TRANSPORT OF ADENOSINE BY RECOMBINANT PURINE-SELECTIVE AND PYRIMIDINE-SELECTIVE SODIUM NUCLEOSIDE COTRANSPORTERS FROM RAT JEJUNUM EXPRESSEDIN XENOPUS-LAEVIS OOCYTES/

Two major Na+-dependent nucleoside transporter subtypes implicated in adenosine transport in mammalian cells are distinguished functionally on the basis of substrate specificity: one is selective for pyrimidine nucleosides but also binds adenosine, and the other has selectivity f or purine nucleosides but also binds uridine. Transportability of aden osine by the purine-selective system has been established by measureme nts of [H-3]adenosine fluxes, whereas the conclusion that adenosine is permeant of the pyrimidine-selective system is based on inhibition as says. We investigated adenosine transport mediated by a recombinant py rimidine-selective rat jejunal/kidney Na+/nucleoside cotransporter (rC NT1) expressed in Xenopus laevis oocytes and compared it with that med iated by a recombinant purine-selective rat jejunal/liver Na+/nucleosi de cotransporter (rCNT2). Adenosine fluxes mediated by rCNT1 were 1 or der of magnitude lower than those mediated by rCNT2. In kinetic studie s, rCNT1 transported adenosine with an apparent K-m value for influx ( 26 mu M) similar to that for uridine but with a very much lower V-max value, and the V-max/K-m ratios were 0.003 and 0.57 for adenosine and uridine, respectively. Recombinant rCNT1 mediated efflux of [H-3]uridi ne from preloaded oocytes, demonstrating a capacity for bidirectional transport of nucleoside permeants. Uridine efflux was stimulated by ex tracellular uridine and inhibited by extracellular adenosine, suggesti ng that the rate of conversion of rCNT1 from its outward-facing confor mation to its inward-facing conformation was increased when the transp orter was complexed with uridine and decreased when it was complexed w ith adenosine. Thus, although rCNT1 binds adenosine and uridine with s imilar affinities, it kinetically favors transport of uridine.