POLYAMINE INHIBITION OF TRANSBILAYER MOVEMENT OF PLASMA-MEMBRANE PHOSPHOLIPIDS IN THE ERYTHROCYTE GHOST

The resting plasma membrane of circulating blood cells demonstrates an asymmetric distribution of the phospholipid classes across the bilayer that is altered during cellular activation. To better understand the mechanisms governing transbilayer distribution of phospholipids, studies were conducted using the erythrocyte ghost, in which plasma membrane leaflet distribution of phospholipids can be readily probed. Preparation of ghosts by hypotonic lysis at increasingly high dilution markedly enhanced (up to 10-fold) calcium-induced (50-500 mu M) transbilayer movement of phospholipids. The enhanced transbilayer movement was assessed by translocation of exogenously added sn-2-[6[(7- nitrobenz-2-oxa-1,3-diazol-4-yl)amino]caproyl]-labeled phosphatidylcholine and phosphatidylserine from the plasma membrane outer leaflet to the inner leaflet and vice versa, as well as transbilayer movement of endogenous phosphatidylserine to the outer leaflet. It was found that phospholipid movement was bidirectional and also nonspecific with regard to polar head group. It was further demonstrated that preparation of ghosts at increasing dilution resulted in depletion of cellular polyamines and that physiologic replenishment of spermine, and to a lesser extent spermidine, resulted in significant inhibition (50 and 25%, respectively) of transbilayer movement of phospholipids. Replenishment of other di- and polyamines demonstrated that inhibition was not simply dependent on total cationic charge but rather on charge density and suggestive of specific interaction of the polyamines, particularly spermine, with the plasma membrane. As most cells demonstrate both a high degree of regulation in maintenance of polyamine levels and the means for facile shifts within cellular polyamine pools, it is suggested that loss of membrane asymmetry during cellular activation may be mediated in part through enhanced transbilayer movement of phospholipids due to altered polyamine-membrane associations.