In the liver, the constituent lipids of chylomicron remnants are repackaged into HDL and VLDL particles (mainly the latter), which are released into the circulation. There is no direct synthesis of LDL in the liver. VLDL contain high concentrations of triglycerides and moderate concentrations of cholesteryl esters and phospholipids. The triglycerides are removed from VLDL by the action of lipoprotein lipase located on the endothelial surface of blood vessels in extrahe-patic tissues, enabling tissue uptake of free fatty acids. When much of the triglyceride has been removed, the VLDL become IDL. Newly released HDL contain a high concentration of protein (about 50 per cent), but very little cholesteryl ester. Circulating HDL take up the excess cholesterol released from cells into the plasma, converting it into cholesteryl esters by the action of lecithin:cholesterol acyltransferase. Much of the HDL cholesteryl ester is transferred to IDL which then become LDL. LDL is the major cholesterol-carrying lipoprotein and high levels of these particles in the circulation are associated with an increased risk of heart disease.
HDL participates in reverse cholesterol transport by acquiring cholesterol from tissues and other lipo-proteins and transferring it to the liver for excretion. Elevated HDL levels in the circulation are associated with reduced risk of heart disease.
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