Hepatocellular transport of bile acids. Evidence for distinct subcellular localizations of electrogenic and ATP-dependent taurocholate transport in rat hepatocytes.
To investigate whether electrogenic and ATP-dependent taurocholate transport activities are both mediated by the same bile acid-transporting polypeptide in rat liver, we further purified isolated canalicular membrane vesicles by free flow electrophoresis. Removal of most of the contaminating endoplasmic reticulum resulted in a complete loss of electrogenic taurocholate transport from an ecto-ATPase-enriched canalicular membrane subfraction. In contrast, ATP-dependent taurocholate transport remained associated with both an ecto-ATPase-enriched and an ecto-ATPase-free canalicular membrane subfraction. Microsomes containing 64% of total endoplasmic reticulum exhibited saturable electrogenic (Km approximately 270 microM), but no ATP-dependent taurocholate uptake. Golgi membrane vesicles were devoid of any taurocholate transport activity. These results indicate that electrogenic taurocholate transport resides entirely in the endoplasmic reticulum, whereas ATP-dependent bile acid transport is an intrinsic function of the canalicular membrane as well as of a so far unidentified intracellular membrane bound compartment. Hence, the two transport activities are most probably mediated by two different bile acid transporting polypeptides. Furthermore, the finding of ATP-dependent taurocholate transport in virtually ecto-ATPase-free vesicles argues against the concept of primary active bile acid transport being exclusively mediated by the canalicular ecto-ATPase.
Kast
C
C
Department of Medicine, University Hospital, Zurich, Switzerland.
Stieger
B
B
Winterhalter
K H
KH
Meier
P J
PJ
D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D001647:Bile Acids and Salts / Q000378:metabolism*; D001692:Biological Transport / Q000187:drug effects; D015415:Biomarkers / Q000032:analysis; D002478:Cells, Cultured; D004721:Endoplasmic Reticulum / Q000378:metabolism; D004798:Enzymes / Q000032:analysis; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D013347:Subcellular Fractions / Q000378:metabolism; D013656:Taurocholic Acid / Q000378:metabolism*
D001647:Bile Acids and Salts; D015415:Biomarkers; D004798:Enzymes; D013656:Taurocholic Acid; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; C027201:ectoATPase