HDL is short for high-density lipoprotein, though it's more commonly called "good" cholesterol. Actually, HDL isn't a kind of cholesterol at all --- instead, it's a cholesterol transporter that helps to clear excess cholesterol out of the body cells and arteries. High levels of HDL increase cardiovascular health. The metabolism of HDL is the mechanism by which you produce these transporters.
Metabolism
Metabolism is the chemical name for the sum of all reactions that take place in the body. With specific regard to HDL, the metabolism of this important lipoprotein refers to the means by which you produce and process HDL particles in the body and in the bloodstream. As with other metabolic processes, HDL metabolism depends intimately upon the activity of various enzymes, which are molecules that help chemical reactions take place faster than they otherwise would, and upon signaling molecules.
HDL Production
HDL particles help carry cholesterol from the body cells to the liver for excretion, but they themselves are made in the liver, explains Dr. Lauralee Sherwood in her book "Human Physiology." The word "lipoprotein" means that HDL particles are a combination of protein and lipids, or fats. The fats are phospholipids, which are phosphorus-containing fatty molecules that the body uses to form membranes. As such, HDL starts off as a hollow ball of protein and phospholipid that gets filled with cholesterol as it travels around the body.
In The Blood
Because HDL particles leave the liver empty, they start off relatively flat. They bind to receptors on cell membranes, where a receptor is a protein-based functional area of a cell that helps cells communicate with one another, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." Once HDL binds to the receptor on a body cell, it can pull cholesterol from the body cell into its interior, causing it to fill and become spherical. HDL particles become larger as they travel through the bloodstream, eventually returning to the liver.
Role in Heart Health
One of the most important features of HDL is its ability to take up cholesterol from macrophages in arteries and return the cholesterol to the liver for metabolism and excretion. Macrophages are part of the immune system, explains Sherwood, and they act a bit like scavengers in their ability to pick up molecules and foreign particles and sequester them. In individuals with too much blood cholesterol, macrophages in the arteries become filled with cholesterol, which leads to atherosclerosis, or hardening of the arteries. By interacting with receptors on macrophages, HDL particles fill with cholesterol, helping to reverse atherosclerosis.
Other Metabolic Reactions
HDL participates in reactions aside from simple removal of cholesterol, making them very metabolically active. The HDL particles have high concentrations of a variety of proteins in their membranes, explain Garrett and Grisham. Among these are proteins that help the blood resist clotting in the vessels and that help prevent so-called oxidative stress. Oxidative stress occurs when body cells are exposed to toxins and various chemical agents and can lead to arterial and cellular damage.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Biochemistry"; Reginald Garrett, Ph.D., and Charles Grisham, Ph.D.; 2007
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