• RNA and DNA monomers
• Energy: ATP
• Physiologic mediators
• cAMP levels → blood flow
• cGMP → second messenge
De novo and salvage pathways
Most organisms can synthesize purine and pyrimidrne
nucleotides from low-molecular-weight precursors in
amounts sufficient for their needs. These so-called de novo
pathways are essentially identical throughout the biological
Salvage pathways involve the utilization of preformed purme
and pyrimidine compounds that would be otherwise lost to
biodegradation. Salvage pathways represent important sites
for manipulation of biological systems.
Nucleic Acid Degradation and the Importance
of Nucleotide Salvage
The salvage, or reuse, of purine and pyrimidine bases involves
molecules released by nucleic acid degradation
Degradation can occur intracellularly, as the result of cell death,
or, in animals, through digestion of nucleic acids ingested in the
In animals, the extracellular hydrolysis of ingested nucleic acids
represents the major route by which bases and nucleosides
become available. Catalysis occurs by endonucleases, which
function to digest nucleic acids in the small intestine. The
products are mononucleotides.
If bases or nucleosides are not reused for nucleic acid synthesis
via salvage pathways, the purine and pyrimidine bases are
further degraded to uric acid or b-ureidopropionate.
PRPP: A Central Metabolite in De Novo and
5-Phospho-a-D-ribosyl-1-pyrophosphate (PRPP) is an
activated ribose-5-phosphate derivative used in both salvage
and de novo pathways.
Purine synthesis from PRPP to inosinic acid
Purines are synthesized at the nucleotide level, starting with
PRPP conversion to phosphoribosylamine and purine ring
assembly on the amino group.
Control over the biosynthesis of inosinic acid is provided
through feedback regulation of early steps in purine
nucleotide synthesis. PRPP synthetase is inhibited by various
purine nucleotides, particularly AMP, ADP, and GDP, and PRPP
amidotransferase is also inhibited by AMP, ADP, GMP, and GDP.
De novo biosynthesis
of the purine ring,
from PRPP to inosinic
1. PRPP amidotransferase
2. GAR synthetase
3. GAR transformylase
4. FGAR amidotransferase
5. FGAM cyclase
6. AIR carboxylase
7. SAICAR synthetase
8. SAICAR lyase
9. AICAR transformylase
10. IMP synthase
Figure 22.6: Pathways from inosinic acid to GMP and AMP.
Purine degradation and
disorders of purine metabolism
Formation uric acid
All purine nucleotide catabolism yields uric acid.
Purine catabolism in primates ends with uric acid, which is
excreted. Most other animals further oxidize the purine ring,
to allantoin and then to allantoic acid, which is either
excreted or further catabolized to urea or ammonia.
Excessive accumulation of uric acid: gout
Uric acid and its urate salts are very insoluble. This is an
advantage to egg-laying animals, because it provides a route
for disposition of excess nitrogen in a closed environment.
Insolubility of urates can present difficulties in mammalian
metabolism. About 3 humans in 1000 suffer from
hyperuricemia, which is chronic elevation of blood uric acid
levels well beyond normal levels. The biochemical reasons
for this vary, but the condition goes by a single clinical name,
which is gout.
Prolonged or acute elevation of blood urate leads to
precipitation, as crystals of sodium urate, in the synovial fluid
of joints. These precipitates cause inflammation, resulting in
painful arthritis, which can lead to severe degeneration of the
Gout results from overproduction of purine nucleotides,
leading to excessive uric acid synthesis, or from impaired uric
acid excretion through the kidney
Several known genetic alterations in purine metabolism lead
to purine oversynthesis, uric acid overproduction, and gout.
Gout can also result from mutations in PRPP amidotransferase
that render it less sensitive to feedback inhibition by purine
nucleotides. Another cause of gout is a deficiency of the
salvage enzyme hypoxanthine-Guanine
Many cases of gout are successfully treated by the
antimetabolite allopurinol, a structural analog of
hypoxanthine that strongly inhibits xanthine oxidase.
This inhibition causes accumulation of hypoxanthine and
xanthine, both of which are more soluble and more readily
excreted than uric acid.
Figure 22.9: Enzymatic abnormalities in three types of gout.
APRT: adenine phosphoribosyltransferase
Lesch-Nyhan syndrome: HGPRT defficiency
Lesch-Nyhan syndrome is a x-linked trait, because the
structural gene for HGPRT is located on the X chromosome.
• Excess uric acid production (“juvenile gout”)
• Excess de novo purine synthesis (↑PRPP, ↑IMP)
Patients with this condition display a severe gouty arthritis,
but they also have dramatic malfunction of the nervous
system, manifested as behavioral disorders, learning
disabilities, and hostile or aggressive behavior, often self-
At present, there is no successful treatment, and afflicted
individuals rarely live beyond 20 years.
Severe combined immune deficiency (SCID)
Patients with a hereditary condition called severe combined
immunodeficiency syndrome are susceptible, often fatally, to
infectious diseases because of an inability to mount an
immune response to antigenic chanllenge.
In this condition, both B and T lymphocytes are affected.
Neither class of cells can proliferate as they must if antibodies
are to be synthesized. In many cases the condition is caused
from a heritable lack of the degradative enzyme adenosine
The deficiency of ADA leads to
accumulation of dATP which is
known to be a potent inhibitor of
A less severe immunodeficiency results from the lack of
another purine degradative enzyme, purine nuceloside
phosphorylase (PNP). Decreased activity of this enzyme
leads to accumulation primarily of dGTP. This accumulation
also affects DNA replication, but less severely than does
Interestingly, the phosphorylase deficiency destroys only
the T class of lymphocytes and not the B cells.