Lipidomics by Paul Wood

Author:Paul Wood
Language: eng
Format: epub
Publisher: Springer New York, New York, NY

2.2 Glycerolipids

Glycerolipids are composed of several classes of neutral lipids including acylglycerols and glycosylated acylglycerols [2]. These neutral lipids serve as a source of energy through the β-oxidation of fatty acids, as the hydroxyl groups of glycerol are esterified with fatty acids.

SFC/MS facilitates superior high-throughput analysis of glycerolipids as compared to LC/MS. Triacylglycerols (TAGs), diacylglycerols (DAGs), monoacylglycerols (MAGs), monogalactosyldiacylglycerols (MGDGs), and digalactosyldiacylglycerols (DGDGs) can be analyzed simultaneously with other lipid categories including glycerophospholipids, sphingolipids, and sterol lipids [12, 13]. With reversed-phase columns such as octadecylsilyl (ODS) and phenyl columns, lipid molecular species are separated based on carbon chain length and degree of unsaturation in their fatty acyl moieties [14]. Specifically, regioisomeric TAGs with the same fatty acid moiety in different positions on the glycerol backbone can be resolved using a C30 column with a shorter analysis time than in LC (Fig. 4) [15]. In addition, normal-phase columns such as silica and 2-ethylpyridine (2-EP) columns can also be utilized for TAG analysis. Since TAGs are highly hydrophobic, they are not retained in normal-phase LC, resulting in poor separation of individual molecular species. On the other hand, TAGs can be separated based on their fatty acyl moieties using a 2-EP column in SFC (Fig. 5) [16]. In this method, TAGs with long carbon chain on their fatty acyl moieties exhibit longer retention times (RTs). This is similar to the case of reversed-phase columns. On the other hand, when the degree of unsaturation in fatty acyl moieties increases, RTs become longer. The elution order is opposite that of reversed-phase columns. In biological systems, the degree of unsaturation usually increases with the increased number of carbon chain length on fatty acyl moieties (the major fatty acid species are FA 18:0, FA 20:4, FA 22:6 among others); therefore, many molecular species in biological samples co-elute when a reversed-phase column is used. As such, 2-EP columns provide favorable selectivity for resolving TAGs from crude mixtures from biological samples.

Fig. 4 MRM chromatograms of six regioisomeric TAG pairs in palm and canola oils. (a) TAG 18:0/18:1/18:0, (b) TAG 18:0/18:0/18:1, (c) TAG 18:0/18:1/16:0, (d) TAG 18:0/16:0/18:1, (e) TAG 18:0/18:3/16:0, (f) TAG 18:0/16:0/18:3, (g) TAG 16:0/18:1/16:0, (h) TAG 16:0/16:0/18:1, (i) TAG 16:0/18:2/16:0, (j) TAG 16:0/16:0/18:2, (k) TAG 16:0/18:3/16:0, (l) TAG 16:0/16:0/18:3. Reproduced from [15] with permission

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