Aspects of fasting metabolism in the desert tortoise Gopherus berlandieri

https://doi.org/10.1016/0305-0491(77)90120-1Get rights and content

Abstract

  • 1.

    1. After 80 days of fasting, liver glucose and glycogen dropped to 33% and 5% of the active feeding values for the desert tortoise, Gopherus berlandieri. As carbohydrate reserves were depleted ureogenesis was increased as evidenced by the tremendous rise in blood (56–363 mg/100 ml) and urine urea (229–600 mg/100 ml), while insignificant changes occurred for uric acid.

  • 2.

    2. Activities of argininosuccinate synthetase (ASS)(E.C. 6.3.4.5.), argininosuccinate lyase (ASL) (E.C. 4.3.2.1.) and arginase (E.C. 3.5.3.1.) were enhanced fourfold, sixfold and threefold, respectively, whereas carbamylphosphate synthetase (CPS)(E.C. 2.7.2.5.) and ornithine transcarbamylase (OTC)(E.C. 2.1.3.3.) were uneffected by fasting.

  • 3.

    3. With the apparent Michaelis constants (Kms) of CPS (ammonia—4.06 mM), OTC (carbamylphosphate—5.3 mM; ornithine—0.64 mM), ASS (citrulline—1.95 mM; aspartate—1.03 mM) and arginase (arginine = 3.2 mM) falling within the range of physiological substrate concentrations, the greater urea production with fasting could have been due to elevated substrate levels associated with fasting gluconeogenesis, and greater concentrations of the last three enzymes of the urea cycle.

References (27)

  • G.W. Brown et al.

    Comparative biochemistry of urea synthesis—I

  • J.W. Campbell et al.

    Tissue distribution of enzymes of arginine biosynthesis in terrestrial snails

    Z. Vergl. Physiol.

    (1968)
  • R.A. Coulson et al.

    The living reptile

  • Cited by (7)

    View all citing articles on Scopus
    View full text