questions 3

Nucleotide Catabolism/Salvage

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Topic updated on 11/08/17 4:50pm

Overview
  •  Purine
    • salvage pathway
      • see figure
    • clinical importance
      • adenosine deaminase (ADA) deficiency 
        • defective purine salvage
          • results in excess ATP and dATP
        • prevents DNA synthesis
          • ATP and dATP feedback negatively on ribonucleotide reductase in the synthesis of purines and pyrimidines for DNA replication
          • ↓ lymphocyte count
            • major cause of SCID (severe combined immunodeficiency disease)
              • lack of both T and B cells
        • AR
      • Lesch-Nyhan disease
        • defective purine salvage
        • lacks hypoxanthine guanine phosphoribosyl pyrophosphate transferase (HGPRT) enzyme
        • XR
        • presentation
          • severe CNS symptoms
            • choreoathetosis
            • mental retardation
          • self-mutilation
          • hyperuricemia
            • degradation of all purines since it cannot salvage
      • gout
        • pathophysiology
          • high urate levels due to
            • ↑ in cell breakdown
              • e.g. treatment of large tumor masses with radiation or chemo
            • ↓ in renal excretion (most common cause)
          • results in precipitation of monosodium urate crystals in joints
            • negative birefringence
              • yellow when parallel to slow ray
            • needle shaped
        • presentation
          • recurrent acute arthritis
            • pain in big toe first (podagra)
          • chronic
            • tophi present
              • granulomatous deposition (multinucleated giant cells) of crystals in soft tissue
          • ↑ frequency in men >30 y/o
        • treatment
          • acute → colchicine or indomethacin
          • chronic due to ↓ in renal excretion → probenecid
          • chronic due to ↑ in cell breakdown → allopurinol
  • Pyrimidine
    • salvage
      • may be salvaged by pyrimidine salvage enzymes
    • degradation
      • completely broken down to ammonia
  • Other causes of hyperuricemia
    • ↑ EtOH intake
      • can precipitate an acute gout attack
    • ↑ nucleic acid in diet
      • red meats, organ meats
    • phosphate "trapping" diseases
      • e.g. glucose-6-phosphate deficiency (G6PD), galactose uridyltransferase deficiency
      • caused by an inability to dephosphorylate common metabolites and therefore leads to trapping of phosphate by these metabolites
      • lack of phosphate prevents synthesis of ATP, GTP, plus other nucleotide phosphates
      • ADP. AMP, and other hypophosphorylated bases are salvaged producing uric acid


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Qbank (2 Questions)

TAG
(M1.BC.1) A six-month-old infant presents with chronic, persistent diarrhea, oral thrush, and a severe diaper rash. The infant was treated four weeks ago for an upper respiratory and ear infection. A family history is significant for a consanguineous relationship between the mother and father. Physical examination demonstrates the absence of palpable lymph nodes. Accumulation of which of the following would lead to this disease phenotype? Topic Review Topic

1. Deoxyadenosine
2. Phenylalanine
3. Galactitol
4. Ceramide trihexoside
5. Sphingomyelin

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TAG
(M1.BC.1) A 3-year-old male was brought to the pediatrician with severe lip lacerations, with a portion of his tongue appearing to be bitten off, as well as missing portions of the fingers on his right hand. A family history is notable for two similar cases in male cousins on the mother's side. A urinalysis revealed a high level of uric acid. Which of the following is the mode of inheritance for this disorder? Topic Review Topic

1. X-linked recessive
2. X-linked dominant
3. Autosomal dominant
4. Autosomal recessvie
5. Maternally inherited mitochondrial defect

PREFERRED RESPONSE ▶
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A patient with Lesch-Nyhan syndrome
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