| THE FREE AND ESTERIFIED CHOLESTEROL
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| Its structure gives cholesterol a low solubility in water. Only about 30% of circulating cholesterol occurs in the free form, the majority is esterified through the hydroxyl group to a wide range of long-chain fatty acids including oleic and linoleic acids. Cholesterol esters are even less soluble in water than free cholesterol and so it is perhaps surprising to discover cholesterol circulating in plasma in concentrations of about 5 mmol/L (200 mg/dL). The apparent paradox is explained by the presence of a range of lipoproteins which incorporate and thereby solubilize the cholesterol molecule (see Chapter 17). Within these lipoproteins, the hydrophobic cholesterol esters are located in the core of the molecule, with free cholesterol in the outside layer.
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| Figure 16.1 Structure of cholesterol. A-D is the conventional notation used to describe the four rings. Numbers 1-27 describe the carbon atoms. |
| The dietary cholesterol brought to the liver is mostly in the form of free cholesterol. On the other hand, the cholesterol present in VLDL and LDL is mostly in the form of cholesteryl esters. Esters are also the tissue storage form of cholesterol
(they are stored in lipid droplets). In the plasma, cholesterol is esterified by the enzyme cholesterol-lecithin acyltransferase (Chapter 17) and in the cells by the acyl-CoA: cholesterol acyltransferase (ACAT, now renamed SOAT) by reacting with fatty acyl-CoA. There are two isoforms of the ACAT present in the endoplasmic reticulum. Sixty to eighty percent of cholesteryl esters present in plasma are taken up by the liver.
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