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SY Yao, CE Cass and JD Young
Department of Physiology, University of Alberta, Edmonton, Canada.
Expression screening in Xenopus oocytes has been used to isolate a cDNA from rat jejunal epithelium encoding an intestinal/kidney Na(+)- dependent nucleoside transporter protein named rCNT1 [J. Biol. Chem. 269:17757-17760 (1994)]. rCNT1 is predicted to have 648 amino acid residues (relative molecular mass, 71,000) with 14 potential transmembrane domains and belongs to a new family of transporter proteins. Recombinant rCNT1 transports physiological pyrimidine nucleosides and adenosine. In the current investigation, functional expression in Xenopus oocytes was used to determine whether recombinant rCNT1 also transports antiviral pyrimidine nucleoside analogs. The recombinant protein mediated Na(+)-dependent transport of both 3'-azido- 3'-deoxythymidine (AZT) and 2',3'-dideoxycytidine (ddC). Apparent K(m) values of '0.5 mM were obtained for both [3H]AZT and [3H]ddC influx compared with 37 microM for [3H]uridine influx, with Vmax/Km ratios of 0.048, 0.039, and 0.57 for AZT, ddC, and uridine, respectively. Extracellular AZT and ddC stimulated rCNT1-mediated efflux of [3H]uridine from preloaded oocytes. These experiments provide direct evidence for Na(+)-dependent transport of AZT and ddC and suggest that members of the cNT family may be involved in the intestinal absorption and renal handling of pyrimidine nucleoside analogs used to treat acquired immune deficiency syndrome.
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