Cholesteryl ester and triglycerides are nonpolar and constitute the hydrophobic core of lipoproteins in varying proportions. Cholesterol is used for the synthesis of bile acids in the liver, the manufacture and repair of cell membranes, and the synthesis of steroid hormones. Triglycerides, which are nonpolar lipids consisting of a glycerol backbone and three fatty acids of varying length and degrees of saturation, are used for storage in adipose tissue and for energy. They are found in liver and adipose cells and also derive from dietary and hepatic sources. The reverse cholesterol transport system, mediated by HDL, is involved in both pathways.
Triglycerides greatly predominate over cholesteryl ester in the core.
The resulting free fatty acids are taken up by adipose tissue for storage and by muscle for energy. During lipolysis, the chylomicron shrinks, and surface components become pinched off and are transferred to HDL in exchange for additional apolipoproteins and cholesteryl ester. Normally, chylomicrons are present in the plasma for only a matter of minutes. Once inside the hepatocyte, the remnant particles are digested by lysosomal enzymes, and the free cholesterol is reesterified and stored. Intracellular cholesteryl ester is then used for bile acid synthesis, cell membrane synthesis, or the synthesis of lipoproteins in the endogenous pathway.