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

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.