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

The transport properties of the human Na+-dicarboxylate cotransporter, (hNa DC-1), expressed in Xenopus laevis oocytes were characterized using the two -electrode voltage clamp technique. Steady-state succinate-evoked inward cu rrents in hNaDC-1 were dependent on the concentrations of succinate and sod ium, 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 consta nt for sodium (K-0.5(sodium)) was 65 mM. The Hill coefficient was 2.3, whic h is consistent with a transport stoichiometry of 3 Na+:1 divalent anion su bstrate. The hNaDC-1 exhibits a high-cation selectivity. Sodium is the pref erred cation and other cations, such as lithium, were not able to support t ransport 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), citr ate (K-0.5 6.8 mM) and alpha-ketoglutarate (K-0.5 16 mM). The hNaDC-1 may a lso transport sodium ions through an uncoupled leak pathway, which is sensi tive to phloretin inhibition. We propose a transport model for hNaDC-1 in w hich the binding of three sodium ions is followed by substrate binding.