Functional characterization of the human high-affinity choline transporter.
Na(+)-dependent, high-affinity choline uptake in cholinergic neurons is the rate-limiting step in acetylcholine synthesis. Here we report the molecular cloning and functional characterization of the human high-affinity choline transporter (hCHT1). The hCHT1 exhibits significant homology with known members of the Na(+)-dependent glucose transporter family, but not with members of the neurotransmitter transporter family. The human CHT1 gene is 25 kb in length with 9 exons and was assigned to chromosome II at position IIq11-12. Northern blot analysis showed that a 5.4 kb hCHT1 transcript was expressed exclusively in tissues containing cholinergic neurons. When expressed in Xenopus oocytes, the human clone induced Na(+)- and Cl(-)-dependent, high-affinity choline uptake, which was sensitive to the specific inhibitor hemicholinium-3, with a K(i) of 1.3 nM. The hCHT1-mediated choline uptake increased with increasing concentrations of choline, Na(+) and Cl(-), with EC(50) values of 2.0 microM, 76 mM, and 48 mM, and with apparent Hill coefficients of 1, 2.5 and 2.3, respectively.
Okuda
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Department of Neurochemistry, Faculty of Medicine, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033, Tokyo, Japan. okuda@m.u-tokyo.ac.jp
Haga
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D000109:Acetylcholine / Q000378:metabolism; D000595:Amino Acid Sequence; D002352:Carrier Proteins / Q000235:genetics* / Q000378:metabolism*; D002794:Choline / Q000378:metabolism*; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000032:analysis; D006801:Humans; D026901:Membrane Transport Proteins; D008969:Molecular Sequence Data; D017386:Sequence Homology, Amino Acid; D014018:Tissue Distribution
D002352:Carrier Proteins; D018076:DNA, Complementary; D026901:Membrane Transport Proteins; C065637:choline transporter; D002794:Choline; D000109:Acetylcholine