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Cholesterol Total, Free and Esterfied
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Free cholesterol is a measure of the circulating cholesterol that has not been esterified. Free cholesterol accounts for about 30% of the total cholesterol in plasma. Cholesterol esterification occurs before or after it is released into the circulation and esterification with any physiologically relevant fatty acid enhances the lipid-carrying capacity of lipoproteins and prevents intracellular toxicity by free cholesterol.
After its synthesis or uptake by the liver, cholesterol is incorporated into very low-density lipoprotein (VLDL) and released into the circulation. Lecithin:cholesterol acyltransferase (LCAT) transfers a fatty acid from position C-2 of phosphatidyl choline to the C-3 hydroxyl group in cholesterol. LCAT is responsible for nearly all of the cholesterol ester that in the circulation. It is synthesized by the liver and is primarily activated by Apolipoprotein A-I. It is important to note that LCAT activity continues to be expressed after a blood sample is drawn. Hence, if cholesterol ester is to be measured, the serum or plasma should be cooled to 4 °C or frozen as soon as possible. A small amount of cholesterol ester is synthesized within the cell. This reaction is catalyzed by acylcholesterol acyltransferase (ACAT) within the cell, which is driven by the conversion of ATP to AMP. Thus, the ACAT pathway to cholesterol ester requires energy while the extracellular LCAT pathway does not.
Measurement of free cholesterol may be used in conjunction with determination of total cholesterol and other lipid and lipoprotein measurements, to assess a patient’s lipoprotein status.
The free cholesterol assay is an automated enzymatic assay. Esterified cholesterol can be determined by measuring total cholesterol and subtracting the free cholesterol measurement.