L31. Drug Detoxification Pathways

Question 1

What kind of solubility do most things require in order to be excreted successfully by the body?

Answer 1

They need to be water soluble

Question 2

What is a xenobiotic? How have they and humans evolved. What kind of solubility do they have?

Answer 2

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

Question 3

Where is the major detoxification system for xenobiotics? What is it called?

Answer 3

Cytochrome p450 and it is in the liver

Question 4

What are the 2 phases of the cytochrome p450 pathway?

Answer 4

1. Detoxification in a CYP dependent manner by several reactions to make it more soluble
2. Cyp independent: addition of a sugar group to make it even more soluble and able to be secreted

Question 5

What are the typical reactions in the cytochrome p450 phase 1 step?

Answer 5

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+

Question 6

What are some examples of xenobiotics?

Answer 6

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

Question 7

There are 49 different cytochrome p450 enzymes in the human species. Within these there are 17 families. Describe the nomenclature

Answer 7

CYP 2B6

Cytochrome of family 2
Subfamily B
Isoform 6

Question 8

Describe caffiene metabolism by the CYP pathway

Answer 8

CYP 1A2 and other cytochrome enzymes demehtylate and hydroxylate the caffiene and makes it soluble enough for excretion in the urine

Question 9

What is the most important CYP in humans in terms of drug metabolism?

Answer 9

CYP 3A4

Question 10

What is the most important CYP in humans in terms of polymorphism (high range of diversity) and metabolism in general?

Answer 10

CYP 2D6

Question 11

What is meant by CYP polymorphisms?

Answer 11

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

Question 12

CYP 3A4 (and others) are inducible. What is meant by this?

Answer 12

On exposure to the appropriate substrates, these CYPs can be activated and expressed. Thus in one individual, the profile of CYPs is in variation

Question 13

What is the main organelle in the hepatocytes for CYP mediated detoxification?

Answer 13

The endoplasmic reticulum

Question 14

How are CYPs studied biochemically?

Answer 14

They are enriched into microsomes

Question 15

Where did the name cytochrome p450 originate from?

Answer 15

The molecule has an absorbance spectrum of 450nm and the haem porphyrin group has a pink pigment (P)

Question 16

What are the major components of the cytochrome p450 complex?

Answer 16

1. Active site (specific for the xenobiotic substrate)
2. Haem prosthetic group with an iron molecule
3. Cysteine anchor (CH2-SH) that anchors the Fe to the haem group
4. Hydrophobic protein “foot” that anchors the complex to the membrane

Question 17

What is the major difference between the haem group haemoglobin and that of cytochrome p450?

Answer 17

The Fe was anchored to Hb haem by His F8 residue while the cytochrome p450 by a cysteine anchor.

Question 18

Where does the energy of the CYP come from?

Answer 18

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.

Question 19

Describe the steps in the P450 reaction

Answer 19

1. NADPH is oxidised by the reductase releasing 2H and 2e
2. 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
3. This highly reactive one forces a reaction with the substrate making it soluble

Question 20

How can Cyp450 generate toxins?

Give an example

Answer 20

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)

Question 21

What proportion of the drugs taken by the body are detoxified by the liver?

Answer 21

Over 75%

Question 22

What are the 4 different types of metabolisers caused by the different CYP isoforms?

Answer 22

1. Poor metabolisers (PM) = homozygous for one deficient allele or heterozygous for two different deficient alleles thus can’t metabolise
2. Intermediate metabolisers (IM) = heterozygous for one deficient allele or carry two alleles that cause reduced activity
3. Extensive metabolisers (EM), who have two wild-type alleles (55%) – these are “normal”
4. Ultrarapid metabolisers (UM) = multiple gene copies and thus have more active CYPs

Question 23

Cyp2d6 has many polymorphisms in the body. Explain how this impacts codeine metabolism in CYP2D6 deficient people

Answer 23

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

Question 24

What happens with CYP2D6 deficiency in drugs that use this pathway?

Answer 24

These drugs aren’t metabolised and thus reach much higher concentrations in the active state. Thus in poor metabolisers = toxic side effects

Question 25

What happens to hyperactive/ultra rapid metabolisers for most drugs that use the pathways?

Answer 25

Hyperactive CYPS (ultra rapid metabolisers) can rapidly destroy some drugs leading to an infective dose in a too low concentration.

Question 26

What is personalised medicine?

Answer 26

CYP genotyping to personalise drug dosage and medications

Question 27

Describe the impact of some drugs that can inhibit CYPs (drug-drug interactions), give an example

Answer 27

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

Question 28

Some drugs compete for certain CYPs for metabolism. What is the effect of this?

Answer 28

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)

Question 29

What is the effect of grapefruit on CYP3A4?

Answer 29

Grapefruit has furanocoumarin which uses the CYP3A4 pathway. Thus other drugs can be poorly metabolised in the presence of this

Question 30

Give examples of inducing CYPs

Answer 30

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)

Question 31

How does dioxin (environmental pollutant) induce CYP 1A2?

Answer 31

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.

Question 32

What are some other endogenous functions of the CYPs?

Answer 32

1. metabolism of vascular autoregulation (esp in the brain)

2. formation of cholesterol, steroids and arachadonic acid metabolites