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 tra nsport activities are both mediated by the same bile acid-transporting polypeptide in rat liver, we further purified isolated canalicular me mbrane vesicles by free flow electrophoresis. Removal of most of the c ontaminating endoplasmic reticulum resulted in a complete loss of elec trogenic taurocholate transport from an ecto-ATPase-enriched canalicul ar membrane subfraction. In contrast, ATP-dependent taurocholate trans port remained associated with both an ecto-ATPase enriched and an ecto -ATPase-free canalicular membrane subfraction. Microsomes containing 6 4% of total endoplasmic reticulum exhibited saturable electrogenic (K- m approximate to 270 mu M), but no ATP-dependent taurocholate uptake. Golgi membrane vesicles were devoid of any taurocholate transport acti vity. These results indicate that electrogenic taurocholate transport resides entirely in the endoplasmic reticulum, whereas ATP-dependent b ile acid transport is an intrinsic function of the canalicular membran e as well as of a so far unidentified intracellular membrane bound com partment. Hence, the two transport activities are most probably mediat ed by two different bile acid transporting poly peptides. Furthermore, the finding of ATP-dependent taurocholate transport in virtually ecto -ATPase-free vesicles argues against the concept of primary active bil e acid transport being exclusively mediated by the canalicular ecto-AT Pase.