Information about the detailed molecular species compositions of phospholipids from adult human tissues is surprisingly haphazard. There have been many isolated reports of extensive characterizations of selected phospholipid classes in individual tissues, but such studies have generally measured compositions of bulk preparations from relatively large tissue samples. Very few clinical or nutritional studies have characterized phos-pholipid compositions in molecular terms. In reality, each cell type contains in excess of 1000 glyceropho-spholipid species, with differential compositions between different membranes in the same cell and even between different regions in the same membrane. Such regions of microheterogeneity may occur either because of the physical properties of the lipids themselves (e.g., forming hexagonal rather than bilayer structures) or because of sequestration by membrane proteins. Phase transitions within the membrane can also exert significant effects, and interactions of cytos-keletal components of the cell have been described with relative solid gel-phase phospholipids in the plasma membrane. One additional important factor is the transmembrane phospholipid distribution between the two leaflets of the cell bilayer. For practically all cell types, PC is relatively more concentrated in the outer leaflet, whereas PE is located primarily in the inner (cytoplasmic) leaflet. Importantly, PS is almost totally restricted to the side of the plasma membrane facing the cytoplasm, where it acts as an activator of protein kinase C. Redistribution of PS to the outer leaflet of the plasma membrane is a signal of cell senescence and is a potent activator of the clotting cascade. Finally, there has been considerable interest in the concept of lipid rafts, subfractions of membranes that are resistant to extraction with detergent and have been extensively implicated in transmembrane signaling particularly in immune cells. The compositional aspects of many such studies must be interpreted with caution; recent analyses have indicated that detergent solubility is more an intrinsic property of individual lipids than a property dependent on membrane organization.
Examples of recent ESI-MS analyses of phospholi-pid molecular species compositions of a variety of human tissues are summarized in Figure 5, which compares ESI-MS spectra of PC from human blood lymphocytes, monocytes, and neutrophils. As for most hematopoeic cell types and in contrast to the mouse compositions shown in Figure 2, the PC composition of these cells is dominated by monounsaturated PC
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