Pathways of Xenobiotic metabolism -L12 Flashcards
What reactions are part of phase 1 metabolism ?
oxidation reduction hydrolysis hydration dethioacetylation isomerisation These are all functionalization reactions, making the molecules more reactive Ideally this metabolism prepares the compound for phase 2 conjugation before excretion
What reactions are part of phase 2 metabolism ?
glucuronidation sulfation methylation acetylation amino acid conjunction glutathione conjunction fatty acid conjunction condensation These are all conjugative reactions
what is metabolism ?
making a substance more water soluble so they are excreted more rapidly
the 2 phases of metabolism dont always necessarily go in order - if chemicals come into contact with enzymes at the right time and in the right conditions then they will react
What is the phase 1 pathway sometimes referred to as?
the activation pathway
Where is the microsomal mixed function oxidase system found?
it is cytochrome p450 dependent and it is present notably within the endoplasmic reticulum of cells within the liver, kidney, lung and intestine
What happens in oxidation reactions ?
initial insertion of oxygen followed by rearrangement and/or decomposition to final products
- this is the most important route due to diversity and importance of reactions catalysed-cytochrome p450 system - it metabolises lots of different small molecules
What is the cytochrome p450 reaction ?
NADPH + H+ + 02 + RH goes to NADP+ + h20 + ROH
- the RH has been hydroxylated to ROH so it is more water soluble
What are cytochrome p450 enzymes like ?
MWt between 45000-55000
haem containing enzymes
family of closely related isoforms in the ER membrane
>50 human cyp450 identified
What cytochrome p450 enzyme is the most prominent in the liver ?
CYP3As quantitatively the major sub-family in human livers - expressed in 30% of total
-CYP3A4 shows considerable inter-individual variation - 1 to 2 orders of magnitude (big difference between individuals), these differences are genetic variation but induction of the enzymes is possible
What are the main CYP450 families?
CYP1= 1a2 CYP2= 2d6, 2c19, 2e1 CYP3= 3a
What are the difference extents of clearance variability between cyp450 enzymes?
1A= about 40 fold 2C= 25-100 fold 2D6= >1000 fold 2E1= about 20 fold 3A4= about 20 fold large differences between individuals for the clearance of drugs and metabolites
What is glutathione-S-transferase?
example of a phase 2 enzyme
causes glutathione conjugation in the liver, kidney, intestine etc
What is glutathione?
its an important protective compound leading to detoxification of many electrophilic xenobiotics
R-CH2-X detoxified by GSH to R-CH2-SG
What characteristic about glutathione-S-transferases is similar to CYP450s?
They also demonstrate inter-individual variation
higher levels of these enzymes may help people remove toxic compounds more easily than those who have lower levels
What type of molecule is GSH?
an endogenous tripeptide
How many different isoforms of glutathione-S-transferase are there in man ?
> 20 consisting of 2 subunits
What are GST polymorphisms?
differential distribution between races
5 gene families : 4 cytosolic= alpha, mu, pi and theta and 2 microsomal
Other than genetic variations how can phase 1 and phase 2 enzymes demonstrate inter-individual variation ?
differences can arise because of induced differences in expression levels
the things you eat can affect your metabolism signature
this can affect your health outcome
What is an individuals metabolic signature?
an individuals exposure to food and environmental compounds leaves a metabolic signature in our body, changing the way we are interacting with our environment
this metabolic signature may serve as a basis for optimising an individuals diet-health relationship
What can contribute to a “metabolic fingerprint map” ?
genetic factors environmental - in utero exposure - lifestyle -diet - eating habits
What correlation is shown between cyp450 enzymes?
levels of activity between different ones shows correlation
if someone has a large amount of one cyp450 then they are likely to have a lower amount of another but this pattern may be opposite in another individual
What effect can a reduced protein diet have on CYP450s?
reduced CYP3A
What effect can vitamin A and metabolites have on CYP450s?
decreased or increased CYP3A
What effect can deficiencies in Fe, Ca, Mg, Zn, Cu, Se, I have on CYP450s?
mostly cause reduced drug metabolism
What effect can a pyrolysis products in your diet have on CYP450s?
increased CYP1A1
What effect can indole from brassicas have on CYP450s?
increased CYP1A2 or increased glucuronidation
What effect can grapefruit juice in your diet have on CYP450s?
increased CYP1A2, CYP3A4 and CYP2A6
- can have a very strong effect and therefore sometimes people are not allowed to take it if they are taking certain drugs
What effect can flavanoids, diallyl sulphide (garlic) and curcumin have on CYP450s?
decreased p450s
What are phase 1 and 2 enzymes unfortunately unable to do ?
cannot tell the difference between beneficial molecules such as vitamins and toxins such as carcinogens
What is aflatoxin?
mycotoxins that are TOXIC, CARCINOGENIC, MUTAGENIC and TERATOGENIC -particularly to LIVER
- weakly carcinogenic because they can intercalate between the DNA base pairs
its an accepted human carcinogen
Why is aflatoxin B1 strongly carcinogenic?
because it can be metabolised to the epoxide form which can covalently link directly to DNA -metabolised by phase 1 enzymes
the epoxide is formed by a oxygen molecule forming across the double bond
heptocarcinogenic mycotoxin
What are the different aflatoxin molecules ?
B1, B2, G1 and G2
B1 and B2 differ by presence of a double bond
Gs differ from Bs due to a 6 helical ring
What organisms are aflatoxins particularly dangerous ?
poultry and fish but appear to be less damaging to humans
Why are there not many directs carcinogens in food?
because lots of the food we eat contains DNA
What is the critical product of aflatoxin B1?
the exo-epoxide
all other products formed from oxidation are at least partially detoxified
What is the half life of the exo-epoxide and what does it mean?
half life= only 1 second
however
the binding and intercalation with DNA is so efficient that a high yield of DNA adducts is obtained
it is still stable enough to migrate from the ER to the DNA in the nucleus
What % of hepatocellular carcinoma is caused by AFB1?
about 50% in parts of the world where AFB1 contaminates food - many showing mutations in codon 248 of p53 gene
transversion of G to T in 3rd position of codon
Why cant aflatoxin produce aspergillus flavus mold in this country?
because the weather isn’t hot enough and humid enough at the same time
Why is it important to understand phase1 and phase2 enzymes?
because these enzymes might be important in food-related cancer - so understanding them might help us protect ourselves against carcinogens such as aflatoxin
Why is the balance between producing the phase 1 epoxide of aflatoxin and the phase 2 producing glutathione conjugate important?
it is a complicated relationship because if you wanted to enhance the beneficial phase 2 component then you would need to increase the dangerous phase 1 component
