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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Functional characterization of high-affinity Na⁺/dicarboxylate cotransporter found in Xenopus laevis kidney and heart

Department of Human Biological Chemistry and Genetics, University of Texas Medical Branch, Galveston, Texas

Submitted 23 June 2004 ; accepted in final form 5 June 2005

The SLC13 gene family includes sodium-coupled transporters for citric acid cycle intermediates and sulfate. The present study describes the sequence and functional characterization of a SLC13 family member from Xenopus laevis, the high-affinity Na⁺/dicarboxylate cotransporter xNaDC-3. The cDNA sequence of xNaDC-3 codes for a protein of 602 amino acids that is 70% identical to the sequences of mammalian NaDC-3 orthologs. The message for xNaDC-3 is found in the kidney, liver, intestine, and heart. The xNaDC-3 has a high affinity for substrate, including a K_m for succinate of 4 µM, and it is inhibited by the NaDC-3 test substrates 2,3-dimethylsuccinate and adipate. The transport of succinate by xNaDC-3 is dependent on sodium, with sigmoidal activation kinetics, and lithium can partially substitute for sodium. As with other members of the family, xNaDC-3 is electrogenic and exhibits inward substrate-dependent currents in the presence of sodium. However, other electrophysiological properties of xNaDC-3 are unique and involve large leak currents, possibly mediated by anions, that are activated by binding of sodium or lithium to a single site.

SLC13 gene family; citric acid cycle intermediate; lithium

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