Plasma lipoproteins are uniquely endowed with the ability to transport large quantities of water-insoluble lipids through an aqueous environment. This because the non-polar lipids (triglyceride and cholesterol ester; Fig. 2) are "buried" in the core of the lipoprotein, surrounded by a monolayer of amphipathic lipids, phospholipid, and unes-terified cholesterol (Fig. 3).

In addition to core and surface lipids, lipoproteins carry proteins termed apolipoproteins (Tables III and IV). These proteins stabilize the lipoprotein particles and carry out particular functions such as receptor recognition and activation of particular enzymes.

Lipids are less dense than water. Consequently, because they are complexed with lipid, plasma lipoproteins tend to float when plasma is subjected to ultracentrifugation (Table I). In contrast, other blood proteins sediment in the centrifuge. Lipoproteins float at distinct buoyant densities and are named according to their flotation behavior. The lipoprotein classes are (Table II) chylomicrons, very-low density lipoprotein (VLDL), low density lipoprotein (LDL), and high density lipoprotein (HDL). Chylomicrons and VLDL are primarily triglyceride carriers, while LDL and HDL are primarily cholesterol (mostly cholesterol ester) carriers.

Key Points about the Distribution of Apolipoproteins

• Only HDL has no apo-B.

• For practical purposes LDL can be considered to have only apo-B.

• The other apolipoproteins overlap and, as described below, readily transfer between the various lipoprotein particles while circulating in the bloodstream.

It is useful to make a distinction between exogenous and endogenous lipid transport. Exogenous lipid transport refers to dietary fat. Endogenous lipids are those synthesized by the liver and adipose tissue from substrates that have already been absorbed and metabolized in these tissues. The exogenous pathway refers to the absorption of lipids in the intestine and the metabolism of chylomicrons. Lipids from chylomicrons can eventually mix with the endogenous lipid pools in the liver and adipose tissue.

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