High Density Lipoproteins

HDL consist of a series of lipoprotein particles of relatively high density, all of which contain apo A-I. A proportion of HDL particles also contain apo A-II. Some HDL species (HDL3) are denser than others (HDL2). HDL particles are composed largely of by-products of catabolism of TAG-rich lipopro-teins. The surface coats of HDL particles contain phospholipids and unesterified cholesterol, apo A-I with or without apo A-II, and other apolipoproteins (apo Cs and apo Es). Their particle cores consist largely of cholesterol esters, although small amounts of TAG are also present. The cholesterol esters of HDL are formed by esterification with a fatty acid through the action of an enzyme, lecithin cholesterol acyl transferase (LCAT); the substrates for this reaction derive either from unesterified cholesterol released during lipolysis of TAG-rich lipoproteins or from the surface of peripheral cells. After ester-ification of cholesterol, the cholesterol esters of HDL are transferred back to TAG-rich lipoproteins and eventually are removed by the liver through direct uptake of remnant lipoproteins or LDL. Whether whole HDL particles can be directly removed from the circulation is uncertain. Some investigators believe that the HDL components are dismantled and removed piecemeal.

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