Apolipoprotein

Apolipoproteins are lipid-binding proteins which are the constituents of the plasma lipoproteins, sub-microscopic spherical particles that transport dietary lipids through the bloodstream from the intestine to the liver, and endogenously synthesized lipids from the liver to tissues that can store them (adipocytes), metabolize them (muscle, heart, lung), or secrete them (breast).

The amphipathic (detergent-like) properties of apolipoproteins solubilize the hydrophobic lipid constituents of lipoproteins, but apolipoproteins also serve as enzyme co-factors, receptor ligands, and lipid transfer carriers that regulate the intravascular metabolism of lipoproteins and their ultimate tissue uptake.

Classes
There are five major classes of apolipoproteins, and several sub-classes:
 * A (apo A-I, apo A-II, apo A-IV, and apo A-V)
 * B (apo B48 and apo B100)
 * C (apo C-I, apo C-II, apo C-III, and apo C-IV)
 * D
 * E

Hundreds of genetic polymorphisms of the apolipoproteins have been described, and many of them alter their structure and function.

Synthesis and regulation
Apolipoprotein synthesis in the intestine is regulated principally by the fat content of the diet.

Apolipoprotein synthesis in the liver is controlled by a host of factors, including dietary composition, hormones (insulin, glucagon, thyroxin, estrogens, androgens), alcohol intake, and various drugs (statins, niacin,and fibric acids).