Differential kinetics of transport of 2',3'-dideoxyinosine and adenosine via concentrative Na+ nucleoside transporter CNT2 cloned from rat blood-brain barrier.

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.