This document discusses the metabolism of xenobiotics, or foreign compounds, in the body. It describes how xenobiotics undergo two phase biotransformation reactions, with phase 1 involving oxidation, reduction, and hydrolysis, and phase 2 involving conjugation to make compounds more water soluble for excretion. The liver plays a key role in these metabolic reactions through cytochrome P450 enzymes and conjugation enzymes. The document also discusses how reactive oxygen species are generated and the antioxidant systems that help prevent cellular damage from free radicals.

1. Metabolism of Xenobiotics
- Dr. Khushbu Soni

2. Xenobiotics
A xenobiotic (Gk xenos "stranger") is a
compound that is foreign to the body.
Xenobiotics can produce a variety of biological
effects including-
Pharmacological responses
Toxicity
Immunological responses
Cancers

4. Biotransformation Reactions
A process whereby a substance is changed from
one chemical to another chemical by a chemical
reaction within the body.
Consequences
o Changes in solubility characteristics
o Detoxification
o Metabolic activation

5. Biotransformation
Potentially toxic xenobiotic
Detoxification
Inactive metabolite
prodrug
Metabolic activation
Reactive intermediate

6. Detoxification Reactions
Biochemical reactions involved in the conversion of
foreign, toxic and water insoluble molecules to non toxic,
water soluble and excretable forms are called
Detoxification reactions
Purpose
o Converts lipophilic to hydrophilic compounds
o Facilitates excretion

7. Metabolism of Xenobiotics
Two phase of metabolism
Phase 1 Phase 2
Compounds produced in
phase 1 or are converted
by specific enzymes to
various polar metabolites
by conjugation.
They become more water
soluble and easily
excretable.
Alteration of xenobiotic
molecule so as to add a
functional group, which can
be conjugated in phase 2.

8. Phase 1 Phase 2
 Oxidation
Reduction
Hydrolysis
Deamination
Dehalogenation
Desulfuration
Epoxidation
Peroxygenation
Glucuronic acid
 Sulfate, acetate,
 Glutathione
 Certain amino acids
 By methylation.

10. Role of Liver
Main organ involved
 Hepatocytes contain wide variety of enzymes to process
xenobiotics
 Enzymes are present in endoplasmic reticulum and to
lesser extent in other organelles
 Each enzyme represents a large family of gene product
 Each gene product may be induced by different
xenobiotics

11. Phase 1 reactions
 Phase I reactions include:
 Oxidation
 Reduction
 Hydrolysis reactions
 They are also called Hydroxylation reactions since
they introduce or expose a functional group (e.g., -OH)
that serves as the active center for sequential
conjugation in a phase II reaction.

12. Oxidation
Oxidation of Alcohols- Primary aliphatic and aromatic alcohols are
oxidized to corresponding acids
Methanol Formaldehyde Formic acid
Ethanol Acetaldehyde Acetic acid
Benzoyal Alcohol Benzaldehyde Benzoic acid

13. It is an example of Entoxification

14. Reduction

15. Hydrolysis

16. Phase 1 reactions- Enzymes
 Mainly Catalyzed by a class of enzymes referred to
as Monooxygenases, Mixed Function oxidases or
Cytochrome P450s.
 Other enzymes of significance are-
o Aldehyde and alcohol dehydrogenase
o Deaminases
o Esterases
o Amidases
o Epoxide hydrolases

17. Cytochrome P450 Enzyme system

18. Properties of Human Cytochrome P450s
o Involved in phase I of the metabolism of innumerable
xenobiotics.
o Involved in the metabolism of many endogenous
compounds.
o They catalyze reactions involving introduction of one atom
of oxygen into the substrate and one into water
o All are haemoproteins.
o Exhibit broad substrate specificity, thus act on many
compounds.
o Extremely versatile catalysts, perhaps catalyze about 60
types of reactions.

19. o Liver contains highest amounts, but found in most if not
all tissues, including small intestine, brain, and lung.
o Located in the smooth endoplasmic reticulum.
o In some cases, their products are mutagenic or
carcinogenic.
o Many have a molecular mass of about 55 kDa.
o Many are inducible, resulting in one cause of drug
interactions.
o Many are inhibited by various drugs or their metabolic
products, providing another cause of drug interactions.
o Some exhibit genetic polymorphisms, which can result
in atypical drug metabolism.

20. Phase 2 - Conjugation
o These reactions involve covalent attachment of small
polar endogenous molecule such as glucuronic acid,
sulfate, or glycine to form water-soluble compounds.
o Conjugation reactions can occur independently or can
follow phase 1(hydroxylation) reactions.
o Conjugation takes place primarily in liver but can occur in
kidney also.
o After conjugation the products are generally rendered
non toxic but in certain conditions they are left unchanged
or become more toxic.

22. 1. Glucuronidation
2. Sulfation
3. Acetylation
4. Methylation
5. Conjugation with Amino acids
6. Conjugation with G-SH

23. o Glucuronidation is the most frequent conjugation reaction.
o UDP-glucuronic acid , is the Glucuronyl donor, which is
formed in the Uronic acid pathway of Glucose metabolism
o Glucuronyl Transferases, present in both the endoplasmic
reticulum and cytosol, are the catalysts.
o The glucuronide may be attached to oxygen, nitrogen, or
sulfur groups of the substrates.
Glucuronidation

25. Sulfation
o The sulfate donor is adenosine 3'-phosphate-5'-
phosphosulfate (PAPS) this compound is called
"active sulfate“
o The enzyme is sulfo transferase
o Compounds which are conjugated with sulphate are
as follows-
• Phenols
• Cresols
• Indole
• Steroids

26. Acetylation
o Acetylation is represented by
where X represents a xenobiotic.
o Acetyl-CoA (active acetate) is the acetyl donor.
o These reactions are catalyzed by acetyltransferases
o Polymorphic types of acetyltransferases exist, resulting
in individuals who are classified as slow or fast
acetylators, and influence the rate of clearance of drugs
from blood.
o Slow acetylators are more subject to certain toxic effects
of drug because the drug persists longer in these

27. o Compounds conjugated by Acetylation
• PABA (Para Amino Benzoic Acid)
• Isoniazid

28. Conjugation with Amino acids
Conjugation with Glutamine
Phenyl Acetic acid + Glutamine
Phenyl Acetyl Glutamine
This reaction is important in patients of Phenyl ketonuria,
since excess of Phenyl acetyl glutamine is excreted in
urine, that imparts a mousy odor to the urine.

29. Conjugation with glutathione
structure

31. Phase III – further modification and
excretion
o Conjugates and their metabolites can be excreted from
cells in phase III of their metabolism.
o A common example is the processing of glutathione
conjugates to acetylcysteine (mercapturic acid)
conjugates.

32. Factors affecting Biotransformation of drugs
o Prior administration of the drug or Co
administration of other drugs
o Diet
o Hormonal status
o Genetics
o Disease (e.g., decreased in cardiac and
pulmonary disease)
o Age and developmental status
o Functional status of Liver and Kidney

33. Effects of Xenobiotics

35. Free Radicals and Antioxidants

36. A free radical is a molecule or molecular fragment that
contains one or more unpaired electrons in its outer orbit.
Represented by a superscript dot, (R•).

37. ROS: Reactive Oxygen Species
o Oxidation reactions ensure that molecular oxygen is
completely reduced to water. The products of partial
reduction of oxygen are highly reactive, called Reactive
oxygen species or ROS

38. o Superoxide anion radical (O2 -•)
o Hydroperoxyl radical (HOO•)
o Hydrogen peroxide (H2O2)
o Hydroxyl radical (OH•)
o Lipid peroxide radical (ROO•)
o Singlet oxygen ( 1O2)
o Nitric oxide (NO•)
o Peroxy nitrite (ONOO-•).

39. Characteristics of ROS
o Extreme reactivity.
o Short life span.
o Generation of new ROS by chain reaction.
o Damage to various tissues.

40. Generation of Free Radicals
Cellular Metabolism
o Leakage of electrons from ETC
o Due to membrane lipid peroxidation
o Peroxisomal generation-oxidation of O2 & H2O2
o During prostaglandin synthesis
o Production of nitric oxide from arginine
o During phagocytosis
o In the oxidation of heme to bile pigments
o Auto-oxidation (e.g. metal ions, Fe2+, Cu2+ glutathione, ascorbic acid)

41. Environmental Sources
o Result of drug metabolism, Cytochrome P450 related reactions.
o Due to damage caused by ionizing radiations on tissues (X-rays)
o Photolysis of O2 by light
o Photo excitation of organic molecules
o Cigarette smoke
o Alcohol promotes lipid peroxidation.

43. Bactericidal activity

44. ROS a RNS
Modification of aa,
fragmentation and
aggregation of proteins
Lipid peroxidation DNA damage
Membrane damage
Loss of membrane
integrity
Damage to Ca2+
and other
ion transport systems
Inability to maintain
normal ion gradients
Activation/deactivation of
various enzymes
Altered gene
expression
Depletion of ATP
Lipids Proteins DNA
Cell injury/
Cell death
Damage by ROS/RNS

46. o Chronic inflammation
o Atherosclerosis
o Cardiac diseases
o Cataract
o Respiratory diseases
o Cancer
o Ageing

47. Lipid peroxidation

48. Antioxidants (Scavenger system)
o According to nature and action (enzymatic or non
enzymatic)
o According to location
o In relation to lipid peroxidation

51. o Non-enzymatic antioxidants:
o Nutrient: carotenoids, α-tocopherol ( vit E), selenium &
vitamin C, glutathione, ceruloplasmin

52. Antioxidants in relation to lipid peroxidation:
o Preventive antioxidants that will block the initial
production of free radicals e.g. catalase, glutathione
peroxidase.
o Chain breaking antioxidants that inhibit the
propagative phase of lipid peroxidation e.g.
Superoxide dismutase, vitamin E.

53. According to location:
o Plasma antioxidants: e.g. β-carotene, ascorbic acid,
ceruloplasmin, transferrin.
o Cell membrane antioxidants: e.g. α-tocopherol.
o Intracellular antioxidants: e.g. SOD, catalase, glutathione
peroxidase.