SUBSTRATE SELECTIVITY, POTENTIAL SENSITIVITY AND STOICHIOMETRY OF NA-NUCLEOSIDE TRANSPORT IN BRUSH-BORDER MEMBRANE-VESICLES FROM HUMAN KIDNEY()

Recently, we demonstrated the presence of a Na+-nucleoside cotransport mechanism that transports both purine and pyrimidine nucleosides in h uman renal brush-border membrane vesicles (BBMV) (Gutierrez et al. (19 92) Biochim. Biophys. Acta 1105, 1-9). The objective of this study was to further elucidate the characteristics of this cotransport system i n terms of electrical potential sensitivity, stoichiometry and substra te selectivity with respect to nucleoside analogs. In BBMV from human kidney, Na+-thymidine uptake was stimulated by an inside negative pote ntial difference created by K+ and valinomycin. A hyperbolic relations hip between initial rate of uridine uptake and Na+ concentration was o btained suggesting a Na+-nucleoside coupling stoichiometry of 1:1. Our previous study had demonstrated that the pyrimidines, thymidine, cyti dine, and uridine and the purines, adenosine, 2'-deoxyadenosine, and g uanosine, but not inosine and formycin B, were substrates of this syst em. To further define the substrate selectivity of the transporter, th e interaction of the drugs, 2-chloroadenosine (2-ClAdo), 5-fluorouridi ne (5-FUrd) and 5-iodo-2'-deoxyuridine (5-IdUrd), nucleoside analogs t hat are modified on the base moiety was studied. The three compounds i nhibited Na+-thymidine uptake in the vesicles via a competitive mechan ism. The IC50 values for 2-ClAdo, 5-FUrd and 5-IdUrd were 75, 49, and 16 muM, respectively. In addition, 5-IdUrd trans-stimulated the initia l uptake of thymidine into the vesicles suggesting that the two compou nds share the same transporter. Collectively, these data suggest that Na+-nucleoside transport in the human renal brush-border membrane is a n electrogenic process and that the kidney may play a role in the disp osition and targeting of clinically important nucleoside analogs.