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Cytosolic phosphorylation potential.

R L Veech, J W Lawson, N W Cornell

    The Journal of Biological Chemistry
    |July 25, 1979
    PubMed
    Summary
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    Enzyme reactions in red blood cells are near equilibrium, with ATP and ADP levels accurately reflecting in vivo activity. In contrast, brain and muscle tissues show lower activity, suggesting significantly lower free cytosolic ADP than initially measured.

    Area of Science:

    • Biochemistry and Cellular Metabolism
    • Enzyme Kinetics and Regulation
    • Physiological Chemistry

    Background:

    • Understanding cellular energy metabolism relies on knowing the activity of key enzymes and their substrates.
    • Previous studies have measured tissue reactant concentrations, but their proximity to equilibrium in vivo remains debated.

    Purpose of the Study:

    • To determine if key metabolic enzyme systems in human blood and rat tissues are at near-equilibrium.
    • To assess the accuracy of measured tissue ATP and ADP concentrations as indicators of in vivo reactant activity.
    • To investigate the cytosolic free ADP concentration in mitochondrial-containing tissues.

    Main Methods:

    • Measured tissue concentrations of reactants for myokinase, glyceraldehyde-3-phosphate dehydrogenase-3-phosphoglycerate kinase, and creatine kinase reactions.

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  • Utilized rapidly inactivated samples from human blood and rat brain, muscle, and liver.
  • Calculated equilibrium constants and compared in vitro values with in vivo measurements.

    • Main Results:

    • Myokinase and glyceraldehyde-3-phosphate dehydrogenase-3-phosphoglycerate kinase reactions are near equilibrium in red blood cells.
    • Measured ATP and ADP levels in red cells closely approximate their in vivo activity.
    • In brain and muscle, calculated equilibrium values were significantly lower than expected, suggesting free cytosolic ADP is approximately 20-fold lower than total measured ADP.

    Conclusions:

    • Key enzyme systems in red blood cells operate under near-equilibrium conditions.
    • The study provides evidence for near-equilibrium in highly active enzyme systems within brain and muscle.
    • Cytosolic free ADP concentration in mitochondrial-rich tissues is likely substantially lower than previously estimated from total cell ADP content.