Plasma lipoprotein classification

Plasma lipoproteins are generally classified by their density and separation achieved with ultracentrifugation. According to this density-based classification system, the major lipoprotein classes are chylomicrons (CH), very low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). 

Cholesterol, which is largely insoluble in aqueous m a, travels through the blood circulation in the form of Upoprotein complexes. The plasma lipoproteins are a family of globular particles that share common structural features. A core of hydrophobic lipid, principally triacylglycerols (triglycerides) and cholesterol esters, is surrounded by a hydrophilic monolayer of phospholipid and protein (the apolipoproteins) [1-3]. Lipid-apolipoprotein interactions, facihtated byi amphi-pathic protein helices that segregate polar from nonpolar surfaces [2,3], provide the mechanism by which cholesterol can circulate in a soluble form. In addition, the apolipoproteins modulate the activities of certain enzymes involved in Upoprotein metabolism and interact with specific cell surface receptors which take up Upopro-teins by receptor-mediated endocytosis. Differences in the Upid and apoUpoprotein compositions of plasma Upoproteins determine their target sites and classification based on buoyant density. 

Table 30.1. Classification and Characteristics of Major Plasma Lipoproteins ...

The compositional and metabolic heterogeneity of operationally defined plasma lipoproteins necessitates the introduction of a classification system based on apolipoproteins as specific markers for identifying discrete lipoprotein families or particles. According to the chemical classification system, plasma lipoproteins consist of discrete simple and complex lipoproteins. Simple lipoproteins contain a single apolipoprotein, whereas complex lipoproteins contain two or more apolipoproteins. 

Classification of Hyperlipemic Disorders by Electrophoretic Separation of Plasma Lipoproteins [36]. 

A classification of the hyperlipoproteinaemias based on the electrophoretic behaviour of the plasma lipoproteins. 

All lipoprotein classes contain proteins, free and esterified cholesterol, TG and phospholipids however, the relative proportion of any component varies so that protein and phospholipid percentages are higher in a-lipoproteins (high-density lipoproteins) and lower in chylomicrons. The reverse is true for TG, while cholesterol circulates mainly as -lipoprotein (low-density lipoprotein). Since lipids circulate as lipoproteins, hyperlipaemias can be more properly defined as hyper-lipoproteinaemias. A classification of human hyperlipoproteinaemias based on chemical determination of plasma lipid classes as well as on paper electrophoretic separation of plasma lipoproteins has been proposed... 

Alaupovic, P., 1972, Classification system of plasma lipoproteins, in "Protides of the Biological Fluids", H. Peeters, ed., Pergamon Press, Oxford, p. 9. 

The classification of hyperUpidaemias is confusing as there are two means of classification - the Fredrickson based on the pattern of plasma lipoproteins in each condition, and the Goldstein based on the underlying enzyme defects. Type I hyperlipidaemia (Fredrickson) is a lipoprotein lipase or apolipoprotein-C2 deficiency. It leads to massive Hpid deposition and presents in childhood. Type Da hyperlipidaemia (familial hypercholesterolaemia) is a severe illness, not to be confused with common hypercholesterolaemia. Familial hypercholesterolaemia presents in childhood with deposits of cholesterol in the skin and ischaemic heart disease in adolescence. 

Koch et al. determined total cholesterol, phospholipids, and fatty acids in CSF samples from 216 individuals in order to establish the lipid and apo-lipoprotein levels in Cerebrospinal Fluid (CSF) in a large group of individuals, on the basis of which a classification of CSF lipoproteins was made. The cholesterol and phospholipids are measured enzymatically by fluorometric detection of the reaction products. Earlier work had shown reduced levels of cholesterol, phospholipids, and free fatty acids in Cerebrospinal Fluid (CSF) of Alzheimer disease patients. Urine levels of F2-isoprostanes or their major metabolite were not significantly different between Alzheimer s disease patients and controls. In addition, urine and CSF F2-isoprostane levels in Alzheimer s disease patients did not correlate. These results indicate that plasma and urine F2-isoprostanes and F2-neuroprostanes do not accurately reflect central nervous system levels of these biomarkers and are not reproducibly elevated in body fluids outside of the central nervous system in Alzheimer s disease patients. 

The lipoproteins present in plasma may be operationally defined according to their density, as low or very low density lipoproteins and high density lipoproteins, but other, more functional definitions may be more appropriate in connection with in vivo metabolism. Examples are the hpoprotein classes chylomicrons and chylomicron remnants. Alaupovic has suggested a classification based on the apolipoprotein composition [1]. As apolipoproteins often determine the metabolic fate of lipoprotein particles this is a logical approach. Lipoprotein particles with specific apolipoprotein compositions exist in all density fractions of human plasma, as well as plasma from a variety of animal species. 

This grouping is possibly not homogenous and may subsequently be subdivided. It includes subjects who have been described as having mixed hyperlipemia by Kuo and Basset (1963), calorie induced hyperlipemia by Kin sell and Schlierf (1965) and probably type V of Fredrickson s and Lees classification (1965). Whatever the homogeneity, it appears that these individuals accumulate very low density lipoproteins in plasma under essentially all dietary conditions except absolutely or relatively low calorie intake, and in some except during the administration of insulin. 

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