What would they sayIf we up and ran away Ran away

2. A Christmas Prayer
by Robert Louis Stevenson
Loving God, help us to remember
the birth of Jesus that we may
share in the songs of the
angels, the gladness of the
shepherds, and the worship of
the wise men.
Close the door of hate and open
the door of love all over the
world. Let kindness come with
every gift and good desires
with every greeting. Deliver us
from evil by the blessing which
Christ brings.
May our minds be filled with
grateful thoughts and our
hearts with forgiveness, for
Jesus‟ sake.
Amen.

3. Nucleotide Metabolism
adenosine triphosphate
Noel Martin S. Bautista, MD, MBAH
Department of Biochemistry, Molecular Biology and Nutrition

4. Introduction
 Overview of nucleotide metabolism and
biological functions of nucleotides
 Metabolism of purine nucleotides
 Clinical disorders of purine metabolism
 Metabolism of pyrimidine nucleotides
 Clinical disorders of pyrimidine metabolism
 Metabolism of deoxyribonucleotides
 Different pharmacologic agents that interfere
with nucleotide metabolism
4

5. Nucleobases
5

6. base + sugar =
Nucleosides
 have either
-D-ribose or
-D-deoxyribose
in an N-glycosidic
linkage between
C-1 of the sugar
and N-9 (purine)
or N-1 (pyrimidine)
Ribonucleosides
6

7. base + sugar =
Nucleosides
 have either
-D-ribose or
-D-deoxyribose
in an N-glycosidic
linkage between
C-1 of the sugar
and N-9 (purine)
or N-1 (pyrimidine)
Deoxyribonucleosides
7

8. Nucleoside + PO4 =
Nucleotides
 have one or more
phosphate groups
esterified to the
sugar
Deoxyribonucleotides
Ribonucleotides
8

9. Metabolic Functions of Nucleotides
Function Selected Examples
1. Energy metabolism ATP (muscle contraction, etc)
2. Monomeric units of
NTPs and dNTPs (substrates for RNA and DNA)
nucleic acids
Adenosine (coronary blood flow); ADP (platelet
3. Physiological
aggregation); cAMP and cGMP (second
mediators
messengers)
GTP (mRNA capping); tetrahydrobiopterin
4. Precursor function
(hydroxylation of aromatic amino acids)
5. Components of
NAD, FAD, FMN and coenzyme A
coenzymes
UDP-glucose (glycogen); CDP-choline
6. Activated
(phospholipids); SAM (methylation); PAPS
intermediates
(sulfation)
ATP (negative effector of PFK-1); AMP (positive
7. Allosteric effectors effector of phosphorylase b); dATP (negative
effector of ribonucleotide reductase)
overview 9

10. Nucleotide Metabolism Overview
10

11. Nucleotide Synthesis: De Novo Pathway
De Novo Pathway
11

12. Nucleotide Synthesis: Salvage Pathway
central 12

13. PRPP: Central Metabolite
 5-phospho-D-ribosyl-1-pyrophosphate (PRPP) is
an intermediate in both the de novo and salvage
synthesis of nucleotides
 synthesized from ribose-5-phosphate
 from pentose phosphate pathway (HMP)
 phosphorolysis of nucleosides by nucleoside
phosphorylase
13

14. PRPP Synthesis
 formed through the
action of PRPP
synthetase, which
activates carbon 1 of
ribose-5-phosphate by
transferring to it the
pyrophosphate moiety
of ATP
14

15. Reactions Requiring PRPP:
 de novo purine nucleotide synthesis:
PRPP + glutamine  5-phosphoribosylamine + glutamate + PPi
 salvage of purine bases:
PRPP + hypoxanthine (guanine)  IMP(GMP) + PPi
PRPP + adenine  AMP + PPi
 de novo pyrimidine nucleotide synthesis:
PRPP + orotate  OMP + PPi
 salvage of pyrimidine bases
PRPP + uracil  UMP + PPi
 NAD+ synthesis
15

16. Synthesis of Nucleotide from PRPP
 PRPP is converted to a nucleotide by addition of
a base
 reaction is reversible, but it proceeds almost
exclusively to the right
 PPi product is readily cleaved by
pyrophosphatases
Guanine
+ Guanine + PPi
purine metabolism 16

17. Purine Metabolism
guanosine
synthesis | degradation | clinical disorders

18. Purine
Nucleotide
Synthesis:
De Novo
Pathway
18

19. Purine Nucleotide Synthesis: De Novo Pathway
 purine ring: utilizes amino acids (glutamine,
glycine, aspartate) as carbon and nitrogen
donors, tetrahydrofolate as a one-carbon donor,
and CO2 as a carbon donor
19

20. Purine Nucleotide Synthesis: De Novo Pathway
 consists of 10 metabolic steps 
inosine 5’-monophosphate (IMP)
 IMP – first ribonucleotide formed; precursor for
both AMP and GMP
 utilizes PRPP as an important substrate
 ATP needed to drive several reactions of the
pathway (6 moles of ATP)
 present in most cells, not all (e.g. RBCs)
 highly-regulated process
20

21. Purine Nucleotide Synthesis: De Novo Pathway
 formation of 5-
phosphoribosylamine
(first step) by
glutamine PRPP
amidotransferase,
is the committed and
regulated step
 the N-C bond formed
will become the N-
glycosidic bond of
the purine nucleotide
 IMP
21

22. Purine Nucleotide Synthesis: De Novo Pathway
 IMP - common precursor for AMP and GMP
22

23. Formation of GMP from IMP
 xanthosine 5‟-monophosphate (XMP) is formed by the
oxidation of IMP catalyzed by IMP dehydrogenase
 guanosine 5‟monophosphate (GMP) is formed by the
addition of an amino group from glutamine to XMP
catalyzed by GMP synthetase
23

24. Formation of AMP from IMP
 adenylosuccinate is formed by the addition of aspartate
to IMP catalyzed by adenylosuccinate synthetase
 adenosine 5‟-monophosphate (AMP) is formed by the
cleavage of fumarate from adenylosuccinate by
adenylosuccinase
24

25. Formation of GMP and AMP from IMP
 IMP  GMP : requires ATP as energy source
 IMP  AMP: requires GTP as energy source
 reciprocal relationship ensures that when there is sufficient ATP in
the cell, GMP will be synthesized from IMP and when there is
sufficient GTP, AMP will be synthesized from IMP
regulation 25

26. De Novo Purine Nucleotide Synthesis Regulation
 formation of 5-
phosphoribosylamine
(PRA) is the committed
step in IMP formation
 glutamine PRPP-
amidotransferase -
rate-limiting; regulated
allosterically by the
substrates and end
products of the pathway
26

27. De Novo Purine Nucleotide Synthesis Regulation
 glutamine PRPP-amidotransferase
 with  IMP, GMP or AMP: the enzyme is dimeric 
inactive
 IMP, GMP and AMP: negative effectors
 with  PRPP: the enzyme is monomeric  active
 PRPP: positive effector
27

28. De Novo Purine Nucleotide Synthesis Regulation
Other points of regulation:
 branch point of IMP to
GMP: IMP
Dehydrogenase
(IMPDH*) is rate-limiting
enzyme and is regulated
by GMP acting as a
competitive inhibitor of
IMDPH
 branch point of IMP to
* ** AMP: adenylosuccinate
synthetase** is rate
limiting in conversion of
IMP to AMP, with AMP
acting as competitive
inhibitor
salvage 28

29. Purine Nucleotide Synthesis: Salvage Pathway
 two specific enzyme systems (phosphoribosyl
transferases) catalyze the transfer of the ribose
phosphate from PRPP to free purine bases
29

30. Purine Nucleotide Synthesis: Salvage Pathway
Hypoxanthine-guanine
phosphoribosyl
transferase (HGPRTase)
 catalyzes the formation of
nucleotides from either
hypoxanthine or guanine
 hypoxanthine and guanine
for salvage arise from
degradation of purine
nucleotides
 IMP and GMP are
competitive inhibitors of
PRPP for HGPRTase
30

31. Purine Nucleotide Synthesis: Salvage Pathway
Adenine
phosphoribosyl
transferase
(APRTase)
 catalyzes the formation
of AMP from adenine
 AMP is a competitive
inhibitor of PRPP for
APRTase
effects 31

32. Effects of Salvage on De No Synthesis Pathway
 salvage pathway synthesis of purine nucleotides
affectively decreases the de novo pathway at the PRPP
amidotransferase step:
 PRPP is consumed  decreasing the rate of
formation of 5-phosphoribosylamine (rate limiting step
of de novo pathway)
 AMP, IMP and GMP serve as feedback inhibitors at
this step
 salvage reactions conserve energy and permit cells to
form nucleotides from free nucleobases or nucleosides
 helpful in cells without glutamine PRPP
amidotransferase (e.g., RBCs)
interconversions 32

33. Purine Nucleotide Interconversions
 occur by indirect steps; no direct one-step
pathway for conversion of GMP to AMP or AMP
to GMP
 AMP or GMP is converted to IMP
purine degradation 33

34. Purine
Nucleotide
Degradation
 degradation of
purine
nucleotides,
nucleosides and
nucleobases
follow a common
pathway that
leads to the
formation of uric
acid
34

35. adenine nucleotides
 hypoxanthine
guanine nucleotides
 xanthine
UA 35

36. Uric Acid Formation
 hypoxanthine and xanthine -
oxidized by xanthine
oxidoreductase (XOR), which
has both dehydrogenase and
oxidase activities
 xanthine dehydrogenase
activity requires NAD+ as the
electron acceptor
 xanthine oxidase activity
utilizes molecular oxygen with
the generation of hydrogen
peroxide
 uric acid is the unique end
product of purine nucleotide
degradation in man
36

37. Uric Acid
 final oxidation product of
purine metabolism
 a very weak organic acid that
is barely soluble in water and
insoluble in alcohol and ether
 urates are its salts
d/o 37

38. Clinical Disorders
of Purine Metabolism

39. Hyperuricemia and Gout
 Gout - manifestation of
acute or prolonged
hyperuricemia
 precipitation of blood urate
as crystals of sodium urate
in the synovial fluid of
joints
 inflammation painful
arthritis, which if untreated,
leads ultimately to severe
degeneration of the joints
 causes
manifestations 39

40. Gout: Manifestations
causes 40

41. Gout: Causes
APRTase
PRPP PRPP
synthetase amido
transferase
1 2
Elevated Loss of
levels feedback HGPRTase Decreased
inhibition Levels
3
41

42. Gout: Causes Elevated activity of PRPP
Synthetase (defect 1)
 insensitivity to feedback
inhibition by purine
nucleotides
  PRPP   activity of
PRPP amidotransferase
reaction (rate-limiting
step in de novo purine
synthesis)
  synthesis of purines
and uric acid
42

43. Gout: Causes Mutations in PRPP
amidotransferase
(defect 2)
 makes the enzyme
less sensitive to
feedback inhibition by
purine nucleotides
  loss of control also
increases flux through
the rate-limiting step
  synthesis of purines
and uric acid
43

44. HGPRTase Deficiency
Gout: Causes 
(defect 3)
 HGPRTase reaction,
when active, consumes
PRPP
 Decreased flux through
this reaction, when
HGPRTase is deficient 
 steady state level of
PRPP;  IMP and GMP
  intracellular activity of
PRPP amidotransferase
  synthesis of purines
and uric acid
other causes 44

45. Gout: Other Causes
 impaired uric acid
excretion (renal pathology)
 increase rate of nucleic
acids degradation
(cancer chemotherapy /
radiotherapy)
tx 45

46. Hyperuricemia and Gout: Treatment
 dietary – decrease intake of
purine rich-foods
 pharmacologic – Allopurinol
 structural analog of
hypoxanthine that strongly
inhibits xanthine
oxidoreductase
 inhibition causes
accumulation of
hypoxanthine and
xanthine, both of which
are more soluble and
more excretable than uric
acid
LNS 46

47. Lesch-Nyhan Syndrome (LNS / LND)
 a rare, inherited disorder caused
by a deficiency of the enzyme
hypoxanthine-guanine
phosphoribosyltransferase
(HGPRTase) (defect 3)
 increase in intracellular PRPP
results in purine overproduction
 manifests as severe gouty arthritis
and some neurologic
manifestations: behavioral
disorders, learning disability,
hostile and aggressive behaviors
towards self (self-mutilation)
47

48. Von Gierke’s Disease
 glucose-6-phosphatase deficiency - increased
availability of G6P for the pentose phosphate
pathway
   production of the PRPP precursor,
ribose-5-phosphate   PRPP
  PRPP accelerates de novo purine synthesis
leading to purine overproduction and
hyperuricemia
 associated lactic acidosis elevates the renal
threshold for urate, elevating total body urates
48

49. Hypouricemia
 associated with increased excretion of
hypoxanthine and xanthine
 caused by deficiency of xanthine oxidoreductase
due to a genetic defect or to severe liver
damage
 patients may exhibit xanthinuria and xanthine
lithiasis
49

50. Adenosine Deaminase (ADA) Deficiency
 Severe Combined
Immunodeficiency Syndrome
(SCID) - both T-cells and B-cells
are deficient and dysfunctional
  deoxyadenosine and
adenosine   deoxyATP which
“Bubble Boy Disease” inhibit ribonucleotide reductase
resulting to inhibition of DNA
synthesis and less proliferation
of lymphocytes
 recurrent or chronic infections,
failure to thrive, fatal before 18
years of age
50

51. What Would They Say (Paul Williams)
What would they say
If we up and ran away
From the roaring crowds
And the worn out city faces
Would they carry on and on
When they found out we were gone
Or would they let us go
Would they tag along
Or would they know to
What would they do
If they found out we were through
With all the little lies
And the downtown aggravations
That we'd traded them away
For a quiet country day
That we had hoped to share
Would they try to find out
Where we were or
What would they say
If we up and ran away
Ran away
51

52. Adenosine Deaminase (ADA) Deficiency
 DNA
 lymphocytes
synthesis
52

53. Purine Nucleoside Phosphorylase (PNP) Deficiency
 less severe than ADA deficiency
 associated with severe
deficiency of T-cells but
apparently normal B-cell
function
 immune dysfunctions appear to
result from accumulation of
dGTP and dATP, which inhibit
ribonucleotide reductase and
thereby deplete cells of DNA
precursors
53

54. Purine Nucleoside Phosphorylase (PNP) Deficiency
 T-cell
lymphocytes
54

55. Pyrimidine Metabolism
uridine
synthesis | degradation | clinical disorders

56. Pyrimidine Nucleotide
Synthesis: De Novo Pathway
 de novo synthesis of the
pyrimidine ring in
mammalian cells utilizes
amino acids and
nitrogen donors in
addition to CO2.
 Uridine 5‟-
monophosphate (UMP)
is synthesized in a six-
step metabolic pathway,
requiring ATP hydrolysis
56

57. Pyrimidine Nucleotide Synthesis: De Novo Pathway
 requires aspartate as carbon and nitrogen
donor, glutamine as a nitrogen donor and CO2
as a carbon donor
 five of the six reactions occur in the cytosol of
the cell, while the other reaction occurs in the
mitochondria
57

58. Pyrimidine Synthesis
 pyrimidine ring is formed
first; ribose 5-phosphate is
added with PRPP as the
ribose 5-phosphate donor
 formation of cytosolic
carbamoyl phosphate
(CPS II) is the regulated step
 formation of N-
carbamoylaspartate is the
committed step in pyrimidine
nucleotide synthesis
58

59. Pyrimidine Synthesis
 nucleotide kinases convert
UMP to UTP
 UTP serves as substrate
for CTP synthetase
59

60. Formation of CTP from UTP
 CTP synthetase catalyzes the
formation of CTP from UTP
with glutamine as the amino
group donor
 the product, CTP is a
negative effector of CTP
synthetase
 this regulation maintains an
appropriate ratio of UTP and
CTP for cellular function and
RNA synthesis
60

61. Pyrimidine Nucleotide
Synthesis Regulation
 regulation of pyrimidine
nucleotide synthesis occurs
at the CPS II step
 CPS II is inhibited by CTP, an
end product of the pathway
 CPS II is activated by PRPP
salvage pathway 61

62. Pyrimidine Nucleotide Synthesis: Salvage Pathway
 pyrimidine bases are „salvaged‟ to reform nucleotides by
pyrimidine phosphoribosyl transferase:
pyrimidine + PRPP  pyrimidine nucleoside 5’-monophosphate + PPi
 these reactions divert the pyrimidine bases from the
degradative pathway to nucleotide formation
 as a pyrimidine base becomes available to cells, there
are competing reactions that will result in either:
a. degradation and excretion of the products
b. reutilization of the bases for nucleotide synthesis
degradation 62

63. Pyrimidine
Nucleotide
Degradation
63

64. Degradation of
Pyrimidine
Nucleobases
 uracil and thymine
are degraded by
analogous reactions
 uracil is degraded to
-alanine, NH4+ and
CO2
 thymine is degraded
to -aminoisobutyric
acid, NH4+ and CO2.
64

65. Clinical Disorders of Pyrimidine Metabolism
Orotic Acidurias
 Reye Syndrome: consequence of the inability of
severely damaged mitochondria to utilize carbamoyl
phosphate, which then becomes available for cytosolic
overproduction of orotic acid
 deficiency of liver mitochondrial ornithine
transcarbamoylase (an enzyme in the urea cycle) 
 excretion of orotic acid, uracil and uridine
 excess carbamoyl phosphate exits from the mitochondria to the
cytosol, where it stimulates pyrimidine nucleotide biosynthesis
dntp 65

66. Deoxyribonucleotide
Metabolism
thymidine
synthesis | interconversions

67. Deoxyribonucleotide Biosynthesis
 synthesis of deoxyribonucleoside triphosphates
(dNTPs) is important in DNA synthesis and
repair
 nucleoside 5’-diphosphate reductase
(ribonucleotide reductase) – catalyzes the
reaction in which ribonucleoside 5‟-diphosphates
are reduced to 2‟-deoxyribonucleoside 5‟-
diphosphates
67

68. Deoxyribonucleotide Biosynthesis
 nucleoside 5’-diphosphate reductase
(ribonucleotide reductase) – catalyzes the
reaction in which ribonucleoside 5‟-
diphosphates are converted to 2‟-
deoxyribonucleoside 5‟-diphosphates
regulation 68

69. Deoxyribonucleotide Biosynthesis: Regulation
 reaction is controlled by the amount of enzyme present in
cells and by a very finely-regulated allosteric control
mechanism (NTPs)
 thioredoxin or glutaredoxin: involved in reduction at the
2‟-position through oxidation of its sulfhydryl groups
 NADPH is used to regenerate free sulfhydryl groups on
thioredoxin or glutaredoxin 69

70. Deoxyribonucleotide Biosynthesis: Regulation
 reduction of a particular substrate requires a specific
nucleoside 5‟-triphosphate as a positive effector
Major Positive
Substrate Major Negative Effector
Effector
CDP ATP dATP, dGTP, dTTP
UDP ATP dATP, dGTP, dTTP
ADP dGTP dATP
GDP dTTP dATP
70

71. Role of Ribonucleotide Reductase in DNA Synthesis
 ribonucleotide reductase (1) catalyzes the rate-limiting reactions in
which 2‟-deoxyribonucleoside 5‟-triphosphates are synthesized de
novo for DNA replication
 inhibitors of ribonucleotide reductase are potent inhibitors
of DNA synthesis and, hence, of cell replication
71

72. Deoxythymidylate
Biosynthesis
 thymidylate synthase
catalyzes the transfer of a
one-carbon unit from
N5,N10-methyleneH4folate to
dUMP and which is
simultaneously reduced to a
methyl group
 N5,N10-methyleneH4folate
serves as a one-carbon
donor and as a reducing
agent
 72

73. Deoxyribonucleotide Interconversions
dCTP and dTTP are major
positive and negative effectors
of the interconversions and
salvage of
deoxyribonucleosides
Rx 73

74. Pharmacologic Agents
that Interfere with
Nucleotide Metabolism
AZT (Zidovudine)
antimetabolites | antifolates | glutamine
antagonists | antivirals | other agents

75. Pharmacologic Agents
 purine and pyrimidine
nucleotides: critical for normal
cell function, maintenance and
replication
 compounds may be specific
inhibitors of enzymes involved
in nucleotide synthesis, inter-
conversions or degradation
 antimetabolites, antifolates.
glutamine antagonists, other
agents
antimetabolites 75

76. Antimetabolites
 structural analogs of purine and pyrimidine
bases or nucleosides that interfere with very
specific metabolic reactions
 6-Mercaptopurine (6-MP)
 5-Fluorouracil (Fura or 5-FU)
 Cytosine Arabinoside (AraC)
76

77. Antimetabolites: 6-Mercaptopurine (6-MP)
 useful antitumor drug in humans
 utilizing PRPP and HGPRTase,
6-mercaptopurine ribonucleoside 5‟-
monophosphate is formed in cells
and serves as a negative effector of
PRPP-amidotransferase
 this nucleotide also act as an
inhibitor of the conversion of IMP to
GMP at the IMP-dehydrogenase step
and IMP to AMP at the
adenylosuccinate synthetase step
77

78. Antimetabolites: 5-Fluorouracil (5-FU)
 analog of uracil
 converted to the active metabolite 5-
fluorouridine 5‟-triphosphate (FUTP)
and 5-fluoro-2‟-deoxyuridine 5‟-
monophosphate (FdUMP).
 FUTP is efficiently incorporated into
RNA  inhibits its maturation
 FdUMP is a potent and specific
irreversible inhibitor of thymidylate
synthase, resulting in the inhibition of
dTMP synthesis and leads to
“thymineless death” for cells
78

79. Antimetabolites: Cytosine Arabinoside (AraC)
 treatment of several forms of
cancer (leukemias)
 metabolized to cytosine
arabinoside 5‟-triphosphate
(araCTP) to exert its cytotoxic
effects
 araCTP competes with dCTP in
the DNA polymerase reaction
and araCMP is incorporated into
DNA, which inhibits the
synthesis of the growing strand
antifolates 79

80. Antifolates
 folate analogs interfere with
metabolic steps in which
tetrahydrofolate is involved as
either as a substrate or product
 purine de novo synthesis
 thymidylate synthase
(dUMP  dTMP)
80

81. Antifolates: Methotrexate (MTX)
 synthetic analog of folic acid
 amino group at C4 instead of OH
 methyl group at N10 instead of an H
81

82. Antifolates:
Methotrexate (MTX)
 interferes with the
formation of H2folate and
H4folate from folate by
specifically inhibiting
H2folate reductase
(DHFR).
82

83. Antifolates: Methotrexate (MTX)
 cells would be
incapable of de
novo synthesis of
purine
nucleotides or
thymidylate
synthesis due to
depletion of
H4folate pools
  lowering of
ribonucleoside 5‟-
triphosphate and
2‟-deoxyribo-
nucleoside 5‟-
triphosphate
“thymineless death” intracellular pools
GA 83

84. Glutamine Antagonists
 many reactions in the cells utilize
glutamine as the amino group donor
 amidation reactions are critical:
 de novo synthesis of purine nucleotide
(N3 and N9)
 synthesis of GMP from IMP
 formation of cytosolic carbamoyl
phosphate
 synthesis of CTP from UTP
 synthesis of NAD+.
 compounds that inhibit these
reactions are referred to as glutamine
antagonists
84

85. Glutamine Antagonists:
O-diazoacetyl-L-serine (Azaserine)
6-diazo-5-oxo-L-norleucine (DON)
 effective inhibitors of enzymes
that utilize glutamine as the
amino donor
 addition of glutamine alone will
not reverse the effects of these
two drugs
 many metabolites (guanine,
cytosine, hypoxanthine/adenine
and nicotinamide): needed to
bypass the many sites blocked
by these glutamine antagonists
(DON) Azaserine AV 85

86. Antivirals
 enzymes of nucleotide synthesis have been
widely studied as target sites for the action of
antiviral or antimicrobial drugs
 purine and pyrimidine analogs can target these
enzymes and thus control or help treat certain
viral diseases
86

87. Antivirals: Acylguanosine (Acyclovir)
 purine analog, commonly used in
the treatment of herpesvirus (HSV)
 converted to the monophosphate
by a specific HSV-thymidine kinase
present only in virally-infected cells
 acycloguanosine monophosphate
is phosphorylated to the di- and
triphosphate forms
 acycloguanosine triphosphate
serves as a substrate for the HSV-
specific DNA polymerase 
incorporated in the growing DNA
chain, causing chain termination
87

88. Antivirals: 3’-azido-3’deoxythymidine (AZT)
 pyrimidine analog,
commonly used in the
treatment of human
immunodeficiency virus
(HIV) infections
 phosphorylated by cellular
kinases to AZT-
triphosphate, which blocks
HIV replication by inhibiting
HIV-DNA polymerase
OA 88

89. Other Agents: Hydroxyurea
 limited use in cancer treatment;
also used in the treatment of
sickle cell anemia
 inhibits ribonucleotide reductase
activity by destroying the tyrosyl
free radical on the small subunit
of the enzyme
 inhibition of the reduction of CDP,
UDP, GDP and ADP to the
corresponding 2‟-
deoxyribonucleoside 5‟-
diphosphates  termination of
DNA replication
89

90. Other Agents: Tiazofurin
 converted by cellular
enzymes to the NAD+
analog tiazofurin adenine
dinucleotide (TAD)
 TAD inhibits IMP
dehydrogenase, the
rate-limiting enzyme in
GTP synthesis
  GTP   dGTP
90

91. Thank you very much!
adenosine triphosphate
Noel Martin S. Bautista, MD, MBAH
Department of Biochemistry, Molecular Biology and Nutrition