THE COMPLEX OF PHOSPHATIDYLINOSITOL 4,5-BISPHOSPHATE AND CALCIUM-IONSIS NOT RESPONSIBLE FOR CA2-INDUCED LOSS OF PHOSPHOLIPID ASYMMETRY IN THE HUMAN ERYTHROCYTE - A STUDY IN SCOTT SYNDROME, A DISORDER OF CALCIUM-INDUCED PHOSPHOLIPID SCRAMBLING()

Elevation of cytoplasmic Ca2+ levels in human erythrocytes induces a p rogressive loss of membrane phospholipid asymmetry. a process that is impaired in erythrocytes from a patient with Scott syndrome. We show h ere that porcine erythrocytes are similarly incapable of Ca2+-induced redistribution of membrane phospholipids. Because a complex of phospha tidylinositol 4,5-bisphosphate (PIP2) and Ca2+ has been proposed as th e mediator of enhanced transbilayer movement of lipids (J Biol Chem 26 9:6347,1994), these cell systems offer a unique opportunity for testin g this mechanism, Analysis of both total PIP2 content and the metaboli c-resistant pool of PIP2 that remains after incubation with Ca2+ ionop hore showed no appreciable differences between normal and Scott erythr ocytes. Moreover, porcine erythrocytes were found to have slightly hig her levels of both total and metabolic-resistant PIP2 in comparison wi th normal human erythrocytes. Although loading of normal erythrocytes exogenously added PIP2 gave rise to a Ca2+-induced increase in prothro mbinase activity and apparent transbilayer movement of nitrobenzoxadia zolyl (NBD)-phospholipids, these PIP2-loaded cells were also found to undergo progressive Ca2+-dependent cell lysis, which seriously hampers interpretation of these data. Moreover, loading Scott cells with PIP2 did not abolish their impaired lipid scrambling, even in the presence of a Ca2+-ionophore. Finally, artificial lipid vesicles containing no PIP2 or 1 mole percent of PIP2 were indistinguishable with respect to transbilayer movement of NBD-phosphatidylcholine in the presence of C a2+. Our findings suggest that Ca2+-induced redistribution of membrane phospholipids cannot simply be attributed to the steady-state concent ration of PIP2, and imply that such lipid movement is regulated by oth er cellular processes. (C) 1995 by The American Society of Hematology.