Dr. S.Chakravarty, MD
A 30 year old man comes with severe
pain in great toe of right foot !!
Learning objectives
• Discuss the components of Nucleotides and molecules contributing
to formation of purine and pyrimidine ring
• Describe the Pyrimidine metabolism and its defects – Orotic
acidurias and Megaloblastic anemias
• Differentiate the features of Denovo and salvage pathways of purine
metabolism
• List the causes of Hyperuricemias, its clinical features and treatment
• Discuss the clinical features and the enzyme defects in SCID and
Lesch - Nyhan syndrome – clinical features
• List the anticancer drugs acting on purine and pyrimidine metabolism
and its mechanism of action
Importance of nucleotides:
• Building blocks of nucleic acids – DNA and RNA
• Act as co-enzymes – FAD, NAD, NADP
• Second messengers – cAMP and cGMP
• Energy currency – ATP and GTP
• Nucleoside and nucleotide analogs – treatment of
cancer.
• Donors of sugar moiety – UDP-Glucuronic acid.
Nomenclature
• Base= (Purines or Pyrimidines)
• Nucleosides = Base + sugar
• Nucleotides = Base + sugar + phosphate
• Sugar = Ribose in RNA
= 2 - Deoxyribose in DNA
Purines
Pyrimidines
Mnemonic : CUT
Pyrimidine biosynthesis
• The donors of C and N atoms
to the pyrimidine ring are
Aspartate, Glutamine and CO2
• The pyrimidine ring is initially
assembled & the ribosephosphate is added later
(compare to purine
biosynthesis)
De
From Kaplan Step 1 lecture notes
CO2 + ATP
Glutamine
-VE
OROTIC ACIDURIA
-VE
-VE
Hydroxyurea
Dihydrofolate reductase
-VE
5-flurouracil
-VE
Methotrexate – Eukaryotes
Trimethoprim – prokaryotes
Pyrimethamine - protozoal
Conversion of the ribonucleotides to
deoxyribonucleotides
Ribonucleotide diphosphate
(ADP, GDP, CDP,
UDP)
Thioredoxin,
NADPH+H+
Ribonucleotide
reductase
(-)
Hydroxyurea
Deoxyribonucleotide diphosphate
(dADP, dGDP, dCDP,
dUDP)
Carbomyl phosphate synthase
•
•
•
•
•
•
Carbamoyl phosphate synthetase I
- involved in urea synthesis
- uses free ammonia as the source of nitrogen
- occurs in liver mitochondria
- activated by N-acetylglutamate
- not affected by UDP or UTP
•
•
•
•
•
•
Carbamoyl phosphate synthetase II
- involved in pyrimidine synthesis
- uses glutamine as the source of nitrogen
- occurs in the cytosol of all nucleated cells
- Inhibited by UDP and UTP, Activated by PRPP, ATP
- not activated by N-acetylglutamate
Orotic aciduria
• Deficiency in UMP synthase activity
– Orotate Phosphoribosyl transferase
– Orotidylate decarboxylase
• Pyrimidine synthesis is decreased, excess orotic acid is
excreted in urine (hence Orotic aciduria)
• Due to demand for nucleotides in red blood cell synthesis
megaloblastic anemia
– Important :- Unresponsive to vit B12 and folic acid,
Orotic aciduria
• Administration of Uridine improves anemia
(Uridine is used for the synthesis of thymidine and
cytidine)
• UTP is feedback inhibitor of CPS II, uridine
administration results in a fall in orotic acid levels.
Orotic aciduria – other causes:
• Urea cycle defect : Ornithine transcarbomylase
(OTC)
• Drugs – Allopurinol competes with orotic acid
for the enzyme orotate phosphoribosyl
transferase .
Question
• A one year old female child is weak and anemic.
The child was found to have megaloblastic
anemia. The height and weight of the child are
less than normal. Urine demonstrates an elevated
level of orotic acid excretion and normal blood
ammonia levels.
• Which of the following enzyme will be deficient ?
– A. Ornithine transcarbamoylase
– B. Orotate Phosphoribosyl transferase
Pyrimidine catabolism
• Dephosphorylated to nucleotide
• Cytosine converted to uracil
• Uracil converted to b-alanine
• Thymine converted to b-aminoisobutyrate
NH3
cytosine
cytosine deaminase
NH2
uracil
CO2
CH2
-alanine
uracil
NH3
CH2
C
-
O
O
NH3+
CO2
thymine
-aminoisobutyrate
NH3
CH2
H
C
CH3
C
-
O
O
Purine Biosynthesis
N10-Formyl
Tetrahydrofolate
Purine synthesis- De novo pathway
Ribose- 5 phosphate
PRPP synthase
Allopurinol
6- Mercaptopurine
Azathioprine
PRPP
(+)
(-) PRPP Amidotransferase
5-phosphoribosylamine
(-)
IMP (Hypoxanthine base)
Aspartate
AMP
Glutamine
GMP
(-)
Purine catabolism
Adenosine
Severe combined Immunodeficiency
ADENOSINE
DEAMINASE
NH3
(-)
Guanosine
T-cell deficiency
Insosine
Ribose 1 P
(-)
(-)
Purine nucleoside
phosphorylase
Purine nucleoside
phosphorylase
Hypoxanthine
Ribose 1 P
Guanine
Guanase
Xanthine Oxidase
Xanthine
Xanthine Oxidase
Uric acid
Urine
(-) Allopurinol
(-) Allopurinol
90%
10%
From Kaplan Step 1 lecture notes
Purine salvage pathway
Inosine
Guanine (90%)
Hypoxanthine (90%)
PRPP
PRPP
Hypoxanthine Guanine
phosphoribosyl transferase
GMP
IMP
(-)
•Purines from Diet
•Nucleic acid turnover
•Other nucleotide metabolism
Adenine
Adenine
phosphoribosyl
transferase
AMP
Lesch Nyhan syndrome
Advantage of purine salvage pathway:
• Reutilization of nucleotides
• Prevents loss of ATPs which are required for denovo
purine synthesis
• Nucleotides formed in the salvage pathway inhibits
denovo pathway at the rate limiting step
• Decreases uric acid formation – end product of
purine catabolism
Conversion of nucleosides to
nucleotides
• NMP + ATP
Nucleoside Mono phosphate kinase
• NDP + ATP
NDP
NTP
Nucleoside DI phosphate kinase
Conversion of Ribonucleotides to De-oxy Ribonucleotides:
• NDP
dNDP
Ribonucletide reductase
(RNA)
(DNA)
Lesch Nyhan syndrome
• X-linked recessive disease
• Complete deficiency of HGPRT – Hypoxanthine guanine
phosphoribosyl transferase.
• Increased uric acid production – pathogenesis:
1. Deficient salvage of nucleosides
2. Decreased IMP and GMP and increased PRPP- REMOVAL OF
FEEDBACK INHIBITON which stimulates PRPP amido
transferase – DeNovo purine synthesis
Clinical features
• Severe heritable form of Gout – due to
increased uric acid production.
• Features of Gout.
• Self mutilation – head banging, biting of
lips and fingers
• Involuntary movements- choreoathetotic
movements.
• Uric acid stones.
Self mutilation
Adenosine deaminase deficiency
• Part of SCID – severe combined immuno
deficiency syndrome.
• Autosomal recessive – def of enzyme Adenosine
deaminase
• Def leads to accumulation of deoxyadenosine
which increases the concentration d-ATP. This
inturn inhibits ribonucleotide reductase which is
required for converting ribonucleotides to deoxyribonucleotides.
Clinical features
• Deficiency of both T and B lymphocytes
• Leads to deficiency of both cell mediated and humoral
immunity.
• Prone to infections.
• Bubble boy – sterile environment.
• Treatment – gene therapy.
• Normal URIC ACID LEVEL2-7mg/dl
• HYPERURICEMIA is not GOUT !!
• HYPERURICEMIA PREDISPOSES TO GOUT !!
GOUT (Podagra)
• Recurrent attacks of inflammatory arthritis
• Metatarso- phalengeal joint of Big toe – MC
• Also presents as tophi, kidneystones and urate
nephropathy.
• Elevation of uric acid levels – over production or
under excretion of uric acid.
Monosodium urate crystals
• Poor solubility and
hypersaturation leads to
formation of needle
shaped monosodium urate
crystals (negatively
birefringent) which are
deposited in joints,
tendons, and
subcutaneous tissues.
• crystals attracts inflammatory cells
inflammation
– inflammatory mediators -
Causes of primary Gout
1. Decreased excretion : Defect in tubular
secretion in kidneys.
2. Increased production :PRPP synthase
mutation – low Km for ribose – 5- PO4 or
high Vmax for PRPP production.
3. HGPRT deficiency- Lesch Nyhan syndrome
Secondary GOUT
•
•
•
•
•
•
•
Lactic acidosis
Von-Gierke’s disease
Cancer chemotherapy
Tumorlysis syndrome
Hereditary Fructose intolerance
Renal failure
Purine rich foods.
Treatment of Gout
• Colchicine – inhibits migration of inflammatory cells
• NSAIDS – Antiiflammatory and analgesics
• Uricosuric drugs –probencid and sulfinpyrazone
– CONTRAINDICATED IN RENAL GOUT
• Xanthine oxidase inhibitors – Allopurinol.
MCQ1
Gout is characterized by elevated uric acid concentrations in
blood and urine due to a variety of metabolic abnormalities
that lead to the overproduction of purine nucleotides.
Allopurinol is used in the treatment of gout because this
drug, and its metabolic product, alloxanthine, act as
inhibitors of:
•
• a)
Xanthine Oxidase
• b) PRPP synthetase
• c)
Adenyl succinate synthase
• d) Hypoxhantine guanine phosphoribosyl transferase
• e) Nucleotides
MCQ2
• Hereditary Orotic aciduria is characterized by severe
anemia, growth retardation, and high levels of orotic
acid excretion. It is produced by deficit of enzymes
related with:
•
• a) synthesis of pyrimidine nucleotides
• b) catabolism of pyrimidine nucleotides
• c) synthesis of purine nucleotides
• d) catabolism of purine nucleotides
• e) synthesis of Heme
• f)
catabolism of Heme