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BUN: end product of pyrimidine metabolism?

Discussion in 'Step I' started by ej37, Apr 1, 2011.

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  1. ej37

    ej37 7+ Year Member

    Jul 2, 2008
    On page 391 of RR Path he says BUN can be increased with pyrimidine breakdown... this totally confused me I thought it was from Purine breakdown, like Lesch-Nyan and stuff...

    Although maybe I am confusing BUN and uric acid? I never really quite understood the distinction...
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  3. nkhan

    nkhan c/o 2013 5+ Year Member

    Feb 16, 2008
    BUN is elevated b/c of increased break down (elevates the level of nitrogen), I think that both purine and pyrimidine breakdown would both elevate nitrogen (they both are nitrogenous bases)

    Uric acid is made from purine degradation products

    Angels and Gods are Pure (Adenine/Guanine)

    Adenine deaminase takes Adenine to Hypoxanthine
    Xanthine Oxidase takes Hypoxanthine to Xanthine
    Guanine deaminase takes Guanine to Xanthine
    Xanthine Oxidase takes Xanthine to Uric Acid
  4. Fuzuli

    Fuzuli New Member 10+ Year Member

    Aug 16, 2004
    Think of the structure of purines and pyrimidines. In pyrimidine metabolism, the ring structure can be cleaved open. As a result, amino acids and ammonia (and some other compounds) are formed. So, both amino acids and pyrimidines can contribute to the ammonia pool of the body. Ammonia enters the urea cycle and is excreted as -not suprisingly- urea. BUN levels reflect this.

    On the other hand, purine ring cannot be cleaved open, so its fate is different. However, ammonia is also formed as a result of purine catabolism (this is what adenosine deaminase does). So, purine catabolism will also contribute to ammonia pool. Ultimately, purines are converted into uric acid and excreted.

    The bottom line is:

    1) Both purine and pyrimidine metabolism yield ammonia, which is converted into urea
    2) Only purine metabolism yields uric acid

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