Lipid molecular species retailoring and membrane fluidity.

on cell morphology, i.e. the formation of stomatocytes, when the native mixed-acid PC species in the monolayer are replaced by (1,2-dilinoleoyl)-PC (Fig. 1). This species has a pronounced cone shape because of a larger cross-section at the end of its hydrophobic tails. Consequently, the outer monolayer, and thereby the whole membrane, will bend inwards. Further support for the above hypothesis was found by studies in which the retailoring of the PC species composition was followed by treatment of the cells with phospholipases. The morphological changes induced by the two different types of lipid manipulations sequentially applied appeared to be additive and predictable, as they indeed can be explained on the basis of changes in molecular geometry (Christiansson et al., 1985). Erythrocytes enriched with (1,2dipalmitoy1)-PC in their outer membrane leaflet tend to form echinocytes (Fig. 1); a non-lytic hydrolysis of PC in the outer layer by phospholipase A, will produce free fatty acid and 1-acyl lyso-PC (inverted cone shape) and as a result further crenation occurs. Conversely, treatment of the intact cells with sphingomyelinase C produces ceramides (cone-shaped) in the outer monolayer, which causes a restoration of the normal discoid morphology in the PC retailored cells, because the molecular geometries of ceramide and ( 1,2-dipaImitoyl)-PC are complementary. On the other hand, a further invagination of stomatocytic cells, as induced by replacement with ( 1,2-dilinoleoyl)-PC, was observed when these cells were subsequently treated with sphingomyelinase C, whereas their discoid morphology could be restored by treatment with phospholipase A, (Christiansson et al., 1985). In conclusion, manipulation of the molecular geometry of phospholipid molecules in the outer monolayer of intact erythrocytes causes a bending of this layer to ensure the maintenance of an optimal packing of the new molecules in this layer. The inner layer has to follow suit and a change in morphological appearance of the cells results.