Apolipoprotein E, Human Plasma

Apolipoprotein E serves as a ligand for low density receptors and participates in the transport and redistribution of cholesterol and other lipids. Other functions include immunoregulation and cell growth modulation and differentiation. Apo E is thought to be involved in tissue repair as increased amounts of the protein are found at sites of peripheral nerve injury and regeneration. A mutant form is associated with familial type III hyperlipoproteinemia. The concentration of Apo E in normal plasma is 5 mg per 100 ml. In addition to facilitating solubilization of lipids, these proteins help to maintain the structural integrity of lipoproteins, serve as ligands for lipoprotein receptors, and regulate the activity of enzymes involved in lipid metabolism. Significant quantities of ApoE are produced in liver and brain and to some extent in almost every organ. ApoE is an important constituent of all plasma lipoproteins. It’s interaction with specific ApoE receptor enables uptake of chylomicron remnants by liver cells, which is an essential step during normal lipid metabolism. It also binds with the LDL receptor (apo B/E). Defects in ApoE are a cause of hyperlipoproteinemia type III. ApoE exists in three major isoforms; E2, E3, and E4, which differ from one another by a single amino-acid substitution.