Lipids total, quantitation

Lipoproteins. The lipid moiety of lipoproteins is quite variable both qualitatively and quantitatively. The a-lipoprotein of serum contains glyceride, phosphatide and cholesterol to about 30 -40% of the total complex. The -lipoprotein of serum contains some glyceride but the phosphatide and cholesterol account for nearly 75% of the total. 

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. 

Phospholipids. The phospholipids comprise approximately 1 % of the total lipid in bovine milk (ca.0.3 to 0.4 g/liter). While quantitatively minor, the ability of the phospholipids to form stable colloidal suspensions or emulsions in aqueous solution cause them to be important in the formation and secretion of milk fat. (Long and Patton 1978 Patton and Keenan 1975). Their physical properties as bipolar molecules and their relatively high concentration of unsaturated fatty acids also make them an important factor to consider during the storage and... 

To quantitate total lipids, defined as the sum of the free and bound lipids, both polar and nonpolar, acid hydrolysis may be necessary to release the bound lipids by dissociating lipid-starch and lipid-protein intermolecular forces. The resultant lipids may then be removed and measured however, the nonlipid components so obtained are not usable for further analysis. Removal of some of the polar lipids may hinder the use of the extracted material for further analysis. 

Chromarod FID peaks of sterols, diglycerides, monoglycerides, and polar lipids are narrower and sharper than peaks of triglycerides and free fatty acids when analyzed using either method described in this unit (see Basic Protocol and Alternate Protocol). Hydrogenation of total lipids (see Support Protocol) results in much sharper and narrower peaks, which in turn substantially improves the resolution between lipid classes. The accuracy and precision in quantitating lipid classes of vegetable oils and animal fats are expected to be better than those from marine lipids. 

The accurate quantitation of minor lipid classes depends not only on the concentration of total lipids in the sample, but also on the profile of lipid classes in the sample. If one lipid class (e.g., triglycerides) dominates all others, it would be difficult to simultaneously quantify the minor lipid classes due to the overloading problem of the dominating lipid class. 

Extraction of fat by supercritical carbon dioxide was investigated as an important option for minimizing the expanded use of frequently flammable and carcinogenic solvents in food analysis. Unfortunately, the presence of moisture in foods has an adverse effect on the quantitative extraction of fat by supercritical fluid extraction (SEE). Hence, samples have to be lyophilized first. The total fat content of freeze-dried meat and oilseed samples was found to be comparable to values derived from Soxhlet-extracted samples (26). Besides, only small amounts of residual lipids could be recovered by an additional extraction of the SFE-extracted matrix by the Bligh and Dyer solvent extraction procedure. As far as the minor constituents are concerned, it was found that the extraction recovery ranged from 99% for PC to 88% for PA. Hence, Snyder et al. concluded that SFE can be used as a rapid, automated method to obtain total fat, including total phospholipids, from foods (26). 

Saito, T. Hakomori, S. Quantitative isolation of total qlycosphinqolipids from animal cells. J. Lipid Res.,1971, 12(2) 257-259. 

Using the new procedure, attempts were made to quantitate the cerebrosides located on the surface of myelin. Myelin is composed of multilamellar bilayers of membrane of approximately 70% lipid and 30% protein (16). About 20% of the total lipid consists of cerebroside and sulfatide. Because of the lipophilic nature of the ceramide moiety and the hydrophilic nature of galactose, it has been postulated that the galactose moiety of myelin cerebrosides is facing the surface while the ceramide moiety is buried within the bilayer. Even considering the multilamellar structure of myelin, at least several percent of the cerebrosides should be present on the myelin surface. The method described in this manuscript should allow us to determine surface cerebrosides to as little as 0.5% of the total cerebrosides. 

---