High-density lipoprotein (HDL) is one of the five major groups of lipoproteins. Lipoproteins are complex particles composed of multiple proteins which transport all fat molecules (lipids) around the body within the water outside cells. They are typically composed of 80–100 proteins per particle and transporting up to hundreds of fat molecules per particle.
A lipoprotein is a biochemical assembly whose primary purpose is to transport hydrophobic lipid molecules in water, as in blood plasma or other extracellular fluids. They have a single-layer phospholipid and cholesterol outer shell, with the hydrophilic portions oriented outward toward the surrounding water and lipophilic portions oriented inward toward the lipids molecules within the particles. Thus, the complex serves to emulsify the fats in extracellular fluids. A special kind of protein, called apolipoprotein, is embedded in the outer shell, both stabilising the complex and giving it a functional identity that determines its fate.
Very-low-density lipoprotein (VLDL), density relative to extracellular water, is a type of lipoprotein made by the liver. VLDL is one of the five major groups of lipoproteins that enable fats and cholesterol to move within the water-based solution of the bloodstream. VLDL is assembled in the liver from triglycerides, cholesterol, and apolipoproteins. VLDL is converted in the bloodstream to low-density lipoprotein (LDL) and intermediate-density lipoprotein (IDL). VLDL particles have a diameter of 30–80 nm. VLDL transports endogenous products, whereas chylomicrons transport exogenous (dietary) products. In the early 2010s both the lipid composition and protein composition of this lipoprotein were characterised in great detail.
Chylomicrons, also known as ultra low-density lipoproteins (ULDL), are lipoprotein particles that consist of triglycerides (85–92%), phospholipids (6–12%), cholesterol (1–3%), and proteins (1–2%). They transport dietary lipids from the intestines to other locations in the body. ULDLs are one of the five major groups of lipoproteins that enable fats and cholesterol to move within the water-based solution of the bloodstream. A protein specific to chylomicrons is ApoB48.
Intermediate-density lipoproteins (IDLs) belong to the lipoprotein particle family and are formed from the degradation of very low-density lipoproteins as well as high-density lipoproteins. IDL is one of the five major groups of lipoproteins that enable fats and cholesterol to move within the water-based solution of the bloodstream. Each native IDL particle consists of protein that encircles various lipids, enabling, as a water-soluble particle, these lipids to travel in the aqueous blood environment as part of the fat transport system within the body. Their size is, in general, 25 to 35 nm in diameter, and they contain primarily a range of triacylglycerols and cholesterol esters. They are cleared from the plasma into the liver by receptor-mediated endocytosis, or further degraded by hepatic lipase to form LDL particles.
A lacteal is a lymphatic capillary that absorbs dietary fats in the villi of the small intestine.
Lipoprotein lipase (LPL) is a member of the lipase gene family, which includes pancreatic lipase, hepatic lipase, and endothelial lipase. It is a water-soluble enzyme that hydrolyzes triglycerides in lipoproteins, such as those found in chylomicrons and very low-density lipoproteins (VLDL), into two free fatty acids and one monoacylglycerol molecule. It is also involved in promoting the cellular uptake of chylomicron remnants, cholesterol-rich lipoproteins, and free fatty acids. LPL requires ApoC-II as a cofactor.
Hyperlipidemia is abnormally elevated levels of any or all lipids or lipoproteins in the blood. Hyperlipidemia is an umbrella term that refers to acquired or genetic disorders that result in high levels of lipids circulating in the blood. This disease is usually chronic and requires ongoing medication to control blood lipid levels.
Tangier disease or hypoalphalipoproteinemia is an extremely rare inherited disorder characterized by a severe reduction in the amount of high density lipoprotein (HDL), often referred to as "good cholesterol", in the bloodstream. Worldwide, approximately 100 cases have even been identified.
Lipid metabolism is the synthesis and degradation of lipids in cells, involving the breakdown or storage of fats for energy and the synthesis of structural and functional lipids, such as those involved in the construction of cell membranes. In animals, these fats are obtained from food or are synthesized by the liver. Lipogenesis is the process of synthesizing these fats. The majority of lipids found in the human body from ingesting food are triglycerides and cholesterol. Other types of lipids found in the body are fatty acids and membrane lipids. Lipid metabolism is often considered as the digestion and absorption process of dietary fat; however, there are two sources of fats that organisms can use to obtain energy: from consumed dietary fats and from stored fat. Vertebrates use both sources of fat to produce energy for organs such as the heart to function. Since lipids are hydrophobic molecules, they need to be solubilized before their metabolism can begin. Lipid metabolism often begins with hydrolysis, which occurs with the help of various enzymes in the digestive system. Lipid metabolism also occurs in plants, though the processes differ in some ways when compared to animals. The second step after the hydrolysis is the absorption of the fatty acids into the epithelial cells of the intestinal wall. In the epithelial cells, fatty acids are packaged and transported to the rest of the body.
Lecithin–cholesterol acyltransferase is an enzyme that converts free cholesterol into cholesteryl ester, which is then sequestered into the core of a lipoprotein particle, eventually making the newly synthesized HDL spherical and forcing the reaction to become unidirectional since the particles are removed from the surface. The enzyme is bound to high-density lipoproteins (HDLs) (alpha-LCAT) and LDLs (beta-LCAT) in the blood plasma. LCAT deficiency can cause impaired vision due to cholesterol corneal opacities, anemia, and kidney damage. It belongs to the family of phospholipid:diacylglycerol acyltransferases.
Acid lipase disease or deficiency is a name used to describe two related disorders of fatty acid metabolism. Acid lipase disease occurs when the enzyme lysosomal acid lipase that is needed to break down certain fats that are normally digested by the body is lacking or missing. This results in the toxic buildup of these fats in the body's cells and tissues. These fatty substances, called lipids, include waxes, oils, and cholesterol.
Lecithin cholesterol acyltransferase deficiency is a disorder of lipoprotein metabolism. The disease has two forms: Familial LCAT deficiency, in which there is complete LCAT deficiency, and Fish-eye disease, in which there is a partial deficiency.
Hepatic lipase (HL), also called hepatic triglyceride lipase (HTGL) or LIPC, is a form of lipase, catalyzing the hydrolysis of triacylglyceride. Hepatic lipase is coded by chromosome 15 and its gene is also often referred to as HTGL or LIPC. Hepatic lipase is expressed mainly in liver cells, known as hepatocytes, and endothelial cells of the liver. The hepatic lipase can either remain attached to the liver or can unbind from the liver endothelial cells and is free to enter the body's circulation system. When bound on the endothelial cells of the liver, it is often found bound to HSPG, heparan sulfate proteoglycans (HSPG), keeping HL inactive and unable to bind to HDL or IDL. When it is free in the bloodstream, however, it is found associated with HDL to maintain it inactive. This is because the triacylglycerides in HDL serve as a substrate, but the lipoprotein contains proteins around the triacylglycerides that can prevent the triacylglycerides from being broken down by HL.
Endothelial lipase (LIPG) is a form of lipase secreted by vascular endothelial cells in tissues with high metabolic rates and vascularization, such as the liver, lung, kidney, and thyroid gland. The LIPG enzyme is a vital component to many biological process. These processes include lipoprotein metabolism, cytokine expression, and lipid composition in cells. Unlike the lipases that hydrolyze Triglycerides, endothelial lipase primarily hydrolyzes phospholipids. Due to the hydrolysis specificity, endothelial lipase contributes to multiple vital systems within the body. On the contrary to the beneficial roles that LIPG plays within the body, endothelial lipase is thought of to play a potential role in cancer and inflammation. Knowledge obtained in vitro and in vivo suggest the relations to these conditions, but human interaction knowledge lacks due to the recent discovery of endothelial lipase. Endothelial lipase was first characterized in 1999. The two independent research groups which are notable for this discovery cloned the endothelial lipase gene and identified the novel lipase secreted from endothelial cells. The anti-Atherosclerosis opportunity through alleviating plaque blockage and prospective ability to raise High-density lipoprotein (HDL) have gained endothelial lipase recognition.
Blood lipids are lipids in the blood, either free or bound to other molecules. They are mostly transported in a protein capsule, and the density of the lipids and type of protein determines the fate of the particle and its influence on metabolism. The concentration of blood lipids depends on intake and excretion from the intestine, and uptake and secretion from cells. Blood lipids are mainly fatty acids and cholesterol. Hyperlipidemia is the presence of elevated or abnormal levels of lipids and/or lipoproteins in the blood, and is a major risk factor for cardiovascular disease.
Scavenger receptor class B type 1 (SRB1) also known as SR-BI is a protein that in humans is encoded by the SCARB1 gene. SR-BI functions as a receptor for high-density lipoprotein.
Sterol O-acyltransferase is an intracellular protein located in the endoplasmic reticulum that forms cholesteryl esters from cholesterol.
Triglyceride lipases are a family of lipolytic enzymes that hydrolyse ester linkages of triglycerides. Lipases are widely distributed in animals, plants and prokaryotes.
Angiopoietin-like 3, also known as ANGPTL3, is a protein that in humans is encoded by the ANGPTL3 gene.
