RECETOX wwwrecetoxcz BIOMARKERS AND TOXICITY MECHANISMS 06 Mechanisms Metabolism amp Detoxification Metabolism and detoxification Chemicals enter body mostly via ID: 655243
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Luděk Bláha, PřF MU, RECETOXwww.recetox.cz
BIOMARKERS AND TOXICITY MECHANISMS06 – Mechanisms Metabolism & DetoxificationSlide2
Metabolism and detoxificationChemicals enter body ... mostly via foodPass directlythrough liver main metabolism organSlide3
DetoxificationBasic principle of detoxification elimination of hydrophobic compounds from body formation of more polar & soluble productsTwo principal phases in metabolism (
Phase I & II)well studied in vertebrates (mammals) liver: major organ involved in detoxificationPlantssimilar oxidating enzymes as described (cytochrom oxidase, phenol oxidase, peroxidase...)
Phase
III
- elimination - both from cell & bodySlide4
Importance of nutrients and vitamins in detoxificationSlide5
Phase I Key enzymes – MFOs = mixed function oxidases / oxygenasesMembrane bound to Endoplasmic Reticulummembrane vesicles "microsomes" = S-9 fraction can be extracted from cells
S9 microsomes used for in vitro metabolization(e.g.during genotoxicity
testing)Slide6
Detoxification - Phase I Key principle enzymes are cytochromes P450 (CYPs) Haem (porfyrin) - containing enzymes superfamily of more than 150 genes
- several classes and subclasses different substrate specificity; structure ...Some examples ... Diverse functionsCytochrome P450 1A (CYP1A) basic for detoxification of hydrophobic environmental contaminantsCytochrome P450 19A (CYP19) "aromatase" involved in synthesis of estradiol
(aromatization of testosterone)
DME
= Drug Metabolism EnzymesSlide7
CYPs and their functionsSlide8
Types of reactions catalyzed by CYPs (and Phase II enzymes)
Highlighted = will be discussed also laterSlide9
CYPs - example: steroid hormone synthesis Slide10
CYP450 overviewSlide11
Hydroxylation (oxidation) mechanism – key in “detoxification” Slide12
Examples of CYP mediated reactionsSlide13
Examples of CYP mediated reactionsSlide14
Examples of CYP mediated reactionsSlide15
Benzo[a]pyreneCYPs and BIOACTIVATION
pro-mutagen (procarcinogen) mutagen (carcinogen)Slide16
CYPs and BIOACTIVATION of procarcinogenSlide17
CYPs and BIOACTIVATION – AFLATOXIN-ASlide18
CYPs and BIOACTIVATION – ethanolSlide19
CYPs and toxicity of drugsExample - PARACETAMOL toxicitySlide20
Detoxification – Phase IIKey reactions = conjugationsReactive xenobiotics or metabolites formed in phase I with endogeneous substratessaccharides and their derivatives – glucuronic acid, aminoacids
(glycine)peptides: glutathione (GSH)Forming water soluble AND “nontoxic” products (conjugates)Phase II enzymes (“transferases”):glutathion S-transferase (GST)UDP-glucuronosyltransferase (UDP-GTS)epoxid hydrolase
(EH
)
sulfotransferase
(ST)Slide21Slide22
major donor of SH (thiol) groups in cells (MW
~ 300 g/mol)- concentrations in tissues and blood up to 5 mM (1.5 g/L)GlutathioneSlide23
Examples of conjugation reactionsSlide24
Xenobiotic conjugations with GSHSlide25
Phase III – elimination / membrane transportPhase 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)Slide26
- MRP (MDR) - multidrug resistance-associated protein family-
OATP - Organic Anion Transporting Polypeptide- P-glycoproteinABC transporters - examplesSlide27
ABC one of the resistance mechanisms of tumour cells to anticancer drugsSlide28
ABC one of the resistance mechanisms of bacteria to antibiotics Slide29
Constitutive vs Induced detoxification enzymesDetoxification enzymes expressionConstitutive – low background levels (always present)May be induced - by substratesCYP1A – induction via Ah-receptor (
AhR)Substrate: hydrophobic organochlorine compounds (PCDDs/Fs, PAHs PCBs ...)[see also: lectures on nuclear receptors]Other CYPs Drugs inductions of specific CYP classesPhase II enzymes Substrates = reactive toxicants, metabolites from Phase I
ABC
transporters
Induction by
respective
chemicals (drugs etc)Slide30
CYP1A induction – role of AhRSlide31
Summary – “toxic consequences” of detoxificationBIOACTIVATION activation of pro-mutagens/pro-carcinogens etc.increasing side adverse effects of certain drugs Increase in oxidative reactions – oxidative stressproduction of Reactive Oxygen Species (ROS)(see oxidative damage and stress
lectures)Side toxic effects (see nuclear receptor lectures)e.g. increased degradation of endogeneous compounds (retinoids – regulatory molecules degraded by CYP1A Crosstalk with other mechanisms & receptors Energy (ATP) depletionchronic inductions of detox enzymes
permanent extra
energetic
demand
Development of resistance
to toxic
compounds
Loss of efficiency
of anticancer drugs, antibiotics etc.