Snímek 1

Snímek 1

BIOMARKERS AND TOXICITY MECHANISMS 06 Mechanisms Metabolism & Detoxification Ludk Blha, PF MU, RECETOX www.recetox.cz Metabolism and detoxification

Chemicals enter body ... mostly via food Pass directly through liver main metabolism organ Detoxification

Basic principle of detoxification elimination of hydrophobic compounds from body formation of more polar & soluble products Two principal phases in metabolism (Phase I & II) well studied in vertebrates (mammals) liver: major organ involved in detoxification Plants

similar oxidating enzymes as described (cytochrom oxidase, phenol oxidase, peroxidase...) Phase III - elimination - both from cell & body Importance of nutrients and vitamins in detoxification Phase I

Key enzymes MFOs = mixed function oxidases / oxygenases Membrane bound to Endoplasmic Reticulum membrane vesicles "microsomes" = S-9 fraction can be extracted from cells

S9 microsomes used for in vitro metabolization (e.g.during genotoxicity testing) 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 functions Cytochrome P450 1A (CYP1A)

basic for detoxification of hydrophobic environmental contaminants Cytochrome P450 19A (CYP19) "aromatase" involved in synthesis of estradiol (aromatization of testosterone) DME = Drug Metabolism Enzymes CYPs and their functions

Types of reactions catalyzed by CYPs (and Phase II enzymes) Highlighted = will be discussed also later CYPs - example: steroid hormone synthesis Cholesterol CYP11A

Pregnenolone CYP17 17-OHPregnenolone 3-HSD

Progesterone CYP21 11-Deoxycorticosterone CYP11B2

Corticosterone CYP11B2 Aldosterone Zona glomerulosa CYP17

3-HSD 17-OHProgesterone CYP21 11-Deoxycortisol CYP11B1

Cortisol Zona fasciculata CYP17 DHEA

3-HSD CYP17 Androstene -dione 17-HSD Testosterone

CYP19 17-Estradiol Zona reticularis CYP450 overview

Hydroxylation (oxidation) mechanism key in detoxification Examples of CYP mediated reactions Examples of CYP mediated reactions Examples of CYP mediated reactions

CYPs and BIOACTIVATION pro-mutagen (procarcinogen) mutagen (carcinogen) Benzo[a]pyrene CYPs and BIOACTIVATION of procarcinogen CYPs and BIOACTIVATION AFLATOXIN-A

CYPs and BIOACTIVATION ethanol CYPs and toxicity of drugs Example - PARACETAMOL toxicity

Detoxification Phase II Key reactions = conjugations Reactive xenobiotics or metabolites formed in phase I with endogeneous substrates saccharides 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) Glutathione

- 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) Examples of conjugation reactions Xenobiotic conjugations with GSH Phase III elimination / membrane transport

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) ABC transporters - examples

- MRP (MDR) - multidrug resistance-associated protein family - OATP - Organic Anion Transporting Polypeptide - P-glycoprotein ABC one of the resistance mechanisms of tumour cells to anticancer drugs

ABC one of the resistance mechanisms of bacteria to antibiotics Constitutive vs Induced detoxification enzymes Detoxification enzymes expression Constitutive low background levels (always present) May be induced - by substrates CYP1A 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 classes Phase II enzymes

Substrates = reactive toxicants, metabolites from Phase I ABC transporters Induction by respective chemicals (drugs etc) CYP1A induction role of AhR Summary toxic consequences of detoxification

BIOACTIVATION activation of pro-mutagens/pro-carcinogens etc. increasing side adverse effects of certain drugs Increase in oxidative reactions oxidative stress production 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) depletion chronic inductions of detox enzymes permanent extra energetic demand

Development of resistance to toxic compounds Loss of efficiency of anticancer drugs, antibiotics etc.

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