This chapter highlights high-density lipoproteins (HDL) metabolism in humans. HDL concentration in plasma is expressed as HDL cholesterol that remains in the supernatant after precipitation of the apolipoprotein (apo)B-containing lipoproteins and represents a minor fraction of cholesterol in plasma. HDL particles have a spherical shape and possess a core of neutral lipids comprising mostly of cholesteryl esters. Variable amounts of triglycerides are present in the core, depending on the plasma triglyceride levels. The hydrophobic core is surrounded by a surface monolayer consisting of phospholipids, cholesterol, and apolipoproteins. HDL particles are heterogeneous with respect to size, composition, and density. The two major apolipoproteins in HDL are apoA-I and apoA-II. The primary translation product of human apoA-I mRNA possesses an additional peptide that contains 18 amino acids that is cleaved intracellularly by a signal peptidase of the endoplasmic reticulum leaving pro-apoA. The apoA-I gene contains six homologous tandemly repeated 66 bp regions indicating internal gene duplication.
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