Classification of Serum Lipoproteins According to Their Apolipoprotein Composition

Recent interest on the study of lipoprotein subfractions has resulted in an increased use of methods of separation based on affinity chromatography, specially those using immunoaffinity. By using columns containing antibodies against specific apolipopro-teins (Table 2), a large number of HDL subpopulations have been resolved. Similarly, this technique

Table 2 Classification and properties of apolipoproteins

Apolipoprotein Amino acids Tissue expression Chromosomal localization Functions apo A-I

apo A-II apo A-IV

apo B-48

apo B-100 apo C-I

apo C-II

apo C-III

apo D

apo E

77 377

2152

4536 57

Variable

Liver Intestine

Liver

Intestine Liver

Intestine

Liver

Liver

Intestine

Liver

Intestine

Liver

Intestine

Most tissues

Liver

Macrophage Liver

1 11

2 19

Major structural component of HDL

Ligand for HDL binding

Activator of LCAT

Reverse cholesterol transport

Structural component of HDL

Activator of hepatic lipase

Regulator of LPL activity

Activator of LCAT

Intestinal lipid absorption

Structural component of TRL

Secretion of chylomicrons

Structural

Activator of LCAT

Inhibitor of the LRP

Activator of LPL

Inhibits LPL

Radical scavenger Reverse cholesterol transport Binding of haem-related compounds Ligand for the LDL receptor Ligand for the LRP

Reverse cholesterol transport ?

allows the separation of several triacylglycerol-rich lipoproteins subfractions.

Lipoproteins containing apo A-I can be separated into two major species: those containing both apo A-I and apo A-II, known as LpAI:AII, and those containing apo A-I but not apo A-II (LpAI). Small numbers of particles containing apo A-II, but not apo A-I, have been detected in normal subjects; however, these particles could become predominant in the presence of rare genetic disorders associated with HDL deficiency. Another HDL species containing apo A-I and apo E is important in reverse cholesterol transport by transporting cholesterol from the cell membranes to the liver for elimination from the body.

Lipoproteins containing apo B consist of four lipoprotein families. Lipoproteins containing apo B only (Lp(B)) are cholesteryl ester-rich and are found primarily within the LDL density range, but they have also been detected within the VLDL range. Particles containing both apo B and apo C (LpB:C), apo B and apo E (LpB:E), and all three apolipoprotein groups (LpB:E:C), are triacylgly-cerol-rich and are found within the VLDL and IDL density range. The apo C and apo E content decreases as density increases.

More recently, the affinity for lectins of Lp(a), a lipoprotein containing apo B-100 as well as an anti-genically unique apolipoprotein [apo(a)], has been used to develop a new technique to measure the levels of this lipoprotein in plasma.

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