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  • Title: The mitochondrial transporter of ascorbic acid functions with high affinity in the presence of low millimolar concentrations of sodium and in the absence of calcium and magnesium.
    Author: Fiorani M, Azzolini C, Cerioni L, Scotti M, Guidarelli A, Ciacci C, Cantoni O.
    Journal: Biochim Biophys Acta; 2015 Jun; 1848(6):1393-401. PubMed ID: 25786874.
    Abstract:
    We recently reported that U937 cell mitochondria express a functional Na+-dependent ascorbic acid (AA) transporter recognised by anti-SVCT2 antibodies. The present study confirms and extends these observations by showing that this transporter is characterised by a Km and a pH-dependence comparable with that reported for the plasma membrane SVCT2. In isolated mitochondria, Na+ increased AA transport rate in a cooperative manner, revealed by a sigmoid curve and a Hill coefficient of 2, as also observed in intact Raw 264.7 cells (uniquely expressing SVCT2). There was however a striking difference on the Na+ concentrations necessary to reach saturation, i.e., 1 or 100 mM for the mitochondrial and plasma membrane transporters, respectively. Furthermore the mitochondrial, unlike the plasma membrane, transporter was fully active also in the absence of added Ca++ and/or Mg++. Taken together, the results presented in this study indicate that the U937 cell mitochondrial transporter of AA, because of its very low requirement for Na+ and independence for Ca++ and Mg++, displays kinetic characteristics surprisingly similar with those of the plasma membrane SVCT2.
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