TRANSVERSE MOVEMENT OF SPIN-LABELED PHOSPHOLIPIDS IN THE PLASMA-MEMBRANE OF A HEPATOCYTIC CELL-LINE (HEPG2) - IMPLICATIONS FOR BILIARY LIPID SECRETION

The redistribution of spin-labeled phospholipid analogs across the pla sma membrane of HepG2 cells, either in suspension or grown as monolaye rs, was investigated. After incorporation into the outer membrane leaf let spin-labeled aminophospholipids phosphatidylserine (PS) and phosph atidylethanolamine (PE) moved rapidly to the inner monolayer, whereas the analog of phosphatidylcholine (PC) disappeared more slowly from th e outer leaflet. The fast, inward movement of the aminophospholipids w as abolished after adenosine triphosphate (ATP)-depletion of cells, su ggesting the presence of an aminophospholipid translocase in the plasm a membrane of these cells. Compared with human red blood cells, the ac tivity of the aminophospholipid translocase is two orders of magnitude higher in HepG2 cells. From these data, a transverse phospholipid asy mmetry can be inferred with the aminophospholipids mainly concentrated on the inner monolayer and the choline-containing phospholipids on th e outer leaflet. The relevance of the enrichment of PC in the outer me mbrane leaflet for the formation and composition of the bile is discus sed.