METABOLISM /

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METABOLISM / BIOTRANSFORMATION of TOXICANTS
Wongwiwat Tassaneeyakul Department of
Toxicology Khon Kaen University
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A D M E
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Hydrophillic cpds
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XENOBIOTIC METABOLISM

• Toxicants (xenobiotics) catalyze by enzymes to
  form metabolite (s) with modified structure
• Several routes of metabolism found in vivo
• May inactivate or bioactivate action
• Liver is the major site of metabolism
• Genetically variation with some enzymes
• Not constant - can be changed by other chemicals
  basic of many interactions

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(Parkinson 2001)
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XENOBIOTIC METABOLISM
metabolism is what the body does to the
toxicants ..

• Toxicants may converted to
• Less toxic chemicals (metabolite) , or
• More toxic chemicals (metabolite) , or
• Chemicals with different type of effect or
  toxicity

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Sites of Metabolism

• where ever appropriate enzymes occur
• plasma,
• kidney,
• lung,
• gut wall and LIVER
• the liver is ideally placed to intercept natural
  ingested toxins (bypassed by injections etc) and
  has a major role in biotransformation

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The Liver
Hepatocytes (liver cells)
smooth endoplasmic reticulum
bile
portal venous blood
Feces
microsomes
contain cytochrome P450 (CYP)
systemic arterial blood
venous blood
Urine, Expiration
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Lipophilic toxicant
Biotransformation
Hydrophilic metabolite
Metabolite excreted
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Types of Biotransformation Reaction
Any structural change in a molecule Phase I -
creates site for phase II oxidation (adds O)
e.g. microsomes reduction hydrolysis (e.g.
by plasma esterases) others Phase II - couples
group to existing (or phase I formed)
conjugation site glucuronide (with glucuronic
acid) sulphate others
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Xenobiotic-Metabolizing Enzymes (XME)
Phase 1 P450s Flavin-containing monoxygenases
(FMO) Epoxide hydrolases Phase 2
Transferases Sulfotransferases
(ST) UDP-glucuronosyltransferases
(UGT) Gluthione S-transferases (GST)
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Relative abundant of human XMEs
Phase II
Phase I
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PHASE 1 reactions
Hydroxylation -CH2CH3 -CH2CH2OH
Oxidation -CH2OH -CHO -COOH
N-dealkylation -N(CH3)2 - NHCH3
CH3OH
Oxidative deamination -CH2CHCH3
-CHCOCH3 NH3


NH2
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Phase I Enzymes

• Cytochromes P450
• Flavin Containing Monooxygenase
• Epoxide Hydrolase
• Alcohol /Aldehyde Dehydrogenases
• Monoamine Oxidases
• Xanthine oxidase

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Cytochromes P450 (CYP)

• Most importance enzyme in xenobiotic metabolism
• Membrane bound enzyme locate in smooth
  endoplasmic reticulum membrane
• All require NADPH and O2
• Divided to Family Subfamily Isoform
• CYP1, CYP2, CYP3 involved in the metabolism of
  xenobiotic

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CYP 102 (Graham Peterson 1999)
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Reduced carbon monoxide difference spectrum of
cytochrome P450
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Inducibility of CYP
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(No Transcript)
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(Parkinson 2001)
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Content of CYP in human liver
CYP2D6
CYP2A6 2B6
CYP1A2
CYP3A
CYP2E1
CYP2C
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Phase I in Action
Imipramine
N
4-hydroxy imipramine (cardiotoxic)
CH2 CH2 N CH3 CH3
desmethyl imipramine (antidepressant)
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Cytochrome P450 dependent Mixed Function Oxidases
DRUG
METABOLITE
DRUGO
O2
Liver microsomes
NADP
NADPH
WATER
H
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Other (non-microsomal) Phase I reactions

• Hydrolysis in plasma by esterases (suxamethonium
  by cholinesterase)
• Alcohol and aldehyde dehydrogenase in liver
  cytosolic (ethanol)
• Monoamine oxidase in mitochondria (tyramine,
  noradrenaline, dopamine, amines)
• Xanthine oxidase (6-mercaptopurine, uric acid
  production)
• Enzymes for particular substrates (tyrosine
  hydroxylase, dopa-decarboxylase etc.)

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PHASE 2 Reactions

• CONJUGATIONS
• -OH, -SH, -COOH, -CONH with glucuronic acid to
  give glucuronides
• -OH with sulphate to give sulphates
• -NH2, -CONH2, amino acids, sulpha drugs with
  acetyl- to give acetylated derivatives
• -halo, -nitrate, epoxide, sulphate with
  glutathione to give glutathione conjugates
• all tend to be less lipid soluble and therefore
  better excreted (less well reabsorbed)

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Cofactors for Phase II
(Source Parkinson 2001)
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BIOACTIVATIONS
(Source Parkinson 2001)
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(Source Parkinson 2001)
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Factors Affecting Metabolism

• Age (reduced in aged children)
• Sex (women more sensitive to ethanol)
• Species (phenylbutazone 3h rabbit, 6h horse, 8h
  monkey, 18h mouse, 36h man)
• Race (fast and slow isoniazid acetylators, fast
  95 Eskimo, 50 Brits, 13 Finns 13 Egyptians)
• Clinical or physiological conditions

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AGE

• Fetus Neonate
• Lower drug metabolizing capacity compare to adult
• Sensitive to drug,
• Long duration of action
• Glucoronosyltransferase (UDPGT)

Enzyme activity
Birth
Adult
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SPECIES

• Rate of Hexobarbital metabolism

Coumarin 7-Hydroxylation found in only human,
rabbit, cat but not in rat, mice Therefore
animal used for toxicity test must has the drug
metabolism process similar to human
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GENETICS

• Activity of xenobiotic metabolizing enzymes can
  be vary between individual
• Population can be divided to RAPID METABOLIZER
  and SLOW METABOLIZERS

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DISEASES

• Liver diseases reduced drug metabolism
• cirrhosis
• Acute alcohol exposure chronic ethanol exposure
  alcoholic cirrhosis
• Endrocrine disorders diabetes

Hormones
Thyroid hormone Insulin Pituitary hormone Sex
hormones
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FOOD

• Nutrients
• Protein
• Cabohydrate
• Fat
• Vitamins

• Non-nutrients
• Pyrolysis products
• some chemical in plants Indole compounds
  (Cabbage/ Brussel sprout)

PYROLYSIS PRODUCTS Break down products of amino
acid after over cook meat at high temp
(fried/charcoal -broiled) Induce drug
metabolizing enzymes P450s and
UDPGT Mutagenecity/ Carcinogenicity
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Chemicals/ Drugs

• INHIBITORS cimetidine
• prolongs action of toxicants or inhibits action
  of those biotransformed to active agents
  (pro-drugs)
• INDUCERS barbiturates, carbamazepine
• shorten action of toxicants or increase effects
  of those biotransformed to active agents