title: Differential kinetics of transport of 2',3'-dideoxyinosine and adenosine via concentrative Na+ nucleoside transporter CNT2 cloned from rat blood-brain barrier.
abstract: The concentrative Na+ nucleoside transporter type 2 (CNT2), cloned from a rat blood-brain barrier cDNA library, yields very high flux ratios for purine nucleosides after expression in frog oocytes. This high activity of the rat CNT2 produced from the blood-brain barrier-derived cDNA, designated clone A-11, enabled a kinetic analysis of 2',3'-dideoxyinosine transport via the rat CNT2. CNT2 transported both adenosine and 2',3'-dideoxyinosine. The 2',3'-dideoxyinosine transport parameters included a Km of 29.2 +/- 8.3 microM, a V(max) of 0.40 +/- 0.11 pmol/oocyte/min, and a constant of nonsaturable transport (KD) of 15.7 +/- 0.6 nl/oocyte/min. The 2',3'-dideoxyinosine Vmax was 27-fold lower than the adenosine Vmax and the 2',3'-dideoxyinosine KD was >15-fold greater than the KD of adenosine transport. Adenosine inhibited both the saturable component of 2',3'-dideoxyinosine transport with a K(I) of 14.8 +/- 1.6 microM, and inhibited the nonsaturable component of 2',3'-dideoxyinosine transport. Both the saturable and nonsaturable components of 2',3'-dideoxyinosine transport were sodium-dependent with a sodium K0.5 of 8.7 +/- 0.9 mM, and a Hill coefficient of 1.00 +/- 0.10. The transport of 2',3'-dideoxyinosine was strongly inhibited by thymidine, whereas thymidine was a weak inhibitor of adenosine transport via rat CNT2. Thymidine was transported by rat CNT2 with a Km = 130 +/- 44 microM and a Vmax = 1.7 +/- 0.5 pmol/oocyte/min. These studies provide evidence for asymmetric transport sites on rat CNT2, where 2',3'-dideoxyinosine and thymidine compete selectively at a low Vmax site on the transporter, whereas adenosine is transported at a high Vmax site.
authors.affiliation: Department of Medicine, UCLA School of Medicine, Los Angeles, California, USA.
authors.forename: J Y
authors.initials: JY
authors.lastname: Li
authors.forename: R J
authors.initials: RJ
authors.lastname: Boado
authors.forename: W M
authors.initials: WM
authors.lastname: Pardridge
mesh_terms: D000241:Adenosine / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport; D001812:Blood-Brain Barrier / Q000502:physiology*; D002999:Clone Cells; D016049:Didanosine / Q000378:metabolism*; D007700:Kinetics; D026901:Membrane Transport Proteins / Q000378:metabolism*; D009865:Oocytes / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D013936:Thymidine / Q000494:pharmacology; D014981:Xenopus
chemical_list: D026901:Membrane Transport Proteins; D012333:RNA, Messenger; C065171:cif nucleoside transporter; D016049:Didanosine; D000241:Adenosine; D013936:Thymidine