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Lipoproteins

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Everyone has heard the term "good" and "bad" cholesterol... But what exactly does that mean? Usually the doctor is not talking about cholesterol per say, but rather about molecules that contain a certain percentage of cholesterol. These molecules are known as "lipoproteins".

If we break the term lipoprotein down we get "lipo", meaning lipid (ie: fatty acids and/or cholesterol) and "protein", which is exactly what a lipoprotein molecule is: a conglomerate of different lipids and proteins.

The lipid component of lipoproteins generally refers to triglycerides (fatty acids connected to a glycerol backbone) and cholesterol. There are numerous protein molecules that associate with lipoproteins. Their function is to bind to different cells in the body so that the fat content of the lipoprotein can be utilized.

There are many different types of lipoprotein molecules. Each type has a different percentage of fat and protein. The more fatty material present (ie: cholesterol and triglycerides), the lower the density of the lipoprotein molecule. We will talk about four different lipoproteins from least dense (chylomicrons) to most dense (HDL).

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Types of Lipoproteins

Chylomicrons || VLDL || LDL || HDL

Chylomicrons

The first and least dense (ie: highest content of fatty material) are lipoproteins known as chylomicrons. Chylomicrons are created in intestinal cells after fat has been absorbed from the diet. From there, they are released into the bloodstream. The fat content (mostly triglycerides) in chylomicrons is absorbed by many different cells in the body such as the liver, heart, skeletal muscle, and adipose tissue. These tissues use the fat as fuel, or store it for later use. In essence, you can think of chylomicrons as a delivery boy for fat.

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Very Low Density Lipoprotein (VLDL)

The next most dense are very low density lipoprotein molecules (VLDL). These molecules are produced by the liver, and like chylomicrons, serve as a reservoir of fat content. They predominately carry triglycerides. Once secreted from the liver they are slowly cleaved into smaller and smaller particles as the fat content is utilized by different body tissues. Once the triglyceride content has been utilized they become LDL particles.

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Low Density Lipoprotein (LDL)

Low density lipoprotein molecules are composed mostly of protein and cholesterol. Unlike chylomicrons and VLDL they do not deliver fat to peripheral tissues, but instead deliver cholesterol.

LDL is "bad"
cholesterol

Cholesterol (in moderation of course) is a vital component of cellular membranes. LDL is known as "bad" cholesterol because it is highly atherogeneic. For more information on why LDL is bad, read the article on atherosclerosis.

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High Density Lipoprotein (HDL)

HDL is "good"
cholesterol

There are several functions that HDL performs. HDL particles serve as a reservoir for proteins used by other lipoprotein molecules such as apo C-II (used by chylomicrons and VLDL) and apo E (used by chylomicron remnants and IDLs). HDL in essence transfers these proteins to the other lipoprotein molecules floating around in the blood stream, which allows each lipoprotein molecule to perform its role more efficiently.

So why is HDL considered the "good" cholesterol? Because of its role in cholesterol metabolism. HDL molecules are capable of removing free cholesterol from the blood, and are also involved in removing cholesterol from peripheral tissues and delivering it back to the liver where it is excreted into the bile. Thus HDL can be thought of as a transporter of "toxic" cholesterol.

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Overview

Lipoproteins are composed of lipids (either fatty acids or cholesterol) and proteins. There are several different types of lipoprotein molecules based on the percentage of lipid to protein. LDL is considered "bad" lipoprotein because it is the most atherogeneic. HDL is considered "good" lipoprotein because it can remove cholesterol from peripheral tissues and blood and deliver it back to the liver for excretion.

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- Acute coronary syndromes and heart attack

- Atherosclerosis

- Cerebrovascular accident (stroke)

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