Apolipoproteins

Apolipoproteins are plasma lipoproteins that consist of a core of triglycerides and cholesterol esters and plays an important role in lipid transport & distribution in various cells and tissues and lipid metabolism (functions as cofactors for enzymes). They also maintain the structural integrity of lipoprotein complex and facilitate the lipoprotein uptake by acting as ligands for specific cell surface receptors. Apolipoproteins are of different types and has different roles which are as follows:

1.  Apolipoprotein A (ApoA): ApoA is the major protein component (70%) of HDL (high density lipoprotein) the "good cholesterol" which promotes cholesterol efflux from tissues to the liver for excretion and plays an important role in reverse cholesterol transport. These lipoproteins also function as a cofactor for lecithin cholesterolacyltransferase  LCAT (enzyme responsible for plasma cholesterol esters formation and in HDL catabolism) [1]. ApoA concentrations are inversely linked to coronary artery disease (CAT)/cardiovascular disease (CVD).

2.  Apolipoprotein B (ApoB): ApoB is the main protein component of LDL (low density lipoprotein) the "bad cholesterol", VLDL (very low density lipoprotein), IDL (intermediate density lipoprotein) and chylomicrons (a type of lipoprotein which carries fat and cholesterol to blood and also helps to absorb vit A and vit E). ApoB are mainly of 2 types: ApoB100 which produced in liver and ApoB48 which is produced in intestine [2]. ApoB100 is an important lipoprotein that is involved in atherosclerosis (chronic inflammation in arterial wall of heart mainly results from lipid metabolism disorder) and cardiovascular disease (CAD). Increased serum ApoB concentration is an important coronary heart disease (where arteries that supply blood to the cardiac muscles become narrowed due to the deposition of cholesterol, fatty cellular waste products in the inner lining of coronary artery) risk factor because ApoB is a major component of all atherogenic particles (LDL, VLDL and IDL) [3]. However several studies found that ApoA1 and ApoB ratio is more accurate predictor of heart attack /acute myocardial infarction risk than ApoB alone. ApoB is also essential for the binding of LDL particles to the LDL receptor for cellular uptake and degradation of LDL particles. Mutation in ApoB gene results in abetalipoproteinaemia (a rare autosomal recessive disorder that affects the fat and fat soluble vitamin absorption in the body and is caused by mutation in microsomal triglyceride transfer protein/MTTP gene) and hypercholesterolaemia (an autosomal dominant disorder caused by a mutation in ApoB100 gene) [4].

3. Apolipoprotein C (ApoC): ApoC is a low molecular weight apolipoprotein which includes Apo C1, Apo CII and Apo CIII. Apo CII is activator of lipoprotein lipase in capillaries and hydrolyzes the triglyceride component of chylomicrons and VLDL. ApoC III inhibits lipoprotein lipase and hepatic lipase and prevent the hepatic uptake of chylomicrons and VLDL [5].

4. Apolipoprotein E (ApoE): This is arginine rich glycoprotein found in plasma lipoproteins and is involved in recognition, regulation and uptake of lipoproteins from plasma. ApoE is also involved in catabolism and metabolism of cholesterol and in receptor recognition of IDLs and chylomicrons remnant by the liver. ApoE4 is linked to atherosclerosis, Alzheimer's disease and impaired cognitive function.

References:

1. https://www.ncbi.nlm.nih.gov/gene/335
2. https://ghr.nlm.nih.gov/gene/APOB#location
3. http://clinchem.aaccjnls.org/content/55/3/407
4. http://patient.info/doctor/apolipoproteins
5. http://onlinelibrary.wiley.com/doi/10.1016/0307-4412(89)90003-4/pdf