Enzymes involved in plasma cholesterol transport.

Regulation of plasma cholesterol transport is to a large extent a function of factors that regulate plasma cholesterol esterification and the transfers of cholesteryl esters between plasma lipoprotein fractions. Plasma cholesterol esterification is catalysed by the action of lecithin: cholesterol acyltransferase on lipids on the surface of HDL, while the transfers of cholesteryl esters require activity of a specific lipid transfer protein. Esterification of the cholesterol on the surface of HDL generates a concentration gradient down which unesterified cholesterol moves from tissues into the plasma. Once within the plasma and esterified, the newly formed cholesteryl esters are incorporated initially into the core of HDL particles before being redistributed to other classes of lipoproteins. The end result of these processes of esterification and transfer is that most of the cholesterol in human plasma is accommodated within the core of LDL, where its transport is a function of the highly regulated uptake by tissues of intact LDL particles. The capacity of HDL to act as substrates for lecithin: cholesterol acyltransferase varies inversely with HDL particle size. Thus, factors such as the concentration of triglyceride-rich lipoproteins and activities of the lipid transfer protein, hepatic lipase, lipoprotein lipase and the HDL conversion protein, which are known to influence HDL particle size, may also be important as regulators of plasma cholesterol esterification.