Important Terms (Lipoprotein/Cholesterol Metabolism)

ABCA1 A membrane lipid transport protein involved in cholesterol and/or phospholipid efflux from cells. Its action is necessary for the extracellular assembly of lipoprotein particles. Apo-A1 An apolipoprotein principally associated with HDL, an activator of lecithin choles-terol:acyltransferase. It interacts with cells to mediate delivery of cholesterol ester from HDL particles. Apo-B100 An apolipoprotein associated with VLDL and LDL particles, synthesized in the liver; it is a ligand for the LDL receptor. Apo-B48 An apolipoprotein associated with chylomi-crons, synthesized in the intestine; it is a truncated form of apo-B100 and does not bind to the LDL receptor.

Apo-E An apolipoprotein principally associated with VLDL and chylomicrons; responsible for the receptor-mediated clearance of IDL and chylomicron remnants. It is a ligand for most members of the LDL receptor superfamily. The apo-E4 isoform is associated with increased risk of Azheimer’s disease. Apolipoproteins The protein components of plasma lipoproteins.

Bile acids Detergent-like molecules formed from cholesterol. They are secreted by the liver and, together with-cholesterol and phospholipids, form bile. Bile forms micelles which emulsify lipids in the intestinal lumen aiding in their absorption.

Cholesterol ester transfer protein (CETP) A blood-borne protein that catalyzes the exchange of triglycerides in VLDL for cholesterol esters in HDL.

Chylomicron remnants Chylomicron particles that have been depleted of triglyceride after the lipoprotein lipase-mediated hydrolysis of their triglycerides.

Chylomicrons Lipoprotein particles produced in the intestine to package and secrete dietary lipids. Chylomi-crons are secreted into the mesenteric lymph.

Familial hypercholesterolemia An elevation in LDL cholesterol due to mutations at the LDL receptor locus.

Familial hypoalphalipoproteinemia A deficiency in HDL due to mutations at the ABCA1 locus.

Fibric acid A class of drugs used to decrease the rate of VLDL triglyceride secretion.

Gallstones Large crystals, usually made of cholesterol, that form in the biliary tract and/or gall bladder when cholesterol levels in bile are too high relative to phos-pholipid and bile acids.

HDL High density lipoprotein; carries about 20% of plasma cholesterol. Its levels negatively correlate with risk of coronary heart disease. Is thought to mediate “reverse cholesterol transport.”

HMG-CoA reductase A major rate-limiting step in the cholesterol biosynthetic pathway; catalyzes the conversion of HMG-CoA to mevalonate.

IDL Intermediate density lipoprotein, also called “VLDL remnant”; the VLDL particle that has been depleted of triglyceride through the action of lipoprotein lipase.

LDL Low density lipoprotein; carries approximately two-thirds of plasma cholesterol. Its levels are positively correlated with risk of coronary heart disease. It is formed in the circulation from the catabolism of VLDL.

LDL receptor Receptor expressed in most tissues and mainly necessary for normal clearance of LDL from the bloodstream.

Lipoprotein lipase Enzyme that catalyzes the hydrolysis of VLDL and LDL triglycerides to free fatty acids and glycerol; present on the luminal surface of the capillary endothelium.

Lipoproteins Particles that transport lipids in the bloodstream.

Lp(a) An LDL-like lipoprotein particle in which apo-B100 is connected through a disulfide linkage to apo(a), a plasminogen-related protein.

Microsomal triglyceride transfer protein (MTP) A protein in the endoplasmic reticulum necessary for secretion of VLDL and chylomicrons.

Nicotinic acid A coenzyme precursor that at pharmacological doses helps to lower triglyceride levels and raise HDL levels.

Reverse cholesterol transport A hypothetical pathway by which cholesterol is transported from extrahepatic-tissues to the liver via HDL. Scavenger receptors Receptors that bind to a wide range of molecules, including modfied forms of LDL. They are involved in the accumulation of cholesterol in macrophages and smooth muscle cells in the arterial wall.

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