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Apolipoproteins are a class of apoproteins, which are proteins that depend on the presence of other small molecules, or cofactors, to function. Thus, apolipoproteins are the protein constituents of lipoproteins, which also consist of phospholipids, triacylglycerols, cholesterol, and cholesterol esters. There are five major types of apolipoproteins: A, B, C, D, and E. Apo E is perhaps the most well studied.
The Apo E protein is 299 amino acids long, and a core apoprotein of the chylomicron, which transports lipoproteins, fat-soluble vitamins, and cholesterol into the lymph system and then into the blood. Apo E is synthesized principally in the liver, but has also been found in other tissues such as the brain, kidneys, and spleen. Apo E was initially recognized for its importance in lipoprotein metabolism and cardiovascular disease. More recently, it has been studied for its role in several biological processes not directly related to lipoprotein transport, including Alzheimer's disease, immunoregulation, and cognition.
The apo E gene, which encodes the Apo E protein,is located on chromosome 19, and consists of four exons and three introns totaling 3597 base pairs. The gene is polymorphic, with three major alleles, apo E-3, apo E-2, and apo E-4, which translate into three isoforms of the protein: E3 (normal), and E2 and E4 (dysfunctional). These isoforms differ from each other only by single amino acid substitutions at positions 112 and 158, but have profound physiological consequences. Apo E2 is associated with the genetic disorder type III hyperlipoproteinemia and with both increased and decreased risk for atherosclerosis. Apo E4 has been implicated in atherosclerosis and Alzheimer's disease, impaired cognitive function, and reducedneurite outgrowth.
Apolipoprotein synthesis is affected by the addition of Nicotinic Acid (a form of the vitamin niacin). It can be prescribed in large doses (2-6g/day) to lower total triglycerides, LDL and total Cholesterol while increasing HDL levels. This is not to be confused with nicotinamide (the form of niacin in supplements).
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