The two-electrode voltage clamp was used to study the currents associated with transport of succinate by the cloned Na ؉ /dicarboxylate cotransporter, NaDC-1, expressed in Xenopus oocytes. The presence of succinate induced inward currents which were dependent on the concentrations of succinate and sodium, and on the membrane potential. At ؊50 mV, the K 0.5 succinate was 180 M and the K 0.5 Na؉ was 19 mM. The Hill coefficient was 2.3, which is consistent with a transport stoichiometry of 3 Na ؉ :1 divalent anion substrate. Currents were induced in NaDC-1 by a range of di-and tricarboxylates, including citrate, methylsuccinate, fumarate, and tricarballylate. Although Na ؉ is the preferred cation, Li ؉ was also able to support transport. The K 0.5 succinate was approximately 10-fold higher in Li ؉ compared with Na ؉ . In the presence of Na ؉ , however, Li ؉ was a potent inhibitor of transport. Millimolar concentrations of Li ؉ resulted in decreases in apparent succinate affinity and in the I max succinate . Furthermore, lithium inhibition under saturating sodium concentrations showed hyperbolic kinetics, suggesting that one of the three cation binding sites in NaDC-1 has a higher affinity for Li ؉ than Na ؉ . We conclude that NaDC-1 is an electrogenic anion transporter that accepts either Na ؉ or Li ؉ as coupling cations. However, NaDC-1 contains a single high affinity binding site for Li ؉ that, when occupied, results in transport inhibition, which may account for its potent inhibitory effects on renal dicarboxylate transport.The active transport of Krebs cycle intermediates, such as succinate and citrate, is mediated by a specific sodium-coupled transporter found on the apical membrane in epithelial cells of the kidney proximal tubule (1, 2). The Na ϩ -dicarboxylate cotransporter reabsorbs a wide range of di-and tricarboxylic acids in the form of divalent anions. This transporter is sensitive to inhibition by lithium (3), and patients receiving therapeutic doses of lithium exhibit increased renal excretion of ␣-ketoglutarate and glutarate (4). The cDNA coding for the rabbit renal Na ϩ /dicarboxylate cotransporter, NaDC-1, 1 has been cloned and sequenced (5), and the protein has been identified in renal brush border membranes (6). NaDC-1 belongs to a distinct gene family of sodium-coupled anion transporters that includes the Na ϩ /dicarboxylate cotransporters, hNaDC-1, from human kidney (7), and NaDC-2, from Xenopus intestine (8), and the renal Na ϩ /sulfate cotransporter, NaSi-1 (9). The transport mechanism of NaDC-1 is thought to involve the ordered binding of four charged substrates: 3 Na ϩ ions and 1 divalent anion substrate (10 -12), resulting in one net inward positive charge across the membrane per cycle. Experiments with rabbit renal brush border membrane vesicles support this hypothesis: sodium-dependent transport of succinate was affected by changes in membrane potential, and transport of succinate also caused a depolarization of membrane potential (12-14). However, the dependence of transport kinetics o...