HDL is synthesised in the liver and intestines and secreted as bilayers of apo AI, apo AII and phospholipid. HDL acquires cholesterol from two major sources. During the catabolic pathways described earlier, lipoprotein surface components (particularly unesterified cholesterol) that become redundant as the particle size decreases are transferred to HDL. Additionally, excess cellular cholesterol is picked up by lipophilic particles. The cholesterol is subsequently esterified by the enzyme lecithin-cholesterol acyltransferase (LCAT). The hydrophobic cholesterol ester that results moves into the core of the HDL particle, leading to the formation of a spherical particle (Fig. 6.7).
