General Veterinary Biochemistry

Functions and classification of lipoproteins

FUNCTIONS AND CLASSIFICATIONS OF LIPOPROTEINS

• Lipoproteins perform three major functions:
• Transport of dietary fat from the intestine to other tissues
• Transport of endogenous triglycerides and cholesterol to other tissues
• Transfer of cholesterol from extrahepatic tissues to the liver.
• Based on the density lipoproteins are classified in
• Chylomicrons 
• VLDL (or pre-beta lipoprotein)
• LDL (or beta-lipoprotein) 
• HDL or (alpha - lipoprotein
• These are, important physiologically and in clinical diagnosis.
• Triacylglycerol is the  predominant  lipid in   chylomicrons   and   VLDL.
  Whereas cholesterol and phospholipids are predominant lipid in LDL and HDL respectively.
• A typical lipoprotein such as chylomicron or VLDL consists of mainly non-polar triacylglycerols and cholesteryl esters, surrounded by a single surface layer of amphipathic phospholipids and cholesterol.
• These are oriented in such a way that their polar groups face outward to the aqueous medium.
• The protein moiety of lipoprotein is known as apoprotein. (apo means detach), which controls the lipid transport in plasma and uptake of lipoproteins into the tissues.
• Chylomicrons transport dietary triacylglycerols (90%) and cholesteryl esters from the intestine to other tissues in the body. Due to the presence TG, it has low density.
• Very low-density lipoprotein transports triacylglycerols and cholesteryl esters synthesized in the liver to other tissues. It is similar to chylomicron.
• Lipoprotein lipase, which is located on the walls of blood capillaries acts on triacylglycerols present in the chylomicrons and VLDL and releases free fatty acids, which are then diffused into extrahepatic tissues.
• The fatty acids may be oxidized for producing energy or stored in the form of triacylglycerols.
• After the removal of triacylglycerols from VLDL the remnant is richer in the content of cholesteryl esters, which is known as LDL, which is finally returned to liver for uptake.
• Excess cholesterol is removed from the extra hepatic tissues by HDL for delivery to liver from there it is excreted in the form of bile salts in to the intestine.
• The transfer of cholesterol from extrahepatic tissues to the liver is called reverse cholesterol transport.