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chapter 27
Nucleotide Metabolism
Inhibition by AMP and GMP, is competitive with respect
to PRPP. The human placental enzyme exists in a small
form (M.W. 133,000) and a large form (M.W. 270,000).
The small form is catalytically active. Ribonucleotides
convert the active form to the large form, whereas PRPP
does the opposite. The regulatory actions of PRPP syn-
thetase and amidophosphoribosyltransferase are coordi-
nated. When there is a decrease in the intracellular con-
centration of adenine ribonucleotides, PRPP synthetase
is activated; this results in increased synthesis of PRPP,
which in turn converts the inactive form of amidophos-
phoribosyltransferase to the active form and increases pro-
duction of purine nucleotides.
Regulation of Formation of AMP and
GMP from IMP
In the formation of AMP and GMP from IMP, ATP is re-
quired for GTP synthesis, and GTP is needed to form ATP
(Figure 27-9). In addition, adenylosuccinate synthetase is
inhibited by AMP, and IMP dehydrogenase is inhibited by
GMP (Figure 27-12).
27.7 Inhibitors of Purine Biosynthesis
A variety of inhibitors of purine biosynthesis function at
different stages and are used as antimicrobial, anticancer,
and immunosuppressive agents.
Inhibitors of Folate Biosynthesis
The
sulfonamide
drugs were the first effective antibacte-
rial agents to be employed systemically in humans. These
drugs resemble p-aminobenzoic acid in structure and in-
hibit utilization of that compound for the synthesis of fo-
late in bacteria. Sulfonamides do not interfere with human
metabolism.
Inhibitors of Formation of IMP
Folate analogues,
such as methotrexate (Figure 27-3),
are folate antagonists. They block production of FH
2
and
FH
4
by dihydrofolate reductase and lead to diminished
purine biosynthesis (inhibition of reactions 3 and 9 in
Figure 27-8). Methotrexate also affects metabolism of
amino acids and pyrimidine (inhibition of thymidylate
synthesis) and inhibits DNA, RNA, and protein synthesis.
It is effective in the treatment of breast cancer, cancer of the
head and neck, choriocarcinoma, osteogenic sarcoma, and
acute forms of leukemia. High doses of methotrexate can
be tolerated provided that the patient also receives folinic
acid (N5-formyl FH4), which decreases damage to bone
marrow and prevents the development of leukopenia and
thrombocytopenia. Methotrexate also produces mucositis,
gastrointestinal symptoms, and liver damage by a direct
toxic effect on hepatic cells. Chronic oral administration
of methotrexate in psoriasis has been associated with an
increased incidence of postnecrotic cirrhosis.
The “rescue” of normal, but not tumor, cells from
methotrexate toxicity by folinic acid is partly explained
by differences in membrane transport. For example, os-
teogenic sarcoma cells (which do not respond to conven-
tional doses of methotrexate treatment) are not rescued by
folinic acid administered after methotrexate, presumably
owing to the absence of transport sites for folinic acid in the
neoplastic cells. The therapeutic effects of administration
of methotrexate and “rescue” with folinic acid are superior
to those of methotrexate alone. Resistance to methotrexate
can develop from increased activity of dihydrofolate re-
ductase, synthesis of an enzyme having a lower affinity for
the inhibitor, decreased transport of the drug into tumor
cells, decreased degradation of the reductase, and genetic
amplification of the gene for dihydrofolate reductase.
Azaserine
(L-serine diazoacetate),
isolated from a
species of
Streptomyces,
is a structural analogue of glu-
tamine
O
♦
II
- n = N = C H — C — O — CH
2
— CH— COOH
î
I
A zaserine
NH
2
O
II
H
2
N— C — CH
2
— CH
2
— CH— COOH
I
G lutam ine
NH
2
that inhibits steps 1
and 4 of inosinic acid synthesis
(Figure 27-8) and the conversion of XMP to GMP
(Figure 27-9). A related substance, 6-diazo-5-oxo-L-
norleucine (DON), is also a glutamine analogue. Both
compounds function as alkylating agents and become
attached to an essential sulfhydryl group (a nucleophilic
group) of the enzyme to form an inactive thioether deriva-
tive. The linkage occurs via the electrophilic carbon atom
(designated by the arrow) of azaserine and release of N2.
Inhibitors of Formation of AMP and GMP
Hadacidin
(N-formyl-N-hydroxyglycine) is an
h
0H
\
I
p
— N — CH2— COOH
O
