Phospholipid Structures

There are two major classes of phospholipid, depending on whether they contain a glycerol or sphingosyl backbone. Glycerophospholipids are molecules based on phosphatidic acid (3-s«-phos-phatidic acid); the nature of the esterified group X defines the class of phospholipid (Figure 1). The most common substituent groups include nitrogenous bases, such as choline and ethanolamine, and polyalcohols, such as myoinositol and glycerol. Sphingophospholipids contain sphingosine (trans-D-erythro-1,3-dihyroxy 2-amino-4-octadecene). Sphin-gomyelin is the most abundant sphingophospholipid class, and it is the phosphorylcholine ester of N-acylsphingosine, also called ceramide. Sphingophos-pholipids are important components of all cell membranes and are structurally and metabolically closely related to glycosphingolipids such as glyco-sylceramides, gangliosides, and cerebrosides. Sphin-gomyelin is recognized as a major substrate for sphingomyelinase enzymes involved in generating intracellular ceramide and sphingosine, which are intimately involved in the regulation of programmed cell death (apoptosis). Sphingophospholipids contain principally saturated and monounsaturated fatty acids; little information is available on the nutritional effects on sphingophospholipid composition, and sphingomyelin metabolism has been recently reviewed.

The distribution of phospholipids is heterogeneous within any cell both between different subcel-lular membranes and within individual membranes. For instance, mammalian cells maintain an enriched distribution of neutral lipids, such as phosphatidyl-choline (PC) and sphingomyelin, in the outer leaflet of the plasma membrane and hence present an uncharged surface to the exterior of the cell (Figure 2). It is critically important to restrict the distribution of uncharged phospholipids to the interior of the cell because increased concentration of phosphatidylserine (PS) in the outer leaflet of the plasma membrane is the initial signal for both programmed cell death (apoptosis) and the clotting cascade.

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