How Does Hdl Perform Its Job?

Cholesterol is carried in the body by two main forms of lipoproteins: low-density lipoproteins (LDL) and high-density lipoproteins (HDL). LDL transports cholesterol to the body's tissues. HDL, on the other hand, is engaged in reverse cholesterol transport and carries about a quarter to one-third of blood cholesterol from the tissues to the liver for excretion. It is therefore considered the "good" cholesterol, as high levels of HDL seem to be cardioprotective.

Function

HDL's role is called reserve cholesterol transport. It removes unesterified cholesterol from cells and other lipoproteins and returns it to the liver to be excreted in bile. Some experts at the American Heart Association also believe that HDL removes excess cholesterol from arterial plaque, preventing its buildup. HDL is able to perform this job because of two key properties: its ability to bind to liver and non-liver cells and the presence of a surface protein that stimulates the activity of an enzyme that pulls cholesterol out of cells.

HDL Binding

HDL can bind to receptors on both liver and non-liver cells. The receptors on these cells may be specific for HDL, but there are other LDL receptors to which HDL can bind, meaning that HDL can compete with LDL at its receptor site. HDL also plays a role in the immune system because of its ability to bind to toxic substances in the blood. As a result, It is sometimes used as a treatment against sepsis.

LCAT Enzymes

The apoA-1 component, which is a protein on the surface of the HDL particle, stimulates an enzyme called lecithin, a cholesterol acyltransferase that forms cholesteryl esters from free cholesterol. This enzyme, simply referred to as LCAT, promotes the net transfer of cholesterol out of non-liver cells and other lipoproteins. These cholesteryl esters are then transported directly to the liver to be excreted in the bile as bile salts.

Particle Self-Assembly

HDL particles are self-assembling, according to the HDL Function and Composition Projects at UC Davis, meaning that they can be synthesized from a variety of combinations of lipids along with various HDL-associated proteins that have different functions. This specific composition enhances HDL's ability to participate in reverse cholesterol transport and increases its potential to protect against pathogens. These reconstituted HDL particles may play a therapeutic role in the treatment of various diseases.