Gout-Biochemical basis

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Biochemical Basis of Gout

Gout is a form of inflammatory arthritis caused by the buildup of uric acid crystals (monosodium urate) in joints, often in the big toe, ankles, knees, or fingers. It leads to sudden, severe pain, swelling, redness, and heat in the affected joint.

Biochemical Basis of Gout

Gout develops due to hyperuricemia — an abnormally high level of uric acid in the blood.

1. Uric Acid Formation (Purine Metabolism)

  • Uric acid is the end product of purine metabolism in humans.
  • Purines (adenine & guanine) from dietary intake, cell turnover, or DNA/RNA degradation or broken down.
  • The degradation pathway for purines involves the following steps:
    • Adenosine and Guanosine Breakdown:
      • Adenosine → Hypoxanthine → Xanthine
      • Guanosine → Xanthine
    • Formation of Uric Acid:
      • Xanthine is converted to uric acid by the enzyme xanthine oxidase.
  • The key enzyme involved is:
    • Xanthine oxidase, which converts:
      • Hypoxanthine → Xanthine → Uric acid

2. Causes of Hyperuricemia

Hyperuricemia, the primary cause of gout, occurs due to:

  1. Overproduction of Uric Acid:
    • Increased purine intake (e.g., meat, seafood, alcohol).
    • Enhanced cell turnover (e.g., in cancer or psoriasis).
    • Genetic mutations affecting enzymes in purine metabolism:
      • Hypoxanthine-Guanine Phosphoribosyltransferase (HGPRT) deficiency in Lesch-Nyhan syndrome.
      • Phosphoribosyl Pyrophosphate Synthetase (PRPP) hyperactivity.
  2. Underexcretion of Uric Acid:
    • Reduced renal clearance (common in kidney dysfunction).
    • Use of medications like diuretics and aspirin.

Conditions like metabolic syndrome or dehydration.

 3. Crystal Deposition and Inflammation

  • When uric acid exceeds its solubility limit, monosodium urate crystals form.
  • These crystals deposit in synovial joints, triggering an immune response.
  • Neutrophils ingest the crystals, releasing inflammatory mediators → causes acute inflammation.

4. Biochemical Basis of Uric Acid Regulation

  1. Key Enzymes:
    • Xanthine Oxidase: Converts hypoxanthine and xanthine to uric acid.
    • HGPRT: Recycles purines by converting hypoxanthine to IMP and guanine to GMP.
    • PRPP Synthetase: Catalyzes the production of PRPP, a precursor in purine biosynthesis.
  2. Kidney Excretion:
    • Uric acid is filtered by the glomerulus, reabsorbed by the proximal tubule, and partially secreted back into the urine.

5. Clinical Manifestations

  • Pain and swelling in joints (especially the big toe, known as podagra).
  • Formation of tophi in chronic cases.
  • Kidney stones due to uric acid crystallization in the urinary tract.

6. Diagnosis

  • Elevated serum uric acid levels (>6.8 mg/dL).
  • Identification of MSU crystals in joint aspirate (needle-shaped and negatively birefringent under polarized light).

7. Treatment (Biochemical Focus)

Drug

Target

Effect

Allopurinol

Inhibits xanthine oxidase

↓ Uric acid production

Febuxostat

Also inhibits xanthine oxidase

Used when allopurinol is not tolerated

Colchicine

Blocks neutrophil activity

Reduces inflammation during acute gout attack

Probenecid

Increases uric acid excretion

Used in underexcretors

Summary

  • Gout = Too much uric acid → crystals in joints → pain and inflammation
  • Caused by purine metabolism imbalance
  • Involves key enzymes like xanthine oxidase and HGPRT
  • Treated by reducing uric acid production or increasing its excretion

 

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