Plasma lipids, lipoproteins and cell membranes.

The major plasma lipids, cholesteryl ester, triacylglycerol, cholesterol and phospholipid, are insoluble in water. They are transported in the circulation as lipoproteins-metabolically diverse macromolecules in which the various lipids are associated with specific polypeptides (the apoproteins). There are four main lipoprotein classes, defined by their size and by the density range within which they float in the ultracentri uge: chylomicrons, very-low-density (VLDL), low-density (LDL) and high-density (HDL) lipoproteins (Jackson et al., 1976). The individual lipoproteins vary widely in size, and in lipid and apoprotein composition, but in normal subjects appear to have a similar structure. They are roughly spherical particles with a hydrophobic core of mainly triacylglycerol and cholesteryl ester. Their surface is covered by a single layer of amphipathic molecules (phospholipid, cholesterol and one or more varieties of apoproteins) with small amounts of cholesteryl ester and triacylglycerol also present. Non-covalent mainly hydrophobic forces stabilize the lipoproteins, but exchange and transfer of their constituent lipids and apoproteins can occur during metabolism. The major physiological function of lipoproteins is transport of lipid for delivery to cells and tissues (Owen & McIntyre, 1982). But lipoproteins also affect cellular metabolism in two other ways: they help maintain a normal cell membrane lipid composition and they regulate certain cellular processes (Owen, 198 1).