Download - BIOTRANSFORMATION
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When toxic chemicals enter the body they eventually end up in the liver for processing
Most of these chemicals are difficult to move out of the body so they need to be transformed in such a away that enables them to be eliminated
This is done through an elaborate
system called PHASE I and PHASE II detoxification mechanism
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Liver SER
smoothendoplasmicreticulum
Two major sets of pathways (enzymatic)
•nonsynthetic reactions - Phase I
•synthetic reactions - Phase II
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XENOBIOTICPRIMARYPRODUCT
SECONDARYPRODUCT
Expose or addfunctional group
OxidationReductionHydrolysis
PHASE I PHASE II
EXCRETION
Conjugation
LIPOPHILIC HYDROPHILIC
Phase I and phase II reactions
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Phase I reactions include: oxidations reductions hydrolysis reactions
they introduce a functional group (e.g., -OH) that serves as the active center for sequential conjugation in a phase II reaction.
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2. Phase II Reactions that include conjugation reactions, which involve the enzyme-catalyzed combination of a drug (or drug metabolite) with an endogenous substance.
Phase II reactions require: a functional group—an active center—as
the site of conjugation with the endogenous substance.
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Drug metabolism reactions :
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• introduction of functional group• hydrophilicity increases slightly• major player is CYP or mixed function
oxygenase (MFO) system in conjunction with NAD(P)H
• location of reactions is smooth endoplasmic reticulum
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oxidation
reduction
hydrolysis
•hydroxylationsaromatic, aliphatic, nitrogen
•dealkylations(N-, S-, P)•deaminations•N-, S-, P- oxidations•S-replacements•epoxidations•others
•azo reduction•nitro reduction•disulfide reduction•others
Nonsynthetic Reactions
oxidoreductases
oxidasesmonoamine oxidasesmixed function oxidases
oxidoreductases
reductases
•esters•amides•hemiacetals,acetals, hemiketals, ketals
esterases
amidases
peptidases
lipases
hydrolases
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Enzymes catalyzing phase I biotransformation reactions include:
cytochrome P-450aldehyde and alcohol dehydrogenase deaminases esterases amidasesepoxide hydratases
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cytochrome P 450
The principal reaction of drug/toxin metabolism is OXIDATION.
The enzymes responsible are oxido-reductases; called mixed-functionoxidases.
Most prominent andimportant among theseis thecytochrome P450system.
consists ofCyt P 450 andCyt P 450 reductase
Fe3+
protoporphryrinring system
BIOTRANSFORMATION - NONSYNTHETIC - OXIDATION
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S
PCH3CH2O
OCH2CH3
O
O PCH3CH2O
OCH2CH3
O
Parathion Paraoxon
S-replacement
CCl2 CH2
ClCl
ClCl
CCl2 CH2
ClCl
ClCl
O
Aldrin Dieldrin
Both compounds areinsecticides.
epoxidation
BIOTRANSFORMATION - NONSYNTHETIC - OXIDATION
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oxidation
reduction
hydrolysis
•hydroxylationsaromatic, aliphatic, nitrogen
•dealkylations(N-, S-, P)•deaminations•N-, S-, P- oxidations•S-replacements•epoxidations•others
•azo reduction•nitro reduction•disulfide reduction•others
Nonsynthetic Reactions
oxidoreductases
oxidases monoamine oxidases mixed function oxidases
oxidoreductases
reductases
•esters•amides•hemiacetals,acetals, hemiketals, ketals
esterases
amidases
peptidases
lipases
hydrolases
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BIOTRANSFORMATION - NONSYNTHETIC - REDUCTION
H2N
NH2
N N SO2NH2
H2N SO2NH2
H2N
H2N
NH2
Prontosil®
sulfanilamide
+
NO2 NO NHOH NH2NO2
CHOH
CHNHCCHCl2HOCH2
O
NH2
CHOH
CHNHCCHCl2HOCH2
O
chloramphenicol(antibacterial)
azo reduction
nitro reduction
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N C
S
SS
S
C NCH3CH2
CH3CH2
CH2CH3
CH2CH3
N C
SCH3CH2
CH3CH2
SH
Disulfiram (Anatabuse®)
2
C C C C
O
C C
OH
As3+As5+
disulfide reduction
other reductions
BIOTRANSFORMATION - NONSYNTHETIC - REDUCTION
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oxidation
reduction
hydrolysis
•hydroxylationsaromatic, aliphatic, nitrogen
•dealkylations(N-, S-, P)•deaminations•N-, S-, P- oxidations•S-replacements•epoxidations•others
•azo reduction•nitro reduction•disulfide reduction•others
•esters•amides•hemiacetals,acetals, hemiketals, ketals
Nonsynthetic Reactions
oxidoreductases
oxidases monoamine oxidases mixed function oxidases
oxidoreductases
reductases
esterases
amidases
peptidases
lipases
hydrolases
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BIOTRANSFORMATION - NONSYNTHETIC - HYDROLYSIS
COOHOCCH3
O
COOHOH
O
acetylsalicylic acid
+HOCCH3
+ H2O
N
S
O
O
CH3
CH3
COOH
N
SH2NO
O
CH3
CH3
COOH
HOOC N
S
O
H
CH3
CH3
COOH
RCOH
+H2O
penicillins RCNH
RCNH
esters
amides
also hemiacetalshemiketalsacetalsketals
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• conjugation with endogenous molecules(GSH, glycine, cystein, glucuronic acid)
• hydrophilicity increases substantially• neutralization of active metabolic intermediates• facilitation of elimination • location of reactions is cytoplasm
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Phase II - combines functional group of compound with endogenous substance
E.g. Glucuronicacid, Sulfuric acid, Amino Acid, Acetyl.
Products usually very hydrophilic The final compounds have a larger
molecular weight.
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Enzymes catalyzing phase II biotransformation reactions include: glucuronyl transferase (glucuronide conjugation) sulfotransferase (sulfate conjugation) transacylases (amino acid conjugation) acetylases ethylasesmethylasesglutathione transferase.
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How We Get To Phase 2
• Most of the drugs do not become polar upon phase 1 reactions.
• Goal of Phase 2 : Make substances more soluble that couldn’t be done in the Phase 1 reactions.
2103-12-2010KLECOP, Nipani
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Synthetic Reactions / Phase II• These reactions usually involves covalent attachments of
small polar endogenous molecules such as Glucoronic acid, Sulfate, Glycine to either unchanged drugs or Phase I product having suitable functional groups as COOH,-OH,-NH2,- SH.
• Thus is called as Conjugation reactions.
• Since the product formed is having high molecular weight so called as synthetic reactions.
• The product formed is hydrophilic in nature with total loss of pharmacologic activity so called as a true detoxification reaction
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Glucuronide Conjugation Methylation Acetylation Sulfate Conjugation Conjugation With Alpha Amino Acids Glutathione Conjugation Glycine Conjugation Cyanide Conjugation
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Very important Synthetic reactions carried out by Uredine Di Phosphate Glucuronosyl Transferase.
• Hydroxyl and Carboxylic acid groups are easily combined with Glucuronic acid.
2403-12-2010KLECOP, Nipani
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Synthetic Reactions
glucuronide formation
O OH
OH
HO
OH
OH
COOH
OH
OH
HO
OH
OH
O OH
COOH
HO
OH
OH
glucose
gluconic acid
glucuronic acid
O
H
HO
H
HO
H
HOHH
OPO32-
CO2H
O
H
HO
H
HO
H
HOHH
O
CO2H
UDP
UTP
PPi
OH
COH
O
salicylic acid
+
a glucuronidederivative
O
H
HO
H
HO
H
HOHH
O
CO2H UDP
UDP-glucuronide
UDP
O
H
HO
H
HO
H
HOHH
O
CO2H
C
O OH
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Major route of biotransformation for aromatic amines, hydrazine. Generally decreases water solubility Enzyme: - N- Acetyltransferase (NAT)
R – NH2 R – NH – COCH3
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BIOTRANSFORMATION - SYNTHETIC - ESTERIFICATION
NH2 SO2NH2
O
CH3C SCoenzyme A HN SO2NH2
high energy bond
sulfanilamide Acetyl Coenzyme A
+ CH3C
O
+ CoenzymeA
acetylation
HO N
N N
N
NH2
O
SO
OHO
POOS
OO
OO
O
P OO
O
O
O
O
The enzymes catalyzingthis type of reaction arecalled sulfotransferases.
Sulfates are carried asphosphoadenosine-phosphosulfate derivatives(PAPS) - a high energy form.
+
sulfate ester formation
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Generates more polar (water soluble), inactive metabolites
Readily excreted from body Metabolites may still have potent biological
activity (or may have toxic properties) Generally applicable to metabolism of all
xenobiotics as well as endogenous compounds such as steroids, vitamins and fatty acids
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Biotransformation is a major mechanism for drug / TOXIN elimination
Biotransformation of drug is defined as the conversion from one chemical form to another
Many drugs undergo
several sequential biotransformation reactions.
Biotransformation is catalyzed by specific enzyme systems
Sites of biotransformation: The liver: the major siteother tissues.
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BLOOD
XENOBIOTIC
BIOTRANS-FORMATION
EX
CR
ET
ION
DNAdamage
Somaticeffect
Detoxification
ActivationNon-toxicmetabolite
Toxicmetabolite
DefinitionBiotransformation is the sum of all processes,whereby a compound is transformed chemicallywithin a living organism
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Metabolism of acetaminophen to harmless conjugates or to toxic metabolites.
nontoxic phase II conjugatesnontoxic phase II conjugates
nontoxic phase II conjugates
toxic
phase I Reaction
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Results of biotransformation
Drug
or Poison
biotransformed Drug or Poison
active
inactive
active
inactive
more potent
TOXIC
less potent
BIOTRANSFORMATION
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ACTIVE analgesicanalgesicSalicylic acidAcetylsalicylic Acid
ACTIVE
OCCH3
CO2H
O
OH
CO2H
ACTIVE (more potent)ACTIVE narcotic analgesicnarcotic analgesicMorphineCodeine
OH3CO OH
H
H N CH3
OHO OH
H
H N CH3
ACTIVE TOXICTOXICCNS depressantFormic AcidFormaldehydeMethanol
() ()
CH3OH HCH
O
HCOH
O
BIOTRANSFORMATION
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Drug metabolizing organs
Drug metabolizing organs
• Liver is the heart of metabolism
• Because of its relative richness of enzymes in large amount.
• Schematic chart of metabolizing organ (decreasing order)
• Liver > lungs > Kidney > Intestine > Placenta > Skin > Brain > Testes > Muscle > Spleen
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One of these chemicals are converted they are carried to the gastrointestinal tract via the bile.
Bile has two major functions in the body:
(1) help the body absorb fats and fat soluble nutrients,
(2) help the body eliminate toxins and wastes.
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Biotransformation of drugs can be affected by many parameters, including:
A. prior administration of the drug in question or of other drugs
B. diet C. hormonal statusD. geneticsE. disease (e.g., decreased in cardiac and
pulmonary disease) F. age and developmental status G. liver function
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• Most drugs are excreted by the kidneys.• For renal excretion drugs should:– have small molecular mass– be polar in nature– not be fully ionised at body pH• Most drugs are complex and do not have these
properties and thus have to be broken down to simpler products.
• Drugs are lipophilic in nature• They have to be converted to simpler hydrophilic
compounds so that they are eliminated and their action is terminated
Why Biotransformation?
03-12-2010KLECOP, Nipani
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Important phase I enzymes
Enzyme Co-factor Substrate
Mixed-function oxidases NADPH Most lipophilic substances(cytochrome P-450) (NADH) with M.wt < 800
Carboxyl esterases Unknown Lipophilic carboxyl esters
’A’ esterases Ca++ Organophosphate esteresEpoxide hydrolases Unknown Organic epoxids
Reduktases NADH Organic nitrous compoundsNADPH Organic halogens
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Mixed function oxidase enzymes (P450) arelocated in the endoplasmic reticulum (SER)
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Enzymes catalyzing phase II biotransformation reactions include: glucuronyl transferase (glucuronide conjugation) sulfotransferase (sulfate conjugation) transacylases (amino acid conjugation) acetylases ethylasesmethylasesglutathione transferase.
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Location of these enzymes: numerous tissues some are present in plasma.
Subcellular locations include: cytosol mitochondria endoplasmic reticulum
Only those enzymes located in the endoplasmic reticulum are inducible by drugs
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a. General features
A large number of families (at least 18 in mammals) of cytochrome P-450 (abbreviated “CYP”) enzymes exists
each member of which catalyzes the biotransformation of a unique spectrum of drugs
some overlap in the substrate specificities.
This enzyme system is the one most frequently involved in phase I reactions.
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Cytochrome P-450 catalyzes numerous reactions, including:
aromatic and aliphatic hydroxylations dealkylations at nitrogen, sulfur, and
oxygen atoms heteroatom oxidations at nitrogen and
sulfur atoms reductions at nitrogen atomsester and amide hydrolysis
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b. Localization
The primary location of cytochrome P-450 is the liver,
Other tissues, including: the adrenals ovaries and testis tissues involved in steroidogenesis and steroid metabolism.
The enzyme's subcellular location is the endoplasmic reticulum.
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c. Mechanism of reaction1. In the overall reaction:
the drug is oxidized oxygen is reduced to water.
Reducing equivalents are provided by nicotinamide adenine dinucleotide phosphate (NADPH), and generation of this cofactor is coupled to cytochrome P-450 reductase.
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Metabolism of phenytoin :