Topology of amino‐phospholipids in the red cell membrane

Abstract

Abstract The red cell membrane has an asymmetric arrangement of phospholipids. The amino‐phospholipids are localized primarily on the inner surface of the membrane and the choline phospholipids are localized to a large extent on the outer surface of the membrane. Evidence is presented based on the use of covalent chemical probes in sequence that the red cell membrane contains heterogeneous domains of PE and PS and that the transport systems for Pi and K + are asymmetrically arranged. Certain amino groups of PE, PS, and/or protein localized on the outer membrane surface are involved in Pi transport and certain amino groups of PE, PS, and/or protein localized on the inner surface of the membrane are involved in K + transport. Cross‐linking studies with DFDNB show that the cross‐linked PE‐PE molecules are rich in plasmalogens. This suggests that clusters of plasmalogen forms of PE occur in the membrane. Both PE and PS are cross‐linked to membrane protein. These PE and PS molecules contain 24–28% 16:0 and 18:0 fatty acids and 12% fatty aldehydes. PE and PS molecules are cross‐linked to a spectrin‐rich fraction. It is proposed that the binding of spectrin to membrane PE and PS may help anchor spectrin to the inner surface of the membrane and regulate shape changes in the cell. K + ‐valinomycin forms a complex with TNBS and converts it from a non‐penetrating proble to a penetrating probe. Valinomycin enhances K + leak and Pi leak in the red cells. SITS inhibits completely the valinomycin‐induced Pi leak and inhibits partially the valinomycin induced K + leak. Valinomycin and IAA have additive effects on Pi leak. Ouabin has no effect on basal or valino‐mycin‐induced Pi leak. These data suggest that Pi leak and K + leak occur by separate transport systems. In summary, the amino‐phospholipids in the red cell membrane are asymmetrically arranged; some occur in clusters and some are closely associated with membrane proteins. Amino‐phospholipids also are believed to bind spectrin to the inner surface of the membrane and also may play a role in cation and anion leak.