Apolipoproteins D: A glycoprotein component of HIGH-DENSITY LIPOPROTEINS that transports small hydrophobic ligands including CHOLESTEROL and STEROLS. It occurs in the macromolecular complex with LECITHIN CHOLESTEROL ACYLTRANSFERASE. Apo D is expressed in and secreted from a variety of tissues such as liver, placenta, brain tissue and others.Apolipoproteins: Protein components on the surface of LIPOPROTEINS. They form a layer surrounding the hydrophobic lipid core. There are several classes of apolipoproteins with each playing a different role in lipid transport and LIPID METABOLISM. These proteins are synthesized mainly in the LIVER and the INTESTINES.Apolipoproteins A: Structural proteins of the alpha-lipoproteins (HIGH DENSITY LIPOPROTEINS), including APOLIPOPROTEIN A-I and APOLIPOPROTEIN A-II. They can modulate the activity of LECITHIN CHOLESTEROL ACYLTRANSFERASE. These apolipoproteins are low in atherosclerotic patients. They are either absent or present in extremely low plasma concentration in TANGIER DISEASE.Apolipoproteins C: A group of apolipoproteins that can readily exchange among the various classes of lipoproteins (HDL; VLDL; CHYLOMICRONS). After lipolysis of TRIGLYCERIDES on VLDL and chylomicrons, Apo-C proteins are normally transferred to HDL. The subtypes can modulate remnant binding to receptors, LECITHIN CHOLESTEROL ACYLTRANSFERASE, or LIPOPROTEIN LIPASE.Apolipoproteins B: Major structural proteins of triacylglycerol-rich LIPOPROTEINS. There are two forms, apolipoprotein B-100 and apolipoprotein B-48, both derived from a single gene. ApoB-100 expressed in the liver is found in low-density lipoproteins (LIPOPROTEINS, LDL; LIPOPROTEINS, VLDL). ApoB-48 expressed in the intestine is found in CHYLOMICRONS. They are important in the biosynthesis, transport, and metabolism of triacylglycerol-rich lipoproteins. Plasma Apo-B levels are high in atherosclerotic patients but non-detectable in ABETALIPOPROTEINEMIA.Apolipoprotein A-I: The most abundant protein component of HIGH DENSITY LIPOPROTEINS or HDL. This protein serves as an acceptor for CHOLESTEROL released from cells thus promoting efflux of cholesterol to HDL then to the LIVER for excretion from the body (reverse cholesterol transport). It also acts as a cofactor for LECITHIN CHOLESTEROL ACYLTRANSFERASE that forms CHOLESTEROL ESTERS on the HDL particles. Mutations of this gene APOA1 cause HDL deficiency, such as in FAMILIAL ALPHA LIPOPROTEIN DEFICIENCY DISEASE and in some patients with TANGIER DISEASE.Apolipoprotein A-II: The second most abundant protein component of HIGH DENSITY LIPOPROTEINS or HDL. It has a high lipid affinity and is known to displace APOLIPOPROTEIN A-I from HDL particles and generates a stable HDL complex. ApoA-II can modulate the activation of LECITHIN CHOLESTEROL ACYLTRANSFERASE in the presence of APOLIPOPROTEIN A-I, thus affecting HDL metabolism.Apolipoproteins E: A class of protein components which can be found in several lipoproteins including HIGH-DENSITY LIPOPROTEINS; VERY-LOW-DENSITY LIPOPROTEINS; and CHYLOMICRONS. Synthesized in most organs, Apo E is important in the global transport of lipids and cholesterol throughout the body. Apo E is also a ligand for LDL receptors (RECEPTORS, LDL) that mediates the binding, internalization, and catabolism of lipoprotein particles in cells. There are several allelic isoforms (such as E2, E3, and E4). Deficiency or defects in Apo E are causes of HYPERLIPOPROTEINEMIA TYPE III.Apolipoprotein C-II: A 9-kDa protein component of VERY-LOW-DENSITY LIPOPROTEINS. It contains a cofactor for LIPOPROTEIN LIPASE and activates several triacylglycerol lipases. The association of Apo C-II with plasma CHYLOMICRONS; VLDL, and HIGH-DENSITY LIPOPROTEINS is reversible and changes rapidly as a function of triglyceride metabolism. Clinically, Apo C-II deficiency is similar to lipoprotein lipase deficiency (HYPERLIPOPROTEINEMIA TYPE I) and is therefore called hyperlipoproteinemia type IB.Apolipoprotein C-III: A 9-kDa protein component of VERY-LOW-DENSITY LIPOPROTEINS and CHYLOMICRON REMNANTS. Apo C-III, synthesized in the liver, is an inhibitor of LIPOPROTEIN LIPASE. Apo C-III modulates the binding of chylomicron remnants and VLDL to receptors (RECEPTORS, LDL) thus decreases the uptake of triglyceride-rich particles by the liver cells and subsequent degradation. The normal Apo C-III is glycosylated. There are several polymorphic forms with varying amounts of SIALIC ACID (Apo C-III-0, Apo C-III-1, and Apo C-III-2).Lipoproteins, HDL: A class of lipoproteins of small size (4-13 nm) and dense (greater than 1.063 g/ml) particles. HDL lipoproteins, synthesized in the liver without a lipid core, accumulate cholesterol esters from peripheral tissues and transport them to the liver for re-utilization or elimination from the body (the reverse cholesterol transport). Their major protein component is APOLIPOPROTEIN A-I. HDL also shuttle APOLIPOPROTEINS C and APOLIPOPROTEINS E to and from triglyceride-rich lipoproteins during their catabolism. HDL plasma level has been inversely correlated with the risk of cardiovascular diseases.Lipoproteins: Lipid-protein complexes involved in the transportation and metabolism of lipids in the body. They are spherical particles consisting of a hydrophobic core of TRIGLYCERIDES and CHOLESTEROL ESTERS surrounded by a layer of hydrophilic free CHOLESTEROL; PHOSPHOLIPIDS; and APOLIPOPROTEINS. Lipoproteins are classified by their varying buoyant density and sizes.Apolipoprotein C-I: A 6.6-kDa protein component of VERY-LOW-DENSITY LIPOPROTEINS; INTERMEDIATE-DENSITY LIPOPROTEINS; and HIGH-DENSITY LIPOPROTEINS. Apo C-I displaces APO E from lipoproteins, modulate their binding to receptors (RECEPTORS, LDL), and thereby decrease their clearance from plasma. Elevated Apo C-I levels are associated with HYPERLIPOPROTEINEMIA and ATHEROSCLEROSIS.Cholesterol: The principal sterol of all higher animals, distributed in body tissues, especially the brain and spinal cord, and in animal fats and oils.Lipoproteins, VLDL: A class of lipoproteins of very light (0.93-1.006 g/ml) large size (30-80 nm) particles with a core composed mainly of TRIGLYCERIDES and a surface monolayer of PHOSPHOLIPIDS and CHOLESTEROL into which are imbedded the apolipoproteins B, E, and C. VLDL facilitates the transport of endogenously made triglycerides to extrahepatic tissues. As triglycerides and Apo C are removed, VLDL is converted to INTERMEDIATE-DENSITY LIPOPROTEINS, then to LOW-DENSITY LIPOPROTEINS from which cholesterol is delivered to the extrahepatic tissues.TriglyceridesLipids: A generic term for fats and lipoids, the alcohol-ether-soluble constituents of protoplasm, which are insoluble in water. They comprise the fats, fatty oils, essential oils, waxes, phospholipids, glycolipids, sulfolipids, aminolipids, chromolipids (lipochromes), and fatty acids. (Grant & Hackh's Chemical Dictionary, 5th ed)Apolipoprotein B-100: A 513-kDa protein synthesized in the LIVER. It serves as the major structural protein of low-density lipoproteins (LIPOPROTEINS, LDL; LIPOPROTEINS, VLDL). It is the ligand for the LDL receptor (RECEPTORS, LDL) that promotes cellular binding and internalization of LDL particles.Apolipoprotein B-48: A 241-kDa protein synthesized only in the INTESTINES. It serves as a structural protein of CHYLOMICRONS. Its exclusive association with chylomicron particles provides an indicator of intestinally derived lipoproteins in circulation. Apo B-48 is a shortened form of apo B-100 and lacks the LDL-receptor region.Cholesterol, HDL: Cholesterol which is contained in or bound to high-density lipoproteins (HDL), including CHOLESTEROL ESTERS and free cholesterol.Lipoproteins, HDL3: Intermediate-density subclass of the high-density lipoproteins, with particle sizes between 7 to 8 nm. As the larger lighter HDL2 lipoprotein, HDL3 lipoprotein is lipid-rich.Tangier Disease: An autosomal recessively inherited disorder caused by mutation of ATP-BINDING CASSETTE TRANSPORTERS involved in cellular cholesterol removal (reverse-cholesterol transport). It is characterized by near absence of ALPHA-LIPOPROTEINS (high-density lipoproteins) in blood. The massive tissue deposition of cholesterol esters results in HEPATOMEGALY; SPLENOMEGALY; RETINITIS PIGMENTOSA; large orange tonsils; and often sensory POLYNEUROPATHY. The disorder was first found among inhabitants of Tangier Island in the Chesapeake Bay, MD.Phosphatidylcholine-Sterol O-Acyltransferase: An enzyme secreted from the liver into the plasma of many mammalian species. It catalyzes the esterification of the hydroxyl group of lipoprotein cholesterol by the transfer of a fatty acid from the C-2 position of lecithin. In familial lecithin:cholesterol acyltransferase deficiency disease, the absence of the enzyme results in an excess of unesterified cholesterol in plasma. EC 184.108.40.206.Lipoproteins, LDL: A class of lipoproteins of small size (18-25 nm) and light (1.019-1.063 g/ml) particles with a core composed mainly of CHOLESTEROL ESTERS and smaller amounts of TRIGLYCERIDES. The surface monolayer consists mostly of PHOSPHOLIPIDS, a single copy of APOLIPOPROTEIN B-100, and free cholesterol molecules. The main LDL function is to transport cholesterol and cholesterol esters to extrahepatic tissues.Nephelometry and Turbidimetry: Chemical analysis based on the phenomenon whereby light, passing through a medium with dispersed particles of a different refractive index from that of the medium, is attenuated in intensity by scattering. In turbidimetry, the intensity of light transmitted through the medium, the unscattered light, is measured. In nephelometry, the intensity of the scattered light is measured, usually, but not necessarily, at right angles to the incident light beam.ATP Binding Cassette Transporter 1: A superfamily of large integral ATP-binding cassette membrane proteins whose expression pattern is consistent with a role in lipid (cholesterol) efflux. It is implicated in TANGIER DISEASE characterized by accumulation of cholesteryl ester in various tissues.Cholesterol Esters: Fatty acid esters of cholesterol which constitute about two-thirds of the cholesterol in the plasma. The accumulation of cholesterol esters in the arterial intima is a characteristic feature of atherosclerosis.Isoelectric Focusing: Electrophoresis in which a pH gradient is established in a gel medium and proteins migrate until they reach the site (or focus) at which the pH is equal to their isoelectric point.Receptors, Lipoprotein: Cell surface proteins that bind lipoproteins with high affinity. Lipoprotein receptors in the liver and peripheral tissues mediate the regulation of plasma and cellular cholesterol metabolism and concentration. The receptors generally recognize the apolipoproteins of the lipoprotein complex, and binding is often a trigger for endocytosis.Lipid Metabolism: Physiological processes in biosynthesis (anabolism) and degradation (catabolism) of LIPIDS.Chylomicrons: A class of lipoproteins that carry dietary CHOLESTEROL and TRIGLYCERIDES from the SMALL INTESTINE to the tissues. Their density (0.93-1.006 g/ml) is the same as that of VERY-LOW-DENSITY LIPOPROTEINS.Cholesterol, LDL: Cholesterol which is contained in or bound to low density lipoproteins (LDL), including CHOLESTEROL ESTERS and free cholesterol.Phospholipids: Lipids containing one or more phosphate groups, particularly those derived from either glycerol (phosphoglycerides see GLYCEROPHOSPHOLIPIDS) or sphingosine (SPHINGOLIPIDS). They are polar lipids that are of great importance for the structure and function of cell membranes and are the most abundant of membrane lipids, although not stored in large amounts in the system.Ultracentrifugation: Centrifugation with a centrifuge that develops centrifugal fields of more than 100,000 times gravity. (McGraw-Hill Dictionary of Scientific and Technical Terms, 4th ed)Hypolipoproteinemias: Conditions with abnormally low levels of LIPOPROTEINS in the blood. This may involve any of the lipoprotein subclasses, including ALPHA-LIPOPROTEINS (high-density lipoproteins); BETA-LIPOPROTEINS (low-density lipoproteins); and PREBETA-LIPOPROTEINS (very-low-density lipoproteins).Lipoproteins, HDL2: Low-density subclass of the high-density lipoproteins, with particle sizes between 8 to 13 nm.Lipoprotein(a): A lipoprotein that resembles the LOW-DENSITY LIPOPROTEINS but with an extra protein moiety, APOPROTEIN (A) also known as APOLIPOPROTEIN (A), linked to APOLIPOPROTEIN B-100 on the LDL by one or two disulfide bonds. High plasma level of lipoprotein (a) is associated with increased risk of atherosclerotic cardiovascular disease.Lymph: The interstitial fluid that is in the LYMPHATIC SYSTEM.Electrophoresis, Polyacrylamide Gel: Electrophoresis in which a polyacrylamide gel is used as the diffusion medium.Apoproteins: The protein components of a number of complexes, such as enzymes (APOENZYMES), ferritin (APOFERRITINS), or lipoproteins (APOLIPOPROTEINS).