Lipid Compositions in Biological Membranes

Proulx [30] summarized the published lipid compositions of BBM isolated from epithelial cells from pig, rabbit, mouse and rat small intestines. Table 3.1 shows the lipid make-up for the rat, averaged from five reported studies [30], On a molar basis, cholesterol accounts for about 50% of the total lipid content (37% on a weight basis). Thus, the cholesterol content in BBM is higher than that found in kidney epithelial (MDCK) and brain endothelial cells (Table 3.1). Slightly different BBM lipid distribution was reported by Alcorn et al. [31] here, the outer (luminal) leaflet of the BBM was seen to be rich in sphingomyelin content, while the inner leaflet (cytosol) was rich in PE and PC. Apical (brush border) and basolateral lipids are different in epithelia. The basolateral membrane content (not reported by 

CBBM = brush border membrane, rat (average of four studies) from Ref. [30], 

Proulx) is high in PC, whereas the BBM has nearly the same PC as PE content. Kramer et al. [37, 38] reported MDCK and BBM lipid composition profiles, listed in Table 3.1, for comparative purposes. 

Different tissues have different lipid compositions. The most common lipid components of membranes are PC and PE. Lipid extracts from brain and lung are also rich in PS heart tissue is rich in PG, and liver is rich in PI [567]. Human blood cells, as ghost erythrocytes (with cytoplasm contents removed), are often used as membrane models. These have different compositions between the inner and outer leaflets of the bilayer membrane. Phospholipids account for 46% of the outer leaflet membrane constituents, with PC and Sph about equal in amount. The inner leaflet is richer in phospholipids (55%), with the mix 19% PE, 12% PS, 7% PC and 5% Sph [567], 

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Rilfors L, Lindblom G. Regulation of lipid composition in biological membranes - biophysical studies of lipids and lipid synthe- 60. sizing enzymes. Colloids Surf. B 2002 26 112-124. 

To begin with, biological membranes are not uniform with respect to membrane sidedness. Both the protein and the lipid composition of these membranes exhibit transbilayer asymmetry. In addition, in polarized mammalian cells, a segregation... 

Lipid domains and rafts in biological membranes are stabilized by several different interactions, including membrane-cytoskeleton, lipid-protein, and lipid-lipid interactions, and the organization can be both equilibrium and non-equilibrium in nature [27]. Lipid-domain formation caused by cooperative phenomena in the lipid bilayers is particularly important for the activation of SPLA2 [32-35]. The cooperative phenomena in lipid bilayers are caused by the fundamental interactions between the lipid molecules and are a consequence of the many-particle character of the supramolecular aggregate. The cooperativity leads to phase transitions and phase equilibria. The key cooperative event in many liposomal membranes is the so-called main phase transition, which takes the bilayer from a solid (gel) phase with conformationally ordered acyl chains to a fluid phase with conformationally disordered chains. The main transition in lipid bilayers is often accompanied by strong lateral density and compositional fluctuations. These fluctuations are manifested as dynamic lipid domains characterized by certain time and length scales that are determined by the thermodynamic conditions and the actual lipid species in question. 

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