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Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston, Texas 77555
The sodium-dicarboxylate cotransporter of the renal
proximal tubule, NaDC-1, reabsorbs filtered Krebs cycle intermediates and plays an important role in the regulation of urinary citrate concentrations.1 Low urinary citrate is a risk factor for
the development of kidney stones. As an initial step in the
characterization of NaDC-1 regulation, the genomic structure and
functional properties of the mouse Na+-dicarboxylate
cotransporter (mNaDC-1) were determined. The gene coding for mNaDC-1,
Slc13a2, is found on chromosome 11. The gene is ~24.9 kb
in length and contains 12 exons. The mRNA coding for mNaDC-1 is found
in kidney and small intestine. Expression of mNaDC-1 in Xenopus
laevis oocytes results in increased transport of di- and
tricarboxylates. The Michaelis-Menten constant
(Km) for succinate was 0.35 mM, and the
Km for citrate was 0.6 mM. The transport
of citrate was stimulated by acidic pH, whereas the transport of
succinate was insensitive to pH changes. Transport by mNaDC-1 is
electrogenic, and substrates produced inward currents in the presence
of sodium. The sodium affinity was relatively high in mNaDC-1, with
half-saturation constants for sodium of 10 mM (radiotracer experiments)
and 28 mM at 
50 mV (2-electrode voltage clamp experiments). Lithium
acts as a potent inhibitor of transport, but it can also partially
substitute for sodium. In conclusion, the mNaDC-1 is related in
sequence and function to the other NaDC-1 orthologs. However, its
function more closely resembles the rabbit and human orthologs rather
than the rat NaDC-1, with which it shares higher sequence similarity.
citrate; succinate; dicarboxylate transport; sodium
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