Drug Metabolism Enzymes and Pathways Explained in Detail

Transformation of Xenobiotics by Biological Systems

IMPLICATIONS FOR DRUG METABOLISM

IMPLICATIONS FOR DRUG METABOLISM
1. Termination of drug action
2. Activation of prodrug
3. Bioactivation and toxication
4. Carcinogenesis
5. Tetratogenesis

Termination of Drug Action
tropic acid and tropine
atropine
propranolol → hydroxypropranolol
(active) (active)

Termination of Drug Action
Conversion of drug to active metabolite to active
metabolite to inactive metabolite

Activation of Prodrug
Dopamine
L-dopa

Inactive Terfenadine is Converted to its Active
Metabolite Fexofenadine
terfenadine
fexofenadine
activation of prodrug

Some Xenobiotics Are Metabolized to Carcinogenic Agents
• 3,4 Benzopyrene
• Aflatoxin
• N-Acetylaminoflluorene
Metabolites of these agents interact with DNA
carcinogenesis

Small Amounts of Acetaminophen is Converted to the
Reactive Metabolite N-Acetylbenzoquinoneimine
Bioactivation of acetaminophen; under certain conditions, the electrophile N-
acetylbenzoquinoneimine reacts with tissue macromolecules, causing liver necrosis.
bioactivation

Thalidomide is a Teratogen
– THALIDOMIDE: Fetal malformations in
humans, monkeys, and rats occur due to
metabolism of the parent compound to a
teratogen. This occurs very early in gestation.
teratogensis

FACTORS AFFECTING DRUG METABOLISM

Factors Affecting Drug Metabolism
• Age
• Diet
• Genetic Variation
• State of Health
• Gender
• Degree of Protein Binding
• Species Variation
• Substrate Competition
• Enzyme Induction
• Route of Drug Administration

• Route of drug administration
– Oral versus systemic administration

Many Drugs Undergo First Pass Metabolism
Upon Oral Administration
• Oral administration
• Drug travels from gut to portal vein to liver
• Vigorous metabolism occurs in the liver. Little drug
gets to the systemic circulation
• The wall of the small intestine also contributes to first
pass metabolism

ORGAN SITES OF DRUG METABOLISM

Organ Sites of Drug Metabolism
• Liver
• Small intestine
• Kidney
• Skin
• Lungs
• Plasma
• All organs of the body

CELLULAR SITES OF DRUG
METABOLISM

Cellular Sites Of Drug Metabolism
• Cytosol
• Mitochondria
• Lysosomes
• Smooth endoplasmic reticulum
(microsomes)

Velocity Of Metabolism Of A Drug
0 10 20 30 40 50 60 70
0
10
20
30
40
50
60
70
80
[Drug] mM
Velocity
(ng/g
tissue/min)
D:summer1Kmx1.pzm

Velocity Of Metabolism Of A Drug
0 5 10 15 20 25 30 35 40 45 50 55 60
0
10
20
30
40
50
60
70
80
first order metabolism
zero order metabolism
[Drug] mM
Velocity
(ng/g
tissue/min)
Kmx2.pzm

First Order Metabolism
v = Vmax [C]
Km + [C]
When Km >>> [C],
then v = Vmax [C] ,
Km
and v α [C]
Metabolism of the drug is a first order process. A constant
fraction of the remaining drug is metabolized per unit time.
Most drugs are given at concentrations smaller than the Km
of the enzymes of their metabolism.
A drug may be given in doses that produce blood
concentrations less than the Km of the enyzme for the drug.

Zero Order Metabolism
v = Vmax [C]
K m + [C]
When [C] >>> Km,
then v = Vmax [C] ,
[C]
and v = Vmax
Metabolism of the drug is a zero order process. A constant
amount of the remaining drug is metabolized per unit time.
Phenytoin undergoes zero order metabolism at the doses
given.
A drug may be given in doses that produce blood concentrations
greater than the Km of the enyzme for the drug.

Velocity Of Metabolism Of Three Drugs
By The Same Enzyme
0 10 20 30 40 50 60 70 80 90
0
10
20
30
40
50
60
70
Drug A
Drug B
Drug C
[Drug] mM
Velocity
(ng/g
tissue/min
)

Phase I Metabolism
R ROH R R
COOH
R RSH R RNH2
Polar groups are exposed on or introduced to a molecule

Phase I Reactions
OXIDATION
REDUCTION
HYDROLYSIS

Phase II Metabolism
D+ENDOX DX+ENDO
A molecule endogenous to the body donates a portion
of itself to the foreign molecule

Patterns of Drug Metabolism
• Parent molecule → Phase 1 metabolism
• Phase 1 metabolite → Phase 2 metabolism
• Parent molecule → Phase 2 metabolism
• Phase 2 metabolite → Phase 1 metabolism
Some drugs are not metabolized, for example, gallamine
and decamethonium. Atracurium undergoes spontaneous
hydrolysis.

Cytochrome P450
fp = NADPH cytochrome P450 reductase, or NADH cytochrome b5
reductase

Oxidation Of Drugs By Cytochrome P450

Aromatic Hydroxylation (1)
acetanilid p-hydroxyacetanilid

N-Hydroxylation of AAF
N-Hydroxylation of AAF is the first metabolic step towards
the development of a carcinogenic agent

ISOENZMYES OF CYTOCHROME P450
CYP1A1
CYP1A2
CYP2A6
CYP2B_
CYP2C9
CYP2C19
CYP2D6
CYP2AE1
CYP3A4
CYP3A5
CYP3A7
CYP4A_

CYP3A4
• CYP3A4 is responsible for metabolism of 60%
of all drugs
• It comprises approximately 28% of hepatic
cytochrome P450
• Metabolizes terfenadine
• Ingestion of grapefruit juice reduces expression
of this enzyme
• Inhibited by some regularly used drugs

Some Drugs That Inhibit CYP3A4
• Macrolide antibiotics
– Erythromycin
– Clarithromycin
– Other such agents
• Antifungal agents
– Ketoconazole
– Itraconazole
– Other such agents
• HIV protease inhibitors

CYP3A4
• Ketoconazole and terfenadine can produce a
drug interaction with fatal consequences.

CONVERSION OF TERFENADINE TO FEXOFENADINE
CYP3A4
O2, NADPH

AN INGREDIENT IN GRAPEFRUIT JUICE
INHIBITS CYP3A4

Grapefruit Juice Increases Felodipine Oral Availability in
Humans by Decreasing Intestinal CYP3A Protein Expression
J.Clin. Invest. 99:10, p.2545-53, 1997
Hours

Grapefruit Juice Consumption Blocks Terfenadine
Metabolism to Fexofenadine
X

CYP3A4 And P-Glycoprotein
• P-Glycoprotein and CYP3A4 control oral bioavailability
of many drugs
• P-Glycoprotein and CYP3A4 share many substrates
and inhibitors

CYP2D6 is an Enzyme with Polymorphisms
• Approximately 70 nucleotide polymorphisms are
known
• Four phenotype subpopulations of metabolizers*
– Poor metabolizers (PM)
– Intermediate metabolizers (IM)
– Extensive metabolizers (EM)
– Ultrarapid metabolizers (UM)
• Variations according to racial background
• More than 65 commonly used drugs are
substrates
• Codeine is a well known substrate
* The Pharmacological Basis of Therapeutics

Codeine is a Substrate of CYP2D6
Consider the variation in codeine’s metabolism among
PM, IM, EM, UM individuals
-CH3
(methyl morphine)

CYP2C9
• Metabolizes some 16 commonly used drugs
• Warfarin and phenytoin are among the substrates
• Two allelic variants are known: metabolizes substrates
5% to 12% of the wild type enzyme
– Warfarin clearance is greatly reduced in individuals
possessing the allelic variants
• Dose adjustments are required for drugs in individuals
who have the mutant enzymes

CYP2C19
• S-mephenytoin is a substrate
– (4-hydroxylation at the phenyl ring)
• As much as eight allelic variants identified
– All are nonfunctional proteins
• Poor metabolizers of S-mephenytoin lack 4-hydroxylase
activity, but N-demethylation to nirvanol is an alternative
but slow metabolic pathway
– Dose adjustments must be made for poor
metabolizers of S-mephenytoin and for other drugs
that are substrates for this enzyme

CYP1A1
• Polycyclic hydrocarbons are among its
substrates
• Inducers include
– Polycyclic hydrocarbons such as 3,4,-benzopyrene,
3-methylcholanthrene, etc.
– Charcoal broiled foods (polycyclic hydrocarbons)

CIMETIDINE Inhibits CYP450 Metabolism Of Many Drugs
Warfarin
Phenytoin
Metoprolol
Labetalol
Quinidine
Caffeine
Lidocaine
Theophylline
Alprazolam
Diazepam
Flurazepam
Triazolam
Chlordiazepoxide
Carbamazepine
Quinidine
Ethanol
Tricyclic
antidepressants
Metronidazole
Calcium channel
blockers
Diazepam
Sulfonylureas

NONMICROSOMAL OXIDATIONS
ALCOHOL DEHYDROGENATION
ALDEHYDE DEHYDROGENATION
XANTHINE OXIDATION
DIAMINE OXIDATION
MONOAMINE OXIDATION

Nonmicrosomal Oxidations
Alcohol dehydrogenation is conducted by the enzyme
alcohol dehydrogenase (cytosolic)
Aldehyde dehydrogenation is conducted by the enzyme
aldehyde dehydrogenase (cytosol and mitochondria)
Xanthine oxidation is conducted by the cytosolic enzyme
xanthine oxidase.
Diamine oxidase (cytosolic) oxidizes histamine and
diamines such as cadaverine and putrescine.
Monoamine oxidation is conducted by mitochondrial
monoamine oxidase (norepinephrine, epinephrine,
dopamine and serotonin are endogenous substrates.

Monoamine Oxidase Metabolism of Serotonin

Some Popular Substrates of Monoamine Oxidase
• Serotonin
• Epinephrine
• Norepinephrine
• Dopamine
• Tyramine (found in certain foods)

Alcohol Dehydrogenase
• A soluble enzyme, found almost exclusively in the
parenchymal cells of the liver
• Converts ethanol to acetaldehyde
• Converts methanol to formaldehyde
• Converts ethylene glycol to its respective aldehyde
metabolites
• Is inhibited by pyrazole

Alcohol Dehydrogenase
CH3CH2OH + NAD+
→ CH3CHO + NADH + H+
ethanol acetaldehyde

Aldehyde Dehydrogenase
CH3CHO + NAD+
→ CH3COOH + NADH + H+
acetaldehyde acetate

Nitro Reduction
NITRO REDUCTION
RNO2 RNH2
MICROSOMES AND CYTOSOL
Microsomes and cytosol

Azo Reduction
RN=NR' RNH2 + R'NH2
AZO REDUCTION

Azo Reduction

Alcohol Dehydrogenation
Cytosol

DIHYDROPYRIMIDINE DEHYDROGENASE
5-Fluorouracil 5-Fluoro-5,6-dihydrouracil
DPYD
• DPYD
– Inactivates 5-fluorouracil by ring reduction
– Inherited deficiency of this enzyme leads to 5-fluorouracil
toxicity
– Enzyme deficiency can be detected by enzymatic or
molecular assays using white blood cells
5-fluorouracil

Amide Hydrolysis
RCONR'R" RCOOH+ HNR'R"
AMIDE HYDROLYSIS

Ester Hydrolysis
RCOOR' RCOOH + R'OH
ESTER HYDROLYSIS

Ester Hydrolysis
Enalaprit

Epoxide Hydrolase
• A microsomal enzyme

D+ENDOX DX+ENDO
PHASE 2 METABOLISM
A molecule endogenous to the body donates a portion
of itself to the foreign molecule

PHASE II REACTIONS
Glucuronidation
Sulfate Conjugation
Acetylation
Glycine Conjugation
Methylation
Transulfuration
Glutathione Conjugation
Mercapturic Acid Synthesis

Uridine-5’-α-D-glucuronic Acid
The microsomal enzyme glucuronyl transferase conducts the
donation of glucuronic acid from the endogenously synthesized
UDPGA to various substrates to form glucuronide conjugates.
Examples of such substrates are morphine and acetaminophen.

UDP-α-D-Glucuronsyltransferase
• Is also called glucuronyl transferase
• A microsomal enzyme
• Substrates are called aglycones
• Conducts phase 2 metabolic reactions
• Products are called glucuronides
• Glucuronides formed
– RN-G; RO-G; RCOO-G; RS-G; RC-G
• Bilirubin is an endogenous substrate
• Induced by phenobarbital

Glucuronidation of Benzoic Acid
UGT= UDP-α-D-Glucuronsyltransferase

Glucuronidation of p-Hydroxyacetanilid

Morphine Metabolism
A small amount of morphine undergoes N-demethylation
Morphine → Morphine -6-glucuronide (active metabolite)
Morphine → Morphine -3-glucuronide (inactive metabolite)

Morphine Metabolism
Morphine -3-glucuronide is the major metabolite

Induction Of UDP-α-D-Glucuronyl Transferase
• Induced by phenobarbital
• Induced by 3-methylcholanthrene

Glucuronidation in the Cat
• The cat can glucuronidate bilirubin but cannot
glucuronidate phenolic compounds such as phenol
and napthol

Sulfate Conjugation
• Conducted by the soluble enzyme sulfotransferase
• Endogenous donor molecule to conjugation is
3’-phosphoadenosine-5’-phosphosulfate (PAPS)
• Conjugates are ethereal in character
• Noninducible

3’-Phosphoadenosine-5’-phosphosulfate (PAPS)
The cytosolic enzyme sulfotransferase conducts the donation of
sulfate from the endogenously synthesized PAPS to various
substrates to form sulfate conjugates. An example of such substrate
is acetaminophen.

Sulfate Conjugation of p-Hydroxyacetanilid
PAP: 3’-phosphoadenosine- 5’-phosphate

MINOXIDIL METABOLISM
MINOXIDIL
(inactive)
MINOXIDIL N-O-SULFATE
(active metabolite)
MINOXIDIL N-O-GLUCURONIDE
(inactive metabolite)

Species Differences in Sulfate Conjugation
• Some species are deficient in the sulfate conjugation
pathway
– Pig
– Opposum

N-Acetyltransferase
• A soluble enzyme
• Isoniazid is a substrate
• Genetic variation occurs
– Some individuals are fast acetylators
– Some individuals are slow acetylators
• Acetyl coenzyme A is the endogenous donor
molecule

Acetyl CoA
Various acetylases, for examples, choline acetylase and N-acetyl
transferase, all soluble enzymes, conduct the transfer of the acetyl
group of acetyl CoA to various substrates. For example, N-acetylation
of isoniazid. Genetic polyporphism occurs with N-acetyltransferase.

N-Acetyltransferase
• The dog cannot acetylate aromatic amino compounds
because it lacks the appropriate isoenzyme of NAT

Conversion of 6-Mercaptopurine to a Nucleotide

S-Adenosylmethionine
Cytosolic enzymes such as catechol-O-methyl transferase (COMT) and
phenylethanolamine-N-methyl transferase (PNMT) conducts the
donation of the methyl group from the endogenously synthesized SAM
to various substrates to form methylated conjugates. Norepinephrine is
N-methylated by PNMT to form epinephrine. Norepinephrine,
epinephrine, dopamine, and L-DOPA are O-methylated by COMT.

Methyltransferases
• A family of soluble enzymes that conducts
– N-methylation; N-CH3
– O-methylation; O-CH3
– S-methylation; S-CH3
• S-adenosylmethionine (SAM)is the endogenous donor
molecule. It is demethylated to S-adenosylhomocysteine

N-Methyltransferases
PNMT- Phenylethanolamine-N-methyltransferase
Norepinephrine Epinephrine
PNMT
SAM

O-Methylation Of Catecholamines
COMT- catechol-O-methyltransferase

O-Methylation of Norepinephrine
COMT- catechol-O-methyltransferase

S-Methylation of 6-Mercaptopurine
TPMT - thiopurinemethyltransferase; some individuals are
deficient in this enzyme that is critically important for the
metabolism of this agent

METABOLISM OF MERCAPTOPURINE (1)
• TMPT -Thiomethylpurinetransferase
– Conducts S-methylation of the substrate
– Found in RBC’s
– Isoforms exist
• active enzyme
• inactive enzyme
6-Mercaptopurine 6-Methylmercaptopurine
TMPT

AMINO ACID CONJUGATION
(mitochondria)

Multiple Metabolic Pathways Exist
for Aspirin’s Metabolism
Hydolysis of aspirin produces salicyclic acid, as
seen in the next slide

Salicyluric Acid is the Glycine Conjugate of Aspirin
Salicyluric acid, the glycine conjugate of salicyclic acid, is the main
metabolite of aspirin. Approximately 76% of aspirin is metabolized
through amino acid conjugation.

Acetyl Salicylic Acid (Aspirin) Metabolism
• Salicylic acid the hydrolytic product of acetyl salicylic
acid. Salicylic acid is further metabolized
• Salicyl uric acid is the glycine conjugate and the main
metabolite of aspirin. About 75% of aspirin is
metabolized by this pathway
• Other metabolites of aspirin
– the acyl glucuronide conjugate of salicylic acid (salicylic acid
glucuronide)
– the phenol glucuronide conjugate of salicylic acid (salicyl phenol
glucuronide)
– the ring hydroxylated product of salicylic acid (gentisic acid)
– the ring hydroxylated product of the glycine conjugate (gentisuric
acid

DRUG INTERACTION WITH GLUTATHIONE
mercapturate
metabolite of drug

MERCAPTURIC ACID FORMATION
• Conjugation of substrate to glutathione by the
enzyme glutathione transferase
• Hydrolytic removal of glutamic acid by glutamyl
transpeptidase
• Hydrolytic removal of glycine by cysteinyl glycinase
• Acetylation of the cysteinyl substrate by
N-acetyltransferase to form the N-acetylated cysteinyl
conjugate of substrate; substrate referred to as a
“mercapturate”

Bioactivation of Acetaminophen

ACETAMINOPHEN AND ITS PHASE II METABOLITES
The sulfate and glucuronide conjugates of acetaminophen are the major
metabolites. High doses of acetaminophen can exhaust the metabolic pathways
that produce these conjugates, allowing more of the parent drug to undergo the
phase I metabolic pathway which is involved in bioactivation and toxication.

ACETAMINOPHEN AND ITS PHASE I METABOLITES

ACETAMINOPHEN AND ITS PHASE I METABOLITES- pt2
The minor metabolite (4% of acetaminophen), N-hydroxyacetaminophen,
is always produced by microsomal cytochrome P450. It rearranges to
the electrophile N-acetylbenzoquinoneimine, which in turn reacts with
the sulfhydryl group of glutathione. Acetaminophen mercapturic acid is
the final metabolite. If tissue glutathione stores are depleted as a result
of fasting, intake of excessive doses of acetaminophen or through
induction of CYP2E1 as a result of chronic intake of ethanol, the
quinone interacts with nucleophilic sites of cellular macromolecules,
such as proteins. Liver necrosis is the result. Regular intake of
acetaminophen during fasting or chronic ethanol intake should be
avoided. N-acetylcysteine is the antidote for acetaminophen poisoning.
It reacts with the electrophile. A small amount of acetaminophen is
reported to undergo deacetylation to the phase 1 metabolite p-
aminophenol.

N-ACETYLCYSTEINE FOR ACETAMINOPHEN TOXICITY

Further Metabolism of N-HydroxyAAF Produces Cancer
N-HydroxyAAF undergoes phase II metabolism to the
ultimate carcingogen. The glucuronide pathway is also
involved in carcinogenesis

CYP1A1 Converts Benzopyrene to a Carcinogen

Aflatoxin is Metabolized to a Carcinogenic Agent

CORRELATION BETWEEN SLEEPING TIME AND PLASMA
T1/2 IN CHRONIC PENTOBARBITAL PRETREATED RABBITS

• Enzyme Induction - increased enzyme protein levels
in the cell
– Phenobarbital type induction by many drugs
– Polycyclic hydrocarbon type induction by
polycyclic hydrocarbons such as 3,4-benzopyrene
and 3-methylcholanthrene

• Age
– Neonates
– Children
– Elderly

• Diet
– Charcoal broiled foods (contain polycyclic
hydrocarbons that increase certain enzyme protein in
cells)
– Grapefruit juice (the active component is the
furancoumarin 6,7-dihydroxybergamottin which
inhibits a certain a group of microsomal enzymes)

Some Enzymes That Exhibit Genetic Variation
– Pseudocholinesterase
• typical enzyme
• atypical enzyme
– N-Acetyltransferase (isoniazid is a substrate)
• fast acetylation
• slow acetylation
– Cytochrome P450 2D6
– Cytochrome P450 2C19
– TMPT -Thiomethylpurinetransferase
– Dihydropyrimidine Dehydrogenase

• State of health
– Hepatitis
– Liver cancer
– Cardiac insufficiency
– Uremia
• degree of protein binding

Changes In Drug Metabolism As A Consequence Of Hepatic Disease
From Principles of Drug Action

• Gender
– Most studies are performed in the rat. In general,
male rats metabolize drugs faster than female rats

• Degree of protein binding
– Conditions that displace bound drug from protein
allows more of the drug to be accessible to the
enzyme for which it serves as a substrate e.g.
uremia, low plasma albumin

• Species variation
– Quantitative
– Qualitative

• Species variation
– Human beings metabolize amphetamine by
deamination; rats and dogs metabolize the drug by
aromatic hydroxylation
– Guinea pigs have very little sulfotransferase
activity, humans have substantial activity
– Guinea pigs do not N-hydroxylate substrates;
mice, rabbits, dogs do
– Hexobarbital is metabolized at different rates by
different species

• Substrate competition
– Two or more drugs competing for the same
enzyme can affect the metabolism of each other;
the substrate for which the enzyme has the
greater affinity would be preferentially metabolized

Drug Metabolism Enzymes and Pathways Explained in Detail

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