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Low density lipoproteins (LDLs) are lipid storage molecules formed by the action of extrahepatic lipoprotein lipase on VLDL and intermediate density lipoprotein molecules. They have a half-life of days. About two-thirds to four-fifths of serum cholesterol is present in LDL.
LDL has relatively sparse triglyceride stores at only 6-8% of its mass. However, it is relatively rich in cholesterol and cholesterol esters at nearly 50% of its mass - it forms the main particle conveying cholesterol from the liver to extrahepatic tissue. Extrahepatic cells recognize LDL by the apoprotein B-100 on its surface. LDL is taken up by receptor-mediated endocytosis. The receptor is termed the LDL receptor and mutations of it lead to familial hypercholesterolaemia.
During transit, LDL interacts with HDL to obtain protein in order to increase stability. In return, LDL passes cholesterol esters to HDL.
Note that the heterogeneity in LDL particle composition, as a result of differences in the amount of cholesterol per LDL particle, means that measurement of LDL cholesterol is not equivalent to measurement of LDL-particles. This is significant because:
A low HDL cholesterol is a clue to the presence of cholesterol-depleted (small, dense) LDL.