DƯỢC LÍ Goodman & Gilman's The Pharmacological Basis of Therapeutics 12th, 2010

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SECTION I
GENERAL PRINCIPLES
reactions for cofactors, (or more correctly, co-substrates)
such as UDP-glucuronic acid (UDP-GA) and
3′-phosphoadenosine-5′-phosphosulfate (PAPS), for
UDP-glucuronosyltransferases (UGT) and sulfotransferases
(SULT), respectively, which react with available
functional groups on the substrates. The reactive functional
groups are often generated by the phase 1 CYPs,
although there are many drugs (e.g., acetaminophen)
where glucuronidation and/or sulfation occur directly
without prior oxidative metabolism. All of the phase 2
reactions are carried out in the cytosol of the cell, with
the exception of glucuronidation, which is localized to
the luminal side of the endoplasmic reticulum. The
catalytic rates of phase 2 reactions are significantly
faster than the rates of the CYPs. Thus, if a drug is targeted
for phase 1 oxidation through the CYPs, followed
by a phase 2 conjugation reaction, usually the rate of
elimination will depend upon the initial (phase 1) oxidation
reaction. Since the rate of conjugation is faster and
the process leads to an increase in hydrophilicity of the
drug, phase 2 reactions are generally considered to
assure the efficient elimination and detoxification of
most drugs.
Glucuronidation. Among the more important of the
phase 2 reactions in the metabolism of drugs is that catalyzed
by UDP-glucuronosyltransferases (UGTs)
(Figure 6–3B). These enzymes catalyze the transfer of
glucuronic acid from the cofactor UDP-glucuronic acid
to a substrate to form β-D-glucopyranosiduronic acids
(glucuronides), metabolites that are sensitive to cleavage
by β-glucuronidase. The generation of glucuronides can
be formed through alcoholic and phenolic hydroxyl
groups, carboxyl, sulfuryl, and carbonyl moieties, as
well as through primary, secondary, and tertiary amine
linkages. Examples of glucuronidation reactions are
shown in Table 6–2 and Figure 6–5. The structural diversity
in the many different types of drugs and xenobiotics
that are processed through glucuronidation assures that
most clinically efficacious therapeutic agents will be
excreted as glucuronides.
There are 19 human genes that encode the UGT proteins. Nine
are encoded by the UGT1 locus and 10 are encoded by the UGT2 family
of genes. Both families of proteins are involved in the metabolism
of drugs and xenobiotics, while the UGT2 family of proteins appears
to have greater specificity for the glucuronidation of endogenous substances
such as steroids. The UGT2 proteins are encoded by unique
genes on chromosome 4 and the structure of each gene includes six
exons. The clustering of the UGT2 genes on the same chromosome,
with a comparable organization of the regions encoding the open reading
frames, is evidence that gene duplication has occurred, a process
of natural selection that has resulted in the multiplication and eventual
12p11p
8
10
13p
Exon 1 sequences
9
UGT1 Locus
13kb 18kb 11kb 23kb 9kb 10kb 19kb 5kb 9kb 17kb 10kb
UGT1A1
UGT1A3
UGT1A4
UGT1A5
UGT1A6
UGT1A7
UGT1A9
UGT1A10
UGT1A8
7
1 2 34 5
198872 bp
Figure 6–6. Organization of the UGT1A Locus. Transcription of
the UGT1A genes commences with the activation of PolII, which
is controlled through tissue-specific events. Conserved exons
2-5 are spliced to each respective exon 1 sequence, resulting in
the production of unique UGT1A sequences. The UGT1A locus
encodes nine functional proteins.
diversification of the potential to detoxify the plethora of compounds
that are targeted for glucuronidation.
The nine functional UGT1 proteins are all encoded by the
UGT1 locus (Figure 6–6), which is located on chromosome 2. The
UGT1 locus spans nearly 200 kb, with over 150 kb of a tandem array
of cassette exonic regions that encode ~280 amino acids of the amino
terminal portion of the UGT1A proteins. Four exons are located at
the 3′ end of the locus that encode the carboxyl 245 amino acids that
combine with one of the consecutively numbered array of first exons
to form the individual UGT1 gene products. Since exons 2-5 encode
the same sequence for each UGT1A protein, the variability in substrate
specificity for each of the UGT1A proteins results from the
significant divergence in sequence encoded by the exon 1 regions.
The 5′ flanking region of each first-exon cassette contains a fully
functional promoter capable of initiating transcription in an inducible
and tissue-specific manner.
From a clinical perspective, the expression of
UGT1A1 assumes an important role in drug metabolism,
since the glucuronidation of bilirubin by UGT1A1
is the rate-limiting step in assuring efficient bilirubin
clearance, and this rate can be affected by both genetic
variation and competing substrates (drugs). Bilirubin is
the breakdown product of heme, 80% of which originates
from circulating hemoglobin and 20% from other
heme-containing proteins such as the CYPs. Bilirubin
is hydrophobic, associates with serum albumin, and
must be metabolized further by glucuronidation to
assure its elimination. The failure to efficiently metabolize
bilirubin by glucuronidation leads to elevated
serum levels and a clinical symptom called hyperbilirubinemia
or jaundice. There are >50 genetic lesions in the
UGT1A1 gene that can lead to inheritable unconjugated
6
5
4
3
2p