Bidirectional transbilayer movement of phospholipid analogs in human red blood cells: Evidence for an ATP-dependent and protein-mediated process

The transbilayer movement of fluorescent and isotopically labeled analogs of phosphatidylserine (PS), phosphatidylethanolamine (PE), and phosphatidylcholine (PC) from the outer to the inner leaflet (flip) and from the inner to the outer leaflet (flop) of human red blood cells (RBC) was examined. The inward movement of 1-oleoyl-2-(N-4-nitrobenzo-2-oxa-1,3-diazole-aminocaproyl)- (C6-NBD-), 1-oleoyl-2-(N-(3-(3-[¹²⁵I]iodo-4-hydroxyphenyl)propionyl)aminoc aproyl)-(C6-¹²⁵I-), or 1-oleoyl-2-N-(3-3-[¹²⁵I]iodo-4-azido-phenyl)propionyl)aminocapr oyl- (C6-¹²⁵I-N₃-) analogs of PC and PE were relatively slow. In contrast, all analogs of PS and PE analogs containing aminododecanoic acid (C 12 lipids) were rapidly transported to the cell's inner leaflet. Analysis of ¹²⁵I-N₃ lipids cross-linked to membrane proteins revealed labeling of 32-kDa Rh polypeptides that was dependent on the lipid's capacity to be transported to the inner leaflet but was independent of lipid species. To investigate whether lipids could also be transported from the inner to the outer leaflet, lipid probes residing exclusively in the inner leaflet were monitored for their appearance in the outer leaflet. Lipid movement could not be detected at 0 °C. At 37 °C, however, approximately 70% of the PC, 40% of the PE, and 15% of the PS redistributed to the cells outer leaflet, thereby attaining their normal asymmetric distribution. Continuous incubation in the presence of bovine serum albumin depleted the cells of the analogs (t_1/2 ∼ 1.5 h) in a manner that was independent of lipid species. Similar to the inward movement of aminophospholipids, the outward movement of PC, PE, and PS was ATP-dependent and could be blocked by oxidation of membrane sulfhydryls and by the histidine reagent bromophenacyl bromide. Evidence is presented which suggests that the outward movement of lipids is an intrinsic property of the cells unrelated to compensatory mechanisms due to an imbalance in lipid distribution.