Membrane skeleton-bilayer interaction is not the major determinant of membrane phospholipid asymmetry in human erythrocytes

Abstract

Transbilayer phospholipid distribution, membrane skeleton dissociation/association, and spectrin structure have been analysed in human erythrocytes after subjecting them to heating at 50°C for 15 min. The membrane skeleton dissociation/association was determined by measuring the Tris-induced dissociation of Triton-insoluble membrane skeletons (Triton shells), the spectrin-actin with normal erythrocyte membrane inside-out vesicles (IOVs). The spectrin structure was ascertained by measuring the spectrin dimer-to-tetramer ratio as well as the spectrin tryptophan fluoresence. Both the Tris-induced Triton shell dissociation and the spectrin-actin extractability under low ionic conditions were considerably reduced by the heat treatment. Also, the binding of heated erythrocyte spectrin-actin to IOVs was significantly smaller than the observed with the normal cell spectrin-actin. Further, the quantity of spectrin dimers was appreciably inreased in heat-treated erythrocytes as compared to the normal cells. This change in the spectrin dimer-to-tetramer ratio was accompanied by marked changes in the spectrin tryptophan fluorescence. Inspite of these heat-induced alterations in structure and bilayer interactions of the membrane skeleton, the inside-outside glycerophospholipid distribution remained virtually unaffected in the heat-treated cells, as judged by employing bee venom and pancreatic phospholipase A₂, fluorescamine and Merocyanine 540 as the external membrane probes. These results strongly indicate that membrane bilayer-skeleton interaction is not the major determining the transbilayer phospholipid asymmetry in human erythrocyte membrane.