740 760 780 800

Molecular mass (mass:charge)

Figure 5 Electrospray ionization mass spectrometry analysis of PC compositions of human blood leukocytes. Total lipids were extracted from lymphocytes, monocytes, and neutrophils and analyzed as described in the legend for Figure 2. (Madden J, Wright S, Clark G and Postle A, unpublished observations.)

species, especially PC16:0/18:1 (m/z = 760), but the distribution of polyunsaturated species is considerably variable between cell types. Both lymphocytes and monocytes are relatively enriched in species containing 20:4n-6 (PC16:0/20:4m/z = 782; PC18:0/20:4m/ z = 810), with an increased content of PC16:0/16:0 (m/z = 734). In contrast, neutrophils are relatively depleted in both PC16:0/16:0 and arachidonoyl species, but they contain considerably higher proportions of sn-1-alkyl-sn-2-acyl species (PC16:01alk/16:0 m/ z = 720; PC16:0alk/18:1 m/z = 746; PC18:0alk/ 18:2 m/z = 772). This comparison illustrates an important role for phenotypic expression as one contributor toward the specificity of cell PC composition.

Different phospholipid classes from the same tissue generally exhibit considerable variation in composition, shown in Figure 2 for mouse tissues and also in the analysis of the white matter of human brain. Although brain PC was highly enriched in monounsaturated species, diacyl PE was enriched in species containing PUFA. The distribution of such species, however, was highly asymmetric, with 22:6n3 and 20:4n-6 species containing 16:0 at the sn-1 position being present in much lower abundance than the same species containing sn-1 18:0. In contrast, both alkenylacyl PE and PS were characterized by a predominance of monounsaturated species. However, whereas PC was enriched in PC16:0/18:1, alkenylacyl PE was enriched in PE18:1alk/18:1 and PS was enriched in PS18:0/18:1. This comparison illustrates the tight regulation of the composition of individual phos-pholipid classes and emphasizes potentially important differences in molecular compositions that could not be predicted from total fatty acid analysis.

Phosphatidylethanolamine molecular species

Figure 6 Dietary lipid and the composition of human erythrocyte phosphatidylethanolamine. Erythrocyte PE species were analyzed from six volunteers before and after consumption of fish oil containing 9g eicosapentaenoic acid (22:5n-3) and 6g docosahaexenoic acid (22:6n-3) per day for 4 weeks. Results are expressed as mean ± SEM; *p < 0.05. (From Knapp HR, Hullin F and Salem N Jr (1994) Asymmetric incorporation of dietary n-3 fatty acids into membrane aminophospholipids of human erythrocytes. Journal of Lipid Research 35: 1283-1291.)

n-3 fatty acids was variable, and the extent of such changes was modest. This comparison illustrates a general observation that although manipulation of cultured cell phospholipid compositions by medium lipid supplementation is relatively easy, phospholi-pid compositions of similar cell types in vivo are considerably more resistant to dietary manipulation.

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