Abnormality of phospholipid transverse diffusion in sickle erythrocytes.
We have used spin-labeled analogues of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine to compare the transverse diffusion rates of lipids in normal and sickle erythrocytes. The beta-chain of the spin-labeled lipids was a short chain (five carbons) providing the spin labels with a relative water solubility, and hence permitting their rapid incorporation into cell membranes. The orientation of the labeled lipids in the membranes was assayed by selective chemical reduction of the nitroxide labels embedded in the outer leaflet. We have found that all three spin-labeled phospholipids are initially incorporated in the outer leaflet. Upon incubation at 4 degrees C the aminophospholipids, not the phosphatidylcholine, diffuse toward the inner leaflet within 3 h. The transverse diffusion rate of aminophospholipids is reduced by 41% (phosphatidylserine) and 14% (phosphatidylethanolamine) in homozygote sickle cells (SS) when compared with normal cells (AA) or heterozygote cells (AS or SC). At equilibrium the asymmetric distribution of spin-labeled phospholipids resulting from this selective diffusion is also reduced in SS cells when compared with AA, SC, or AS cells. This reduced asymmetry was not found in a reticulocyte-rich blood sample (hemoglobin A), indicating that the age of the cell cannot be responsible for this phenomenon. Moreover, because at low temperatures the sickling process does not occur, the observed perturbations in phospholipid organization reflect preexisting membrane abnormalities in sickle cells. Ghosts loaded with ATP give the same results. Varying the concentration of intracellular calcium had no effect on lipid diffusion, except at very high free calcium concentrations (3 microM) when diffusion was practically abolished. We suggest that membrane protein alterations may be part of the explanation of the observed abnormalities.