Metabolism of foreign compounds that are strangers to us

1. Metabolism of Xenobiotics
R. C. Gupta
Professor and Head
Department of Biochemistry
National Institute of Medical Sciences
Jaipur, India

2. Xenobiotics are foreign compounds that
are strangers to our body
They may be divided into:
A. Pharmacological xenobiotics
B. Environmental xenobiotics

3. Pharmacological xenobiotics are the
drugs that we take intentionally
Environmental xenobiotics enter our body
with or without our knowledge

4. Environmental xenobiotics
include:
• Food additives
• Food adulterants
• Pollutants present in food
• Pollutants present in water
• Pollutants present in air

5. Human beings may be exposed to more
than 100,000 foreign chemicals
The list of manufactured chemicals is
getting longer every year

6. Foreign compounds, which are antigenic,
are dealt with by our immune system
The non-antigenic foreign chemicals
(xenobiotics) are metabolized

7. The purpose of metabolism is to convert:
Less soluble xenobiotics
into more soluble ones
Toxic xenobiotics into non-
toxic or less toxic ones
The relatively soluble metabolites are
excreted, mainly in urine or bile

8. Liver is the major site for metabolism of
xenobiotics
But metabolism can occur in some other
tissues as well
The metabolic reactions can be divided
into two phases – phase 1 and phase 2

9. Phase 1 reactions are non-synthetic while
phase 2 reactions are synthetic
Some xenobiotics are excreted directly
after phase 1 reactions
In most cases, phase 1 reactions are
followed by phase 2 reactions

10. Phase 1 reactions
The main phase 1
reactions are:
Hydroxylation
Oxidation
Reduction
Hydrolysis

11. Hydroxylation
Hydroxylation is the major reaction in
phase 1
It is catalyzed by the microsomal
hydroxylase system
This system is associated with the
membrane of endoplasmic reticulum

12. Microsomal hydroxylase system consists of:
Cytochrome P-450 (abbreviated as CYP)
NADPH-CYP reductase
FAD
FMN
Cytochrome b5 (sometimes)

13. Substrate and an O2 molecule bind to CYP
One oxygen atom is introduced into the
substrate
The other oxygen atom reacts with two
hydrogen atoms to form water
The hydrogen atoms are provided by
NADPH, and pass via FAD and FMN

15. The overall reaction catalyzed by the
microsomal hydroxylase system may be
summed up as:
The chemical groups that are hydroxylated
include methyl group, phenyl group, amino
group, unsaturated aliphatic group etc
XH + O2 + NADPH + H+ → X–OH + H2O + NADP+

20. Cytochrome P-450 is not a single protein
More than 200 isoforms of CYP, encoded
by different genes, have been identified
CYP isoforms are divided into families
Cytochrome P-450

21. Important families of CYP in human
beings are CYP1, CYP2, CYP3, CYP4,
CYP11, CYP17, CYP19 and CYP21
Members of a particular family show at
least 40 percent homology in amino acid
sequence

22. Many families have subfamilies
For example, CYP2 has five subfamilies
(CYP2A, CYP2B, CYP2C, CYP2D and
CYP2E)
Members of a particular subfamily have
at least 55 percent homology in amino
acid sequence

23. Subfamilies are divided into individual
members
For example, CYP1A1 and CYP1A2 are
members of CYP1A subfamily

24. In the abbreviation:
The digit after CYP shows
the number of the family
The letter after the digit
shows the subfamily
The last digit shows the
individual member

25. CYP1, CYP2 and CYP3 are the major
families metabolizing xenobiotics
Other families of CYP hydroxylate endo-
genous substrates
The largest number of xenobiotics are
metabolised by CYP3A subfamily followed
by CYP2D and CYP2C subfamilies

26. Different isoforms of CYP differ in substrate
specificity but some overlapping is also seen
Some xenobiotics are metabolized by more
than one isoforms of CYP
Some isoforms of CYP metabolize more
than one xenobiotics

27. Some isoforms are induced by one
compound but metabolize some other
compound also
Some isoforms are inhibited by certain
compounds; this can alter the metabolism of
some other compounds
Some isoforms are genetically deficient in
some individuals or in some racial groups

28. One drug can increase or decrease the
metabolism of some other drug
Hence, alertness is required when
prescribing a combination of drugs
Overlapping specificities are particularly
important in the metabolism of drugs

29. Metabolism of drugs
may be affected by:
CYP induction
CYP inhibition
CYP deficiency

30. CYP2C9 metabolizes phenobarbital as well
as warfarin
Example 1 – Phenobarbital and warfarin
Phenobarbital induces CYP2C9

31. Upon taking phenobarbital
and warfarin together -
Phenobarbital induces
CYP2C9
Metabolism of warfarin is
accelerated
Dose of warfarin has to be
increased

32. If phenobarbital is subsequently withdrawn,
concentration of CYP2C9 decreases
This necessitates a decrease in the dose
of warfarin

33. CYP3A4 metabolizes rifampicin as well as
contraceptive steroids
Example 2 – Rifampicin and contraceptive
steroids
CYP3A4 is induced by rifampicin

34. If rifampicin and contraceptive
steroids are taken together -
Rifampicin induces
CYP3A4
Metabolism of contraceptive
steroids is accelerated
This may result in failure of
contraception

35. Quinidine, chlorpromazine and haloperidol
are inhibitors of CYP2D6
They can impair the metabolism of timolol,
fluoxetine, nortryptiline, perphenazine etc
which are metabolized by CYP2D6

36. CYP3A subfamily is inhibited by erythro-
mycin, ketoconazole and verapamil
Inhibition of CYP3A impairs the metabolism
of drugs metabolised by CYP3A subfamily
These include lidocaine, lovastatin, cyclo-
sporine, nifedipine, carbamazepine etc

37. Inherited deficiency of CYP2D6 can
decrease the metabolism of several drugs
CYP2D6 converts codeine into its active
metabolite morphine by hydroxylation
Therefore, codeine is less effective in
persons deficient in CYP2D6

38. CYP2D6 metabolizes timolol, fluoxetine,
nortryptiline, perphenazine etc
Their metabolism is impaired by deficiency
of CYP2D6

39. Some mutations in CYP genes have a
racial distribution
For example, nearly 20% of Asian people
are genetically deficient in CYP2C19
CYP2C19 metabolizes mephenytoin,
diazepam and omeprazole

40. Ethanol and methanol are oxidized to
acetaldehyde and formaldehyde respec-
tively by alcohol dehydrogenase
Acetaldehyde and formaldehyde are
oxidized to acetic acid and formic acid
respectively by aldehyde dehydrogenase
Oxidation

42. Methanol is oxidized to more toxic
formaldehyde and formic acid
These can cause atrophy of optic
nerves

43. Metabolites of ethanol are non-toxic;
hence ethanol is used to treat methanol
poisoning
The common enzymes that metabolize
these two have a greater affinity for
ethanol than for methanol

47. Nitrobenzene and p-nitrophenol are
metabolized by reduction
Nitrobenzene is reduced to amino-
benzene
p-Nitrophenol is reduced to p-amino-
phenol
Reduction

50. Chloramphenicol and metyrapone are
partly metabolized by reduction
Picric acid is metabolized by reduction
to picramic acid

54. Phase 2 reactions include methylation
and conjugation
Conjugation is more common than
methylation
Phase 2 reactions

55. Some xenobiotics (e.g. thiouracil) under-
go methylation during phase 2
The methyl group is transferred from S-
adenosyl methionine to the xenobiotic by
a methyl transferase
Methylation

56. Conjugation
Conjugation is the major phase 2 reaction
The xenobiotic may be conjugated with:
Glucuronic acid
Sulphate
Acetate
Glycine
Glutathione

57. Glucuronyl moiety of UDP-glucuronic
acid is conjugated with the xenobiotic
The reaction is catalyzed by UDP-
glucuronyl transferase
Conjugation with glucuronic acid

58. Xenobiotics conjugated with glucuronic
acid are:
• Acetylaminofluorene
• Acetaminophen
• Diazepam
• Meprobamate
• Morphine
• Phenytoin
• Aniline
• Salicylic acid
• Benzoic acid

59. Some alcohols and phenols are
conjugated with sulphate
The sulphate is provided by phospho-
adenosine phosphosulphate (PAPS)
It is added to xenobiotics by sulpho-
transferases
Conjugation with sulphate

61. Sulphanilamide, hydralazine, caffeine and
isoniazid are conjugated with acetate
N-Acetyl transferase-2 transfers acetyl
group from acetyl CoA to the xenobiotic
Metabolism of these drugs is decreased
when N-acetyl transferase-2 is deficient
Conjugation with acetate

62. Benzoic acid, after its conversion into
benzoyl CoA, is conjugated with glycine
to form hippuric acid
Salicylic acid is partly conjugated with
glycine to form salicyluric acid
Conjugation with glycine

64. Acetaminophen and some carcinogens
are conjugated with glutathione
Conjugation occurs with the sulphydryl
group of cysteine residue of glutathione
Conjugation with glutathione

65. The glutamate and glycine residues of
glutathione are split off
Cysteine residue is acetylated to a
mercapturic acid residue
The mercapturic acid derivative of
xenobiotic is excreted

66. Majority of the xenobiotics first undergo
a phase 1 reaction followed by a phase
2 reaction
But some may be directly conjugated or
methylated

67. Some xenobiotics are metabolized by a
single route
Some are metabolized by several routes
forming a number of metabolites

68. Our metabolic machinery is adapted to
metabolize, detoxify and excrete a wide
range of xenobiotics
Yet there are some foreign chemicals
which are either not metabolized or are
metabolized to a limited extent
Environmental health hazards

69. The poorly metabolized xenobiotics are
mostly environmental pollutants
These may be present in air, water or
food
We may encounter them at home, at
workplace or in the community

70. The environmental pollutants include:
• Industrial chemicals
• Insecticides and pesticides
• Lead, mercury, cadmium etc
• Arsenic
• Asbestos
• Silica
• Oxides of nitrogen and sulphur
• Carbon monoxide
• Solid particulate matter (spm) etc

71. Exposure to environmental pollutants
beyond certain limits can cause disease
Such diseases may affect respiratory
system, cardiovascular system, nervous
system, reproductive system, immune
system, liver, kidneys etc

72. Exposure to smoke (amongst
firemen), welding fumes, epoxy
resins and asbestos can cause
respiratory diseases
Respiratory diseases

73. Chronic exposure to lead, carbon
disulphide, methylene chloride and
carbon monoxide (from furnaces
and motor vehicle exhausts) can
cause cardiovascular diseases
Cardiovascular diseases

74. Prolonged exposure to lead,
arsenic, mercury, toluene, organo-
phosphates, organochlorines,
polychlorinated biphenyls,
perchloroethylene etc can cause
neurological diseases
Nervous diseases

75. Chronic exposure to insecticides,
herbicides, polychlorinated
biphenyls, polybrominated biphenyls,
dibromochloropropane, ethylene
oxide, lead, arsenic, mercury,
cadmium etc can cause reproductive
diseases
Reproductive diseases

76. Prolonged exposure to polybrominated
biphenyls, methylcholanthrene,
dieldrin, mercury etc can disturb the
functioning of immune system
Immune diseases

77. Hepatocellular or cholestatic liver
disease can result from exposure to
carbon tetrachloride, methylene
diamine, arsenic etc
Hepatic diseases

78. Chronic exposure to lead, mercury,
cadmium etc can damage the kidneys
Renal diseases

79. Safe limits for most environmental
pollutants in air, water and food have
been established
Continuous monitoring and remedial
action, when required, are necessary to
safeguard the health of the community