HDL

High-density lipoprotein (HDL) is one of the five major groups of lipoproteins, which, in order of size, are chylomicrons, very-low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL), and HDL. Lipoproteins are complex particles composed of multiple proteins, typically 80–100 proteins per particle (organized by one, two, or three apolipoprotein molecules) which transport all fat molecules (lipids) around the body within the water outside cells. They are typically classified by their density: the more lipid, the less dense the particle.

Structure

HDL particles are the smallest and densest of the lipoproteins because they contain the highest proportion of protein. They are synthesized in the liver and intestines. HDL particles are heterogeneous in size and composition and can be divided into subfractions. The major protein component of HDL is apolipoprotein A-I (ApoA-I), which plays a critical role in the metabolism of HDL.

Function

HDL is often referred to as "good cholesterol" because high levels of HDL cholesterol are associated with a lower risk of cardiovascular disease. HDL particles are involved in the process of reverse cholesterol transport, which is the transport of cholesterol from the tissues back to the liver for excretion. This process helps to remove excess cholesterol from the bloodstream and tissues, including the walls of arteries, thereby reducing the risk of atherosclerosis.

Metabolism

HDL metabolism involves several steps:

• Nascent HDL particles are synthesized in the liver and intestines.
• These particles acquire cholesterol and phospholipids from cells and other lipoproteins through the action of the enzyme lecithin–cholesterol acyltransferase (LCAT).
• Mature HDL particles transport cholesterol to the liver, where it can be excreted in the bile or converted to bile acids.

Clinical Significance

Low levels of HDL cholesterol are associated with an increased risk of cardiovascular disease. Factors that can lower HDL levels include smoking, being overweight, and physical inactivity. Conversely, factors that can raise HDL levels include regular physical activity, weight loss, and a healthy diet. Certain medications, such as statins, can also help to increase HDL levels.

Related Pages

• Lipoprotein
• Cholesterol
• Cardiovascular disease
• Apolipoprotein
• Atherosclerosis
• Lecithin–cholesterol acyltransferase
• Statin