Any complex or compound containing both lipid and protein. Lipoproteins are important constituents of biologic membranes and of myelin. Conjugation with protein facilitates transport of lipids, which are hydrophobic, in the aqueous medium of the plasma. Plasma lipoproteins can be separated by ultracentrifugation, electrophoresis, or immunoelectrophoresis. They migrate electrophoretically with )- and *-globulins, but are usually classified according to their densities (flotation constants). The principal classes by density are chylomicrons, which transport dietary cholesterol and triglycerides from the intestine to the liver and other tissues; very low density lipoproteins (VLDL), which transport triglycerides from the intestine and liver to muscle and adipose tissue; low density lipoproteins (LDL), which transport cholesterol to tissues other than the liver; and high density lipoproteins (HDL), which transport cholesterol to the liver for excretion in bile. The properties of these and other plasma lipoproteins are set forth in the accompanying table (see this page). The protein moiety of a lipoprotein is called an apolipoprotein (or apoprotein). Besides rendering lipids soluble in an aqueous solution, some apolipoproteins perform biochemical functions such as enzyme activation. The apolipoproteins of plasma lipoproteins are synthesized by the liver and intestinal mucosal cells and vary in molecular weight from 7,000 to 500,000. Protein makes up more than 50% of some HDLs but only 1% of chylomicrons. As the proportion of lipid in a lipoprotein increases, its density decreases. A plasma lipoprotein particle is typically spheric, with a hydrophobic core of triacylglycerol, cholesteryl esters, and apolar amino acid residues surrounded by hydrophilic protein structures and phospholipids.
The concentrations of certain serum lipoproteins correlate closely with the risk of atherosclerosis. Although dietary factors are important in some people with hyperlipidemia, basal levels of lipoprotein, cholesterol, and triglycerides depend chiefly on heredity. Several phenotypes of familial hyperlipoproteinemia associated with risk of premature cardiovascular disease and death have been identified. SEE hyperlipoproteinemia. An HDL cholesterol level below 35 mg/dL (0.90 mmol/L), an LDL cholesterol level above 160 mg/dL (4.15 mmol/L), and a fasting triglyceride level above 250 mg/dL (2.83 mmol/L) are all independent risk factors for coronary artery disease. Medical management of patients with coronary artery disease (myocardial infarction, angina pectoris, history of coronary artery bypass graft or coronary angioplasty) and other atherosclerotic disorders (peripheral arterial disease, abdominal aortic aneurysm, carotid artery disease) includes detection and correction of hypercholesterolemia and hyperlipoproteinemia. Reducing elevated LDL cholesterol diminishes the risk of coronary artery disease. The currently recommended target serum levels for treatment of disorders of lipid metabolism are LDL cholesterol less than 100 mg/dL (2.6 mmol/L), HDL cholesterol more than 40 mg/dL (1 mmol/L), and triglyceride less than 150 mg/dL (1.7 mmol/L). Dietary recommendations for patients with hyperlipidemia include a total fat intake providing less than 35% of all calories (saturated fat less than 7%, polyunsaturated fat less than 10%, and monounsaturated fat less than 20%), carbohydrate (mostly complex carbohydrates from fruits and vegetables) 50–60% of total calories, fiber 20–30 g/day, and cholesterol less than 200 mg/day. Dietary saturated fat raises LDL cholesterol more than any other dietary component, cholesterol itself not excepted. Of people with elevated LDL cholesterol, 75% can achieve normal levels with diet, weight reduction, and exercise; the others need drug treatment. Factors besides familial hyperlipoproteinemias that can elevate LDL cholesterol include diabetes mellitus, hypothyroidism, nephrotic syndrome, obstructive liver disease, and drugs (progestogens, anabolic steroids, corticosteroids, thiazide diuretics).
Reference: Stedman's Medical Dictionary