Transfers and exchanges of esterified cholesterol between plasma lipoproteins.

Knowledge concerning the formation and transport of esterified cholesterol in plasma is fundamental to an understanding of the mechanisms by which cholesterol is delivered to peripheral tissues from the liver and removed from these tissues for recycling or excretion through the liver. Hepatic cholesterol enters the plasma in man predominantly in the unesterified, or free, form as a component of lipoproteins secreted by the liver. In the plasma most of this cholesterol is converted to cholesteryl esters which have their origin within the plasma high density lipoproteins (HDL) in a transesterification reaction catalysed by the enzyme lecithin:cholesterol acyltransferase (LCAT) (Glomset, 1968). The newly formed cholesteryl esters are subsequently incorporated into the hydrophobic core of lipoprotein particles in all plasma lipoprotein fractions. In the postabsorptive state, esterified cholesterol accounts for about 70% of the cholesterol in human plasma and is transported primarily in low density lipoproteins (LDL) (Goodman, 1965). In most other mammalian species, however, HDL are the major transport vehicle for plasma esterified cholesterol (Goodman, 1965). This process of esterification and incorporation of cholesteryl esters into the hydrophobic core of lipoproteins enables the delivery of cholesterol to peripheral tissues to be controlled by the regulated uptake of plasma lipoproteins by tissues. An esterification of plasma cholesterol is required also for the removal of cholesterol from peripheral tissues. Free cholesterol equilibrates rapidly between cell membranes and the surface monolayer of plasma lipoproteins (Hagerman & Gould, 1951). A continuing esterification of the free cholesterol on the surface (, plasma lipoproteins maintains a concentration gradient which can drive a net movement