw with paracetamol).
Transferase enzymes are responsible for most phase II reactions, e.g. uridine diphosphoglucuronosyl transferase (UGT), N-acetyl transferase (NAT), glutathione S-transferase (GST), and sulphotransferase (ST).
Microsomal enzymes
Location - smooth endoplasmic reticulum
Sites - liver then kidney, lungs, intestinal mucosa.
Enzymes – mono-oxygenases, (CYPs,FMOs); UGTs etc.
Reactions - majority of drug biotransformation reaction ;
oxidative, reductive & hydrolytic and glucuronidation
Note - they are inducible by drugs, diet etc…
Non microsomal enzymes
Location - cytoplasm and mitochondria of hepatocytes, other tissues
Enzymes - protein oxidases, esterases, amidases, conjugases
Reactions - non specific enzymes that catalyze few oxidative,
a number of reductive & hydrolytic reactions and also conjugation reactions other than glucuronidation.
Note - Not inducible but having polymorphism
(acetyltransferase & pseudocholinesterase)
Ethanol – role of ADH
- Metabolism of ethanol doesn’t fit the category of phase I and Phase II.
- Ethanol doesn’t need to be conjugated for excretion.
- Only between 2% and 10% is usually excreted in the urine, because it is used in the liver as a dietary fuel.
- The major route is via alcohol dehydrogenase (ADH)
Ethanol – role of microsomal system
The other route is via the so-called Microsomal Ethanol Oxidising System MEOS, which also produces acetaldehyde as a toxic intermediate.
• A cytochrome P450 isoenzyme has high affinity for alcohol. A number of cytochrome P450 isoenzymes make up the MEOS.
• In the context of moderate alcohol intake this system accounts for only 10 to 20 % of ethanol metabolism.
• However, because the cytochrome P450 enzymes are inducible, chronic alcohol use increases CYP2E1 levels 5 or 10-fold (as well as inducing other cytochrome enzymes).
Ethanol – liver damage
If the liver’s ability to oxidise acetaldehyde is overwhelmed, acetaldehyde accumulates in the liver.
• It contributes to alcohol-induced hepatitis by forming adducts with amino acids.
• It also binds to glutathione and therefore predisposes cells to damage by peroxide and other free radicals.
