The most common way for lipoproteins to be classified is by density, although there are also systems of classification according to the type and diagnostic tests. This article will outline the different types of lipoprotein by density, or size, and then discuss the details of other classification systems.
As a general rule, lipoproteins with a higher fat to protein ratio are larger and less dense. Different sizes and types can be identified through processes of electrophoresis and ultracentrifugation.
High-Density Lipoproteins (HDL)
HDL has a density greater than 1.063 g/mL and ranges in diameter from 5 to 15 nm. In a young and otherwise healthy individual, it is composed of approximately 33% protein, 30% cholesterol, 29% phospholipid and 4% triaglycerol.
The role of HDL is to collect hat molecules such as phospholipids, cholesterol and triglycerides in the cells of the body and transport it to the liver to be broken down. HDL is sometimes known as “good” cholesterol because high concentrations of this lipoprotein usually correspond to healthier blood vessels and lower risk of atherosclerosis.
Low-Density Lipoproteins (LDL)
The density of LDL is between 1.019 and 1.063 g/mL and ranges in diameter from 18 to 28 nm. In a young and otherwise healthy individual, it is composed of approximately 50% cholesterol, 25% protein, 21% phospholipid and 8% triaglycerol.
LDL is important for the transport of fat molecules such as phospholipids, cholesterol and triglycerides around the body. It is sometimes known as “bad” cholesterol because raised concentrations tend to be associated with progression of atherosclerosis.
There are further subtypes of LDL, including large buoyant LDL (lb LDL) and small dense LDL (sd-LDL).
Intermediate Density Lipoproteins (IDL)
The density of IDL is between 1.006 and 1.019 g/mL and ranges in diameter from 25 to 50 nm. In a young and otherwise healthy individual, it is composed of approximately 31% triaglycerol. 29% cholesterol, 22% phospholipid and 18% protein. This type of lipoprotein is not usually present in the blood when fasting.
Very Low-Density Lipoproteins (VLDL)
VLDL has a density between 0.950 and 1.006 g/mL and ranges in diameter from 30 to 80 nm. In a young and otherwise healthy individual, it is composed of approximately 50% triaglycerol. 22% cholesterol, 18% phospholipid and 10% protein.
This lipoprotein is responsible for the transportation of synthesized triglyceride from the liver to the adipose tissue where it can be stored.
The density of chylomicrons is less than 0.95 g/mL and ranges in diameter from 100 to 1000 nm. In a young and otherwise healthy individual, it is composed of approximately 84% triaglycerol. 8% cholesterol, 7% phospholipid and less than 2% protein.
These lipoproteins are responsible fro the transpotation of triglycerides in the gastrointestinal tract throughout the body, such as to the liver, skeletal tissue and adipose tissue.
Classification by Type: Alpha and Beta
In addition to classification by density, lipoproteins can be categorized according to their type as alpha or beta. This can be identified through serum protein electrophoresis and is often utilized in the description of certain lipid disorders.