Cholesterol is an essential structural component of cell membranes and of the myelin sheaths that insulate the axons of nerve cells. Cholesterol is also a precursor of steroid hormones and of the bile acids necessary for digestion. The liver produces approximately 70% of the cholesterol used by the body, and the other 30% comes from the diet.
Lipoproteins are small spherules that transport fats in the body and consist of protein, cholesterol, triglycerides, and phospholipids. The terms "good" and "bad" cholesterol refer to High Density Lipoproteins (HDL) and Low Density Lipoproteins (LDL), respectively. High levels of LDL are associated with coronary atherosclerosis, whereas high levels of HDL appear to protect against cardiovascular diseases.
Classification of Lipoproteins
There are five main classes of lipoproteins:
Lipoprotein particles range in size from 10 to 1000 nanometers. The largest lipoproteins are about one tenth the size of a red blood cell. The density of lipoproteins increases in proportion to their ratio of proteins to lipids. In general, as the density of a lipoproteins increases, the size of the particles decreases. The outer layer of a lipoprotein consists of a water-soluble (hydrophilic) layer of apolipoproteins, phospholipids and cholesterol. The center of a lipoprotein is composed of cholesteryl esters, triglycerides, fatty acids and fat-soluble vitamins like Vitamin E.
|apolipoprotein||a protein that binds to lipids|
|cholesteryl ester||a compound of cholesterol and a fatty acid|
|triglyceride||a compound of glycerol and three fatty acids,
an ordinary fat molecule
|phospholipid||a compound of glycerol, two fatty acids, and choline phospate,
an emulsifier like lecithin
Chylomicrons are the largest and least dense of the lipoproteins. These 1000-nanometer particles originate in the intestinal mucosa. Their function is to transport dietary triglycerides and cholesterol absorbed by the intestinal epithelial cells. Chylomicrons contain about 1-2% protein, 85-88% triglycerides, ~8% phospholipids, ~3% cholesteryl esters and ~1% cholesterol. The high triglyceride content of chylomicrons gives them a density of less than 0.95.
The lymphatic system transports chylomicrons to the plasma where they acquire additional apolipoproteins from HDL. Triglycerides contained in chylomicrons are hydrolyzed in the tissues and the particle remnants are processed by the liver.
Very Low Density Lipoproteins (VLDL)
Very low density lipoproteins are approximately 25-90 nanometers in size, and have a density of ~0.98. VLDL contains 5-12% protein, 50-55% triglycerides, 18-20% phospholipids, 12-15% cholesteryl esters and 8-10% cholesterol. VLDL also acquires several apolipoproteins from plasma HDL and is a source of triglycerides for the cells.
Intermediate Density Lipoproteins (IDL)
Intermediate density lipoproteins are smaller than VLDL, approximately 40 nanometers, and have a density of ~1.0. IDLs are composed of 10-12% protein, 24-30% triglycerides, 25-27% phospholipids, 32-35% cholesteryl esters and 8-10% cholesterol. IDLs are derived from VLDL by triglyceride depletion and therefore contain the same apolipoproteins as VLDL. IDL becomes LDL as its triglycerides are transferred to the cells.
Low Density Lipoproteins (LDL) - "Bad" Cholesterol
Low density lipoproteins are smaller than IDL, approximately 26 nanometers, and have a density of ~1.04. LDL contains 20-22% protein, 10-15% triglycerides, 20-28% phospholipids, 37-48% cholesteryl esters, and 8-10% cholesterol. One of the protein components of LDL is apolipoprotein B100 which serves to bind the lipoprotein particles to LDL-specific receptors on the surface of many cells. LDL particles bound to the surface of a cell are engulfed and the cholesterol in the LDL particles is used as a structural component of cell membranes or converted to steroid hormones. Apoprotein B is the major protein in all lipoproteins, except high density lipoprotein (HDL).
LDL and HDL transport both dietary and endogenous cholesterol in the plasma, but LDL is the main transporter of cholesterol and cholesteryl esters and makes up more than half of the total lipoprotein in plasma.
High Density Lipoproteins (HDL) - "Good" Cholesterol
High density lipoproteins are the smallest of the lipoproteins. HDL particles have a size of 6-12.5 nanometers and a density of ~1.12. HDL contains approximately 55% protein, 3-15% triglycerides, 26-46% phospholipids, 15-30% cholesteryl esters, and 2-10% cholesterol. HDL contains a large number of different proteins including apolipoproteins such as apo-AI (apolipoprotein A1), apo-CI, apo-CII, apo-D, and apo-E. The HDL proteins serve in lipid metabolism, complement regulation, and participate as proteinase inhibitors and acute phase response to support the immune system against inflammation and parasitic diseases.
HDL is produced in the liver and intestine and acts like a scavenger of cholesterol. HDL can bind to cholesterol in cell membranes by using the apo-AI protein to mediate the formation of cholesteryl esters. The apo-D protein in HDL then activates the transfer of cholesteryl esters to VLDL and LDL. HDL also transfers apo-CII and apo-E proteins to chylomicrons and other low density lipoproteins. In the liver, the apo-E protein is used to recognize and absorb the remants of lipoproteins so that excess cholesterol can be removed and converted to bile acids that are excreted into the duodenum (small intestine) through the bile duct.