The nine classes of apoproteins listed in the table are synthesized in the mucosal cells of the intestine and in the liver, with the liver accounting for about 80 percent of production.

Chylomicrons are synthesized in the intestinal mucosa. The cells of this tissue, although able to make most apoproteins, are the principal source of apoB (the B-48 form) and apoA-I. The apoC-II component of chylomicrons is an activator for a plasma enzyme that hydrolyzes the triglyceride of these complexes. This enzyme, called lipoprotein lipase, resides on the cell surface and makes the fatty acids of triglycerides available to the cell for energy metabolism. To some degree, the enzyme is also activated by apoC-II, present in minor amounts in chylomicrons.

VLDL, the lipoprotein carrier for triglycerides synthesized in the liver and destined for use in the heart and muscle, has a complement of five apoproteins. Among them are apoB-100, a protein performing a structural role in the complex, and apoC-I, -II, and -III. The first two of these activate the enzymes lecithin cholesterol acyltransferase (LCAT) and lipoprotein lipase. Curiously, apoC-III, a minor component of both chylomicrons and VLDL, inhibits lipoprotein lipase. Following discharge of the triglycerides, the remnants of VLDL return to the liver.

LDL contains a single apoprotein and is the principal carrier of cholesterol to the peripheral tissue as both the free sterol and esters. The discharge of the lipid contents of this complex requires the recognition of the LDL B-100 apoprotein by a receptor located on the surface of recipient cells. When the protein is bound to the receptor, the receptor-LDL complex is engulfed by the cell in a process known as endocytosis. The endocytosed LDL discharges its contents within the cell, and B-100 is degraded to free amino acids that are used to synthesize new proteins or are metabolized as an energy source. The elucidation of the process of cellular uptake of LDL by Michael Brown and Joseph Goldstein earned them the Nobel Prize for Physiology or Medicine in 1985.

The primary function of HDL with its complement of apoproteins is to take up cholesterol from the cells of the body and deliver it to the liver for its ultimate excretion as bile acids and salts. The major apoproteins are A-I, an LCAT activator, and A-II. All of the HDL apoproteins have their biosynthetic origin in the liver. When HDL is secreted by this organ, it is a small, flattened discoid devoid of cholesterol but containing phospholipids and the apoproteins. In the peripheral tissues, HDL picks up cholesterol from the surface membranes of cells and, through the agency LCAT, converts it into esters using acyl chains from phosphatidylcholine.

Cellular energy source

Fatty acids that are stored in adipose tissue as triglycerides are a major energy source in higher animals, as is glucose, a simple six-carbon carbohydrate. In healthy, well-fed humans only about 2 percent of the energy is derived from the metabolism of protein. Large amounts of lipids are stored in adipose tissue. In the average American male about 25 percent of body weight is fat, whereas only 1 percent is accounted for by glycogen (a polymer of glucose). In addition, the energy available to the body from oxidative metabolism of 1 gram of triglyceride is more than twice that produced by the oxidation of an equal weight of carbohydrate such as glycogen.