metabolism of xenobiotics xiao li. ※ ※ xenobiotics: is a compound that is foreign to the body ;...
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Metabolism of XenobioticsMetabolism of Xenobiotics
Xiao Li
※Xenobiotics: is a compound that is foreign
to the body ; is a chemical which is found in a
n organism but which is not normally produce
d or expected to be present in body.
IntroductionIntroduction
※Endogenous: Pigments , hormone
※nonendogenous : Such as drugs , food ad
ditives, pollutants, toxin, etc Most of these compounds are subject to
metabolism (biotransformation) in human body.
Definition of theDefinition of the biotransformationbiotransformation
Conversion of lipophilic xenobiotics to water-soluble chemicals by a process catalyzed by enzymes in the liver and other tissues.
In most cases, biotransformation lessens the toxicity of xenobiotics, but many must undergo the process to exert their toxic effects.
1. facilitates excretion: Converts lipophilic to h
ydrophilic compounds
2. Detoxification/inactivation: converts chemic
als to less toxic forms
3. Metabolic activation: converts chemicals to
more toxic active forms
Purpose of biotransformation
Spider lentigoHepatic palm
Chronic hepatitis and cirrhosis of liver
Increase of estrogen, aldosterone and antidiuretin
benzpyrene
Detoxification ≠ biotransformation
3,4-Benzypyrene
Sites of Sites of bbiotransformationiotransformation
Liver– Primary site! Rich in enzymes– Acts on endogenous and exogenous compounds
Extrahepatic metabolism sites– Intestinal wall
Sulfate conjugation Esterase and lipases - important in prodrug metaboli
sm
– Lungs, kidney, placenta, brain, skin, adrenal glands
Knowledge of the metabolism of xenobiotics:
rational understanding of pharmacology and therapeutics , pharmacy, toxicology, cancer research , and drug addiction .
It is convenient to consider the metabolism of xenobiotics in two phases– phase Ⅰand phase Ⅱ
General Metabolic PathwaysGeneral Metabolic Pathways
Approximately 30 different enzymes catalyze reactions involved in xenobiotic metabolism; however, this note will only cover a selected group of them.
Phase I reactionsPhase I reactions
♣♣ Functionalization– Oxidation– Reduction – Hydrolytic reactions
Purpose
Introduction of polar functional groups in a molecules
♣ Increase a molecule’s polarity
♣ Does provide a site for phase II metabolism
Phase II reactionsPhase II reactions
★ Purpose– Introduce highly polar conjugates: ☻☻Glucuronic acid ☻☻ Sulfate
– Detoxification Glycine or other Amino Acids (some solubility),
Acetyl , Methylations , Glutathione
★ Site of attachment often introduced in Phase I
Hydroxyl , Carboxylate , Amino
♣♣ Conjugation
Comparing Phase I & Phase IIComparing Phase I & Phase II
Enzyme Phase I Phase II
Types of reactions HydrolysisOxidationReduction
Conjugations
Increase inhydrophilicity
Small Large
General mechanism Exposes functionalgroup
Polar compound addedto functional group
Consquences May result inmetabolic activation
Facilitates excretion
Phase : OxidationⅠ1. Hydroxylation
RH: drugs, cacinogens, pesticides, petroleum products,
pollutants, steroids, eicosanoids, fatty acids, retinoids, etc. Enzyme:
Cytochrome P450s-dependent monooxygenase
Addition of an oxygen atom or bond
Require NADH or NADPH and O2 as cofactors
RH + O2 + NADPH + H+ R-OH + H2O + NADP+
HydroxylationHydroxylation :: OO22
* It has been shown by the use of O2 that one
atom of oxygen enters R-OH and one atom enters H2O.
* This dual fate of the oxygen accounts for the former naming of monooxygenases as “mixed-function oxidases”.
RH + O2 + NADPH + H+ R-OH + H2O + NADP+
★Microsomal drug oxidations require
cytochrome P450,
cytochrome P450 reductase,
NADPH , & O2
The actual reaction mechanism is as follows:
Cytochrome P450s-dependent monooxygenase
-----the most versatile biocatalyst
----Works on a large number of diverse compounds
CYP or Cytochrome P-450CYP or Cytochrome P-450
Cytochrome P450s-dependent monooxygenase
★ Heme proteins
★ Iron containing porphyrin - binds O2
★ The name cytochrome P450 is derived from the
spectral properties of this hemoprotein in its reduced
(ferrous, Fe2+) form, it binds CO to give a complex that
absorbs light maximally at 450 nm
What is a Heme Protein?
Cytochrome P-450, Hemoglobin, & Myoglobin ALL Heme Proteins!
Cytochrome P-450Cytochrome P-450
Enzymes isolated by disruption of the liver cells– Endoplasmic reticulum - microsomes when disr
upted– Enzymes are membrane bound– Explains why lipophilic drugs are processed
– Catalytic process heme binds O2
CCytochrome P450: ytochrome P450: IsozymesIsozymes
★ Isozymes - multiple forms of an enzyme ★ Supergene family- More than 8,000 P450 genes as of November/2007- More than 368 gene families, 814 subfamilies- Human: 18 families, 43 subfamilies, 57 sequenced
genes
★ Nomenclature
CYP1A2
family subfamily individual member of that subfamily
CCytochrome P450ytochrome P450
This enzyme is very important. approximately
50% of the drugs that humans ingest are
metabolized by isoforms of cytochrome P450.
These enzymes also act on various carcinogens
and pollutants.
SubstratesSubstrates
H2C
CH
Aromatic Benzyl
Vinyl
Allyl
Aliphatic Alicyclic
2. Monoamine oxidase, MAO
RCH2NH2+O2+H2O2RCHO+NH3+H2O
★ MAO catalyze the oxidative deamination of monoamines.
★ Oxygen is used to remove an amine group from a molecule, resulting in the corresponding aldehyde and ammonia.
★ MAO are found bound to the outer membrane of mitochondria in most cell types in the body. They belong to protein family of flavin containing amine oxidoreductases.
3. ADH and 3. ADH and ALDH
Alcohol Dehydrogenase belongs to the oxidored
uctase family of enzymes.
high concentrations within the liver and kidney.
ALDH aldehyde dehydrogenase
ADHADH alcohol dehydrogenasealcohol dehydrogenase
FunctionFunction The primary and most common role of ADH in hum
ans is to detoxify incoming ethanol by converting it into aldehyde.
The resulting aldehyde, a more toxic molecule than ethanol, is quickly converted into acetate by aldehyde dehydrogenase (ALDH) and other molecules easily utilized by the cell.
R CH2OH R CHO R CO2H
NAD+ NADH NAD+ NADH
ADHADH ALDHALDH
R CH2OH R CHO R CO2H
NAD+ NADH NAD+ NADH
During this reaction, hydrogen is removed from the alcohol and transferred to a molecule called nicotinamide adenine dinucleotide (NAD), converting it to reduced NAD (NADH).
ADHADH ALDHALDH
NADH participates in numerous other metabolic reactions, passing on the hydrogen to other compounds or electron transfer chain.
AbsorptionAbsorption Soluble in waterSoluble in water
Small size - Small size - penetrates everywhere, penetrates everywhere, easily crosses all bio membraneseasily crosses all bio membranes
Rapidly absorbed from GIRapidly absorbed from GI
20%
80%
In people who consume alcohol at moderate levels and/or only occasionally, most of the alcohol is broken down by ADH and ALDH.
after higher alcohol consumption, The MEOS plays a role in alcohol metabolism.
CH3CH2OH + NADPH + O2 + H+ CH3CHO
+ NADP+ + 2H2O
MEOS
CH3CHOALDHALDH
CH3COOH
MEOS: Microsomal Ethanol-Oxidizing System , is also called Cytochrome P450-dependent Microsomal Ethanol Oxidizing System. converts alcohol to acetaldehyde
◆consume oxygens of liver and NADPH
◆ As byproducts of these reactions, oxygen radicals or reac
tive oxygen species (ROS) are generated. These ROS can cont
ribute to liver damage through a variety of mechanisms.
CH3CH2OH + NADPH + O2 + H+ CH3CHO
+ NADP+ + 2H2O
MEOS
This reaction also relies on oxygen and NADPH, and results in the formation of NADP and water.
Although the rate at which ADH breaks down alcohol generally stays the same, the activity of the MEOS can be increased (induced) by alcohol consumption.
Because the MEOS metabolizes not only alcohol but also
other compounds (certain medications), enhanced MEOS activ
ity resulting from high alcohol consumption also can alter the
metabolism of those medications.
This may contribute to harmful interactions between alcoho
l and those medications or otherwise influence the activity of t
hose medications.
Alcoholism leads to fat accumulation in the liver, hyperlipidemia, and ultimately cirrhosis.
4. Nitro and Azo Reduction4. Nitro and Azo Reduction
• NADPH dependent microsomal and nitro-reductase enzymes.
• Bacterial reductases play a role in enterohepatic recirculation of nitro or azo containing drugs.
Ar NO
O
+Ar N O Ar NHOH Ar NH2
N Ar'NAr NH
Ar NH
Ar' NH2 Ar NH2 Ar'+
Phase : ReductionⅠ
sudan red
2005 , sudan red incident2005 , sudan red incident
chili patse
chloromycetin
nitroreductase
5. Hydrolysis5. Hydrolysis
Catalyzed by widely distributed hydrolytic enzymes
Hydrolysis of esters major metabolic pathway for ester drugs
☻Non-specific esterases (liver, kidney, and intestine)
☻Plasma pseudocholinesterases also participateO CH3
CO2H
O
OH
CO2H CH3
O
OH
ASA
Acetylsalicylic Acetylsalicylic AcidAcid, ASA, ASA
esterase
salicylic acidsalicylic acid
Substrates: esters , amide , glycoside , amide , glycoside, etc.
Phase : ReductionⅠ
PPhasehase II: II: ConjugationConjugation
In phase Ⅰ reactions, xenobiotics are generally
converted to more polar, hydroxylated derivatives.
In phase Ⅱ reactions, these derivatives are conjugated
with molecules such as glucuronic acid, sulfate, or
glutathione.
This renders them even more water-soluble, and they
are eventually excreted in the urine or bile.
xenobiotic
Phase II
nontoxic metabolite
ProtectionElimination
Phase I
excrectionexcrectionReactive metabolite
Cell injury Antibody product mutation
cancerCell injury
FFive types of phase ive types of phase IIII reactions reactions
A. Glucuronidation
B. Sulfation
C. Conjugation with glutathione
D. Acetylation
E. Methylation
1. G1. Glucuronidationlucuronidation the most frequent conjugation reaction. UDP-glucuronic acid (UDPGA) is the glucuronyl
donor UDP-glucuronyl transferases (UGT), present in
both the endoplasmic reticulum(ER) and cytosol, are the catalysts. – Liver, lung, kidney, skin, brain and intestine
Attachment sites are hydroxyls– Alcohols, phenols, enols, N-hydroxyls,
acids
Oxygen site often from Phase I
Oxygen glucuronides cont…Oxygen glucuronides cont…
Alcohol hydroxyl example
Phenol hydroxyl example
NH
CH3
OH
O
NH
CH3
O
O
O
HOHOH
HO2C
OH
AcetaminophenAPAPN-acetyl-p-aminophenol
NH
ClO2N
OH
OHO
Cl NH
ClO2N
OH
OO
Cl
O
HOHOH
HO2C
OHChloramphenicol - Chloromycetin® -
2. Sulfate Conjugation2. Sulfate Conjugation Some alcohols, arylamines, and phenols are sulfated.Catalyzed by sulfotransferases
– liver, kidney and intestine
Sulfate donor: adenosine 3’-phosphate-5’-phosphosulfate (PAPS); this compound is called “active sulfate.”
Leads to inactive water-soluble metabolites Glucuronate conjugation often more competitive process
OHH2O3PO
OAdenine
OP
O
OH
OS
O
O
-O
HX Drug
X DrugS
O
O
-O
PAPS
sulfotransferase
Sulfate ConjugationSulfate Conjugation
estrone
O
OH
+ PAPS+PAP
estrone sulfate
O
HO3SO
OH
OH
CH3
NH2
CO2H
O
OH
CH3
NH2
CO2H
S
O
O
-O
alpha-MethyldopaAldomet® - MerckAntihypertensive
3. Conjugation with glutathione3. Conjugation with glutathione
glutamic acid, cysteine, glycine
R + GSH R-S-GGST
R: epoxides and halogenides
GST: Glutathione S-Transferases (Liver and kidney)
where R= an electrophilic xenobiotics
Glutathione (GSH) ConjugationGlutathione (GSH) Conjugation
DETOXIFICATION of electrophiles!Electrophilic chemicals cause:
– Tissue necrosis– Carcinogenicity– Mutagenicity– Teratogenicity
The thiol (SH group) ties up potent electrophiles
GST + glutathione
Inactive DNA DNA reactive;reactive;
lung and skin lung and skin
tumorstumors
Glutathione S-transferase Glutathione S-transferase
HO
HOHO
OH
(+)benzo[a]pyrene-7,8-dihydrodiol-9-10-epoxide
glu
tathio
ne
DETOXIFICATION
O
oxidation
Epoxide or anArene Oxide
OH
SGHSG
H+
Benzene
aflatoxin b-1 epoxide aflatoxin b-1 epoxide
Glutathione (GSH) ConjugationGlutathione (GSH) Conjugation
X + Acetyl-CoA - - - - >Acetyl-X + CoS where X represents a xenobiotics. (for: aromatic amines)
4. Acetylation4. Acetylation
• Enzyme: acetyltransferases
• present in the cytosol of various tissues, particularly in liver.
isoniazid
sulfanilamidesulfanilamide
Important for drugs with primary amino groupsGenerally, metabolites are nontoxic and inacti
veAcetylation does NOT increase water solubilityDetoxification or termination of drug activity
MethylationMethylationA few xenobiotics are subject to
methylation by methyltransferase,
emplyoing S-adenosylmethione(SAM) as the methyl donor.
SAM
catecholcatechol
Metabolism via MethylationMetabolism via Methylation
Key for biosynthesis of many compounds Important in the inactivation of physiologically active biogenic amines
neurotransmitters – norepinephrine, dopamine, serotonin, histamine
Minor pathway in the metabolism of drugsMethylation does NOT increase water solubilityMost methylated products are inactive
Factors that influence Factors that influence metabolismmetabolism
Age– older people less efficient at metabolism
Sex– Linked to hormonal differences
Heredity– Genetic differences can influence amounts and
efficiency of metabolic enzymes
Disease states– Liver, cardiac, kidney disease
Summary Summary Xenobiotic, Biotransformation Phase I reactions:
– Purpose: Enhances elimination Converts chemical to less toxic forms (detoxification) Converts chemicals to more toxic active forms (activation)
– Functionalization: Oxidation: monooxygenase, CYP450 Reduction: ADH, ALDH Hydrolytic reactions: esterase
Phase II: Conjugation Rx– Purpose: more water-soluble, excreted in the urine or bile– Functionalization:
Glucuronidation, Sulfation, Conjugation with glutathione, Acetylation, Methylation
ALCOHOLALCOHOLMechanism of Fatty LiverMechanism of Fatty Liver
The likelihood of hypoglycemia is also increased in alcoholics when they fast, as they often have low hepatic stores of glycogen because of poor nutrition.
The shift in the NADH/NAD+ ratio also inhibits β-oxidation of fatty acids and promotes triglyceride synthesis; this increases hepatic synthesis of VLDL, and the remaining excess triglyceride is deposited in the liver.