Purine catabolism

Catabolism of Purines &

GOUT

Dr. N. SivaranjaniAsst. Prof

Dr. N. Sivaranjani 1

• Nucleotides of cell undergo continual turnover.

Nucleotides

Nucleosides

Free bases + R-1-P

• Some of bases are reused to form nucleotides by Salvage pathway.

• Others are degraded to products that are excreted.

• Uric acid is end product of purine catabolism

Nucleotidase

Nucleoside Phosphorylase

Dr. N. Sivaranjani 2

AMP IMP

Nucleotidase NucleotidaseH2O

H2O

PiPi

Adenosine Inosine

H2O

Adenosine deaminase(ADA)

NH4

Hypoxanthine

PiPurine nucleoside phosphorylase (PNP)Ribose 1 phosphateRibose

N

N

N

N

NH2

Ribose

N

N

N

HN

O

NH

N

N

HN

O

R-5-P

N

N

N

N

NH2

Adenosine is not degraded by PNP, but converted to inosine & further metabolized Dr. N. Sivaranjani 3

Hypoxanthine

Xanthine

Xanthine Oxidase

H2O + O2

H2O2

H2O + O2

H2O2Xanthine Oxidase

URIC ACID

GMP

Nucleotidase

Guanosine

Guanine

Guanine deaminase

H2ONH3

Purine nucleoside phosphorylase

H2O

Pi

Pi

Ribose 1 phosphate

2,6,8 –trioxy purine

N

N

N

HN

O

NH

N

NH

HN

O

NH

NH

NH

HN

O

O

O

O

NH

N

N

HN

O

NH2

Dr. N. Sivaranjani 4

AMP

Adenosine

Hypoxanthine

Guanosine

Guanine

GMP

Xanthine

URIC ACID

Purine Bases

IMP

Inosine

XMP Xanthosine

Adenine nucleotide are degraded as Hypoxanthine

Guanine nucleotide are degraded as Xanthine

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Xanthine oxidase

• Found in LIVER & Small intestine

• Metallo flavoprotein

• Contains FAD, Molybdenum and Iron

• This reaction produces H2O2 (reactive oxygen species)

H2O2 H2O + O2Catalase

Hypoxanthine

Xanthine

URIC ACID

XO

XO

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• The end product of purine catabolism is uric acid in humans.

• N excreted as uric acid is very little in humans, as humans are ureotelic (nitrogen is excreted as urea).

• In birds, amphibians and reptiles are uricotelic – they excrete uric acid as major end product of purine and amino acid catabolism.

• Lower primates and some mammals have the enzyme uricase which converts uric acid to allantoin (which is more soluble).

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Sources and excretion of Uric acid

Gut

Breakdown of Endogenous purine De novo synthesis – 400 mg Diet -300 mg

Purine nucleotides

URIC ACID

UricolysisRenal Excretion

Catabolized

Body uric acid pool in Men 1200 mg,Female – 600 mg

2/3 CO2 + NH3

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Normal serum Uric acid concentration :

• 3 - 7 mg /dl in males , 2 - 5 mg/dl in females

• Uric acid pool –It is on average of 1200 mg

Excretion - 500 to 700 mg /day excreted

• Uric acid is cleared by both glomerular filtration and tubular secretion.

• Uric acid acts as Natural ANTIOXIDANT Dr. N. Sivaranjani 9

Hyperuricemia and gout:

• Hyperuricemia –

• increased serum uric acid levels above 7 mg/dl in Men & above 6 mg/dl in women.

Causes – Excessive Alcohol consumption, CRF, inherited metabolic disorders, Malignancies, Pre-eclampsia.

Gout is a metabolic disorder of purine catabolism, resulting in overproduction of uric acid.

• At physiological pH , uric acid is found in a minimal soluble form as Mono sodium urates – easily ppt at lower temperature.

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• Types of gout :

Primary gout :

Inherited - 90% ,due to an Inborn error of metabolism caused by defective enzymes of Purine synthesis.

Idiopathic - 10 % cases

1) Variant form of PRPP synthetase- not subject to allosteric control.

2) Variant of PRPP glutamyl amidotransferase - not sensitive to feedback control.

3) Glucose 6 phosphatase deficiency - Von Gierke’s disease G-6-P enters HMP shunt produces excess R-5-P & PRPP – purine overproduction. Lactic acidosis in Von Gierke’s disease – impairs UA excretion.

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4) Deficiency of enzymes of salvage pathway –HGPRT deficiency leading to Lesch-Nyhan syndrome.

Dec. utilization of purines by salvage pathway – diverts PRPP to purine synthesis

Dec. salvage pathway – dec. IMP & GMP – impairs feedback reg. of denovosynthesis of purine – leads to overproduction of purines.

5) Elevation of Glutathione reductase

It coverts oxidized Glutathione to reduced form by utilizing NADPH from HMP shunt.

Abnormal activity of GR - Inc. NADP+ - Inc. HMP shunt – which rises R-5-P and PRPP synthesis – overproduction of purines.

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Glucose 6 phosphate

HMP shunt

Ribose 5 phosphate

NADP+

NADPH

2 GSH

G-S-S-G

PRPPPRPP synthetase

Glutamine

5-Phosphoribosylamine

Inosine monophosphate

GMP AMP Adenine

Hypoxanthine XanthineURIC ACID XO XO

Guanine HGPRT

Hypoxanthine

APRT

PRPP Glutamylamidotransferase

Glutathione reductase

Glucose

G-6-phosphatase

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Secondary gout:

• Due to various disease causing increased synthesis or decreased excretion of uric acid.

a) Overproduction of uric acid – due to enhanced turn over rate of nucleic acids

i) Increased tissue turn over due to psoriasis.

ii) Rapidly growing malignant tissues - CANCER - Leukemias,

polycythemia, lymphomas.

iii) Increased tissue break down – after treatment for large tumor

masses –radiotherapy & chemotherapy, trauma and starvation.Dr. N. Sivaranjani 14

b) Reduced excretion of uric acid

i) Chronic Renal failure due to reduced GFR.

ii) Increased alcohol consumption leads to lactic acidosis - Lactic acid decreases tubular excretion of uric acid.

iii) Ketoacidosis – decreases the tubular excretion of uric acid

iv) Thiazide diuretics inhibits tubular secretion of uric acid.Dr. N. Sivaranjani 15

Clinical features:

• Due to the low solubility of uric acid.

• More common in Males, post menopausal women.• Typical gouty arthritis affects first metatarso

phalangeal joint (GREAT TOE) – Classical site

• In Gout , serum urate levels exceed solubility limit, leading to formation of MSU crystals and get deposited in joints.

These deposits are called Tophi.

inflammation of joints

painful acute gouty arthritis chronic gouty arthritis.

• Other complications like urolithiasis and renal damage.Dr. N. Sivaranjani 16

increased serum uric acid

Mono-sodium urate crystals

Deposited in areas where body temperature is lower – Tophi

Gouty arthritis

Deposited in Kidney

Renal calculi /stone Renal damage

” redness, swelling, and pain“-hallmarks of a gout attack.

Severe form – body uric acid pool reach 20,000 -30,000 mg

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Often HISTORY is - patient have few drinks at night , go to sleep symptomless , but are awakened during early hrs by severe joint pains.

• Hyperuricemia doesn't always cause gout. Over the course of yrs, sharp uratecrystals build up in the synovial fluid of the joints.

Precipitating event - infection, surgery, stress or often heavy ALCOHOL drink.

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Investigations :

• Serum uric acid level -increased

• Microscopic Examination of Synovial

fluid reveals uric acid crystals - rod / needle –shaped crystals.

• Birefringent crystals under polar microscope is diagnostic.

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Treatment of gout :

• Low intake of Purine diet- like red meat, acidy fruits and vegetables, lentils

• Restrict Alcohol

• Consume plenty of Water

Drugs:

1. Anti-inflammatory drugs - Colchicine is used for treatment of gouty arthritis. NSAID - indomethacin , ibuprofen. Corticosteroids also useful for acute attacks.

2. Uricosuric drugs – Probenecid.

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NH

C

CN

N

C

C

HN

O

NH

C

CC

N

N

C

HN

O

NH

C

CN

N

C

C

HN

OHypoxanthine

Allopurinol Alloxanthine

OH

XO

3. Xanthine oxidase inhibitors – ALLOPURINOL

drug of choice for treatment of Gout.

It is structural analog of hypoxanthine.

Competitively inhibits XO enzyme.

Suicide inhibition

Hypoxanthine , Xanthine are more soluble and excreted in urine.

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Pseudogout :

• Serum uric acid level normal.

• Symptoms as seen in gout.

• But it is characterized by deposition of calcium – pyrophosphate crystals in joints.

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Lesch-Nyhan syndrome:

• Inherited X-linked recessive disorder, affects only males

• Enzyme defect – hypoxanthine guanine phoshoribosyl transferase (HGPRT)

• Characterized by excess production of uric acid leads to GOUT.

• Self mutilation – bite their fingers and lips

• Neurological abnormalities like mental -retardation, aggressive behavior , learning disabilities occur.

• Neurological symptoms may be due to dependence of brain on the salvage pathway.

• Nephrolithiasis – leads to renal failure.Dr. N. Sivaranjani 23

Hypouricemia

Decreased in uric acid level

Xanthinuria

• Xanthine oxidase deficiency, due to either genetic defect or due to severe LIVER damage.

• Inc. excretion of xanthine & hypoxanthine

• So decreased uric acid

• Xanthine lithiasis occur in severe XO def.

Hypoxanthine Xanthine URIC ACID XO XO

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Adenosine deaminase deficiency

• Leads to Both T and B cells are dysfunctional – Severe Combined Immunodeficiency (SCID )

Adenosine Inosine , dAdenosine dInosine

• Immune dysfunction is due to high levels of deoxy Adenosine

• Deoxyadenosine is converted to dAMP, dADP, dATP.

• dATP allosterically inhibits Ribonucleotide reductase - decreases DNA synthesis.

ADA ADA

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Purine-nucleoside phosphorylase deficeincy

• PNP deficiency

Inosine Hypoxanthine , Guanosine Guanine

uric acid formation is decreased.

• Impaired T-cells function with normal B cells function.

• Here dGTP accumulates which inhibits Ribonucleotide reductase.

PNP PNP

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ADA and PNP deficiency

• both are inherited as autosomal recessive

• Hypouricemia seen

• Both associated with symptoms of recurrent and chronic infections

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