Phospholipid Composition

The glycerophospholipid composition of most cell types in the body is regulated within relatively restricted limits and is often specialized for the function of the cell involved. Moreover, most cells maintain distinct and different compositions of the various phospholipid classes. For example, ESI-MS analysis of total lipid extracts from a variety of mouse tissues shows that PC exhibits a wide variation of composition but that of phosphatidylinositol (PI) is relatively constant (Figure 3). PC species range from predominantly polyunsaturated in liver (Figure 3A) to disaturated and monounsaturated in lungs (Figure 3B) and brain (Figure 3C). Liver PC contains substantial amounts of species with either n-6 fatty acids (20:4n-6) or n-3 fatty acids (22:6n-3), whereas PC from pancreas, for instance, is essentially composed solely of species containing n-6 fatty acids (Figure 3D). This inherent variation in PC composition is emphasised by that of spleen (Figure 3E), which, like lung, is dominated by PC16:0/16:0 but also contains increased concentrations of monounsa-turated and polyunsaturated species. In contrast, PI from all tissues measured was dominated by the single polyunsaturated stearoyl arachidonoyl species (PI18:0/20:4) (Figures 3F-3J). This diversity of composition is mirrored for the distinctive and different compositions of all the other phospholipid classes in most cell types and emphasizes the highly specific mechanisms that regulate phospholipid compositions in vivo. PE is typically enriched in arachidonoyl-containing species, whereas PS is generally dominated by the monounsaturated species PS18:0/18:1. It is important to recognize, for nutritional studies in vitro, that many of these tissue-specific distributions are lost or reduced for cells maintained in cell culture supplemented with fetal calf serum.

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