section 27.9
Biosynthesis of Pyrimidine Nucleotides
641
C D P ---------------- d C D P ---------------- d C T P ---------------* D N A
T h y m in e
F IG U R E 2 7 -2 9
Pathway for TTP synthesis. The key intermediate is dUTP, which is
converted to dUMP by dUTPase.
N-glycosylase that removes U from DNA. Figure 27-29
summarizes the pathway for TTP synthesis.
Because there is only one pathway for synthesis of
TMP, it can be used to synthesize radioactively labeled
DNA or to inhibit DNA synthesis selectively. In bacte-
rial thymidylate synthase mutants (thy- ), DNA synthe-
sis is not possible without added thymine or thymidine,
both of which can be utilized by salvage pathways. Nei-
ther thymine nor thymidine engages in synthetic reactions
other than production of TMP, so radioactive thymine
or thymidine can serve as unique precursors for synthe-
sis of radioactive DNA. The radioactivity usually is in
the methyl group; since this group is not metabolized,
the appearance of radioactivity in any other compound
is avoided.
Thymidylate synthase is competitively inhibited by flu-
orodeoxyuridylate (FUDRP), with formation of a sta-
ble ternary complex with methylene FH
4
. FUDRP is
generated by
salvage from exogenous
5-fluorouracil
(FU) or fluorodeoxyuridine (FUDR). FUDR is a use-
ful drug in cancer chemotherapy because it inhibits
TMP formation in proliferating cells. Thymidine nu-
cleotide deficiency can also be induced by competitive
inhibitors of dihydrofolate reductase, e.g., aminopterin
(4-aminofolate) and methotrexate (4-amino-10-methyl-
folate; see Figure 27-3).
Fluoropyrimidines are catabolized by enzymes that
normally participate in the breakdown of endogeneous
pyrimidines uracil and thymine (see page 643). The de-
ficiency of the initial rate-limiting enzyme dihydropyri-
midine dehydrogenase can cause adverse drug reactions
in patients receiving standard chemotherapy. This is
another use of pharmacogenomics.
Pyrimidine Analogues
This group of compounds (Figure 27-30) includes sev-
eral drugs that are clinically useful in the treatment
of neoplastic disease, psoriasis, and infections caused
by
fungi
and
DNA-containing
viruses.
The mecha-
nism of action of
5-fluorouracil,
a halogenated pyrimi-
dine, was mentioned above. An antifungal agent,
flucy-
tosine (5-ftuorocytosine),
acts through conversion to
5- fluorouracil by cytosine deaminase in the fungal cells.
Idoxuridine
(iododeoxyuridine),
another halogenated
pyrimidine derivative, is used in viral infections. It is a
competitive inhibitor (via phosphorylated derivatives) of
the incorporation of thymidylic acid into DNA.
Cytarabine (cytosine arabinoside)
is an analogue of
2'-deoxycytidine. The 2'-hydroxyl group of the arabinose
moiety is in a trans position with respect to the 3'-hydroxyl
group (instead of in a cis position, as in the ribose) and
causes steric hindrance to rotation of the base around the
nucleoside bond (Chapter 23). Phosphorylated derivatives
of cytarabine inhibit nucleic acid synthesis as well as being
incorporated into nucleic acids.
6-Azauridine
and its triacetyl derivative, azaribine, in-
hibit pyrimidine biosynthesis. Azaribine is better absorbed
than
6
-azauridine, but it is converted in the blood to
6
- azauridine, which undergoes intracellular transforma-
tion to
6
-azauridylic acid (
6
-AzUMP).
6
-AzUMP, a
competitive inhibitor of orotidylate decarboxylase, blocks
formation of UMP (Figure 27-26), resulting in high
rates of excretion of orotic acid and orotidine in the
urine.
5-Azacytidine,
after conversion to a phosphorylated
derivative, arrests the DNA synthesis phase of the cell
cycle. It also affects methylation of certain bases, which
leads to hypomethylation (Chapter 26).
Azathymidine,
3'-azido-3'-dcoxythymidine (AZT), af-
ter conversion to the corresponding 5'-triphosphate, in-
hibits viral reverse transcriptase. Hence, AZT is used in
the treatment of AIDS (Chapter 25).
Regulation of
de Novo
Pyrimidine Biosynthesis
Regulation of the
de novo
pathway is complex. In prokary-
otic cells, aspartate transcarbamoylase, an allosteric pro-
tein, is inhibited by the end products of pyrimidine
