Apolipoprotein

Apolipoproteins are proteins that bind to lipids (oil-soluble substances such as fat and cholesterol) to form lipoproteins, which transport the lipids through the lymphatic and circulatory systems.

The lipid components of lipoproteins are not soluble in water; however, because of their detergent-like (amphipathic) properties, apolipoproteins and other amphipathic molecules (such as phospholipids) can surround the lipids, creating the lipoprotein particle that is itself water-soluble, and can thus be carried through water-based circulation (i.e., blood, lymph).

Apolipoproteins also serve as enzyme cofactors, receptor ligands, and lipid transfer carriers that regulate the metabolism of lipoproteins and their uptake in tissues.

Functions

 * They are enzyme coenzymes (C-II for lipoprotein lipase and A-I for lecithin-cholesterol acyltransferase)
 * Lipid transport proteins
 * Ligands for interaction with lipoprotein receptors in tissues ( apoB100 and apoE for LDL-receptors, apoA-I for HDL receptors)

Classes
There are six 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
 * H

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). Apo B is an integral apoprotein whereas the others are peripheral apoproteins.