Classification of Serum Lipoproteins According to Their Ultracentrifugal Characteristics

The presence of lipids within the lipoprotein particles confers these macromolecular complexes with a lower density compared with other serum proteins. With the arrival of the analytical ultracentrifugation in the 1940s, this characteristic allowed its initial separation as a discrete peak using this technique. During the following years, it was demonstrated that this fraction was made up of a wide spectrum of particle sizes and densities (d) ranging from 0.92 to 1.21 gml-1.

Lipoproteins were classically separated into four major classes designated as chylomicrons (exogenous triacylglycerol-rich particles of d <0.94 gml-1), very low-density lipoproteins (VLDL, endogenous triacyl-glycerol-rich particles of d = 0.94-1.006 g ml-1), LDL

(cholesteryl ester-rich particles of d = 1.0061.063 gml-1), and HDL (particles containing approximately 50% protein of d = 1.0631.21 g ml-1). With subsequent improvements to the ultracentrifugation techniques, further heterogeneity was detected within each of those major lipoprotein classes; this resulted in the need for further subdivision into several density subclasses such as HDL2a (d = 1.10-1.125gml-1), HDL2b (d = 1.063-1.10gml-1), and HDL3 (d = 1.125-1.21 gml-1).

There is no doubt that the separation of lipopro-teins by ultracentrifugation has been esential for the advances in this field; however, this technique is very labor intensive and the isolated lipoproteins are usually modified due to the high g force and salt concentrations used in this process. The development of new vertical and near vertical rotors has shortened considerably the runs and thus diminished some of these negative effects.

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