Purine Metabolism and Disorders

Purine Synthesis - Building Blocks of Life

• De novo pathway: Primary route for purine nucleotide formation, mainly in the liver.
• Starting material: Ribose-5-phosphate, converted to PRPP (Phosphoribosyl pyrophosphate).
• Key intermediate: IMP (Inosine Monophosphate) - the parent purine nucleotide.
• Purine ring sources:
  • Aspartate (N1)
  • N10-Formyl-THF (C2, C8)
  • Glutamine (N3, N9)
  • Glycine (C4, C5, N7)
  • $CO_2$ (C6) 📌 Mnemonic for sources: "Glycine, Aspartate, Glutamine, $CO_2$, Formyl-THF" (GAG-CF).
• Rate-limiting enzyme: Glutamine PRPP amidotransferase.
  • Subject to feedback inhibition by AMP, GMP, and IMP.
  • Activated by PRPP.

⭐ IMP is the common precursor from which AMP and GMP are synthesized, representing a critical branch point in purine metabolism.

Purine Degradation - The Uric Acid Trail

• Endogenous (cell turnover) and dietary purine nucleotides (AMP, GMP) are catabolized to uric acid.
• Key Enzymes & Conversions:
  • Adenosine Deaminase (ADA): Converts Adenosine $\rightarrow$ Inosine. (📌 ADA deficiency causes Severe Combined Immunodeficiency - SCID).
  • Purine Nucleoside Phosphorylase (PNP): Inosine $\rightarrow$ Hypoxanthine; Guanosine $\rightarrow$ Guanine. (Deficiency impairs T-cell immunity).
  • Guanase: Guanine $\rightarrow$ Xanthine.
  • Xanthine Oxidase (XO): Catalyzes Hypoxanthine $\rightarrow$ Xanthine, then Xanthine $\rightarrow$ Uric Acid. A molybdenum-containing enzyme.
• Uric acid is the final product, excreted mainly by kidneys. Elevated levels cause gout.

⭐ Xanthine Oxidase (XO) is the primary target for urate-lowering therapy in gout; Allopurinol and Febuxostat are key inhibitors.

Purine Disorders - When Metabolism Goes Wrong

• Gout:
  • Cause: Hyperuricemia (↑ uric acid > 6.8 mg/dL).
  • Patho: Monosodium urate (MSU) crystal deposition in joints (podagra) & soft tissues (tophi).
  • Rx: Allopurinol (xanthine oxidase inhibitor).
• Lesch-Nyhan Syndrome (LNS):
  • Defect: HGPRT deficiency (X-linked recessive).
  • Features: Hyperuricemia, gout, dystonia, choreoathetosis, self-mutilation, intellectual disability. 📌 HGPRT: He's Got Purinic Rage & Tics.
• Adenosine Deaminase (ADA) Deficiency:
  • Defect: ADA enzyme.
  • Result: dATP accumulation, toxic to lymphocytes.
  • Clinical: Severe Combined Immunodeficiency (SCID) - B & T cell defect.

  ⭐ ADA deficiency is a major cause of autosomal recessive SCID.

• Purine Nucleoside Phosphorylase (PNP) Deficiency:
  • Defect: PNP enzyme.
  • Result: dGTP accumulation.
  • Clinical: Impaired T-cell function; recurrent infections.
• Xanthinuria:
  • Defect: Xanthine oxidase deficiency.
  • Result: ↑ xanthine, ↓ uric acid.
  • Clinical: Xanthine kidney stones.

Purine Pharmacology - Drug Targets & Therapies

• Xanthine Oxidase Inhibitors:
  • Allopurinol, Febuxostat: ↓ Uric acid production. For gout, tumor lysis syndrome.
• Uricosuric Agents:
  • Probenecid, Lesinurad: ↑ Renal uric acid excretion.
• Recombinant Uricases:
  • Rasburicase, Pegloticase: Convert uric acid to allantoin. For refractory gout.
• Antimetabolites (Inhibit Purine Synthesis):
  • Azathioprine (prodrug), 6-Mercaptopurine (6-MP), 6-Thioguanine (6-TG):
    • 📌 Thiopurines; inhibit de novo synthesis. Used in ALL, IBD.
  • Mycophenolate Mofetil: Inhibits IMP Dehydrogenase. For transplant rejection.
• Other Purine Analogs:
  • Cladribine, Fludarabine (anticancer).
  • Acyclovir, Ganciclovir (antiviral).

⭐ Allopurinol inhibits xanthine oxidase, reducing uric acid. It also inhibits metabolism of azathioprine/6-MP, requiring their dose reduction to prevent toxicity.

High‑Yield Points - ⚡ Biggest Takeaways

• De novo purine synthesis needs PRPP, glycine, aspartate, glutamine, CO2, THF; mainly in liver.
• HGPRT deficiency: Lesch-Nyhan syndrome (Hyperuricemia, Gout, Pissed off, Retardation, DysTonia).
• ADA deficiency: SCID due to ↑dATP, toxic to lymphocytes.
• Xanthine oxidase (hypoxanthine → xanthine → uric acid) inhibited by allopurinol, febuxostat.
• Gout: hyperuricemia; acute: NSAIDs, colchicine, steroids; chronic: allopurinol.
• Tumor Lysis Syndrome: ↑uric acid, ↑K+, ↑PO43-, ↓Ca2+; manage: hydration, allopurinol/rasburicase.