Nomenclature and General Structure of Lipoproteins

Hydrophobic lipids (triacylglycerols and cholesteryl esters) are virtually completely insoluble in water they are solubilized for transport in plasma by incorporation into lipoproteins. Lipoproteins are spherical complexes containing triacylglycerol (triglyceride) and cholesteryl ester surrounded by a layer containing phospholipids, unesterified cholesterol, and specific apolipoproteins. 

This general model is now universally accepted, though there are minor reservations [e.g., both cholesteryl ester (S39) and triglyceride (H4) have limited solubility in phospholipid, and hence a fraction of these hydrophobic core lipids is probably found in the outer coat of lipoprotein lipid]. 

It should also be remembered that plasma lipoproteins are constantly in a state of metabolic flux. Some large particles are converted to smaller particles, and vice versa. Lipids and most apolipoproteins exchange or are transferred between particles and particles are released into and removed from the circulation. 

Lipoproteins interact with enzymes (e.g., lecithin cholesterol acyltrans-ferase, or lipases), with lipid transfer proteins, or with receptors on cell surfaces. The composition of a lipoprotein class depends upon the results of these kinds of interactions. Ultracentrifugation itself may result in minor changes in lipoprotein composition (see Sections 6 and 6.1). 

Chylomicrons d 0.95 g/ml very low-density lipoproteins (VLDL) d = 0.95-1.006 g/ml intermediate-density lipoproteins (IDL) d= 1.006-1.019 g/ml low-density lipoproteins (LDL) d = 1.019-1.063 g/ml high-density lipoprotein subfraction 2 (HDLa) d= 1.063-1.125 g/ml high-density lipoprotein subtraction 3 (HDL3) d = 1.125-1.21 g/ml very high-density lipoproteins (VHDL) d 1.21 g/ml. 

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