Journal of Biological Chemistry
Volume 273, Issue 33, 14 August 1998, Pages 20972-20981
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MEMBRANES AND BIOENERGETICS
Characterization of a Rat Na+-Dicarboxylate Cotransporter*

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The metabolism of Krebs cycle intermediates is of fundamental importance for eukaryotic cells. In the kidney, these intermediates are transported actively into epithelial cells. Because citrate is a potent inhibitor for calcium stone formation, excessive uptake results in nephrolithiasis due to hypocitraturia. We report the cloning and characterization of a rat kidney dicarboxylate transporter (SDCT1). In situ hybridization revealed that SDCT1 mRNA is localized in S3 segments of kidney proximal tubules and in enterocytes lining the intestinal villi. Signals were also detected in lung bronchioli, the epididymis, and liver. When expressed inXenopus oocytes, SDCT1 mediated electrogenic, sodium-dependent transport of most Krebs cycle intermediates (Km = 20–60 μm), including citrate, succinate, α-ketoglutarate, and oxaloacetate. Of note, the acidic amino acids l- and d-glutamate and aspartate were also transported, although with lower affinity (Km = 2–18 mm). Transport of citrate was pH-sensitive. At pH 7.5, the Km for citrate was high (0.64 mm), whereas at pH 5.5, theKm was low (57 μm). This is consistent with the concept that the −2 form of citrate is the transported species. In addition, maximal currents at pH 5.5 were 70% higher than those at pH 7.5, and our data show that the −3 form acts as a competitive inhibitor. Simultaneous measurements of substrate-evoked currents and tracer uptakes under voltage-clamp condition, as well as a thermodynamic approach, gave a Na+to citrate or a Na+ to succinate stoichiometry of 3 to 1. SDCT1-mediated currents were inhibited by phloretin. This plant glycoside also inhibited the SDCT1-specific sodium leak in the absence of substrate, indicating that at least one Na+ binds to the transporter before the substrate. The data presented provide new insights into the biophysical characteristics and physiological implications of a cloned dicarboxylate transporter.

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*

This work was supported by the International Human Frontier Science Program Long-Term Fellowship (to X.-Z. C.) and by National Institutes of Health Grant DK43171 (to M. A. H.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank™/EMBL Data Bank with accession number(s) AF058714.