Drug Metabolising Enzymes
Drug Metabolising Enzymes
METABOLISM OR BIOTRANSFORMATION
The conversion from one chemical form of a substance to another.
The term metabolism is commonly used probably because products of drug
transformation are called metabolites.
Metabolism is an essential pharmacokinetic process, which renders lipid soluble and
non-polar compounds to water soluble and polar compounds so that they are
excreted by various processes.
This is because only water-soluble substances undergo excretion, whereas lipid
soluble substances are passively reabsorbed from renal or extra renal excretory sites
into the blood by virtue of their lipophilicity.
Metabolism is a necessary biological process that limits the life of a substance in the
Biotransformation: It is a specific term used for chemical transformation of
xenobiotics in the body/living organism.
• a series of enzyme-catalyzed processes—that alters the physiochemical properties of
foreign chemicals (drug/xenobiotics) from those that favor absorption across biological
membranes (lipophilicity) to those favoring elimination in urine or bile (hydrophilicity )
Metabolism : It is a general term used for chemical
transformation of xenobiotics and endogenous
nutrients (e.g., proteins, carbohydrates and fats) within
or outside the body.
Xenobiotics : These are all chemical substances that
are not nutrient for body (foreign to body) and which
enter the body through ingestion, inhalation or dermal
They include :
drugs, industrial chemicals, pesticides, pollutants,
plant and animal toxins, etc.
Functions of Biotransformation
It causes conversion of an
active drug to inactive or
less active metabolite(s)
called as pharmacological
It causes conversion of an
active to more active
metabolite(s) called as
• It causes conversion of an
inactive to more active
toxic metabolite(s) called
as lethal synthesis
Functions of Biotransformation….contd
• It causes conversion of an
inactive drug (pro-drug) to
active metabolite(s) called
• It causes conversion of an
active drug to equally active
metabolite(s) (no change in
• It causes conversion of an
active drug to active
(change in pharmacological
Site/Organs of drug metabolism
The major site of drug metabolism is the liver
(microsomal enzyme systems of hepatocytes)
Secondary organs of biotransformation
• kidney (proximal tubule)
• lungs (type II cells)
• testes (Sertoli cells)
• skin (epithelial cells); plasma. nervous tissue
Sites of Biotransformation…contd
The primary site for metabolism of almost all drugs because it is relatively
rich in a large variety of metabolising enzymes.
Metabolism by organs other than liver (called as extra-hepatic metabolism)
is of lesser importance because lower level of metabolising enzymes is
present in such tissues.
Within a given cell, most drug metabolising activity is found in the smooth
endoplasmic reticulum and the cytosol.
Drug metabolism can also occur in mitochondria, nuclear envelope and
A few drugs are also metabolised by non-enzymatic means called as nonenzymatic metabolism.
For example, atracurium, a neuromuscular blocking drug, is inactivated in
plasma by spontaneous non-enzymatic degradation (Hoffman elimination)
in addition to that by pseudocholinesterase enzyme.
ENDOPLASMIC RETICULUM (microsomes): the primary location for the
(a) Phase I: cytochrome P450, flavin-containing monooxygenase, aldehydeoxidase,
carboxylesterase, epoxide hydrolase, prostaglandin synthase, esterase.
(b) Phase II uridine diphosphate-glucuronosyltransferase, glutathione Stransferase, amino acid conjugating enzymes.
CYTOSOL (the soluble fraction of the cytoplasm): many water-soluble enzymes.
(a) Phase I: alcohol dehydrogenase, aldehyde reductase, aldehyde dehydrogenase,
epoxide hydrolase, esterase.
(b) Phase 11: sulfotransferase, glutathione S-transferase, N-acetyl transferase,
catechol 0-methyl transferase, amino acid conjugating enzymes.
(a) Phase I: monoamine oxidase, aldehyde dehydrogenase, cytochrome P450.
(b) Phase II: N-acetyl transferase, amino acid conjugating enzymes.
LYSOSOMES. Phase I: peptidase.
Phase II: uridine diphosphate-glucuronosyltransferase (nuclear membrane of
Drug Metabolising Enzymes
A number of enzymes in animals are capable of metabolising
drugs. These enzymes are located mainly in the liver, but may
also be present in other organs like lungs, kidneys, intestine,
brain, plasma, etc.
Majority of drugs are acted upon by relatively non-specific
enzymes, which are directed to types of molecules rather than
to specific drugs.
The drug metabolising enzymes can be broadly divided into two
groups: microsomal and non-microsomal enzymes.
Microsomal enzymes: The endoplasmic reticulum (especially
smooth endoplasmic reticulum) of liver and other tissues
contain a large variety of enzymes, together called microsomal
(microsomes are minute spherical vesicles derived from
endoplasmic reticulum after disruption of cells by
centrifugation, enzymes present in microsomes are called
They catalyse glucuronide conjugation, most oxidative
reactions, and some reductive and hydrolytic reactions.
The monooxygenases, glucuronyl
important microsomal enzymes.
(microsomes) are called non-microsomal enzymes.
These are usually present in the cytoplasm, mitochondria, etc.
and occur mainly in the liver, Gl tract, plasma and other tissues.
They are usually non-specific enzymes that catalyse few
oxidative reactions, a number of reductive and hydrolytic
reactions, and all conjugative reactions other than
None of the non-microsomal enzymes involved in drug
biotransformation is known to be inducible.
Extrahepatic microsomal enzymes
Hepatic microsomal enzymes
Hepatic non-microsomal enzymes
Factors Affecting Drug Metabolism
1. Species differences : eg in phenylbutazone, procaine and
2. Genetic differences – variation exist with species
3. Age of animal –feeble in fetus,aged, newborn.
4.sex: under the influence of sex hormones.
5. Nutrition: starvation and malnutrition
6. Patholigical conditions: Liver/Kidney dysfunction
TYPES OF BIOTRANSFORMATION
Phase 1 reaction. (Non synthetic phase).
• a change in drug molecule. generally
results in the introduction of a
functional group into molecules or the
exposure of new functional groups of
: Phase I (non-synthetic or nonconjugative phase) includes reactions
which catalyse oxidation, reduction and
hydrolysis of drugs.
In phase I reactions, small polar
functional groups like-OH, -NH2. -SH, COOH, etc. are either added or
unmasked (if already present) on the
lipid soluble drugs so that the resulting
products may undergo phase II
• result in activation, change or
inactivation of drug.
Phase II reaction. (Synthetic phase)
• Last step in detoxification reactions
and almost always results in loss of
biological activity of a compound.
• May be preceded by one or more of
phase one reaction
• Involves conjugation of functional
groups of molecules with hydrophilic
endogenous substrates- formation
of conjugates - is formed with (an
endogenous substance such as
carbohydrates and amino acids. )with
drug or its metabolites formed in
phase 1 reaction.
Involve attachment of small polar
endogenous molecules like glucuronic
acid, sulphate, methyl, amino acids,
etc., to either unchanged drugs or
phase I products.
Products called as 'conjugates' are
water-soluble metabolites, which are
readily excreted from the body.
Phase I metabolism is sometimes called a •
Results in the introduction of new
hydrophilic functional groups to compounds. •
Function: introduction (or unveiling) of
functional group(s) such as –OH, –NH2, –SH,
–COOH into the compounds.
Reaction types: oxidation, reduction, and
Oxygenases and oxidases: Cytochrome P450 (P450
or CYP), flavincontaining
monooxygenase (FMO), peroxidase, monoamine
oxidase(MAO), alcohol dehydrogenase, aldehyde
dehydrogenase, and xanthine 0xidase. Reductase:
Aldo-keto reductase and quinone reductase.
Hydrolytic enzymes: esterase, amidase, aldehyde
oxidase, and alkylhydrazine
Enzymes that scavenge reduced oxygen:
Superoxide dismutases, catalase,
glutathione peroxidase, epoxide hydrolase, yglutamyl transferase,
dipeptidase, and cysteine conjugate β-lyase
Phase II metabolism includes what are known
as conjugation reactions.
Generally, the conjugation reaction with
endogenous substrates occurs on the
metabolite( s) of the parent compound after
phase I metabolism; however, in some cases,
the parent compound itself can be subject to
phase II metabolism.
Function: conjugation (or derivatization) of
functional groups of a compound or its
metabolite(s) with endogenous substrates.
Reaction types: glucuronidation, sulfation,
methylation and conjugation with amino acids
(e.g., glycine, taurine, glutamic acid).
Enzymes: Uridine diphosphate-Glucuronosyltransferase
(UDPGT): sulfotransferase (ST), N-acetyltransferase,
glutathione S-transferase (GST),methyl transferase, and
amino acid conjugating enzymes.
diphosphateglucuronosyltransferase; Sulfation by sulfotransferase
3. Acetylation by N-acetyltransferase;
conjugation by glutathione S-transferase;. Methylation by
methyl transferase; Amino acid conjugation
PHASE I BIOTRANSFORMATION
• Oxidation by cytochrome P450 isozymes (microsomal
• Oxidation by enzymes other than cytochrome P450s—most of these
• (a) oxidation of alcohol by alcohol dehydrogenase,
• (b) oxidation of aldehyde by aldehyde dehydrogenase,
• (c) N-dealkylation by monoamineoxidase.
Phase I Reactions
• Oxidative reactions are most important metabolic reactions, as
energy in animals is derived by oxidative combustion of organic
molecules containing carbon and hydrogen atoms.
• The oxidative reactions are important for drugs because they
increase hydrophilicity of drugs by introducing polar functional
groups such as -OH.
• Oxidation of drugs is non-specifically catalysed by a number of
enzymes located primarily in the microsomes. Some of the
oxidation reactions are also catalysed by non-microsomal enzymes
(e.g., aldehyde dehydrogenase, xanthine oxidase and monoamine
The most important group of oxidative enzymes are microsomal
monooxygcnases or mixed function oxidases (MFO).
These enzymes are located mainly in the hepatic endoplasmic
reticulum and require both molecular oxygen (02) and reducing
NADPH to effect the chemical reaction.
Mixed function oxidase name was proposed in order to
characterise the mixed function of the oxygen molecule, which is
essentially required by a number of enzymes located in the
The term monooxygenses for the enzymes was proposed as they
incorporate only one atom of molecular oxygen into the organic substrate
with concomitant reduction of the second oxygen atom to water.
The overall stoichiometry of the reaction involving the substrate RH which
yields the product ROH, is given by the following reaction:
RH+02+NADPH+H+ ----------------► R0H+H20+NADP+
The most important component of mixed function oxidases is the
cytochrome P-450 because it binds to the substrate and activates oxygen.
The wide distribution of cytochrome P-450 containing MFOs in varying
organs makes it the most important group of enzymes involved in the
biotransformation of drugs.
PHASE I ENZYMES
(Cytochrome P450, P450,
• Alcohol Dehydrogenase
PHASE II ENZYMES
• Sulfotransferase (ST)
• N-Acetyltransferase (NAT)
• Methyl Transferase
• Amino Acid Conjugation
The cytochrome P-450 ENZYMES
• Superfamily of haem-thiolate proteins that are widely distributed
across all living creatures.
• The name given to this group of proteins because their reduced
form binds with carbon monoxide to form a complex, which has
maximum absorbance at 450 nm.
• Depending upon the extent of amino acid sequence homology, the
cytochrome P-450 (CYP) enzymes superfamily contains a number of
isoenzymes, the relative amount of which differs among species and
among individuals of the same species.
• These isoenzymes are grouped into various families designated by
Arabic numbers 1, 2, 3 (sequence that are greater than 40% identical
belong to the same family), each having several subfamilies
designated by Capital letter A, B, C, while individual isoenzymes are
again allotted Arabic numbers 1.2,3 (e.g., CYP1A1, CYP1A2, etc.).
ROLE OF CYP ENZYMES IN HEPATIC DRUG METABOLISM
In human beings, of the 1000 currently known cytochrome P-450s, about 50 are functionally
active. These are categorised into 17 families, out of which the isoenzymes CYP3A4 and CYP2D6
carry out biotransformation of largest number of drugs.
RELATIVE HEPATIC CONTENT
OF CYP ENZYMES
% DRUGS METABOLIZED
BY CYP ENZYMES
Participation of the CYP Enzymes in Metabolism of Some
Clinically Important Drugs
CYP Enzyme Examples of substrates
Caffeine, Testosterone, R-Warfarin
Acetaminophen, Caffeine, Phenacetin, R-Warfarin
Cyclophosphamide, Erythromycin, Testosterone
Acetaminophen, Tolbutamide (2C9); Hexobarbital, SWarfarin (2C9,19); Phenytoin, Testosterone, R- Warfarin,
Acetaminophen, Caffeine, Chlorzoxazone, Halothane
Acetaminophen, Codeine, Debrisoquine
Acetaminophen, Caffeine, Carbamazepine, Codeine,
Cortisol, Erythromycin, Cyclophosphamide, S- and RWarfarin, Phenytoin, Testosterone, Halothane, Zidovudine
2. Reduction :
Enzymes responsible for reduction of xenobiotics require NADPH as a cofactor.
Substrates for reductive reactions include azo- or nitrocompounds, epoxides,
heterocyclic compounds, and halogenated hydrocarbons:
(a) Azo or nitroreduction by cytochrome P450;
(b) Carbonyl (aldehyde or ketone) reduction by aldehyde reductase, aldose
reductase, carbonyl reductase, quinone reductase
(c) other reductions including disulfide reduction, sulfoxide reduction, and
The acceptance of one or more electron(s) or their equivalent from another
Reductive reactions, which usually involve addition of hydrogen to the drug
molecule, occur less frequently than the oxidative reactions.
Biotransformation by reduction is also capable of generating polar functional
groups such as hydroxy and amino groups, which can undergo further biotransformation.
Many reductive reactions are exact opposite of the oxidative reactions
(reversible reactions) catalysed cither by the same enzyme (true reversible
reaction) or by different enzymes (apparent reversible reactions).
Such reversible reactions usually lead to conversion of inactive metabolite
into active drug, thereby delaying drug removal from the body.
3. Hydrolysis :
Esters, amides, hydrazides, and carbamates can be hydrolyzed by
The hydrolytic reactions, contrary to oxidative or reductive
reactions, do not involve change in the state of oxidation of the
substrate, but involve the cleavage of drug molecule by taking up
a molecule of water.
The hydrolytic enzymes that metabolise drugs are the ones that
act on endogenous substances, and their activity is not confined
to liver as they are found in many other organs like kidneys,
intestine, plasma, etc.
A number of drugs with ester, ether, amide and hydrazide
linkages undergo hydrolysis. Important examples are
cholinesters, procaine, procainamide, and pethidine.
PHASE II REACTIONS
Phase II or conjugation (Latin, conjugatus = yoked together)
reactions involve combination of the drug or its phase I
metabolite with an endogenous substance to form a highly polar
product, which can be efficiently excreted from the body.
In the biotransformation of drugs, such products or metabolites
have two parts:
Exocon, the portion derived from exogenous compound or
Endocon, the portion derived from endogenous substance.
Conjugation reactions have high energy requirement and they
often utilise suitable enzymes for the reactions.
The endogenous substances (endocons) for conjugation
reactions are derived mainly from carbohydrates or amino acids
and usually possess large molecular size.
They are strongly polar or ionic in nature in order to render the
substrate water-soluble. The molecular weight of the conjugate
(metabolite) is important for determining its route of excretion.
High molecular weight conjugates are excreted predominantly in
bile (e.g., glutathione exclusively, glucuronide mainly),
while low molecular weight conjugates are excreted mainly in the
As the availability of endogenous conjugating substance is limited,
saturation of this process is possible and the unconjugated
drug/metabolite may precipitate toxicity.
1. Conjugation with glucuronic a./ Glucuronidation
Conjugation with glucuronic acid (glucuronide conjugation or
glucuronidation) is the most common and most important
phase II reaction in vertebrates, except cats and fish.
The biochemical donor (cofactor) of glucuronic acid is uridine
diphosphate«-D-glucuronicacid (UDPGA) and the reaction is
carried out by enzyme uridine diphosphate-glucuronyl
Glucuronyl transferase is present in microsomes of most
tissues but liver is the most active site of glucuronide
Glucuronidation can take place in most body tissues because
the glucuronic acid donor UDPGA is present in abundant
quantity in body, unlike donors involved in other phase II
In cats, there is reduced glucuronyl transferase activity, while
in fish there is deficiency of endogenous glucuronic acid donor.
The limited capacity of this metabolic pathway in cats may
increase the duration of action, pharmacological response and
potential of toxicity of several lipid-soluble drugs (e.g., aspirin)
in this species.
A large number of drugs undergo glucuronidation including
morphine, paracetamol and desipramine. Certain endogenous
substances such as steroids, bilirubin, catechols and thyroxine
also form glucuronides.
Deconjugaiion process: Occasionally some glucuronide
conjugates that are excreted in bile undergo deconjugation
process in the intestine mainly mediated by β glucuronidase
This releases free and active drug in the intestine, which may be
reabsorbed and undergo entero-hepatic cycling.
Deconjugation is an important process because it often prolongs
the pharmacological effects of drugs and/or produces toxic
2. Conjugation with sulphate/ Sulphation:
sulphoconjugation orsulphation) is similar to glucuronidation
but is catalysed by non-microsomal enzymes and occurs less
The endogenous donor of the sulphate group is 3'phosphoadenosine-5'-phosphosulphate (PAPS), and enzyme
catalysing the reaction is sulphotransferase
The conjugates of sulphate are referred to as sulphate ester
conjugates or ethereal sulphates. Unlike glucuronide
conjugation, sulphoconjugation in mammals is less important
because the PAPS donor that transfers sulphate to the substrate
is easily depleted.
Capacity for sulphate conjugation is limited in pigs. However
in cats, where glucuronidation is deficient, sulphate conjugation
is important. Functional groups capable of forming sulphate
conjugates include phenols, alcohols, arylamines, Nhydroxylamines and N-hydroxyamides.
chloramphenicol, phenols, and adrenal and sex steroids.
3. Conjugation with methyl group/ Methylation :
Conjugation with methyl group (methyl conjugation or
methylation) involves transfer of a methyl group (-CH3) from the
cofactor S-adenosyl methionine (SAM) to the acceptor substrate
by various methyl transferase enzymes.
Methylation reaction is of lesser importance for drugs, but is
more important for biosynthesis (e.g., adrenaline, melatonin)
and | Inactivation (e.g., histamine) of endogenous amines.
Occasionally, the metabolites formed are not polar or watersoluble and may possess equal or greater activity than the
parent compound (e.g., adrenaline synthesised from
4. Conjugation with glutathione and mercapturic acid formation.
Conjugation with glutathione (glutathione conjugation) and mercapturic acid formation
is a minor but important metabolic pathway in animals.
Glutathione (GSH, G=glutathione and SH = active-SH group) is a tripeptide having
glutamic acid, cysteine and glycine.
It has a strong nucleophilic character due to the presence of a -SH (thiol) group in its
structure. Thus, it conjugates with electrophilic substrates, a number of which are
potentially toxic compounds, and protects the tissues from their adverse effects.
The interaction between the substrate and the GSH is catalysed by enzyme glutathioneS-transferase, which is located in the soluble fraction of liver homogenates.
The glutathione conjugate either due to its high molecular weight is excreted as such in
the bile or is further metabolised to form mercapturic acid conjugate that is excreted in
5. Conjugation with acetyl group/ Acetylation :
Conjugation with acetyl group (acetylation) is an important
metabolic pathway for drugs containing the amino groups.
The cofactor for these reactions is acetyl coenzyme A and the
enzymes are non-microsomal N-acetyl transferases, located in
the soluble fraction of cells of various tissues.
Acetylation is not a true detoxification process because
occasionally it results in decrease in water solubility of an amine
and. thus, increase in its toxicity (e.g., sulphonamides).
Acetylation is the primary route of biotransformation of
sulphonamide compounds. Dogs and foxes do not acetylate
the aromatic amino groups due to deficiency of arylamine
Conjugation with amino acids : Conjugation with amino acids
occurs to a limited extent in animals because of limited
availability of amino acids. The most important reaction
involves conjugation with glycine.
Conjugation with other amino acids like glutamine in man and
ornithine in birds is also seen.
Examples of drugs forming glycine or glutamine conjugates are
salicylic acid, nicotinic acid and cholic acid.
Conjugation with thiosulphate : Conjugation with thiosulphate is
an important reaction in the detoxification of cyanide. Conjugation
of cyanide ion involves transfer of sulphur atom from the
thiosulphate to the cyanide ion in presence of enzyme rhodancse
to form inactive thiocyanate.
Thiocyanate formed is much less toxic than the cyanide (true
detoxification) and it is excreted in urine.
INDUCTION OF METABOLISM
Administration of certain xenobiotics sometimes results in a
selective increase in the concentration of metabolizing enzymes in
both phase I and II metabolism, and thereby in their activities
Enzyme induction becomes important especially when
polypharmacy involves drugs with narrow therapeutic windows, since
the induced drug metabolism could result in a significant decrease in
its exposure and therapeutic effects.
In addition, enzyme induction may cause toxicity, associated with
increased production of toxic metabolites.
Mechanisms of Induction
Stimulation of transcription of genes and/or translation of proteins,
and/or stabilization of mRNA and/or enzymes by inducers, resulting in
elevated enzyme levels.
Stimulation of preexisting enzymes resulting in apparent
enzyme induction without an increase in enzyme synthesis (this
is more common in vitro than in vivo).
In many cases, the details of the induction mechanisms are
TWO receptors have been identified for CYPlA1/2 and
(a) Ah (aromatic hydrocarbon) receptor in cytosol, which
regulates enzyme (CYP1 A1 and 1A2) induction by polycyclic
aromatic hydrocarbon (PAH)-type inducers;
(b) Peroxisome proliferator activated receptor (PPAR), where
hypolipidemic agents cause peroxisome proliferation in rats
(CYP4A1 and 4A2);-humans have low PPAR and show no
effects from hypolipidemic agents.
Characteristics of Induction
Induction is a function of intact cells and cannot be achieved by treating
isolated cell fractions such as microsomes with inducers.
Evaluation of enzyme induction is usually conducted in ex vivo experiments,
ie., treating animals in vivo with potential inducers and measuring enzyme
activities in vitro or in cell-based in vitro preparations such as hepatocytes, liver
slices, or cell lines.
Recent studies have demonstrated that primary cultures of hepatocytes can
be used for studying the inducibility of metabolizing enzymes such as P450 under
certain incubation conditions
Enzyme induction is usually inducer-concentration–dependent. The extent of
induction increases as the inducer concentration increases; however, above
certain values, induction starts to decline.
In general, inducers increase the content of endoplasmic reticulum within
hepatocytes as well as liver weight.
In some cases, an inducer induces enzymes responsible for its own
metabolism (so-called “autoinduction”).
Induction of Drug Metabolising Enzymes
Several drugs and chemicals have ability to increase the drug
metabolising activity of enzymes called as enzyme induction.
These drugs known as enzyme inducers mainly interact with DNA and
increase the synthesis of microsomal enzyme proteins, especially
cytochrome P-450 and glucuronyl transferase.
As a result, there is enhanced metabolism of endogenous substances
(e.g., sex steroids) and drugs metabolised by microsomal enzymes.
Some drugs (e.g., carbamazepine and rifampicin) may stimulate their
own metabolism, the phenomenon being called as auto-induction or
Since different cytochrome P450 isoenzymes are involved in the
metabolism of different drugs, enzyme induction by one drug affects
metabolism of only those drugs, which are substrate for the induced
However, some drugs like Phenobarbitone may affect metabolism of a
large number of drugs because they induce isoenzymes like CYP3A4 and
CYP2D6 which act on many drugs.
Enzyme inducers are generally lipid-soluble compounds with relatively long
Repeated administration of inducers for a few days (3 to 10 days) is often
required for enzyme induction, and on stoppage of drug administration,
the enzymes return to their original value over 1 to 3 weeks.
Non-microsomal enzymes are not known to be induced by any drug or
Clinical importance of enzyme induction
It reduces efficacy and potency of drugs metabolised by these
It reduces plasma half-life and duration of action of drugs.
It enhances drug tolerance.
It increases drug toxicity by enhancing concentration of
metabolite, if metabolite is toxic.
It increases chances of drug interactions.
It alters physiological status of animal due to altered metabolism
of endogenous compounds like sex steroids.
Inhibition of Drug Metabolising Enzymes
Contrary to metabolising enzyme induction, several drugs or
chemicals have the ability to decrease the drug metabolising activity of
certain enzymes called as enzyme inhibition.
Enzyme inhibition can be either non-specific of microsomal enzymes
or specific of some non-microsomal enzymes (e.g., monoamine oxidase,
cholinesterase and aldehyde dehydrogenase).
The inhibition of hepatic microsomal enzymes mainly occurs due to
administration of hepatotoxic agents,
which cause either rise in the rate of enzyme degradation (e.g., carbon
tetrachloride and carbon disulphide) or fall in the rate of enzyme synthesis
(e.g., Puromycin and Dactinomycin).
Nutritional deficiency, hormonal imbalance or hepatic
dysfunction, etc.also inhibit microsomal enzymes indirectly.
Inhibition of non-microsomal enzymes with specific function
usually results when Structurally similar compounds compete for
the active site on the enzymes.
Such an inhibition is usually rapid (a single dose of inhibitor
may be sufficient) and clinically more important than the nonspecific microsomal enzyme inhibition.
Enzyme inhibition generally results in depressed metabolism
As a result, the plasma hall-life, duration of action, and efficacy
as well toxicity of the object drug (whose metabolism has been
inhibited) are significantly enhanced.
In case the drug undergoes hepatic first-pass effect, the
bioavailability and toxicity Of the drug will be markedly increased
in presence of enzyme inhibition. Enzyme inhibition may also
produce undesirable drug interactions.
In therapeutics, some specific enzyme inhibitors like
monoamine oxidase inhibitors, cholinesterase inhibitors and
angiotensin converting enzyme (ACE) inhibitors are purposely
used for producing desirable pharmacological actions
In general, enzyme inducers are lipophilic at physiological pH and
exhibit relatively long t 1/2 with high accumulation in the liver.
Different classes of enzyme inducers.
1. Barbiturates: Phenobarbitone, Phenobarbital.
2. Polycyclic aromatic hydrocarbons (PAH): 3-methylcholanthrene (3-MC),
2,3,7,8,-tetrachlorodibenzo-p-dioxin (TCDD), β-naphthoflavone β ( -NF).
3. Steroids: Pregnenalone 16-α -carbonitrile (PCN), Dexamethasone.
4. Simple hydrocarbons with aliphatic chains: Ethanol (chronic), Acetone,
5. Hypolipidemic agents: Clofibrate, lauric acids.
6. Macrolide antibiotics: Triacetyloleandomycin (TAO).
7. A wide variety of structurally unrelated compounds: e.g., Antipyrine,
Carisoniazid. Bamazepine, Phenytoin, and Rifampicin
Most tissues have some metabolic activity; however,
quantitatively the liver is by far the most important organ for drug
Important organs for extrahepatic metabolism include the
intestine (enterocytes and intestinal microflora), kidney, lung,
plasma, blood cells, placenta, skin, and brain.
In general, the extent of metabolism in the major extrahepatic
drug-metabolizing organs such as the small intestine, kidney, and
lung is approximately 10–20% of the hepatic metabolism.
Less than 5% of extrahepatic metabolism compared to hepatic
metabolism can be considered low with negligible
First-Pass Effect/First-Pass Metabolism
First-pass effect (first-pass metabolism or pre-systemic metabolism) may be defined
as the loss of drug through biotransformation before it enters systemic circulation.
This may occur during passage of drug for first time (therefore called first-pass
effect/metabolism) through intestine or liver after oral administration.
Intestinal first-pass effect: In this type, drugs are metabolised in the gastrointestinal
tract by enzymes present in either gut mucosa or gut lumen before they are
Recent studies have indicated that P450 isoforms such as CYP2C19 and 3A4 in
enterocytes might play an important role in the presystemic intestinal metabolism of
drugs and the large interindividual variability in systemic exposure after oral
The cytochrome P450 content of the intestine is about 35% of the hepatic content
in the rabbit, but accounts for only 4% of the hepatic content in the mouse.
Cytochrome P450 levels and activities are highest in the duodenum near the
pyrolus, and then decrease toward the colon
A similar trend in regional activity levels along the intestine has been observed for
glucuronide, sulfate, and glutathione conjugating enzymes.
Microorganisms present in the GI tract also inactivate some drugs.
Such drugs are not suitable by oral administration due to poor
bioavailability (e.g., catecholamines).
Hepatic first-pass effect: In this type, drugs are suitably absorbed
across the GI tract and enter portal circulation, but they are rapidly
and significantly metabolised during the first passage through the
(Normally, when a drug is absorbed across GI tract, it first enters the
portal vein and passes through liver before it reaches the systemic
Such drugs are also not/less suitable by oral administration due to
their poor bioavailability. Examples of drugs undergoing significant
hepatic first-pass effect include Propranolol, Lignocaine and Nitroglycerine.
The rate and extent of first-pass intestinal metabolism of a drug
after oral administration are dependent on various physiological
1. Site of absorption: If the absorption site in the intestine is different
from the metabolic site, first-pass intestinal metabolism of a drug
may not be significant.
2. Intracellular residence time of drug molecules in enterocytes: The
longer the drug molecules stay in the enterocytes prior to entering
the mesenteric vein, the more extensive the metabolism.
3. Diffusional barrier between splanchnic bed and enterocytes: The
lower the diffusibility of a drug from the enterocytes to the
mesenteric vein, the longer its residence time.
4. Mucosal blood flow: Blood in the splanchnic bed can act as a sink
to carry drug molecules away from the enterocytes, which reduces
intracellular residence time of drug in the enterocytes
In addition to physiological functions of homeostasis in water and
electrolytes and the excretion of endogenous and exogenous compounds
from the body, the kidneys are the site of significant biotransformation
activities for both phase I and phase II metabolism.
The renal cortex, outer medulla, and inner medulla exhibit different
profiles of drug metabolism, which appears to be due to heterogeneous
distribution of metabolizing enzymes along the nephron.
Most metabolizing enzymes are localized mainly in the proximal tubules,
although various enzymes are distributed in all segments of the nephron
The pattern of renal blood flow, pH of the urine, and the urinary
concentrating mechanism can provide an environment that facilitates the
precipitation of certain compounds, including metabolites formed within the
The high concentration or crystallization of xenobiotics and/or their
metabolites can potentially cause significant renal impairment in specific
regions of the kidneys.
Metabolism in Blood
Blood contains various proteins and enzymes.
As metabolizing enzymes, esterases, including cholinesterase,
arylesterase, and carboxylesterase, have the most significant effects
on hydrolysis of compounds with ester, carbamate, or phosphate
bonds in blood .
Esterase activity can be found mainly in plasma, with less activity in
red blood cells.
Plasma albumin itself may also act as an esterase under certain
For instance, albumin contributes about 20% of the total hydrolysis
of aspirin to salicylic acid in human plasma.
The esterase activity in blood seems to be more extensive in small
animals such as rats than in large animals and humans. Limited, yet
significant monoamine oxidase activities can be also found in blood.