A nucleoside transporter is functionally linked to ectonucleotidases in rat liver canalicular membrane

Prevention of nucleoside loss in bile is physiologically desirable because hepatocytes are the main source of nucleosides for animal cells which lack de novo nucleoside biosynthesis. We have demonstrated a Na+ gradient-energized, concentrative nucleoside transport system in canalicular membrane vesicles (CMV) from rat liver by studying [3H]adenosine uptake using a rapid filtration technique. The Na(+)-dependent nucleoside transporter accepts purine, analogues of purine nucleosides and uridine; exhibits high affinity for adenosine (apparent Km, 14 microM); is not inhibited by nitrobenzylthioinosine or dipyridamole, and is present in CMV but not in rat liver sinusoidal membrane vesicles. Adenosine transport in right side-out CMV was substantially greater than with inside-out CMV. CMV also contain abundant ecto-ATPase and ecto-AMPase (5'-nucleotidase). These ectoenzymes were shown to degrade nucleotides into nucleosides which were conserved by the Na(+)-dependent nucleoside transport system.