title: Sodium-dependent nucleoside transport in the human intestinal brush-border membrane.
abstract: The objective of the study was to determine the identity and kinetic characteristics of nucleoside transporters present in the brush-border membrane of the human jejunum. With use of brush-border membrane vesicles, uptake of [3H]uridine was stimulated two- to threefold by an inwardly directed Na+ gradient and was inhibited by both 100 microM thymidine and 100 microM guanosine nucleosides, which serve as model substrates for purine (N1, cif) and pyrimidine (N2, cit) transporters, respectively. [3H]thymidine and [3H]guanosine transport exhibited an overshoot phenomenon only in the presence of a Na+ gradient. Na(+)-thymidine uptake was inhibited by 100 microM cytidine or thymidine but not by guanosine, inosine, formycin B, or hypoxanthine. [3H]guanosine uptake was inhibited by 100 microM inosine, guanosine, or formycin B but not by thymidine or cytidine. Both adenosine and uridine inhibited uptake of [3H]thymidine and [3H]guanosine to a similar extent, indicating that both N1, cif and N2, cit Na(+)-nucleoside transporters are expressed in human jejunum. Enhanced uptake of Na(+)-thymidine by an inside-negative potential difference generated by K+ and valinomycin provides evidence that nucleoside transport is rheogenic, involving net transfer of a positive charge. The Hill coefficient was unity for all three substrates, indicating a Na(+)-nucleoside coupling stoichiometry of 1:1. At saturating Na+ concentration (150 mM) the kinetic parameters (n = 3-4) Michaelis-Menten constant and maximum velocity for uridine, thymidine, and guanosine uptake were 4.15 +/- 1.79, 2.74 +/- 0.58, 12.02 +/- 1.34 microM and 25.93 +/- 7.38, 16.10 +/- 3.64, 63.92 +/- 10.23 pmol.mg-1.10 s-1, respectively. These results suggest that, in contrast to the human kidney that expresses the N4 nucleoside transporter, the human jejunum expresses both N1 and N2 Na(+)-nucleoside transporters.
authors.affiliation: Department of Pharmaceutics, School of Pharmacy, University of Washington, Seattle 98195, USA.
authors.forename: S D
authors.initials: SD
authors.lastname: Patil
authors.forename: J D
authors.initials: JD
authors.lastname: Unadkat
mesh_terms: D000328:Adult; D000368:Aged; D001692:Biological Transport / Q000187:drug effects; D002414:Cations, Monovalent / Q000494:pharmacology; D005260:Female; D006151:Guanosine / Q000378:metabolism; D006801:Humans; D007413:Intestinal Mucosa / Q000378:metabolism*; D007421:Intestine, Small; D007700:Kinetics; D008297:Male; D008871:Microvilli / Q000378:metabolism*; D008875:Middle Aged; D009705:Nucleosides / Q000378:metabolism* / Q000494:pharmacology*; D012964:Sodium / Q000494:pharmacology*; D013936:Thymidine / Q000378:metabolism; D014019:Tissue Donors; D014529:Uridine / Q000378:metabolism
chemical_list: D002414:Cations, Monovalent; D009705:Nucleosides; D006151:Guanosine; D012964:Sodium; D013936:Thymidine; D014529:Uridine