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  • Title: Chemical versus isotopic equilibrium and the metabolic fate of glycolytic end products in the heart.
    Author: Damico LA, White LT, Yu X, Lewandowski ED.
    Journal: J Mol Cell Cardiol; 1996 May; 28(5):989-99. PubMed ID: 8762037.
    Abstract:
    Recent studies of isotope exchange across lactate dehydrogenase (LDH) and alanine aminotransferase (AAT) in hearts call into question whether both reactions are in equilibrium. To compare the oxidative and non-oxidative fates of glycolytic end products, isolated rabbit hearts were perfused with 5 mM [2-13C] glucose and 2.5 mM [3-13C] pyruvate: with (n = 6) and without (n = 7) stimulation of pyruvate oxidation using dichloroacetate (DCA), and during normal perfusion or hypoxia (n = 7/n = 6, +/- DCA). 13C NMR spectroscopy of intact hearts confirmed a steady-state enrichment level in both alanine and lactate. 1H- and 13C-NMR spectroscopy of tissue extracts identified the fractions of lactate, alanine and glutamate pools formed from each exogenous substrate. Glycolysis from glucose accounted for 22 +/- 7% of lactate formed and 10 +/- 2% of alanine formed in control hearts, and 16 +/- 2% lactate and 15 +/- 2% alanine in hypoxic hearts (mean +/- S.E.M.). In contrast, exogenous pyruvate formed 36 +/- 5% of the lactate pool, and 86 +/- 3% of the alanine pool in controls and 47 +/- 3% of lactate and of 67 +/- 3% alanine during hypoxia. [2(-13)C] glucose did not contribute to oxidative energy production via the TCA cycle as determined from low 13C enrichment of glutamate C5 from glucose (< 2%), while [3-13C] pyruvate accounted for 84 +/- 7% of labeled glutamate C4. Thus, exogenous pyruvate out-competed the metabolism of glucose, indicating low glycolytic activity. At 40 min, 96 +/- 2% of the total alanine was labeled from either glucose or pyruvate, confirming equilibrium at AAT. However, only 55 +/- 10% of total lactate was labeled, suggesting that the LDH reaction is not in rapid equilibrium within the myocardium.
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