124 - Drug Detoxification Pathways Flashcards
Type of drug that caffeine is
Methylxanthine psychoactive drug
Reason for having CYPs
1
2
3
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.
CYP-dependent stage of hepatic detoxification
Phase I detoxification
Phase I detoxification
- CYPs catalyse a range of reactions: hydroxylation,
epoxidation, dealkylation & oxidation - => makes toxins more soluble
Typical phase I reaction
- A typical cytochrome P450 catalysed reaction is:
RH + NADPH + H+ + O2 → ROH + H2O + NADP+
Phase II detoxification
Addition of a sugar (eg glucuronate) to the –OH group makes the foreign molecule even more soluble.
CYP-independent
Number of CYPs
Thousands, in ~74 families
Number of human CYPS
49 CYPs from 17 families
Subfamilies of CYPs
A, B, C, D, E
Naming of CYPs
CYP - (family), (subfamily), (form)
EG: CYP 2B6 (family 2, subfamily B, form 6)
Example of CYP involved in caffeine metabolism
CYP1A2
Most important CYPs in humans
CYP3A4
CYP2D6
Most-important CYP in drug metabolism
CYP3A4
Most-important CYP in terms of polymorphisms
CYP2D6
Significance of CYP2D6 polymorphisms
Metabolise drugs differently.
Example of an inducible CYP
CYP3A4
Main organ for CYP-mediated detoxification
Liver
Organelle involved in CYP detoxification
Endoplasmic reticulum
How can CYPs be enriched for biochemical studies?
Microsomes (pink ER-derived vesicles)
Structure in all CYPs
Haem
What does ‘P450’ mean?
P = pigment.
Carbon monoxide complex has a max wavelength of 450nm
Things that cytochromes P450 have
1
2
3
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
What changes charge of Fe in haem prosthetic group in CYPs?
Donation of e- to oxygen
What feeds e- to CYPs?
NADPH
How are e- transferred from NADPH to CYP?
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
P450 reaction
1
2
3
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.
CYP toxin generation
Amine oxidation, aldehyde formation, epoxide formation can yield reactive groups that can be toxic
Example of CYP toxin generation 1 2 3 4
1) Aspergillus fungus on corn and peanuts makes aflatoxin,
2) Converted to aflatoxin B1 epoxide by P450
3) Reacts with DNA
4) Mutagen
Proportion of administered drugs oxidised by CYP450 system
~50%
Classifications of metabolism 1 2 3 4
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 %).
CYP2D6 problems
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.
Example of a drug that CYP2D6 can alter the metabolism of
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.
Example of a drug that inhibits some CYPS
Some SSRI medications (contraindicated with other drugs metabolised by CYP2D6, eg tricyclics, antipsychotics)
How do some drugs induce CYPs? 1 2 3 4
1) Receptor in cytoplasm bound
2) Complex translocates to the nucleus.
3) Binds Xenobiotic response element
4) Upregulates a particular CYP
Ways to measure CYP450 effects in vitro
1
2
3
1) Microsomes used
2) Can use liver microsomes of Supersomes
3) Identify possible problems with differential rates of metabolism and drug-drug interactions
Other functions of CYPs
1
2
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)
Normal level of CYP metabolism
Extensive metabolisers
