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Lipoproteins Metabolism

Lipids are not water soluble, which means that they can't enter the blood stream in their natural state. Among other roles, lipoproteins have the job of carrying lipids through the blood. Their role in metabolism is to enable lipids to be carried to the liver where they can be broken down for energy.

  1. Function

    • Lipoproteins contain lipids and protein. They are used for insulation and are also used in the structure of cell membranes and in the creation of hormones, but they primarily have the role of carrying lipids through the blood stream. Lipoproteins are classified according to their density, which is determined by the ratio of lipid to protein. The classes are chylomicrons, or very low-density lipoproteins (VLDL); low-density lipoproteins (LDL); and high-density lipoproteins (HDL). The editors of the MediLexicon state that protein makes up more than 50 percent of HDLs, while it is only 1 percent of chylomicrons. In other words, HDLs contain the least amount of lipids while chylomicrons and VLDLs contain the most.

    Metabolism Definition

    • Metabolism is a term that includes the breaking down of lipoproteins so that they can be used in the body as well as the creation of lipoproteins. Their breakdown, which is also called catabolism, begins with digestion.

    Digestion

    • During digestion dietary fats are broken down into their chemical parts--glycerol and fatty acids--so that they can be absorbed in the intestine. But they can't be carried in the blood because they're not water soluble, so they're mixed with cholesterol and covered in a layer of protein. The result is the lipoprotein called a chylomicron. The job of a chylomicron is to transport dietary lipids--cholesterol and triglycerides--to the liver and other organs.

    Catabolism

    • Once the chylomicrons enter the blood, they are broken into fragments. Most of the fragments go to the liver but larger fragments may continue to circulate in the blood. The liver breaks them down again into glycerol and fatty acids. The glycerol is easily converted into glucose and quickly used for energy. The breakdown of the fatty acids is a little more complicated. The liver breaks the fatty acids into fragments and uses a chemical process to convert the pieces into acetyl-coenzyme A. In this form they can enter more chemical reactions and be turned into energy. Any excess lipids are released back into the blood in the form of a very low density lipoprotein.

    Anabolism

    • If the body needs more lipids, the liver can rebuild them out of glucose or amino acids. Converting glucose is essentially the reverse of the catabolic process. Amino acids are converted into glucose and follow the same path.

    Lipoprotein Roles

    • Very low density lipoproteins transport triglycerides from the liver to muscle and adipose tissue for storage. Low density lipoproteins transport cholesterol to tissues other than the liver, which can contribute to heart disease. High density lipoproteins reverse the effects of cardiovascular buildup by collecting cholesterol and carrying it to the liver to be excreted in the bile.

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