The transport properties of the human renal Na+- dicarboxylate cotransporter under voltage-clamp conditions

The transport properties of the human Na(+)-dicarboxylate cotransporter, (hNaDC-1), expressed in Xenopus laevis oocytes were characterized using the two-electrode voltage clamp technique. Steady-state succinate-evoked inward currents in hNaDC-1 were dependent on the concentrations of succinate and sodium, and on the membrane potential. At -50 mV, the half-saturation constant for succinate (K(0.5)(succinate)) was 1.1 mM and the half-saturation constant for sodium (K(0.5)(sodium)) was 65 mM. The Hill coefficient was 2.3, which is consistent with a transport stoichiometry of 3 Na(+):1 divalent anion substrate. The hNaDC-1 exhibits a high-cation selectivity. Sodium is the preferred cation and other cations, such as lithium, were not able to support transport of succinate. The preferred substrates of hNaDC-1 are fumarate (K(0.5) 1.8 mM) and succinate, followed by methylsuccinate (K(0.5) 2.8 mM), citrate (K(0. 5) 6.8 mM) and alpha-ketoglutarate (K(0.5) 16 mM). The hNaDC-1 may also transport sodium ions through an uncoupled leak pathway, which is sensitive to phloretin inhibition. We propose a transport model for hNaDC-1 in which the binding of three sodium ions is followed by substrate binding.