ATP-dependent phosphatidylcholine translocation in rat liver canalicular plasma membrane vesicles

Phosphatidylcholine (PC) translocation was stud- ied in rat liver canalicular plasma membrane vesicles using a fluorescent PC analogue that permitted the quantitation of asymmetric PC distribution in the outer and inner leaflet of the vesicles. PC translocation to the outer leaflet of the canali- cular membrane was stimulated by ATP and an ATP-regener- ating system in a time- and temperaturedependent manner resulting in 200 pmol PC translocated/mg protein per 30 min. A non-hydrolyzable ATP analogue did not support translocation. Translocating activity was observed with PC but not with phosphatidylethanolamine and was specific for in- side-out oriented canalicular membrane vesicles. Addition of taurocholate (10 p~), a micelle-forming bile acid, enhanced ATPdependent PC translocation 1.5 k 0.1-fold, whereas ad- dition of taurodehydrocholate ( 10 PM), a non-micelle-forming bile acid, did n0t.l These results indicate the presence of an ATPdependent transporter that "flips" phosphatidyl- choline from the inner to the outer leaflet of the rat bile canalicular plasma membrane from where it can become associated with bile acids in the canalicular lumen, thereby enhancing ATP-dependent flipping activity. Several lines of evidence suggest that the transporter is Mdr2 P-glycopro- tein.-Nies, A. T., Z. Catmaitan, and I. M. Arias. ATPde- pendent phosphatidylcholine translocation in rat liver canali- cular plasma membrane vesicles. J. Lipid Res. 1996. 37: 1125-1136.