1. XENOBIOTIC METABOLISM
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2. Background
Increasingly, humans are subjected to exposure to various foreign
chemicals (xenobiotics)— drugs, food additives, pollutants, etc.
A xenobiotic (Gk xenos “stranger”) is a compound that is
foreign to the body.
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3. The principal classes of xenobiotics of
medical relevance
-Drugs
-Chemical carcinogens
-Various compounds that have found their way into our
environment by one route or another, such as
polychlorinated biphenyls (PCBs) and certain insecticides.
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4. Xenobiotic metabolism
Metabolism of xenobiotics include two phases:
A-Phase 1, e.g, hydroxylation.
B-Phase 2, conjugation (e.g. glucronic acid).
A-Phase 1, the major reaction involved is hydroxylation, catalyzed
by members of a class of enzymes referred to as monooxygenases
or cytochrome P450s isoforms .
The other reaction include reduction and hydrolysis, which
perform by same enzymes species.
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5. B-Phase 2, the hydroxylated or other compounds produced in
phase 1 are converted by specific enzymes to various polar
metabolites by conjugation with glucuronic acid, sulfate, acetate,
glutathione, or certain amino acids, or by methylation.
-The overall purpose of the two phases of metabolism of
xenobiotics is to increase their water solubility (polarity) and thus
excretion from the body.
-Very hydrophobic xenobiotics would persist in adipose tissue
almost indefinitely if they were not converted to more polar forms.
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6. Hydroxylation reaction
Hydroxylation is the chief reaction involved in phase 1. The
responsible enzymes are called monooxygenases or cytochrome
P450s.
Example of the reaction:
RH + O2 +NADPH +H+ →R-OH +H2O +NADP
(RH) represent a very wide variety of xenobiotics, including
drugs, carcinogens, pesticides, petroleum products, and
pollutants (such as a mixture of PCBs), as well as endogenous
compounds, such as certain steroids, eicosanoids, fatty acids,
and retinoids.
-In the above reaction one atom of oxygen enters R-OH and
one atom enters water.
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7. -This dual fate of the oxygen accounts for the former naming of
monooxygenases as (mixed function oxidases).
The reaction catalyzed by cytochrome P450 can also be
represented as follows:
Reduced cytochrome P450 Oxidized cytochrome P450
RH + O2→R — OH + H2O
The major monooxygenases in the endoplasmic reticulum
are cytochrome P450s (CYP450)—so named because the enzyme
was discovered when it was noted that preparations of
microsomes that had been chemically reduced and then exposed
to carbon monoxide exhibited a distinct peak at 450 nm.
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8. -This enzyme is important is the fact that approximately 50% of
the drugs humans ingest are metabolized by isoforms of
cytochrome P450.
Important points concerning cytochrome P450s.
(1) Like hemoglobin, they are hemoproteins.
(2) They are widely distributed across species.
(3) They are present in highest amount in liver and adrenal.
In liver, in the membranes of the smooth endoplasmic reticulum,
which constitute part of the microsomal fraction, In the adrenal,
in mitochondria as well as in the endoplasmic reticulum.
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9. (4) At least six isoforms of cytochrome P450 are present.
(5) NADPH, not NADH, is involved in the reaction mechanism
of cytochrome P450.
(6) Lipids are also components of the cytochrome P450
system..The preferred lipid is phosphatidylcholine,
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10. (7) Most isoforms of cytochrome P450 are inducible. For
instance, the administration of phenobarbital or many other
drugs causes three to four fold increase in the amount of
cytochrome P450.
(8) Certain isoforms of cytochrome P450 (e.g, CYP1A1) are
particularly involved in the metabolism of polycyclic aromatic
hydrocarbons (PAHs).
(9) Certain cytochrome P450s exist in polymorphic forms (genetic
isoforms), some of which exhibit low catalytic activity.
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11. CONJUGATION REACTIONS
-In phase 1 reactions; xenobiotics are generally converted to more
polar, hydroxylated derivatives.
-In phase 2 reactions; these derivatives are conjugated with
molecules such as glucuronic acid, sulfate, or glutathione.
This renders them even more water-soluble, and they are eventually
excreted in the urine or bile.
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12. Five Types of Phase 2 Reactions Are Described
A. GLUCURONIDATION:
The glucuronidation of bilirubin is similar reactions whereby
xenobiotics are glucuronidated.
UDP-glucuronic acid glucuronyl donor (e.g phenol, benzoic
acid).
B. SULFATION:
Some alcohols, arylamines, and phenols are sulfated. The sulfate
donor in these and other biologic sulfation reactions (eg, sulfation
of steroids, glycosaminoglycans glycolipids, and glycoproteins) is
adenosine 3-phosphate- 5-phosphosulfate (PAPS)
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13. C. CONJUGATION WITH GLUTATHIONE
Glutathione (γ-glutamyl-cysteinylglycine) is a tripeptide
consisting of glutamic acid, cysteine, and glycine. Glutathione is
commonly abbreviated GSH (because of the sulfhydryl group of
its cysteine, which is the business part of the molecule).
A number of potentially toxic electrophilic xenobiotics (such as
certain carcinogens) are conjugated to the nucleophilic
GSH in reactions that can be as follows:
R + GSH→R—S—G
where R = an electrophilic xenobiotic. The enzymes catalyzing
these reactions are called glutathione S transferases.
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14. Some common notes about GHS.
1-If the potentially toxic xenobiotics were not conjugated to GSH,
they would be free to combine covalently with DNA, RNA, or cell
protein and could thus lead to serious cell damage.
2-GSH is therefore an important defense mechanism against
certain toxic compounds, such as some drugs and carcinogens.
3-If the levels of GSH in a tissue such as liver are lowered
then that tissue can be shown to be more susceptible to injury by
various chemicals that would normally be conjugated to GSH. 14

15. **Glutathione has other important functions in human
cells apart from its role in xenobiotic metabolism.
1. It participates in the decomposition of potentially toxic hydrogen
peroxide in the reaction catalyzed by glutathione peroxidase.
2-It is an important intracellular reductant, helping to maintain
essential SH groups of enzymes in their reduced state.
3-Metabolic cycle involving GSH as a carrier has been implicated in
the transport of certain amino acids across membranes in the
kidney.
Amino acid+ GSH → γ-Glutamyl amino acid + Cysteinylglycine
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16. 4. Acetylation:
Acetylation is represented by
X + Acetyl - CoA→Acetyl -X + CoA
where X represents a xenobiotic. As for other acetylation
reactions, acetyl-CoA (active acetate) is the acetyl donor. These
reactions are catalyzed by acetyltransferases.
e.g. The drug isoniazid, used in the treatment of tuberculosis, is
subject to acetylation
5. Methylation:
A few xenobiotics are subject to methylation by methyltransferases,
employing S-adenosylmethionine as methyl donor.
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17. ** THE ACTIVITIES OF XENOBIOTIC
METABOLIZING ENZYMES ARE AFFECTED BY
AGE, SEX, & OTHER FACTORS.
There are significant differences in enzyme activities among
individuals, many of which appear to be due to genetic factors. The
activities of some of these enzymes vary according to age and sex.
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18. **RESPONSES TO XENOBIOTICS INCLUDE
PHARMACOLOGIC, TOXIC, IMMUNOLOGIC,
& CARCINOGENIC EFFECTS
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19. -When the xenobiotic is a drug, phase 1 reactions may produce its
active form or may diminish or terminate its action if it is
pharmacologically active in the body without prior metabolism.
-Certain xenobiotics are very toxic even at low levels
(e.g, cyanide). On the other hand, there are few xenobiotics,
including drugs, that do not exert some toxic effects
if sufficient amounts are administered.
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20. 1-The first is cell injury (cytotoxicity), which can be severe enough
to result in cell death.
-There are many mechanisms by which xenobiotics injure cells.
The one considered here is covalent binding to cell macromolecules
of reactive species of xenobiotics produced by metabolism. These
macromolecular targets include DNA, RNA, and protein.
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21. 2-Second, the reactive species of a xenobiotic may bind to a
protein, altering its antigenicity. The xenobiotic is said to act as a
hapten, i.e, a small molecule that by itself does not stimulate
antibody synthesis but will combine with antibody once formed.
The resulting antibodies can then damage the cell by several
immunologic mechanisms that grossly perturb normal cellular
biochemical processes.
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22. 3-Third, reactions of activated species of chemical carcinogens
with DNA are thought to be of great importance in chemical
carcinogenesis. Some chemicals (eg, benzo[α]pyrene) require
activation by monooxygenases in the endoplasmic reticulum to
become carcinogenic (they are thus called indirect carcinogens).
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23. -The enzyme epoxide hydrolase is of interest because it can exert a
protective effect against certain carcinogens. The products of the
action of certain monooxygenases on some procarcinogen substrates
are epoxides.
-Epoxides are highly reactive and mutagenic or carcinogenic or
both.
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24. -Epoxide hydrolase—like cytochrome P450, also present in the
membranes of the endoplasmic reticulum—acts on these
compounds, converting them into much less reactive dihydrodiols.
The reaction catalyzed by epoxide hydrolase can be represented as
follows:
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