124 - Drug Detoxification Pathways

Question 1

Type of drug that caffeine is

Answer 1

Methylxanthine psychoactive drug

Question 2

Reason for having CYPs
1
2
3

Answer 2

1) Plants have generated alkaloids and other toxins to avoid being eaten by animals.
2) Urinary, digestive systems are good at removing water-soluble compounds, but not at hydrophobic compounds.
3) Animals developed a way to metabolise xenobiotics (hydrophobic toxins) to make them more soluble, excretable by kidneys.

Question 3

CYP-dependent stage of hepatic detoxification

Answer 3

Phase I detoxification

Question 4

Phase I detoxification

Answer 4

• CYPs catalyse a range of reactions: hydroxylation,
  epoxidation, dealkylation & oxidation
• => makes toxins more soluble

Question 5

Typical phase I reaction

Answer 5

• A typical cytochrome P450 catalysed reaction is:

RH + NADPH + H+ + O2 → ROH + H2O + NADP+

Question 6

Phase II detoxification

Answer 6

Addition of a sugar (eg glucuronate) to the –OH group makes the foreign molecule even more soluble.
CYP-independent

Question 7

Number of CYPs

Answer 7

Thousands, in ~74 families

Question 8

Number of human CYPS

Answer 8

49 CYPs from 17 families

Question 9

Subfamilies of CYPs

Answer 9

A, B, C, D, E

Question 10

Naming of CYPs

Answer 10

CYP - (family), (subfamily), (form)

EG: CYP 2B6 (family 2, subfamily B, form 6)

Question 11

Example of CYP involved in caffeine metabolism

Answer 11

CYP1A2

Question 12

Most important CYPs in humans

Answer 12

CYP3A4

CYP2D6

Question 13

Most-important CYP in drug metabolism

Answer 13

CYP3A4

Question 14

Most-important CYP in terms of polymorphisms

Answer 14

CYP2D6

Question 15

Significance of CYP2D6 polymorphisms

Answer 15

Metabolise drugs differently.

Question 16

Example of an inducible CYP

Answer 16

CYP3A4

Question 17

Main organ for CYP-mediated detoxification

Answer 17

Liver

Question 18

Organelle involved in CYP detoxification

Answer 18

Endoplasmic reticulum

Question 19

How can CYPs be enriched for biochemical studies?

Answer 19

Microsomes (pink ER-derived vesicles)

Question 20

Structure in all CYPs

Answer 20

Haem

Question 21

What does ‘P450’ mean?

Answer 21

P = pigment.

Carbon monoxide complex has a max wavelength of 450nm

Question 22

Things that cytochromes P450 have
1
2
3

Answer 22

1) Active site that is specific for a particular class of substrates
2) A heam prosthetic group that alternates between Fe2+ and 3+
3) A cysteine anchor from the protein that forms a ligand to the Fe in haem

Question 23

What changes charge of Fe in haem prosthetic group in CYPs?

Answer 23

Donation of e- to oxygen

Question 24

What feeds e- to CYPs?

Answer 24

NADPH

Question 25

How are e- transferred from NADPH to CYP?

Answer 25

In the ER the enzyme is NADPH cytochrome P450 reductase

- catalyses the transfer of electrons from NADPH to FAD to FMN and then to heme in CYP450

Question 26

P450 reaction
1
2
3

Answer 26

Step 1 - NADPH is oxidised by cytochrome P450 reductase (releases H+ and 2 e-)

Step 2 - The H+ and 2 e- are used by CYP to reduce one of the two atoms of molecular oxygen to water.

Step 3 - The other oxygen atom is retained in a highly reactive form, and used to force a reaction (eg hydroxylation) on the substrate.

Question 27

CYP toxin generation

Answer 27

Amine oxidation, aldehyde formation, epoxide formation can yield reactive groups that can be toxic

Question 28

Example of CYP toxin generation 
1
2
3
4

Answer 28

1) Aspergillus fungus on corn and peanuts makes aflatoxin,
2) Converted to aflatoxin B1 epoxide by P450
3) Reacts with DNA
4) Mutagen

Question 29

Proportion of administered drugs oxidised by CYP450 system

Answer 29

~50%

Question 30

Classifications of metabolism 
1
2
3
4

Answer 30

1) Poor metabolisers (PM), who are homozygous for one deficient allele or heterozygous for two different deficient alleles (4% of caucasians)
2) Intermediate metabolisers (IM), who are heterozygous for onedeficient allele or carry two alleles that cause reduced activity (32%);
3) Extensive metabolisers (EM), who have two wild-type alleles (55%);
4) Ultra rapid metabolisers (UM), who have multiple gene copies (9 %).

Question 31

CYP2D6 problems

Answer 31

Inactive in ~5-10% of the population.
Can fail to activate prodrugs, can result in high concentrations of drug if drug isn’t cleared properly.
If a hyperactive metaboliser, drugs are ineffective because metabolised too quickly.

Question 32

Example of a drug that CYP2D6 can alter the metabolism of

Answer 32

Codeine.
Codeine has no intrinsic analgesic activity unless it is converted to morphine by CYP2D6 activity.
Poor metabolisers with impaired CYP2D6 activity get little relief from codeine.

Question 33

Example of a drug that inhibits some CYPS

Answer 33

Some SSRI medications (contraindicated with other drugs metabolised by CYP2D6, eg tricyclics, antipsychotics)

Question 34

How do some drugs induce CYPs?
1
2
3
4

Answer 34

1) Receptor in cytoplasm bound
2) Complex translocates to the nucleus.
3) Binds Xenobiotic response element
4) Upregulates a particular CYP

Question 35

Ways to measure CYP450 effects in vitro
1
2
3

Answer 35

1) Microsomes used
2) Can use liver microsomes of Supersomes
3) Identify possible problems with differential rates of metabolism and drug-drug interactions

Question 36

Other functions of CYPs
1
2

Answer 36

1) Involved in vascular autoregulation, particularly in the brain
2) Involved in the formation of cholesterol, steroids and arachidonic acid metabolites (mostly in the mitochondria, in adrenals)

Question 37

Normal level of CYP metabolism

Answer 37

Extensive metabolisers