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METABOLISM /
BIOTRANSFORMATION
of TOXICANTS
Wongwiwat Tassaneeyakul
Department of Toxicology
Khon Kaen University
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Dosage
Plasma
Concen.
Pharmacokinetic
Toxicokinetics
Site
of
Action
Effects
Pharmacodynamics
Toxicodynamics
ADME
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สารเคมี
(Xenobiotics)
Highly lipophillic
compounds
Lipophillic compounds
สะสมในเซลล์
ไขมัน
Phase I biotransformation
(oxidation, reduction,
hydrolysis)
Polar compounds
Phase II biotransformation
(conjugation)
ถูกขับออกจากร่างกาย
(Renal excretion, Biliary excretion)
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Hydrophillic cpds
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
1.
Less
toxic chemicals (metabolite) , or
2. More toxic chemicals (metabolite) , or
3. Chemicals with
or toxicity
different type of effect
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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)
portal
venous
blood
smooth
endoplasmic
reticulum
microsomes
systemic
arterial
blood
bile
Feces
contain cytochrome P450
(CYP)
venous blood
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Urine,
Expiration
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
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PHASE 1 reactions
Hydroxylation -CH2CH3
Oxidation -CH2OH
-CHO
N-dealkylation -N(CH3)2
Oxidative deamination
-CH2CH2OH
- NHCH3 + CH3OH
-CH2CHCH3
|
NH2
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-COOH
-CHCOCH3 + NH3
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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(Parkinson 2001)
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Content of CYP in human liver
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Phase I in Action
Imipramine
4-hydroxy
imipramine
(cardiotoxic)
N
CH2
CH2
N
CH3 CH3
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desmethyl
imipramine
(antidepressant)
Cytochrome P450 dependent
Mixed Function Oxidases
DRUG
O2
NADPH
METABOLITE
=DRUG+O
Liver
microsomes
NADP+
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
CP
Fetus & Neonate :
UDPGT
Lower drug metabolizing capacity
compare to adult
Sensitive to drug,
Long duration of action
Conjugated-CP
Glucoronosyltransferase (UDPGT)
Excretion
GREY BABY SYNDROME
(toxicity of chloramphenicol
in newborn baby)
Volmiting
irregular & rapid respiration
Cyanosis
Birth
Adult
SPECIES
Rate of Hexobarbital metabolism
Species
Mice
Rat
Dog
Sleeping Time (min)
12
90
315
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
GENETICS
Activity of xenobiotic metabolizing enzymes
can be vary between individual
Population can be divided to RAPID
METABOLIZER and SLOW METABOLIZERS
Variation in toxicant metabolism
Variation of toxicant level in the body
Variation of toxicant response/toxicity
DISEASES
Liver diseases : reduced drug metabolism
 cirrhosis
 Acute alcohol exposure chronic ethanol
exposure alcoholic cirrhosis
Endrocrine disorders : diabetes
Hormones
Thyroid hormone
Pituitary hormone
hormones
Insulin
Sex
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
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