Terence (Detoxification of xenobiotics) Flashcards
Detoxification of xenobiotics
- Xenobiotics relates to a substance, typically a synthetic chemical, that is foreign to the body.
- Xenobiotics include drugs, poisons, chemicals in the diet (e.g. food additives), environmental pollutants (e.g. pesticides).
- Xenobiotics can be directly toxic or their metabolites can be toxic.
- Xenobiotics are frequently lipophilic molecules that partition into lipid-rich environments such as cell membranes where their chemical reaction leads to cellular damage and death.
- We have evolved enzymes that chemically modify xenobiotics to less reactive, more water soluble compounds that are then readily excreted.
Detoxification involved the elimination of toxins from the body.
Drug elimination
Drug elimination is the irreversible loss of a drug from the body.
This can be through:
- biotransformation (chemical)- drug/toxin metabolised is transformed to a less toxic/benign molecule
- excretion (physical)- removal of the toxin/drug or its metabolites from the body
Excretion
Excretion can occur via the:
- Kidneys (in urine)- the majority of drugs/drug metabolites leave the body via the kidneys in urine
- Hepatobiliary system- biliary excretion involves active secretion of drug molecules or their metabolites from hepatocytes into the bile and excreted in faeces. Tends to be drugs that are highly lipophilic or too large to be filtered by the glomerulus. Drugs/metabolites in bile may be reabsorbed in the intestine leading to prolonged action and then further metabolised (called enterohepatic cycling)
- Lungs- volatile gaseous anaesthetics
- Milk and sweat- negligible amount excreted via this route
Elimination of drug via the kidney
Water soluble drugs/drug metabolites are efficiently eliminated by the kidneys in urine.
Lipophilic drugs cannot be eliminated by the kidney.
The inability to eliminate lipophilic drugs can lead to their accumulation and toxicity. These drugs must be metabolised to more polar (water soluble) products for elimination. This transformation of a chemical by the body is called biotransformation.
Biotransformation occurs mainly in the liver and the intestine but can also occur in the plasma of any cell in the body.
Xenobiotic transformation
Commonly this process converts lipophilic (fat soluble) drugs into hydrophilic (water soluble) drugs that can be readily excreted via the kidneys in the urine.
The liver and small intestine contain the highest concentrations of xenobiotic biotransforming enzymes.
During this process drugs can undergo one of four potential biotransformations:
- Active drug to an inactive metabolite
- Active drug to an active metabolite
- Inactive drug to an active metabolite
- Active drug to a toxic metabolite (biotoxification)
Biotransformation is commonly divided into two phases that ultimately reduces lipid solubility and thus increases elimination
- Phase I reactions- increase reactivity, addition of low molecular weight functional group
- Phase II reactions- increase solubility, conjugation with a higher weight water soluble group
Phase I biotransformation
Phase I reactions expose or introduce a polar functional group (e.g. OH, NH2, SH, COOH). This is called functionalisation. Most commonly these reactions lead to the production of a nucleophile.
Phase I reactions are most commonly”
- Oxidation- gain of oxygen, loss of hydrogen, loss of electrons
- Hydrolysis- a reaction where water is used to break chemical bonds in the other reactant
- Reduction- loss of oxygen, gain of hydrogen, gain of electrons
These reactions slightly increase hydrophilicity but more importantly often increases the reactivity of the compound to allow for a Phase II reaction to occur.
If the drug is sufficiently polar after phase I metabolism, the drug may be excreted at this stage
Phase II biotransformation
Products of phase I often undergo phase II reactions which involves the covalent attachment of a small hydrophilic endogenous molecule to a pre-existing or acquired functional group.
- These are conjugation reactions
- They are anabolic reactions
- They mainly take place in the liver
- They increase hydrophilicity/water solubility
- They generally result in an inactive product (exceptions are pro-drugs)
Not all drugs undergo biotransformation and not all drugs are fully biotransformed
Drugs NOT biotransformed, excreted unchanged:
- Benzylpenicilin
- Aminoglycosides
- Metformin
- Turbocurarine
- Amantadine
Excreted due to hydrophobicity, need to be excreted so toxicity is avoided.
Drugs excreted both unchanged and as metabolites:
- Paracetamol
- Salicylates (aspirin)
- Phenobartital
Can be excreted in urine and are metabolised
Drugs FULLY biotransformed, excreted only as metabolites:
- TCADs (Tricyclic antidepressants)
- Phenothiazines
- Chloramphenicol
Only the metabolites are excreted as they have been fully transformed.
Enzymes that carry out Phase I oxidation reactions
Microsomal catalysed by:
- Cytochrome P-450
- Microsomal flavine-containing monooxigenase (FMO)
Non-microsomal catalysed by:
- Monoamino-oxidases (MAO)- mitochondrial
- Molybdemun-containing oxidases
- Alcohol and aldehyde dehydrogenases
(Microsomes are heterogenous vesicle-like structures formed from pieces of the endoplasmic reticulum after eukaryotic cells are broken up in the laboratory. They can be isolated from a non-microsomal membrane fraction by centrifugation).
Cytochrome P450 enzymes
Superfamily of enzymes.
74 families and over 400 isozymes (different enzymes that carry out similar reactions).
Conserved in eukaryotic cells from yeast to man.
The majority of xenobiotic biotransformations are carried out by CYP3A4, CYP2D6 and CYP2C9.
Cytochrome P450 enzymes are responsible for the biotransformation of majority of drugs (CYP2D6 and CYP3A4 metabolise over 50% of orally effective drugs in current use).
They are a catalytically versatile family of enzymes that metabolise a large number of xenobiotics.
Apart from xenobiotic biotransformation they play a vital role in the biosynthesis or catabolism of endogenous molecules such as steroid hormones, bile acids, fat-soluble vitamins, fatty acids, and eicosanoids,
Most CYPs are predominantly located in the liver and the gut but can be found elsewhere.
CYPs are located in the microsomes (smooth ER).
CYPs are embedded in the phospholipid bilayer of the endoplasmic reticulum.
Works in concert with the enzyme NADPH-CYP450 oxidoreductase.
NADPH-CYP transfers electrons to the CYP where it can, in the presence of O2, oxidise xenobiotic substrates, many of which are hydrophobic and dissolved in the ER.
A single NADHP-CYP oxidoreductase species transfers electrons to all CYP isoforms in the ER.
Each CYP contains a molecule of iron-protoporphyrin IX that functions to bind and activate O2.
CYP nomenclature
CYP nomenclature is based on shared homology of amino acid sequence.
e.g. CYP3A4*1:
CYP = human cytochrome
3 = family name
A = sub-family
4= isoform
*1 = allele
(Over 40% amino acid homology (shared) with other family members
Cytochrome P450 enzymes carry out these oxidation reactions
- Hydroxylation of an aliphatic or aromatic carbon
- Epoxidation of a double bond
- Heteroatom (S, N, and I) oxygenation and N-hydroxylation
- Heteroatom (O, S, N, Si) dealkylation
- Oxidative group transfer
- Cleavage of esters
- Dehydrogenation
Alcohol dehydrogenase
Cytosolic enzyme
Highly expressed in liver, kidney, lung, gastric mucosa
Oxidises ethanol (alcohol) to acetaldehyde
Can also be oxidised by CYP2E1.
Non-P450 oxidation reactions
Methanol is not toxic.
Methanol is metabolised to toxic metabolites formaldehyde and formate.
Formate inhibits cytochrome enzymes of the electron transport chain.
Causes retinal damage and blindness.
Treated with ethanol or formepizole to competitively inhibit alcohol dehydrogenase. (Ethanol uses up alcohol dehydrogenase so methanol isn’t metabolised into toxic metabolites)
Non-P450 oxidation reactions- hydrolysis
In a reaction with water, a bond in the compound is broken, resulting in two compounds. At the same time the water molecule splits in two, with a hydrogen transferring to one of the compounds and a hydroxide to the other compound.
Enzymes that carry out these reactions are often found in plasma. These include:
- Peptidases
- Esterases
- Alkaline phosphatases
Hydrolysis reactions often lead to activation of a drug i.e. prodrugs (e.g. hydrolysis of aspirin by an esterase Ibn the plasma to form active salicylic acid)
