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Departments of 1 Biochemistry and Molecular Biology, and 3 Obstetrics and Gynecology, Medical College of Georgia, Augusta, Georgia 30912; and 2 Department of Genetics, Yale University School of Medicine, New Haven, Connecticut 06510
We have cloned
and functionally characterized the human Na+-dependent
high-affinity dicarboxylate transporter (hNaDC3) from placenta. The
hNaDC3 cDNA codes for a protein of 602 amino acids with 12 transmembrane domains. When expressed in mammalian cells, the cloned
transporter mediates the transport of succinate in the presence of
Na+ [concentration of substrate necessary for half-maximal
transport (Kt) for succinate = 20 ± 1 µM]. Dimethylsuccinate also interacts with hNaDC3. The
Na+-to-succinate stoichiometry is 3:1 and concentration of
Na+ necessary for half-maximal transport
(KNa+0.5) is 49 ± 1 mM as determined by uptake studies with
radiolabeled succinate. When expressed in Xenopus
laevis oocytes, hNaDC3 induces Na+-dependent inward
currents in the presence of succinate and dimethylsuccinate. At a
membrane potential of
50 mV,
KSuc0.5 is 102 ± 20 µM and
KNa+0.5 is 22 ± 4 mM as determined by the electrophysiological approach. Simultaneous measurements of succinate-evoked charge transfer and
radiolabeled succinate uptake in hNaDC3-expressing oocytes indicate a
charge-to-succinate ratio of 1:1 for the transport process, suggesting
a Na+-to-succinate stoichiometry of 3:1. pH titration of
citrate-induced currents shows that hNaDC3 accepts preferentially the
divalent anionic form of citrate as a substrate. Li+
inhibits succinate-induced currents in the presence of Na+.
Functional analysis of rat-human and human-rat NaDC3 chimeric transporters indicates that the catalytic domain of the transporter lies in the carboxy-terminal half of the protein. The human
NaDC3 gene is located on chromosome
20q12-13.1, as evidenced by fluorescent in situ hybridization. The
gene is >80 kbp long and consists of 13 exons and 12 introns.
human placenta; electrophysiology; chromosomal localization; exon-intron organization
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