Specificity of human and rat orthologs of the concentrative nucleoside transporter, SPNT

To understand the roles that nucleoside transporters play in the in vivo distribution of clinically important nucleoside analogs, the substrate specificity of each transporter isoform should be determined. In the present work, we studied the substrate specificities of the human and rat orthologs of the Na+-dependent purine-selective nucleoside transporter (SPNT; concentrative nucleoside transporter 2), for nucleosides, nucleobases, and base- and ribose-modified nucleoside analogs. The two-electrode voltage-clamp technique in Xenopus laevis oocytes expressing these transporters was used. Purine nucleosides and uridine induced currents in oocytes expressing rat SPNT (rSPNT) or human SPNT1 (hSPNT1). The rank order of magnitude of nucleoside-induced currents was guanosine > uridine > adenosine > inosine and guanosine > uridine > inosine > adenosine for rSPNT- and hSPNT1-expressing oocytes, respectively. Uridine analogs (modified at the 5-position of the base) induced little or no current, suggesting that these compounds are only poorly transported by either transporter. Cladribine induced currents in oocytes expressing rSPNT (K-0.5 = 57 +/- 12 muM) but not hSPNT1. The ribose-modified nucleoside analogs, adenine arabinoside, and 2', 3'-dideoxyadenosine induced currents in rSPNT- expressing, but not in hSPNT1-expressing, oocytes. These data suggest that there are notable species differences in the specificity of SPNT for synthetic nucleoside analogs.