Fractionation into Main Lipid Classes

Separation of a mixture of different lipids into its components can be achieved by several chromatographic procedures. Older methods, e.g. solvent fractionation. 

FFA = free fatty acids, CH = cholesterol, DO = diglycerides, MO = monoglycerides, PL = phospholipids 

Silicic acid is the adsorbent most frequently used for column chromatography of lipids. It was introduced by Borgstrom (1952) with hexane-benzene mixtures as eluting solvents. Fillerup and Mead (1953) used silicic acid with mixtures of diethyl ether-petroleum ether and hexane-benzene respectively. Reproducible results are obtained with the standardized method of Hirsch and Ahrens (1958). 

Separation of the lipids is 200r achieved by either stepwise or mq gradient elution. With unpolar solvents such as petroleum ether or hexane hydrocarbons are first eluted. By addition of various amounts of ether or benzene cholesterol esters, triglycerides, free fatty acids, free cholesterol, di-and monoglycerides are eluted in this order. Phospholipids are adsorbed stronger and can be eluted with a polar solvent such as methanol (figure 1). 

Fractionation of phospholipids by column chromatography is more difficult than the separation of neutral lipids. By using chloroform methanol mixtures and increasing the concentration of methanol up to 100% some fractionation is possible, although each fraction is still contaminated by other phospholipids (Hirsch and Ahrens 1958, Zollner and Kirsch 1960). 

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The choice of methods for separation of lipid classes depends on the desired degree of separation. Simpler methods permit concentration or separation of only a few fractions, whereas newer methods based mainly on chromatographic procedures permit fractionation into all main lipid classes. The selection is further dependent on the amoimts of material available. 

Complexes of lipid and protein which transport lipids in the blood. There are two main systms of nomenclature, one based on their electrophoretic mobility, the other on their behaviour during ultracentrifugation. These class the lipoproteins into four fractions ... 

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