L31. Drug Detoxification Pathways Flashcards
What kind of solubility do most things require in order to be excreted successfully by the body?
They need to be water soluble
What is a xenobiotic? How have they and humans evolved. What kind of solubility do they have?
They are molecules that are completely foreign to the body.
Most have evolved by plants and other animals to prevent being eaten by prey and humans. Humans have thus evolved mechanisms (CYP) to combat and detoxify them.
They are all insoluble/hydrophobic
Where is the major detoxification system for xenobiotics? What is it called?
Cytochrome p450 and it is in the liver
What are the 2 phases of the cytochrome p450 pathway?
- Detoxification in a CYP dependent manner by several reactions to make it more soluble
- Cyp independent: addition of a sugar group to make it even more soluble and able to be secreted
What are the typical reactions in the cytochrome p450 phase 1 step?
Hydroxylation, epoxidation, dealkylation and oxidation
= makes the toxin more soluble essentially by adding an OH group to it
RH+ NADPH+ H+ + O2 → ROH + H2O + NADP+
What are some examples of xenobiotics?
Caffeine: a methylxanthine psychoactive drug
Theobromine & Theophylline: bitter alkaloids of the cacao and tea plants
Limonene: a cyclic terpene - Oranges and tangerines
Indole-3-carbinol: found in broccoli, cabbage, brussels sprouts and kale
Dioxins (Eg. PCDDs): polycyclic hydrocarbons - environmental pollutants
DDT: organochloride – insecticide
Phthalate esters: plasticisers
There are 49 different cytochrome p450 enzymes in the human species. Within these there are 17 families. Describe the nomenclature
CYP 2B6
Cytochrome of family 2
Subfamily B
Isoform 6
Describe caffiene metabolism by the CYP pathway
CYP 1A2 and other cytochrome enzymes demehtylate and hydroxylate the caffiene and makes it soluble enough for excretion in the urine
What is the most important CYP in humans in terms of drug metabolism?
CYP 3A4
What is the most important CYP in humans in terms of polymorphism (high range of diversity) and metabolism in general?
CYP 2D6
What is meant by CYP polymorphisms?
Within the isoforms of different CYPs there are subsets or polymorphisms that vary in the population (ie. different people have different activity of different CYPs) - this enables the diversity in drug reactions
CYP 3A4 (and others) are inducible. What is meant by this?
On exposure to the appropriate substrates, these CYPs can be activated and expressed. Thus in one individual, the profile of CYPs is in variation
What is the main organelle in the hepatocytes for CYP mediated detoxification?
The endoplasmic reticulum
How are CYPs studied biochemically?
They are enriched into microsomes
Where did the name cytochrome p450 originate from?
The molecule has an absorbance spectrum of 450nm and the haem porphyrin group has a pink pigment (P)
What are the major components of the cytochrome p450 complex?
- Active site (specific for the xenobiotic substrate)
- Haem prosthetic group with an iron molecule
- Cysteine anchor (CH2-SH) that anchors the Fe to the haem group
- Hydrophobic protein “foot” that anchors the complex to the membrane
What is the major difference between the haem group haemoglobin and that of cytochrome p450?
The Fe was anchored to Hb haem by His F8 residue while the cytochrome p450 by a cysteine anchor.
Where does the energy of the CYP come from?
Cytochrome p450 recieves electrons from associated enzymes called the NADPH CYTOCHROME P450 REDUCTASE via an electron transport chain.
The reductase catalyses transfer of electrons from NADPH to FAD, electrons then move to FMN (called flavins) and then to the haem in cyp450.
Describe the steps in the P450 reaction
- NADPH is oxidised by the reductase releasing 2H and 2e
- These are used by the CYP to reduce molecular oxygen - one of the atoms becomes water with the 2e and one of them becuase a highly reactive oxygen species
- This highly reactive one forces a reaction with the substrate making it soluble
How can Cyp450 generate toxins?
Give an example
Some xenobiotics and molecules are non-toxic in the body until CYP acts on them and makes them toxic.
Aspergillus toxinis acted on by CYP - the result aflatoxin is highly toxic and intergrates with the DNA (mutagen)
What proportion of the drugs taken by the body are detoxified by the liver?
Over 75%
What are the 4 different types of metabolisers caused by the different CYP isoforms?
- Poor metabolisers (PM) = homozygous for one deficient allele or heterozygous for two different deficient alleles thus can’t metabolise
- Intermediate metabolisers (IM) = heterozygous for one deficient allele or carry two alleles that cause reduced activity
- Extensive metabolisers (EM), who have two wild-type alleles (55%) – these are “normal”
- Ultrarapid metabolisers (UM) = multiple gene copies and thus have more active CYPs
Cyp2d6 has many polymorphisms in the body. Explain how this impacts codeine metabolism in CYP2D6 deficient people
Morphine is a prodrug that requires metabolism by the CYP 2D6 pathway into codeine in order to have an effect.
PM (CYP D26 deficient) = get no drug relief from codeine because it stays as morphine in the body
What happens with CYP2D6 deficiency in drugs that use this pathway?
These drugs aren’t metabolised and thus reach much higher concentrations in the active state. Thus in poor metabolisers = toxic side effects
What happens to hyperactive/ultra rapid metabolisers for most drugs that use the pathways?
Hyperactive CYPS (ultra rapid metabolisers) can rapidly destroy some drugs leading to an infective dose in a too low concentration.
What is personalised medicine?
CYP genotyping to personalise drug dosage and medications
Describe the impact of some drugs that can inhibit CYPs (drug-drug interactions), give an example
Some drugs can inhibit CYPs making the extensive metabolisers (normal) appear as “poor metabolisers” because their activity of CYP is impacted by the drug.
Eg. Prozac can inhibit tricyclic antidepressants
Some drugs compete for certain CYPs for metabolism. What is the effect of this?
Slows metabolism of the other drug (the one with less affinity ends up having a higher action in the body as it can’t be metabolised)
What is the effect of grapefruit on CYP3A4?
Grapefruit has furanocoumarin which uses the CYP3A4 pathway. Thus other drugs can be poorly metabolised in the presence of this
Give examples of inducing CYPs
Drugs and other xenobiotics can induce specific forms of CYP450 - Eg. Dioxin (environmental pollutant) induces CYP1A; phenobarbital induces CYP2B2 & CYP3A1
Cruciferous vegetables: brussels sprouts, Kale, cabbage etc. - induce the expression of CYP1A2 May decrease bioavailability of drugs such as haloperidol (antipsychotic)
How does dioxin (environmental pollutant) induce CYP 1A2?
Dioxin binds to hydrocarbon in the cytosol
This translocates to nucleus and binds to response elements and this upregulates expression of the cytochrome.
This is beneficial to removing the toxin but also upregulates the action.
What are some other endogenous functions of the CYPs?
- metabolism of vascular autoregulation (esp in the brain)
2. formation of cholesterol, steroids and arachadonic acid metabolites
