A lipoprotein is a biochemical assembly whose primary function is to transport hydrophobic lipid (also known as fat) molecules in water, as in blood plasma or other extracellular fluids. They consist of a triglyceride and cholesterol center, surrounded by a phospholipid outer shell, with the hydrophilic portions oriented outward toward the surrounding water and lipophilic portions oriented inward toward the lipid center. 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 role.
Plasma lipoprotein particles are commonly divided into five main classes, based on size, lipid composition, and apolipoprotein content: HDL, LDL, IDL, VLDL and chylomicrons. Subgroups of these plasma particles are primary drivers or modulators of atherosclerosis. [1]
Many enzymes, transporters, structural proteins, antigens, adhesins, and toxins are sometimes also classified as lipoproteins, since they are formed by lipids and proteins.
Some transmembrane proteolipids, especially those found in bacteria, are referred to as lipoproteins; they are not related to the lipoprotein particles that this article is about. [2] Such transmembrane proteins are difficult to isolate, as they bind tightly to the lipid membrane, often require lipids to display the proper structure, and can be water-insoluble. Detergents are usually required to isolate transmembrane lipoproteins from their associated biological membranes.
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Because fats are insoluble in water, they cannot be transported on their own in extracellular water, including blood plasma. Instead, they are surrounded by a hydrophilic external shell that functions as a transport vehicle. The role of lipoprotein particles is to transport fat molecules, such as triglycerides, phospholipids, and cholesterol within the extracellular water of the body to all the cells and tissues of the body. The proteins included in the external shell of these particles, called apolipoproteins, are synthesized and secreted into the extracellular water by both the small intestine and liver cells. The external shell also contains phospholipids and cholesterol.
All cells use and rely on fats and cholesterol as building blocks to create the multiple membranes that cells use both to control internal water content and internal water-soluble elements and to organize their internal structure and protein enzymatic systems. The outer shell of lipoprotein particles have the hydrophilic groups of phospholipids, cholesterol, and apolipoproteins directed outward. Such characteristics make them soluble in the salt-water-based blood pool. Triglycerides and cholesteryl esters are carried internally, shielded from the water by the outer shell. The kind of apolipoproteins contained in the outer shell determines the functional identity of the lipoprotein particles. The interaction of these apolipoproteins with enzymes in the blood, with each other, or with specific proteins on the surfaces of cells, determines whether triglycerides and cholesterol will be added to or removed from the lipoprotein transport particles.
Characterization in human plasma [3]
Chylomicrons | VLDL | LDL | HDL | |
---|---|---|---|---|
Electrophoretic mobility | Origin | Pre-Beta | Beta | Alpha |
Density | less than 0.96 | 0.96-1.006 | 1.006-1.063 | 1.063-1.21 |
Diameter (nm) | 100-1000 | 30-90 | 20-25 | 10-20 |
Apolipoproteins | B48, Al, All | B100 CI, CII | B100 | AI, AII, CI |
Composition (% of total content) | ||||
· Protein | 2 | 10 | 20 | 40 |
· Lipid | 98 | 90 | 80 | 60 |
Lipid component (% of total lipid content) | ||||
· Triglycerides | 88 | 55 | 12 | 12 |
· Cholesteryl esters | 4 | 24 | 59 | 40 |
· Phospholipids | 8 | 20 | 28 | 47 |
· Free fatty acids | - | 1 | 1 | 1 |
Lipoproteins are complex particles that have a central hydrophobic core of non-polar lipids, primarily cholesteryl esters and triglycerides. This hydrophobic core is surrounded by a hydrophilic membrane consisting of phospholipids, free cholesterol, and apolipoproteins. Plasma lipoproteins, found in blood plasma, are typically divided into five main classes based on size, lipid composition, and apolipoprotein content: HDL, LDL, IDL, VLDL and chylomicrons. [4]
The handling of lipoprotein particles in the body is referred to as lipoprotein particle metabolism. It is divided into two pathways, exogenous and endogenous, depending in large part on whether the lipoprotein particles in question are composed chiefly of dietary (exogenous) lipids or whether they originated in the liver (endogenous), through de novo synthesis of triglycerides.
The hepatocytes are the main platform for the handling of triglycerides and cholesterol; the liver can also store certain amounts of glycogen and triglycerides. While adipocytes are the main storage cells for triglycerides, they do not produce any lipoproteins.
Bile emulsifies fats contained in the chyme, then pancreatic lipase cleaves triglyceride molecules into two fatty acids and one 2-monoacylglycerol. Enterocytes readily absorb the small molecules from the chymus. Inside of the enterocytes, fatty acids and monoacylglycerides are transformed again into triglycerides. Then these lipids are assembled with apolipoprotein B-48 into nascent chylomicrons . These particles are then secreted into the lacteals in a process that depends heavily on apolipoprotein B-48. As they circulate through the lymphatic vessels, nascent chylomicrons bypass the liver circulation and are drained via the thoracic duct into the bloodstream.
In the blood stream, nascent chylomicron particles interact with HDL particles, resulting in HDL donation of apolipoprotein C-II and apolipoprotein E to the nascent chylomicron. The chylomicron at this stage is then considered mature. Via apolipoprotein C-II, mature chylomicrons activate lipoprotein lipase (LPL), an enzyme on endothelial cells lining the blood vessels. LPL catalyzes the hydrolysis of triglycerides that ultimately releases glycerol and fatty acids from the chylomicrons. Glycerol and fatty acids can then be absorbed in peripheral tissues, especially adipose and muscle, for energy and storage.
The hydrolyzed chylomicrons are now called chylomicron remnants. The chylomicron remnants continue circulating the bloodstream until they interact via apolipoprotein E with chylomicron remnant receptors, found chiefly in the liver. This interaction causes the endocytosis of the chylomicron remnants, which are subsequently hydrolyzed within lysosomes. Lysosomal hydrolysis releases glycerol and fatty acids into the cell, which can be used for energy or stored for later use.
The liver is the central platform for the handling of lipids: it is able to store glycerols and fats in its cells, the hepatocytes. Hepatocytes are also able to create triglycerides via de novo synthesis. They also produce the bile from cholesterol. The intestines are responsible for absorbing cholesterol. They transfer it over into the blood stream.
In the hepatocytes, triglycerides and cholesteryl esters are assembled with apolipoprotein B-100 to form nascent VLDL particles. Nascent VLDL particles are released into the bloodstream via a process that depends upon apolipoprotein B-100.
In the blood stream, nascent VLDL particles bump with HDL particles; as a result, HDL particles donate apolipoprotein C-II and apolipoprotein E to the nascent VLDL particle. Once loaded with apolipoproteins C-II and E, the nascent VLDL particle is considered mature. VLDL particles circulate and encounter LPL expressed on endothelial cells. Apolipoprotein C-II activates LPL, causing hydrolysis of the VLDL particle and the release of glycerol and fatty acids. These products can be absorbed from the blood by peripheral tissues, principally adipose and muscle. The hydrolyzed VLDL particles are now called VLDL remnants or intermediate-density lipoproteins (IDLs). VLDL remnants can circulate and, via an interaction between apolipoprotein E and the remnant receptor, be absorbed by the liver, or they can be further hydrolyzed by hepatic lipase.
Hydrolysis by hepatic lipase releases glycerol and fatty acids, leaving behind IDL remnants, called low-density lipoproteins (LDL), which contain a relatively high cholesterol content [5] (see native LDL structure at 37°C on YouTube). LDL circulates and is absorbed by the liver and peripheral cells. Binding of LDL to its target tissue occurs through an interaction between the LDL receptor and apolipoprotein B-100 on the LDL particle. Absorption occurs through endocytosis, and the internalized LDL particles are hydrolyzed within lysosomes, releasing lipids, chiefly cholesterol.
Plasma lipoproteins may carry oxygen gas. [6] This property is due to the crystalline hydrophobic structure of lipids, providing a suitable environment for O2 solubility compared to an aqueous medium. [7]
Inflammation, a biological system response to stimuli such as the introduction of a pathogen, has an underlying role in numerous systemic biological functions and pathologies. This is a useful response by the immune system when the body is exposed to pathogens, such as bacteria in locations that will prove harmful, but can also have detrimental effects if left unregulated. It has been demonstrated that lipoproteins, specifically HDL, have important roles in the inflammatory process. [8]
When the body is functioning under normal, stable physiological conditions, HDL has been shown to be beneficial in several ways. [8] LDL contains apolipoprotein B (apoB), which allows LDL to bind to different tissues, such as the artery wall if the glycocalyx has been damaged by high blood sugar levels. [8] If oxidised, the LDL can become trapped in the proteoglycans, preventing its removal by HDL cholesterol efflux. [8] Normal functioning HDL is able to prevent the process of oxidation of LDL and the subsequent inflammatory processes seen after oxidation. [8]
Lipopolysaccharide, or LPS, is the major pathogenic factor on the cell wall of Gram-negative bacteria. Gram-positive bacteria has a similar component named Lipoteichoic acid, or LTA. HDL has the ability to bind LPS and LTA, creating HDL-LPS complexes to neutralize the harmful effects in the body and clear the LPS from the body. [9] HDL also has significant roles interacting with cells of the immune system to modulate the availability of cholesterol and modulate the immune response. [9]
Under certain abnormal physiological conditions such as system infection or sepsis, the major components of HDL become altered, [9] [10] The composition and quantity of lipids and apolipoproteins are altered as compared to normal physiological conditions, such as a decrease in HDL cholesterol (HDL-C), phospholipids, apoA-I (a major lipoprotein in HDL that has been shown to have beneficial anti-inflammatory properties), and an increase in Serum amyloid A. [9] [10] This altered composition of HDL is commonly referred to as acute-phase HDL in an acute-phase inflammatory response, during which time HDL can lose its ability to inhibit the oxidation of LDL. [8] In fact, this altered composition of HDL is associated with increased mortality and worse clinical outcomes in patients with sepsis. [9]
Lipoproteins may be classified as five major groups, listed from larger and lower density to smaller and higher density. Lipoproteins are larger and less dense when the fat to protein ratio is increased. They are classified on the basis of electrophoresis, ultracentrifugation and nuclear magnetic resonance spectroscopy via the Vantera Analyzer. [11]
For young healthy research subjects, ~70 kg (154 lb), these data represent averages across individuals studied, percentages represent % dry weight:
Density (g/mL) | Class | Diameter (nm) | % protein | % cholesterol & cholesterol ester | % phospholipid | % triglyceride |
>1.063 | HDL | 5–15 | 33 | 30 | 29 | 4-8 |
1.019–1.063 | LDL | 18–28 | 25 | 46-50 | 21-22 | 8-10 |
1.006–1.019 | IDL | 25–50 | 18 | 29 | 22 | 31 |
0.95–1.006 | VLDL | 30–80 | 10 | 22 | 18 | 50 |
<0.95 | Chylomicrons | 75-1200 | 1-2 | 8 | 7 | 83-84 |
[12] [13] However, these data are not necessarily reliable for any one individual or for the general clinical population.
It is also possible to classify lipoproteins as "alpha" and "beta", according to the classification of proteins in serum protein electrophoresis. This terminology is sometimes used in describing lipid disorders such as abetalipoproteinemia.
Lipoproteins, such as LDL and HDL, can be further subdivided into subspecies isolated through a variety of methods. [14] [15] These are subdivided by density or by the protein contents/ proteins they carry. [14] While the research is currently ongoing, researchers are learning that different subspecies contain different apolipoproteins, proteins, and lipid contents between species which have different physiological roles. [14] For example, within the HDL lipoprotein subspecies, a large number of proteins are involved in general lipid metabolism. [14] However, it is being elucidated that HDL subspecies also contain proteins involved in the following functions: homeostasis, fibrinogen, clotting cascade, inflammatory and immune responses, including the complement system, proteolysis inhibitors, acute-phase response proteins, and the LPS-binding protein, heme and iron metabolism, platelet regulation, vitamin binding and general transport. [14]
High levels of lipoprotein(a) are a significant risk factor for atherosclerotic cardiovascular diseases via mechanisms associated with inflammation and thrombosis. [16] The links of mechanisms between different lipoprotein isoforms and risk for cardiovascular diseases, lipoprotein synthesis, regulation, and metabolism, and related risks for genetic diseases are under active research, as of 2022. [16]
Cholesterol is the principal sterol of all higher animals, distributed in body tissues, especially the brain and spinal cord, and in animal fats and oils.
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. HDL particles enlarge while circulating in the blood, aggregating more fat molecules and transporting up to hundreds of fat molecules per particle.
Low-density lipoprotein (LDL) is one of the five major groups of lipoprotein that transport all fat molecules around the body in extracellular water. These groups, from least dense to most dense, are chylomicrons, very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL) and high-density lipoprotein (HDL). LDL delivers fat molecules to cells. LDL has been associated with the progression of atherosclerosis.
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 nanometers (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, such as fats and cholesterol, from the intestines to other locations in the body, within the water-based solution of the bloodstream. ULDLs are one of the five major groups lipoproteins are divided into based on their density. 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 triglycerides 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.
Fatty acid metabolism consists of various metabolic processes involving or closely related to fatty acids, a family of molecules classified within the lipid macronutrient category. These processes can mainly be divided into (1) catabolic processes that generate energy and (2) anabolic processes where they serve as building blocks for other compounds.
Apolipoproteins are proteins that bind lipids to form lipoproteins. They transport lipids in blood, cerebrospinal fluid and lymph.
Hyperlipidemia is abnormally high levels of any or all lipids or lipoproteins in the blood. The term hyperlipidemia refers to the laboratory finding itself and is also used as an umbrella term covering any of various acquired or genetic disorders that result in that finding. Hyperlipidemia represents a subset of dyslipidemia and a superset of hypercholesterolemia. Hyperlipidemia is usually chronic and requires ongoing medication to control blood lipid levels.
Apolipoprotein B (ApoB) is a protein that in humans is encoded by the APOB gene. It is commonly used to detect risk of atherosclerotic cardiovascular disease.
Lipid metabolism is the synthesis and degradation of lipids in cells, involving the breakdown and 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 and 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 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 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.
Apolipoprotein C-III also known as apo-CIII, and apolipoprotein C3, is a protein that in humans is encoded by the APOC3 gene. Apo-CIII is secreted by the liver as well as the small intestine, and is found on triglyceride-rich lipoproteins such as chylomicrons, very low density lipoprotein (VLDL), and remnant cholesterol.
Hepatic lipase (HL), also called hepatic triglyceride lipase (HTGL) or LIPC (for "lipase, hepatic"), 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 heparan sulfate proteoglycans (HSPG), keeping HL inactive and unable to bind to HDL (high-density lipoprotein) or IDL (intermediate-density lipoprotein). 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 processes. 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 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 phospholipid capsule, and the type of protein embedded in this outer shell determines the fate of the particle and its influence on metabolism. Examples of these lipids include cholesterol and triglycerides. The concentration of blood lipids depends on intake and excretion from the intestine, and uptake and secretion from cells. 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.
Apolipoprotein A-V is a protein that in humans is encoded by the APOA5 gene on chromosome 11. It is significantly expressed in liver. The protein encoded by this gene is an apolipoprotein and an important determinant of plasma triglyceride levels, a major risk factor for coronary artery disease. It is a component of several lipoprotein fractions including VLDL, HDL, chylomicrons. It is believed that apoA-V affects lipoprotein metabolism by interacting with LDL-R gene family receptors. Considering its association with lipoprotein levels, APOA5 is implicated in metabolic syndrome. The APOA5 gene also contains one of 27 SNPs associated with increased risk of coronary artery disease.
The vertical auto profile (VAP) test is a cholesterol, lipid and lipoprotein test.
Remnant cholesterol, also known as remnant lipoprotein, is a very atherogenic lipoprotein composed primarily of very low-density lipoprotein (VLDL) and intermediate-density lipoprotein (IDL). Stated another way, remnant cholesterol is all plasma cholesterol that is not LDL cholesterol or HDL cholesterol, which are triglyceride-poor lipoproteins. However, remnant cholesterol is primarily chylomicron and VLDL, and each remnant particle contains about 40 times more cholesterol than LDL.
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