Lipid classes

Four Major Lipid Classes Are Present in Lipoproteins... 

The major lipid classes are phospholipids and cholesterol the major phospholipids are phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS) along with sphingomyelin (Sph). 

Some Selected Solvents Used for the Separation of Different Lipid Classes... 

Simple lipids such as CE, WE, EFA, cholesterol, alcohols, ketones, TG, DG, and MG are usually separated on silica gel plates. Depending on the complexity of the lipid material and the variety of lipid classes present in a single sample, either single-or multiple-solvent systems can be used (Figure 12.4a). Although benzene [45] or... 

FIGURE 12.4 (A) Diagrammatic representation of the separation of major simple lipid classes on silica gel TLC — solvent system hexane diethylether formic acid (80 20 2) (CE = cholesteryl esters, WE = wax esters, HC = hydrocarbon, EEA = free fatty acids, TG = triacylglycerol, CHO = cholesterol, DG = diacylglycerol, PL = phospholipids and other complex lipids). (B) Diagrammatic representation of the separation of major phospholipids on silica gel TLC — solvent sytem chloroform methanol water (70 30 3) (PA = phosphatidic acid, PE = phosphatidylethanolamine, PS = phosphatidylserine, PC = phosphatidylcholine, SPM = sphingomyelin, LPC = Lysophosphatidylcholine). 

After the extrachon of total lipids from four different genotypes of flax seed (Linum usitassimum) differing markedly in their acyl composihon, PTLC was used for the isolahon of different lipid classes in the neutral lipid frachon [69]. Application of planar chromatographic methods, including PTLC, in the separahon of food lipids has been reviewed with 40 references by Olsson [70]. The polar lipid fraction of niger seed (Guizotia abyssinica Cass.) collected from different regions of Ethiopia could be separated by PTLC on silica gel [71]. 

PTLC was also used for the separation of lipid components in pathogenic bacteria. Mycobacterium avium has a requirement for fatty acids, which can be fulfilled by palmitic or oleic acid, and these fatty acids are then incorporated into triagylglycerols [80]. PTLC was used for the separation of fatty acids and triacylglycerols in the extracts of these bacterial cells to study the lipid classes in the bacterial cells cultured under different growth conditions. 

After the extraction of lipid and nonlipid components from the leaves of mandarin orange Citrus reticulata, the lipid fraction was further separated by PTLC to determine different lipid classes that affect the chemical deterrence of C. reticulata to the leaf cutting ecat Acromyrmex octopinosus. These lipids seem to be less attractive to the ants [81a]. The metabolism of palmitate in the peripheral nerves of normal and Trembler mice was studied, and the polar lipid fraction purified by PTLC was used to determine the fatty acid composition. It was found that the fatty acid composition of the polar fraction was abnormal, correlating with the decreased overall palmitate elongation and severely decreased synthesis of saturated long-chain fatty acids (in mutant nerves) [81b]. 

Kramer, S. D., Hurley, J. A., Abbott, N. J., Begley, D. J., Lipids in blood-brain barrier models in vitro I TLC and HPLC for the analysis of lipid classes and long polyunsaturated fatty adds, submitted. 

This technique was used by Delmas et al. [404] to separate lipid extracts in seawater into various classes. Lipid classes that have been eluted away from the point of application may be burnt off the rod in a partial scan, allowing those lipids remaining near the origin to be developed into the place that has just been simultaneously scanned and reactivated. By analysis of complex mixtures of neutral lipids in this stepwise manner it is possible to be more selective about lipid class separations as well as to be more confident about assigning identities to peaks obtained from a seawater sample. In addition, this approach also reduces the possibility of peak contamination by impurities which would normally coelute with marine lipid classes (e.g., phthalate esters [403]). 

Table 9.2. Seawater lipid classes and standards used for their identification and calibration... 

Figure 9.4. Shipboard analysis of particulate lipid classes on the same Chromarod. a Nonpolar neutral lipids and internal standard, partial scan, b Remaining neutral lipids and polar lipids, full scan, increased attenuation at X. Source [404]...

Recently, a quantitative electrospray ionization/mass spectrometry method (ESI/MS) has been developed to analyze the molecular profile, or hpidome of different lipid classes in very small samples. In this method, total lipid extracts from tissues or cultured cells can be directly analyzed. By manipulating the ionization method, the mass spectrographs of polar or even non-polar lipids can be obtained [8]. This method and the use of lipid arrays allow precise and quantitative identification of the lipid profile of a given tissue, and map functional changes that occur. 

The partition of different lipids between two immiscible solvents (countercurrent distribution) is useful for crude fractionation of lipid classes with greatly differing polarities. Repeated extractions in a carefully chosen solvent pair increase the effectiveness of the separation but in practice mixtures of lipids are still found in each fraction. A petroleum ether-ethanol-water system can be used to remove polar contaminants (into the alcoholic phase) when interest lies in the subsequent analysis of neutral glycerides, which may be recovered from the ether phase. Carbon... 

Table 12.9 Lipid classes in order of increasing polarity...

Rhodamine 6G (0.1 g l-1 in water) Spray or dip plate and view under UV light A versatile stain for all lipid classes. Pink-purple-red spots. Most useful with alkaline solvents. The dye may be incorporated in the plate... 

Garcia-Fuentes, E., A. Gil-Villarino, M. F. Zafra, and E. Garcia-Peregrin. Differential changes in the fatty acid composition of the main lipid classes of chick plasma induced by dietary coconut oil. Comp Biochem Physiol B Biochem Mol Biol 2002 133(2) 269-275. 

Chen, X., and B. R. T. Simoneit, Epicuticular Waxes from Vascular Plants and Particles in the Lower Troposphere Analysis of Lipid Classes by Iatroscan Thin-Layer Chromatography with Flame Ionization Detection, . /. Atmos. Chem., 18, 17-31 (1994). 

Lipids Class of biomolecules that includes fats and oils all lipids are insoluble in water and similar substances. 

To facilitate the reader, the nomenclature, structure and properties of the principal fatty acids and of the principal lipid classes are summarized in Appendices 3A, 3B and 3C. The structure and properties of the fat-soluble vitamins, A, D, E and K, are discussed in Chapter 6. 

Droplets of different density and lipid protein ratios ranging from about 1.5 1 to 40 1 have been isolated from bovine mammary gland. Triglycerides are the major lipid class in droplets of all sizes and represent increasingly greater proportions of total droplet mass in increasingly less dense droplet preparations. Surface coat material of droplets contains cholesterol and the major phospholipid classes found in milk, i.e. sphingomyelin, phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and phosphatidyl-serine. 

Milk lipids have attracted the interest of and have frustrated investigators. The lipids are readily available, for example in butter, but are exceptionally complex, both with respect to lipid classes and to component fatty acids. Furthermore, the latter have been difficult to analyze because of the shortchain fatty acids present and the large number of fatty acids in general. Jenness and Patton (1959) listed 16 fatty acids found in milk lipids. The list had grown to about 150 by 1967 (Jensen et al. 1967) and is now over 400 (Table 4.1). 

Column and thin-layer chromatography (TLC) came into use at about the same time as GLC, with the latter widely accepted because of its speed, ease of use, versatility, resolving power, and, probably most important, ease of visualization. Thin-layer chromatography has been particularly useful in the separation and nondestructive recovery of lipid classes. Tentative identifications can be made by comparison with known compounds, and purity can be checked. Jensen et al. (1961) may well have been the first group to separate milk lipid classes with TLC when they used the technique to obtain diacylglycerols from lipo-lyzed milk lipids. 

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