lipoprotein, any organic compound that is composed of both protein and the various fatty substances classed as lipids, including fatty acids and steroids such as cholesterol. The lipoprotein complex of proteins and steroids is usually provided by a weak, noncovalent interaction; proteins complexed with some other lipids do so by the information of covalent chemical bonds. There are several types of lipoproteins present in human blood, including low-density lipoproteins (LDLs)—molecules with a larger molecular weight and a relatively low percentage of protein—and high-density lipoproteins (HDLs)—molecules with a smaller molecular weight and a relatively high percentage of protein. LDLs are the main transport for cholesterol through the body. HDLs appear to carry excess cholesterol to the liver for processing. Studies have found that high levels of HDLs, which seem to retard or even reverse the formation of cholesterol plaque in the arteries (see arteriosclerosis), reduce the risk of cardiovascular disease. Cell membranes are essentially lipoprotein in nature; the membrane is a continuous sheet of lipid molecules, largely phospholipids, in close association with proteins that either face one side of the membrane or penetrate all the way through the membrane.

Any of a class of organic compounds that contain both lipid (fat) and protein. They may be soluble (those in egg yolk and blood plasma) or insoluble (those in cell membranes) in water and water solutions. Lipoproteins in blood plasma are the mode of transport for cholesterol, insoluble by itself. Low-density lipoproteins (LDLs) carry cholesterol from the liver, where it is made, to the cells, where it is used; high-density lipoproteins (HDLs) may carry excess cholesterol back to the liver for breakdown and excretion. LDL-bound cholesterol is primarily responsible for deposits in arteries (see arteriosclerosis) that can lead to coronary heart disease, angina pectoris, myocardial infarction, or stroke. HDL does not form such deposits and may actually retard or reduce their buildup.

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