Detoxification
Chemicals
entering body
(mostly via food)
must pass
through liver




Principle of detoxification
- elimination of hydrophobic compounds from body
- formation of polar / soluble products
Two principal phases (phase I & II)
- well studied in vertebrates (mammals)
- liver: major organ involved in detoxification
- plants: similar oxidating enzymes:
cytochrom oxidase, phenol oxidase, peroxidase
Phase III - elimination - both from cell & body
Detoxification

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MFO enzymes
(mixed function oxidase, mixed function oxygenase)
- membrane enzymes bound to Endoplasmic reticulum
- membrane vesicles "microsomes" = S-9 fraction can be extracted from cells
MFO: principle enzymes: cytochromes P450 (CYPs)
- haem-containing enzymes
(superfamily of more than 150 genes)
- several classes and subclasses
(different substrate specificity; structure ...)
Cytochrome P450 1A (CYP1A)
– basic for detoxification of hydrophobic environmental contaminants
Cytochrome P450 19A (CYP19)
– "aromatase" involved in synthesis of estradiol (aromatization of testosterone)
Phase I










CYPs - example: steroid hormone synthesis



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CYPs & Phase I of detoxification - major reactions
oxidation
hydrolysis
(reductions and others)

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Conjugation reactions:
reactive xenobiotics or metabolites formed in phase I
+
endogeneous substrates
- saccharides and their derivatives – glucuronic acid, - aminoacides (glycine)
- peptides: glutathione (GSH)
Phase II enzymes:
glutathion S-transferase (GST)
epoxid hydrolase (EH)
UDP-glucuronosyltransferase (UDP-GTS)
sulfotransferase (ST)
-
+ Excretion of conjugates in urine, sweat or bile
Phase II




Glutathione:
- major donor of SH (thiol) groups in cells (MW ~ 300 g/mol)
- concentrations ~ 5 mM (1.5 g/L)

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Phase  III - transporters
ATP-binding cassette transporters
(ABC transporters)
- protein superfamily (one of the largest, and most ancient in all extant phyla from prokaryotes to
humans)
- transmembrane proteins - transport across extra- and intracellular membranes (metabolic products,
lipids, sterols, drugs)

Phase  III - transporters
- MRP (MDR) - multidrug resistance-associated protein family
- OATP: Organic anion transporting polypeptide
- P-glycoprotein
- …  many others

Phase  III - transporters



Detoxification enzymes
may be induced by substrates
- CYP1A – induction via AhR
-Substrate: hydrophobic organochlorine compounds
(PCDDs/Fs, PAHs PCBs ...)
[see also: lectures on nuclear receptors]
-
- Other CYPs - substrate-induced
- Phase II enzymes - by reactive toxicants
- ABC transporters - by respective chemicals

AhR dependent CYP1 induction



Induction of detoxication enzymes
-> increased energetic demand (ATP, metabolism)
-> may lead to resistance to toxic compounds
-> activation of pro-mutagens/pro-carcinogens
-> increase of oxidative reactions
production of Reactive Oxygen Species (ROS)
[see oxidative damage and stress lectures]
-> side toxic effects [see nuclear receptor lectures]
- increased degradation of endogeneous compounds
(retinoids – regulatory molecules degraded by CYP1A
- crosstalk with other mechanisms & receptors

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Activation of promutagens by CYPs
Benzo[a]pyrene




Aflatoxin B1