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chapter 27
Nucleotide Metabolism
M eth ion ine
f h 4
-
N s-M eth yl F H 4-
V.
A T P
PR + R
______ .
S -A d enosylm ethio nin e
M eth yl a c c e p to r ^
M eth yltran sferase
M eth ylated product
**
Transm eth ylatio n
reaction
S-A d enosylhom ocystein e (S A H )
S A H hydrolase
(deoxyad en o sin e
inhibits this
,, en zy m e )
-H o m o c y s te in e + A denosine
I
C ystein e
F IG U R E 2 7 -2 2
Formation and degradation of S-adenosylhomocysteine (SAH). In ADA
deficiency, accumulation of deoxyadenosine, a suicide substrate, inhibits
SAH hydrolase. This inhibition is accompanied by a higher steady-state
level of SAH, which in turn inhibits several methyltransferase reactions.
Normally, homocysteine is reconverted to methionine or cysteine.
leads to the hydrolysis of the substrate and to the reoxi-
dation of NADH, (Figure 27-23). Deoxyadenosine binds
to the enzyme and undergoes initial oxidation to a ke-
tosugar intermediate, which is unstable and decomposes
with elimination of adenine, leaving the enzyme in the
reduced (NADH) state (Figure 27-23). The catalysis is
stopped, and exogenous NAD
1
/NADH + H+ cannot af-
fect the reaction. Thus, inactivation of SAH hydrolase by
conversion of enzyme-bound NAD+ to NADH is a sui-
cide inactivation, in which no covalent modification of the
enzyme is involved.
Immune system dysfunction in ADA deficiency has also
been ascribed to the inhibition of pyrimidine nucleotide
synthesis by adenosine, known as
pyrimidine starvation.
This may arise from inhibition of conversion of orotic acid
to orotidine 5'-monophosphate or from inhibition of PRPP
synthesis by excessive synthesis of adenine nucleotides.
ADA deficiency causes death from massive infection
before the patient reaches the age of 2 years. Some chil-
dren with ADA or PNP deficiency have benefited from pe-
riodic infusions of irradiated erythrocytes (which contain
ADA and PNP). Irradiation of erythrocytes is necessary
to inactivate any white blood cells that may be present
and thereby to reduce the risk of graft-versus-host disease
(Chapter 35). PNP deficiency usually causes hypouricemia
and hypouricosuria and excretion of inosine, guanosine,
deoxyinosine, and deoxyguanosine.
Another mode of enzyme replacement therapy is pe-
riodic administration of a polyethylene glycol-modified
form of bovine intestinal ADA by intramuscular injec-
tion. The modified enzyme, which is prepared by conjugat-
ing polyethylene glycol with purified ADA (PEG-ADA),
possesses a longer half-life as well as reduced immuno-
genicity. As a consequence of this treatment, the correction
of two biochemical abnormalities, namely, accumulation
of toxic phosphorylated metabolites and inhibition of SAH
hydrolase, preceded clinical improvement (i.e., the ab-
sence of infection and resumption of weight gain).
Gene replacement therapy should be possible in both
ADA and PNP deficiency, and such a trial was attempted
with two patients having ADA deficiency. The patient’s
T cells were removed and a normal ADA gene was in-
serted into the T cells by means of a retroviral vector.
The modified T cells were reintroduced into the patient’s
bloodstream. Patients were followed for clinical improve-
ment while they continued to receive PEG-ADA treatment.
No permanent cure was achieved.
Myoadenylate Deaminase Deficiency
Myoadenylate deaminase (or AMP deaminase) deficiency
is a relatively benign muscle disorder characterized by fa-
tigue and exercise-induced muscle aches. This disorder is
presumably inherited as an autosomal recessive trait. The
relationship between the exercise-induced skeletal muscle
dysfunction and AMP deaminase deficiency is explained
by an interruption of the purine nucleotide cycle.
The purine nucleotide cycle of muscle consists of the
conversion of AMP —> IMP -» AMP and requires AMP
deaminase, adenylosuccinate synthetase, and adenylosuc-
cinate lyase (Figure 27-24). Flux through this cycle in-
creases during exercise. Several mechanisms have been
proposed to explain how the increase in flux is responsi-
ble for the maintenance of appropriate energy levels during
exercise (Chapter 21).
1. During muscle contraction AMP deaminase activity
increases. Nucleoside triphosphates are negative
modulators, whereas nucleoside di- and
monophosphates are positive modulators of the
enzyme. The increased AMP deaminase activity
prevents accumulation of AMP so that the adenylate
kinase reaction favors the formation of ATP: ADP +
ADP ^ ATP + AMP.
2. The NH
3
produced in the AMP deaminase reaction
and the decreased levels of ATP as a result of exercise
stimulate phosphofructokinase to enhance the rate of
glycolysis (Chapter 13).
3. The increased concentration of IMP may activate
glycogen phosphorylase and further enhance
glycolysis.
4. The production of fumarate (Figure 27-24; see also
Chapter 13) may enhance the TCA cycle when
demand for ATP production increases.
5. The formation of IMP may provide a means by which
the intracellular purine nucleotide pool is maintained.
AMP deaminase deficiency disrupts the purine
