LECTURE NOTES ON XENOBIOTICS METABOLISM-2.ppt

Medical biochemistry

1. LECTURE NOTES ON : LECTURE NOTES ON :
Xenobiotics Metabolism
By
Ajilore B. S. (MBChB,
PhD)

2. FOREIGN BODY METABOLISMFOREIGN BODY METABOLISM
Toxic substances and potentially toxic substances may enter
human body. E.g. poisons, food additives, toxins, certain
drugs, chemicals, pesticides, environmental pollutants, etc
These substances are called xenobiotics
“Xenos” is a Greek word meaning strange or foreign
These foreign molecules may be accidentally ingested or
otherwise.
In the body, xenobiotics undergo changes that reduce their
toxicities.

3. The conversion of highly toxic xenobiotics to less toxic
substances is called detoxification or detoxication or
biotransformation.
Detoxification protects the body from harmful effects of
toxins, drugs etc by removing them from the body.
Occasionally, detoxification may generate toxic substances
from relatively non-toxic substances.
Generally, detoxification converts less soluble toxic
substance to more polar water soluble, and excrete it in
urine.
Some detoxified compounds may be excreted in faeces
through the bile.

4. Liver is the organ involved in detoxification reactions.
Several theories have been proposed to explain
detoxication:
i.Theory of Sherwin: Detoxication mechanisms render so
called toxic compounds less toxic by transforming them
into more soluble derivatives, which are then more easily
excreted.
ii.Theory of Berczeller: Proposed that toxic compounds are
made less toxic by transformation into compounds having a
surface tension nearly like water than the parent
compound.

5. In this way, toxic compounds are prevented from
accumulating at the surface of cells, since the non-toxic
forms are swept into body fluids and excreted.
iii.Theory of Quick: Proposed that the important factor is
conversion of a weakly acidic substance to a strongly acidic
one. Kidney can excrete stronger acids and their salts
more readily than weaker acids.
Foreign molecules may be exogenous or endogenous
Those that are endogenous are produced in the body by
synthesis or as metabolites of various processes in the
body
Detoxification of xenobiotics occurs in 2 stages/ phases.

6. In the first phase, xenobiotics undergo 3 types of chemical
reactions namely: oxidation, reduction (hydroxylation) and
hydrolysis. Phase 1 is also known as biotransformation
The second phase involves conjugation of xenobiotics with
variety of substances. Introduces a highly hydrophilic group
to promote excretion. Compounds produced in phase 1
are converted by specific enzymes to various polar
metabolites by conjugation
Occasionally, the detoxified products are more toxic than
the original substance. Biotoxification is the appropriate
word used to explain such process.
Therefore mechanisms of detoxification are oxidation,
reduction, hydrolysis and conjugation.

8. THE BIOTRANSFORMATION OF DRUGSTHE BIOTRANSFORMATION OF DRUGS

9. Sites of Drug MetabolismSites of Drug Metabolism

10. OXIDATION
-A large number of foreign substances are destroyed in the
body by oxidation.
-Addition of oxygen/ negatively charged radical or removal of
hydrogen/ positively charged radical.
-Reactions are carried out by group of monooxygenases in
the liver.
-Final step: Involves cytochrome P-450 haemoprotein,
NADPH, cytochrome P-450 reductase and O2

11. Cytochrome P450 enzymes Cytochrome P450 enzymes
Monooxygenase enzyme family

Major catalyst: Drug and endogenous compound oxidations
in liver, kidney, G.I. tract, skin and lungs
Oxidative reactions require: CYP heme protein, the
reductase, NADPH, phosphatidylcholine and molecular
oxygen
Location: smooth endoplasmic reticulum
The reductase serves as the electron source for the
oxidative reaction cycle

12. Cytochrome P family Cytochrome P family
Multiple CYP gene families have been identified in humans,
and the categorized based on protein sequence homology
 Most of the drug metabolizing enzymes are in CYP 1, 2, &
3 families .
Frequently, two or more enzymes can catalyze the same
type of oxidation, indicating redundant and broad substrate
specificity.
CYP3A4 is very common to the metabolism of many drugs;
its presence in the GI tract is responsible for poor oral
bioavailability of many drugs

13. Cytochromes: Metabolism of DrugsCytochromes: Metabolism of Drugs

14. Non-CYP Drug Oxidations Non-CYP Drug Oxidations
Monoamine Oxidase (MAO), Diamine Oxidase (DAO)
-MAO (mitochondrial) oxidatively deaminates endogenous
substrates including neurotransmitters
-Dopamine, serotonin, norepinephrine, epinephrine
Alcohol & Aldehyde Dehydrogenase
-Non-specific enzymes found in soluble fraction of liver
- Ethanol metabolism
Flavin Monooxygenases
-Require molecular oxygen, NADPH, flavin adenosine
dinucleotide (FAD)

15. -Aliphatic as well as aromatic alcohols may be oxidised to
corresponding acids.
-Also, certain amines, anilides and drugs also can undergo
oxidation.
-Examples are:
1.Methyl groups: These groups can be oxidised to form
-COOH group through formation of aldehyde.

16. 2.Primary aliphatic and aromatic alcohols: They are oxidised to
corresponding acids, e.g.
3.Aromatic hydrocarbons: Aromatic hydrocarbons are
oxidised to Phenol and other phenolic compounds.
- They are conjugated with glucuronic acid or sulphuric acid
and excreted as corresponding glucuronides and sulphates.

17. 4.Aldehydes: Aldehydes are oxidised to form the
corresponding acids.

18. 5.Anilides: Anilides are oxidised to the corresponding
Phenols, e.g. Acetanilide is present as a constituent of
analgesic drugs, which relieves pain. It is oxidised in the
body to form p-acetyl amino phenol.
6.Amines: Many primary aliphatic amines undergo oxidation
to the corresponding acids and N is converted to urea.
7.Sulphur compounds: The sulphur present in organic
sulphur compounds is oxidised to SO4 – which in turn
may be excreted in inorganic or organic form or as neutral
(unoxidised) sulphur.

19. 8.Drugs: Certain drugs can be oxidised in the body and are
excreted as hydroxy derivative or salts.
Examples are:
i.Meprobamate: A tranquilizer used in psychiatric disorders is
excreted largely as the oxidation product hydroxy
meprobamate.

20. ii.Chloral: Used as a hypnotic. Most of the chloral
undergoes reduction and conjugation; but partly it can be
oxidised to form trichloroacetic acid which is excreted as
its salt.

21. REDUCTION
-Reduction usually does not occur extensively in man
-Drugs primarily reduced are chloralhydrate,
chloramphenicol, halothane.
i.Certain aldehydes, e.g. chloral, a hypnotic, principally
undergoes reduction in the body to form corresponding
alcohol, which is then conjugated with D-glucuronic acid and
excreted as corresponding glucuronides.

22. ii.Aromatic nitro-compounds, e.g. p-nitrobenzaldehyde is
reduced to corresponding amines and excreted after
conjugation
-Note that some of the reduced metabolites, instead of
being less toxic, may be more toxic.

23. HYDROLYSIS
-There are quite a number of therapeutic compounds, used as
drugs, which undergo hydrolysis, usually in liver.
-Cleavage of drug molecule by taking up a molecule of water.
• Sites: Liver, intestines, plasma and other tissues
• Examples: Choline esters, Procaine, Isoniazid, pethidine,
oxytocin.

24. CONJUGATION
-A process by which the foreign molecules or its metabolites
are coupled with a conjugating agent and converted to
soluble, nontoxic derivatives which are easily excreted in
urine.
The following are features of conjugation:
-Various conjugating agents are available in the body and
some of them are synthesised in the body, e.g. D-glucuronic
acid formed from glucose by uronic acid pathway
-Certain amino acids as glycine, cysteine, can be available
from dietary proteins/or breakdown of tissue proteins or
synthesised.

25. -Conjugation reaction principally occurs in liver and to some
extent it can occur in kidneys as well
-Conjugation produces less toxic and more soluble
compounds which are excreted.
-Conjugation can occur independently or it can follow
oxidation, reduction or hydroxylation of a compound.
Examples/ types of conjugation reactions are:
i.Methylation
-Methylation as a detoxication process though limited in the
body, at the same time is quite important.
-Usual methyl donor is “S-adenosyl methionine” (“active”
methionine).

26. -Methylation of heterocyclic N-atom of compounds of the
Pyrimidine and Quinoline types, e.g. Nicotinamide.
-This occurs also with other heterocyclic aromatic
compounds, e.g. Histamine
-Methylation of p-aminomethyl amino azo benzene to p-
dimethyl amino azo benzene (butter yellow), a potential
hepatic carcinogen.

27. -O-methylation of certain naturally occurring amines (with
phenolic hydroxyl group), e.g. epinephrine and
norepinephrine and their metabolites are methylated at the
phenolic hydroxyl group.
-O-methylation of natural estrogens.
ii.Acetylation Reactions
-In detoxication reactions, conjugation with acetic acid
occurs only with aromatic NH2 group.
Exception to this rule : Acetylation of –OH group takes place
in physiological compounds like formation of acetyl choline.
-

28. -Acetic acid helps in conjugation of aromatic compounds
along with cysteine to form corresponding mercapturic
acids
-In humans, certain drugs, e.g. sulpha drugs are conjugated
by acetylation. As much as 50 per cent of excreted sulpha
drugs may be acetylated and excreted as acetylated
derivatives.
-Similarly, like sulphonamide drugs, PABA is also acetylated
and excreted as acetyl derivative.

29. Acetylation is done by active acetate (Acetyl-CoA): It is
catalysed by the enzyme acetyl transferase present in the
cytosol of various tissues.
Clinical Examples:
-The drug “Isoniazid”, used in treatment of TB is
detoxicated by acetylation.

30. iii.Conjugation with Sulphuric Acid
-Sulphuric acid is used by human beings for detoxication
of various compounds having phenolic or hydroxyl
groups
-Substances like phenol, cresol, indole and skatole formed
in the gut by the action of intestinal bacteria are
absorbed and transported to liver, where they are
conjugated with sulphate to form Ethereal sulphates,
which are excreted in urine, being less toxic and more
acidic

31. Other compounds which are conjugated in the body to
form corresponding esters are tyrosine to form tyrosine-
O-SO4 required for fibrinogen molecule, the amino sugars,
certain hormones like oestrogens and androgens
iv.Conjugation with D-Glucuronic Acid
-Most important and commonest detoxication reaction.
-D-Glucuronic acid participates in its detoxication reaction
as its active form “UDP-glucuronic acid” which is formed in
“uronic acid” pathway of glucose oxidation.
- Enzyme required is Glucuronyl transferase.

32. -Thus, formation of glucuronides play an important role in
detoxication mechanisms of exogenous and endogenous
compound and their excretion as corresponding
glucuronides.
- In the process of conjugation, the glucuronic acid can form
two types of linkages: An “ether” (glucosidic) linkage and an
“ester” linkage

33. v.Conjugation with Amino Acids
Examples of the amino acids used in the conjugation
reactions are: glycine, L-cysteine and glutamine.

34. Reaction with aromatic –COOH group separated from the aromatic
ring by a “Vinyl” group, e.g. Cinnamic acid

35. -In man, a few aromatic compounds are conjugated with L-
cysteine in presence of acetic acid to form mercapturic acids

36. -In man and in primates (chimpanzee), glutamine conjugates
phenyl acetic acid to form ‘phenylacetyl glutamine’ and
excreted in urine.
-This accounts for “mousy” odour of urine in phenyl-
ketonurics
vi. Conjugation with Glutathione
-A number of potentially toxic electrophilic xenobiotics, e.g.
certain carcinogens are conjugated to the nucleophilic G-SH,
in reactions that can be represented as follows:
-The reaction is catalysed by the enzyme glutathione S-
Transferases.

37. The enzyme is present in high amounts in liver cytosol and
in lower amounts in other tissues
Note: If the potentially toxic xenobiotics are not
conjugated with G-SH, they would be free to combine
covalently with DNA, RNA or cell proteins and can
produce serious cell damage.
G-SH is thus an important defence mechanism against
certain toxic compounds, such as some drugs and
carcinogens.