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Sodium and Lithium Interactions with the Na+/Dicarboxylate Cotransporter*

A. Pajor,B. Hirayama,D. Loo

Published 1998 in Journal of Biological Chemistry

ABSTRACT

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 inXenopus 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, theK 0.5 succinate was 180 μm 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. TheK 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.

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