Functional Characterization of a Human Purine-Selective, Na+-Dependent Nucleoside Transporter (hSPNT1) in a Mammalian Expression System

Nucleosides and nucleoside analogs are actively transported in the human kidney. With the recent cloning of a purine-selective, Na + -dependent, nucleoside transporter (hSPNT1, also termed hCNT2) from human kidney, it is now possible to study the interaction of nucleosides and nucleoside analogs with this transport protein and gain a more detailed knowledge of the underlying mechanisms of nucleoside transport in the human kidney. In this study we examined the substrate selectivity of hSPNT1 for nucleosides and nucleoside analogs. We determined that the naturally occurring nucleosides adenosine, inosine, and uridine are substrates for this carrier, whereas thymidine is not. The nucleoside analogs (0.5 mM) 2′,3′-dideoxyadenosine; 2′,3′-dideoxyinosine; and 2-chloro-2′deoxyadenosine (2CdA), significantly inhibited the uptake of [ 3 H]inosine in HeLa cells transiently transfected with hSPNT1. However, there was no significant Na + -dependent uptake of [ 3 H]2′,3′-dideoxyinosine or [ 3 H]2CdA in the transfected cells, suggesting that these nucleoside analogs are not permeants of hSPNT1. Interestingly, 2CdA was considerably less potent in inhibiting [ 3 H]inosine uptake in HeLa cells expressing hSPNT1 than in cells expressing the rat homolog rSPNT (IC 50 = 371 μM versus 13.8 μM), suggesting that there may be notable species differences in the kinetic interactions of some nucleoside analogs with purine- selective nucleoside transporters.