This document discusses purine and pyrimidine metabolism. It covers the biosynthesis of purines through 11 steps, degradation of purines to uric acid, medical conditions related to purine metabolism like Lesch-Nyhan and ADA deficiency, the causes and treatment of gout, and drugs used to treat gout like colchicine, probenecid, and allopurinol.

1. Metabolism of
Purines and Pyrimines
Dr Muhammad Mustansar

2. Origin of atoms in pyrimidine ring

4. Biological significance of
nucleotidemake up nucleic acids (DNA
• Nucleotides
metabolism
and RNA)
• Nucleotide triphosphates are the “energy
carriers” in cells (primarily ATP)
• Many metabolic pathways are regulated by
the level of the individual nucleotides
– Example: cAMP regulation of glucose
release
• Adenine nucleotides are components of
many of the coenzymes
+ +

5. Dietary nucleotides
• do not contribute energy as do carbs,
proteins and fats
• are not incorporated into RNA or
DNA unless given I.V.
• normally metabolized to individual
components (bases, sugar and
phosphate)
• purines are converted to uric acid
which is then excreted

6. Medical significance of
nucleotide metabolism
• Anticancer agents:
• Rapidly dividing cells biosynthesize lots of
purines and pyrimidines, but other cells
reuse them. Cancer cells are rapidly
dividing, so inhibitor of nucleotide
metabolism kill them
• Antiviral agents
– Zidovudine (Retrovir)
– Lamivudine (Epivir)
– Valacyclovir (Valtrex)

8. Structures of nucleotide
building blocks and nucleotides
6 7 4
5
N 5
1 N 3N
8
2 2 6
N 4 N N
9
3 H 1
PURINE PYRIMIDINE

9. Structures of nucleotide building
NH2
blocks and nucleotides
O O O
NH 2
CH 3
N N HN HN
N N N
N H2N N N O N O N O N
N
H H H H H
ADENINE GUANINE THYMINE URACIL CYTOSINE
guanine: comes from guano; thymine –thymus gland

10. OH OH
Base Base
OH P O CH 2 OH P O CH 2
O O
O H H O H H
H H H H
OH OH OH H
RIBONUCLEOTIDE DEOXYRIBONUCLEOTIDE
Ribonucleotide – phosphate = ribonucleoside

11. Biosynthesis of the purine nucleotide system

12. The big picture

13. Steps 1 thru 3
• Step 1:Activation of ribose-5-
phosphate
– enzyme: ribose phosphate
pyrophosphokinase
– product: 5-phosphoribosyl-α-
pyrophosphate (PRPP)
– PRPP is also a precursor in the
biosynthesis of pyrimidine nucleotides
and the amino acids histidine and

14. Step 1: purine synthesis

15. Steps 1 thru 3
• Step 2: acquisition of purine atom 9
– enzyme: amidophosphoribosyl
transferase
– displacement of pyrophosphate group
by glutamine amide nitrogen (inversion
of configuration – α to β
– product: β-5-phosphoribosylamine
Steps 1 and 2 are tightly regulated by feedback inhibition

16. Step 2: purine synthesis:
commited step

17. Steps 1 thru 3
• Step 3: acquisition of purine atoms
C4, C5, and N7
– enzyme: glycinamide synthetase
β-phosphoribosylamine reacts with ATP
and glycine
– product: glycinamide ribotide (GAR)

18. Step 3 : purine synthesis

19. Steps 4 thru 6
• Step 4: acquisition of purine atom C8
– formylation of free α-amino group of GAR
– enzyme: GAR transformylase
– co-factor of enzyme is N10-formyl THF
• Step 5: acquisition of purine atom N3
– The amide amino group of a second glutamine is
transferred to form formylglycinamidine ribotide (FGAM)
• Step 6: closing of the imidazole ring or formation of 5-
aminoimidazole ribotide

21. Step 6: purine synthesis

22. Step 7
• Step 7: acquisition of C6
– C6 is introduced as HCO3-
– enzyme: AIR carboxylase
(aminoimidazole ribotide carboxylase)
– product: CAIR (carboxyaminoimidazole
ribotide)
– enzyme composed of 2 proteins: PurE
and PurK (synergistic proteins)

23. Step 7: purine synthesis

24. Steps 8 thru 11
• Step 8: acquisition of N1
– N1 is acquired from aspartate in an
amide condensation reaction
– enzyme: SAICAR synthetase
– product: 5-aminoimidazole-4-(N-
succinylocarboxamide)ribotide
(SAICAR)
– reaction is driven by hydrolysis of ATP

25. Step 8: purine synthesis

26. Steps 8 thru 11
• Step 9: elimination of fumarate
– Enzyme: adenylosuccinate lyase
– Product: 5-aminoimidazole-4-
carboxamide ribotide (AICAR)
• Step 10: acquisition of C2
– Another formylation reaction catalyzed
by AICAR transformylase
– Product: 5-formaminoimidazole-4-
carboxamide ribotide (FAICAR)

27. Step 9: purine synthesis

28. Step 10: purine
synthesis

29. Step 11
• cyclization or ring closure to form
IMP
• water is eliminated
• in contrast to step 6 (closure of the
imidazole ring), this reaction does not
require ATP hydrolysis
• once formed, IMP is rapidly
converted to AMP and GMP (it does
not accumulate in cells

30. Step 11: purine
synthesis

33. Lesch-Nyhan syndrome
• there is a defect or lack in the
HGPRT enzyme
• the rate of purine synthesis is
increased about 200X
• uric acid level rises and there is gout
• in addition there are mental
aberrations
• patients will self-mutilate by biting lips
and fingers off

34. Lesch-Nyhan syndrome

35. Adenosine deaminase

36. ADA deficiency
• In the absence of ADA lymphocytes are
destroyed
• deoxyadenosine is not destroyed, is
converted to dAMP and then into dATP
• dATP is a potent feedback inhibitor of
deoxynucleotide biosynthesis
• this leads to SCID (severe combined
immunodeficiency disease)
• Infants with this deficiency have a high
fatality rate due to infections

37. ADA deficiency
• treatment consists of administering
pegylated ADA which can remain in
the blood for 1 – 2 weeks
• more efficient is gene therapy:
replacing the gene that is missing or
defective
• gene therapy has been performed on
selected patients

38. Adenosine deaminase

39. ADA deficiency
• In the absence of ADA lymphocytes are
destroyed
• deoxyadenosine is not destroyed, is
converted to dAMP and then into dATP
• dATP is a potent feedback inhibitor of
deoxynucleotide biosynthesis
• this leads to SCID (severe combined
immunodeficiency disease)
• Infants with this deficiency have a high
fatality rate due to infections

40. ADA deficiency
• treatment consists of administering
pegylated ADA which can remain in
the blood for 1 – 2 weeks
• more efficient is gene therapy:
replacing the gene that is missing or
defective
• gene therapy has been performed on
selected patients

41. O O
H N H N
N N
H 2N N N O N N
Ribose-P H Ribose-P
H 2O H2O
nucleotidase nucleotidase
Pi Pi
Degradation of O
O
H
GMP and XMP N
N H
N
N
H 2N N N
O N N
Pi Ribose
Ribose
PNP H
Ribose-1P Pi
PNP
Ribose-1P
O
O
H N
N H N
N
H2N N N
H2O NH3 N
O N
H
H H

42. NH2 OH
N N
N N N
N
N N H 2N N N
N N
H H
H
ADENINE GUANINE
(6-AMINOPURINE) 2-AMINO-6-OXYPURINE) PURINE
OH OH OH
N N N
N N N
OH
N HO N N N
N HO N
H H H
HYPOXANTHINE XANTHINE URIC ACID
(6-OXYPURINE) (2,6-DIOXYPURINE) (LACTIM FORM)

43. CATABOLISM OF
PURINES
ADENINE + H O
2
adenase
HYPOXANTHINE + AMMONIA
guanase XANTHINE + AMMONIA
GUANINE + H2O
xanthine
HYPOXANTHINE + O2 + H2O oxidase XANTHINE + H2O2
xanthine
XANTHINE + O2 + H20 URIC ACID + H2O2
oxidase

44. O O
H N H
N N
N
N N N
O N
O2 H202
H H
H
HYPOXANTHINE
XANTHINE
O2
H202
O O
H
H N H
N N
N
OH O
O N N N
O N
H H
H H
acidic proton
URIC ACID

45. GOUT
• a disorder associated with abnormal
amounts of urates in the body
• early stage: recurring acute non-
articular arthritis
• late stage: chronic deforming
polyarthritis and eventual renal
complication
• disease with rich history dating back
to ancient Greece

46. GOUT
• once fashionable to associate gout
with intelligence
• people with gout:
– Isaac Newton
– Benjamin Frankin
– Martin Luther
– Charles Darwin
– Samuel Johnson

47. Gout
• prevails mainly in adult males
• rarely encountered in premenopausal
women
• symptoms are cause by deposition of
crystals of monosodium urate
monohydrate (can be seen under
polarized light)
• usually affect joints in the lower
extremities (the big toe is the classic

48. Gout

49. Four Stages of Gout
1. asymptomatic hyperuricemia
2. acute gouty arthritic attacks
3. asymptomatic intercritical period
4. tophaceous gout (characterized by the
formation of tophi in joints)
– podagra (big toe)
– cheiagra (wrist) according to Hippocrates
– gonadra (knee)

50. Diagnostic features
• usually affect joints in the lower
extremities ( 95%)
• onset is fast and sudden
• pain is usually severe; joint may be
swollen, red and hot
• attack may be accompanied by fever,
leukocytosis and an elevated ESR

51. Drugs which may induce
hyperuricemia
• niacin
• thiazides and other diuretics
• low dose aspirin
• pyrazinamide
• ethambutol
• cyclosporine
• cytotoxic drugs

52. Non-pharmacological
approaches foods:
• Avoid purine rich
– red meat and organ meat (liver,
kidneys)
– shellfish, anchovies, mackerel, herring
– meat extracts and gravies
– peas and beans, aspargus, lentils
– beer, lager, other alcoholic beverages
• Weight loss
• Control alcohol (binge drinking)

53. Pharmacological management of
• gout on the premise that the
based
hyperuricemia is due to both
overproduction and underexcretion of uric
acid
• symptomatic relief of pain is also
achieved with analgesics (i.e.
indomethacin)
• drugs used:
– analgesics (NSAIDs)
– uricosuric agents
– xanthine oxidase inhibitors

54. Therapy of acute gout
• treat with colchicine or NSAIDs
• avoid aspirin
• do not treat with allopurinol or
uricosuric drugs
• uric acid lowering agents should
never be started or stopped during
acute attack
• pain resolution occurs within 48-72

55. Colchicine
CH3O H
N
CH3O C CH3
O
OCH3
O
OCH3
COLCHICINE
a non-basic alkaloid from the seeds and corms of Colchicum autumnale
(Meadow Safron)

56. COLCHICINE
• used in the symptomatic treatment of
acute attacks of gout
• decreases leukocyte motility,
decreases phagocytosis and lactic
acid production
• not used in other forms of arthritis
• a very potent drug
• can cause severe GI distress and
abdominal pain

57. Probenecid (Benemid)
O C3H7
HO2C S N
O C3H7
PROBENECID (BENEMID)
A uricosuric agent

58. Probenecid (Benemid)
• inhibits the tubular reabsorption of uric
acid
• it can also inhibit the tubular excretion of
certain organic acid via the transporter
• used in gout to promote the elimination of
uric acid (not effective in acute attack)
• also used to enhance plasma
concentration of certain antiinfectives
(beta lactams)

59. ALLOPURINOL (Zyloprim)
• prevention of attacks of gouty arthitis
and nephropathy
• also used during chemotherapy of
cancer and to prevent recurrent calcium
oxalate calculi
• metabolized to oxypurinol (also an
inhibitor of xanthine oxidase)
• inhibits the metabolism of certain
anticancer drugs (6-MP, azathioprine)

60. Allopurinol (Zyloprim)
OH
N
N
N N
H
ALLOPURINOL (ZYLOPRIM)
An inhibitor of xanthine oxidase; prevents the formation of uric acid from
precursorial purines

62. Fate of uric acid
• in human and other primates uric acid is the final
product of purine degradation and is excreted in
the urine
• the same is true in bird, reptiles and many insects
• in other mammals uric acid is oxidized to allantoin
(urate oxidase)
• teleost (bony) fish convert allantoin to allantoic
acid
• cartilaginous fish and amphibian further degrade
allantoic acid to urea
• and finally marine invertebrates decompose urea
to ammonia

63. O
H
H
H N urate oxidase O
N N
O NH2
O
O N N
CO2 O N N
ALLANTOIN
H H
H H
H
URIC ACID
allantoinase
O
allantoicase H2N COOH NH2
H2N C NH2
UREA glyoxylic acid O N N O
H
H H
ALLANTOIC ACID

64. Rasburicase (Elitek)
A recombinant form of uric
acid oxidase. Used for initial
management of plasma uric
acid levels in pediatric
patients with leukemia,
lymphoma, and solid tumor
malignancies who are
receiving anticancer therapy
expected to result in tumor
lysis and subsequent
elevation of plasma uric acid.

65. Catabolism of a pyrimidine