metabolism and biotransformation of xenobiotics/drugs

1. Metabolism
and
Biotransformation
Submitted by : Chintan K. Patel
Roll no. : 21
M.Sc. Zoology Sem–2
Paper – 409 (Unit–3)
Submitted to : Dr. K. R. Desai ma’m
Department of Zoology, Biomedical Technology & HG, University School of
Sciences, Gujarat University, Navrangpura, Ahmedabad.

2.  Introduction
• The conversion from one chemical form of a substance to another.
• Metabolism is an essential pharmacokinetic process, which renders
lipid soluble & non-polar compounds to water soluble & polar
compounds so that they are excreted by various processes.
DRUG MORE POLAR DRUG EXCRETION
• Metabolism is a necessary biological process that limits the life of
substance in the body.
metabolism

3.  Definitions
 Biotransformation : It is a specific term used for chemical
transformation of xenobiotics in the body or living organism.
 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 exposure. Xenobiotics includes drugs,
industrial chemicals, pesticides, pollutants, plant and animal toxin,
etc.
 Metabolism : It is a general term used for chemical transformation of
xenobiotics & endogenous nutrients (e.g. proteins, carbohydrates &
fats) within or outside the body.

4.  Site/Organs of Drug Metabolism :
• The major site of drug metabolism is the liver (microsomal
enzyme systems of hepatocytes).
• Secondary organs of biotransformation are following :
- Kidney (proximal tubule)
- Lungs (type-II cells)
- Testes (sertoli cells)
- Skin (epithelial cells)
- Plasma
- Nervous tissue (brain)
- Intestine

5.  Drug Metabolising Enzymes :
• A number of enzymes in animals are capable of metabolising drugs.
• These enzymes can be divided into two groups :
(i) Microsomal enzymes
(ii) Non-microsomal enzymes
(i) Microsomal enzymes : The endoplasmic reticulum (especially
SER) of liver and other tissues contain a large variety of enzymes,
together called microsomal enzymes.
• They catalyze: Phase-I reactions - Oxidation, Reduction,
Hydrolysis
Phase-II reactions - Glucuronide conjugation

6. (ii) Non-microsomal enzymes : These are usually present in the
cytoplasm, mitochondria, etc. and occur mainly in the liver, GI
tract, Plasma and other tissues.
• They catalyze all Phase-II reactions except glucuronide
conjugation.
• They are non-inducible enzymes.
• None of the non-microsomal enzymes involved in the drug
biotransformation is known as Inducible.

7. Types of Biotransformation :
Phase-I (Non synthetic phase)
• A change in drug molecule,
generally introduction of new
functional group(-OH,
-SH,-COOH etc.) into
molecule.
• It results in activation, change
or inactivation of drug.
• It includes reactions which
catalyse oxidation, reduction
and hydrolysis of drugs.
Phase-II (Synthetic phase)
• Last step in detoxification reactions
& almost always results in loss of
biological activity of a compound.
• 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, which are excreted
from the body.

8. PHASE-I ENZYMES
• Cytochrome P450
Monooxygenase
• Flavin containing
Monooxygenase(FMO)
• Esterase
• Alcohol
Dehydrogenase(ADH)
• Aldehyde
Dehydrogenase(ALDH)
• Monoamine Oxidase(MAO)
PHASE-II ENZYMES
• Uridine Diphosphate
Glucuronosyltransferase
(UDPGT)
• Sulfotransferase(ST)
• N-Acetyltransferase(NAT)
• Glutathione S-Transferase
(GST)
• Methyl Transferase
• Amino Acid Conjugation

9.  Phase - I Reactions :
(1) Oxidation :
• The oxidative reactions are important for drugs because they
increase hydrophilicity of drugs by introducing polar functional
groups such as -OH.
• The most important group of oxidative enzymes are microsomal
monooxygenases or mixed function oxidases (MFO).
• These enzymes are located mainly in the hepatic ER and require
both molecular oxygen (O2) and reducing NADPH to effect the
chemical reaction.

10. • The substrate RH which yields the product ROH, is given by the
following reaction :
RH + O2 + NADPH + H+ ROH + H2O + NADP+
• The most important component of mixed function oxidases is the
cytochrome P-450 because it binds to the substrate and activates
oxygen.
• Heme-containing proteins complex formed between Fe2+ and CO
absorbs light maximally at 450nm .
MFO

12. (2) Reduction :
• Enzymes responsible for reduction of xenobiotics require NADPH
a cofactor.
• Substrates for reductive reactions include azo- or nitrocompounds,
epoxides, heterocyclic compounds and halogenated
• Reductive reactions, which usually involve addition of hydrogen to
the drug molecule, occur less frequently than the oxidative
reactions.
• Many reductive reactions are exact opposite of the oxidative
reactions (reversible reactions) catalyzed either by the same
or by different enzymes.
• Such reversible reactions usually lead to conversion of inactive
metabolite into active drug, thereby delaying drug removal from
body.

14. (3) Hydrolysis :
• The hydrolytic reactions, opposite 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
molecule of water.
• The hydrolytic enzymes that metabolise drugs are the ones that
on endogenous substances and their activity is not confined to
as they found in many other organs like kidneys, intestine, plasma,
etc.
• A number of drugs with ester, ether, amide and hydrazide linkages
undergo hydrolysis.

16.  Phase - II Reactions :
• Phase-II or conjugation reactions involve combination of drug or its
phase-I metabolite with an endogenous substances to form a highly
polar product, which can be excreted from the body.
 EXOCON : The portion derived from exogenous compound or
xenobiotic.
 ENDOCON : The portion derived from endogenous substance.
• High molecular weight conjugates are excreted predominantly in
bile.
• Low molecular weight conjugates are excreted mainly in the urine.

17. (1) Glucuronide conjugation :
• This is the most common and most important phase-II reaction in
vertebrates, except cats & fish.
• The biochemical donor (cofactor) of glucuronic acid is uridine
diphosphate D-glucuronic acid (UDPGA) and the reaction is carried
by enzyme uridine diphosphate-glucuronyl transferase.
• In cats, there is reduced glucuronyl transferase activity.
• while in fish, there is deficiency of endogenous glucuronic acid donor.
• Drugs - morphine, paracetamol and desipramine
Endogenous substances – steroids, bilirubin, catechols & thyroxine
undergo glucuronide conjugation.

18. (2) Conjugation with sulphate :
• It is similar to glucuronidation but is catalyzed by non-microsomal
enzymes and occurs less commonly.
• The endogenous donor of the sulphate group is 3’-
phosphoadenosine - 5’- phosphosulphate (PAPS), and enzyme
catalyzing the reaction is sulphotransferase.
• In cats, where glucuronidation is deficient, sulphate conjugation is
important.
• Drugs undergoing sulphate conjugation include chloramphenicol,
phenols, adrenal & sex steroids.

20. (3) Conjugation with methyl group :
• Conjugation with methyl group involves transfer of a –CH3 from the
cofactor S-adenosyl methionine (SAM) to the acceptor substrate by
various methyl transferase enzymes.
• Methylation reaction is less important for drugs, but is more
important for biosynthesis & inactivation of endogenous amines.

21. (4) Conjugation with glutathione :
• Glutathione is a tripeptide having glutamic acid, cysteine and
glycine.
• It conjugates with electrophilic substrates, a number of which are
potentially toxic compounds and protects the tissue from their
adverse effects.
• The reaction is catalyzed by enzyme glutathione-S-transferase
(GST), which is located in the soluble fraction of liver
homogenates.

23. (5) 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
enzyme are non-microsomal N-acetyl transferases, located in the
soluble fraction cells of various tissues.
• It is not a true detoxification process because it results in decrease
in water solubility of an amine and thus increase in its toxicity.

24.  Conjugation with 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.

25.  Conjugation with thiosulphate :
• It is an important reaction in the detoxification of cyanide.
• Conjugation of cyanide ion involves transfer of Sulphur atom from
thiosulphate to the cyanide ion in presence of enzyme rhodancse to
form inactive thiocyanate.
• Thiocyanate formed is much less toxic than cyanide and it is
excreted in urine.

27.  Enzyme Induction :
• Several drugs (inducers) induce the growth of smooth ER leading to
enhanced microsomal enzyme activity.
• Accelerates metabolism and decrease pharmacological response of
not only the inducer itself but also of the substrate.
• It occurs gradually over 1-2 weeks.

28.  Clinical Importance of Enzyme Induction :
• It reduces efficiency and potency of drugs metabolized by these
enzymes.
• 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 alters physiological status of animal due to altered metabolism of
endogenous compounds like sex steroids.

29.  Enzyme Inhibition :
• “Opposite to inducers, several drugs or chemicals have the ability to
decrease the drug metabolizing activity of certain enzymes called as
enzyme inhibition.”
• Nutritional deficiency, hormonal imbalance or hepatic disfunction, 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.
• Enzyme inhibition results in depressed metabolism of drugs. As a result,
the plasma half-life, duration of action and efficiency as well toxicity of
the object drug are significantly enhanced.
• Cimetidine, Erythromycin, Monoamine Oxidase, etc. are inhibitors.

30. LIVER DETOXIFICATION

31.  References :
(1) Casarett & Doull’s Toxicology The Basic Science of Poisons. 7th
Edition.
(2) quora.com/Pharmacology
(3) researchgate.net
(4) odomhealthandwellness.com
(5) basicmedicalkey.com/pharmacokinetics

32. THANK
YOU