In as much as most lipids are insoluble in aqueous media transport of the substances in blood plasma is achieved differently from water soluble molecules. Lipids are not transported in free form in blood but bound to protein in form of lipoprotein. Lipoproteins are lipids related with specific proteins. These lipoproteins are of different characteristics, kinds and chemical composition.
Definition of lipoproteins:
The lipoprotein is the multicomponent complex of protein and lipids of characteristic density, molecular weight, size and chemical composition. These complexes of protein and lipids are held together by non covalent forces. While the certain typical chemical composition and molecular weight exists for each kind of lipoprotein complex, there may exist no exact stoichiometry among components of complex.
Classification of lipoproteins (plasma):
Categorization of plasma lipoproteins is difficult as physical and chemical characteristics of these complexes are frequently heterogenous. Though, most popular system for categorization of plasma lipoprotein particles is based on criterion of density that is a reflection of their lipid content. Four density classes of plasma lipoprotein are known widely in humans. They comprises high density lipoproteins (HDL) the low density lipoproteins (LDL) the very low density lipoproteins (VLDL) and chylomicrons. LDL is further classified in LDL1 and LDL2 or Intermediate density lipoproteins (IDL) and chylomicrons.
Composition of lipoproteins (plasma):
Lipid fraction of plasma lipoproteins has significant amount of triacylglycerols, phospholipids, free cholesterols, and cholesterol esterified with long chain fatty acids and other lipids present in small amount. When protein components of lipoprotein are separated from their lipid components by extraction of lipid with the organic solvent, isolated protein (apolipoproteins) can be illustrated by immunological and chemical characterization to be at least seven distinct kinds. Apolipoprotein isolated from plasma HDL is majorly apolipoprotein A. Apolipoprotein B (ApoB) is major protein for LDL fraction . The third, apolipoprotein C (Apo C), is found predominatly in VLDL. Other chemically distinct apolipoprotein are apolipoproteins D & E (ApoD) and (ApoE). ApoD is found in HDL, where it is minor component. Apo E is found in chylomicrons, also in low concentration.
Structure of lipoproteins:
Structure of lipoprotein molecules have been investigated with the wide range of methods comprising electron microscopy, X-ray diffraction and spectrophotometric methods. Though these methods were unable to give the definitive structures of plasma lipoproteins. Structure of lipoproteins can be effortlessly visualized as the complex of organic matter having a composite of lipids and proteins with polar side of complex facing outside towards water solvents and apolar parts embedded inside complex
Function of lipoproteins:
Each class of lipoprotein has the specific function, determined by its point of synthesis, lipid composition and apolipoprotein content. Chylomicrons are engaged in movement of dietary triacylgycerols from intestine to other tissues, they are the largest but least in density and have high proportion of triacylglycerols.
VLDL When diet has more fatty acids than are required immediately as fuel, they are converted to triacylglycerols and exported from liver as VLDL. In addition, VLDL have some cholesterol, cholesteryl ester.
LDL Loss of triacylglycerol converts some VLDL to VLDL remnants (also known as IDL) and with further removal of triacylglycerols, to LDL. LDL is very rich in cholesterol and cholesteryl esters and thus transports cholesterol to extrahepatic tissues which have specific plasma membrane receptors.
HDL Fourth class of major lipoprotein is HDL that essentially picks up cholesterols stored in extrahepatic tissues and carries it to liver. It can also pick up cholesterol from liver for conversion in bile salts. Additionally HDL particles convert cholesterol and phosphatidylcholine of chylomicron and VLDL remnants to cholesteryl esters.
Plasma lipoprotein particles:
Role of lipoprotein particles is to transport triacylglycerols (a.k.a. triglycerides) and cholesterol in blood between all tissues of the body. The most common being liver and adipocytes of adipose tissue. Particles are synthesized in small intestine and liver, but interestingly not in adipocytes.
All cells employ and rely on fats and cholesterol as building-blocks to create multiple membranes which cells employ both to manage internal water content and internal water-soluble elements and to systematize internal structure and protein enzymatic systems.
Lipoprotein particles contain hydrophilic groups of phospholipids, cholesterol and apoproteins directed outward. These characteristics make them soluble in salt water-based blood pool.
Interaction of proteins forming surface of particles (with enzymes in blood; with each other; and with specific proteins on surfaces of cells) finds whether triglycerides and cholesterol will be added to or eliminated from lipoprotein transport particles.
Regarding atheroma development and progression as opposed to regression, main issue has always been cholesterol transport patterns, not cholesterol concentration itself.
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