13.9 Cholesterol

Cholesterol

•Most abundant steroid in the human body•Two main functions:

▫Plasma membrane in all cells Example: red blood cells

▫Raw material for synthesis of other steroids Example: sex and adrenocorticoid hormones

•Exists in free form and esterified with fatty acids▫Example: gallstones contain free

cholesterol

•Necessary for human life•A body’s liver manufactures cholesterol to

satisfy needs without dietary intake•Reduces production rate by half if

cholesterol level exceeds 150mg/100mL•Cholesterol is constantly circulating in

blood▫Need water-soluble carrier to circulate in

the aqueous medium of blood

*Lipoproteins: carriers of cholesterol

•Cholesterol is transported by lipoproteins•4 kinds of lipoproteins

▫High-density lipoprotein (HDL) “Good Cholesterol” Consists about 33% protein & 30%

cholesterol▫Low-density lipoprotein (LDL) “Bad

Cholesterol” Contains only 25% protein, but 50%

cholesterol

▫Very-low-density Lipoprotein (VLDL) Carries triglycerides (fats) synthesized by the

liver▫Chylomicrons

Carry dietary lipids synthesized in the intestines

Transportation of Cholesterol in LDL

•Transports cholesterol from liver starts as VLDL▫Contains: triglycerides, cholesterol esters,

cholesteryl linoleate▫Surrounded by phospholipids and proteins

•Carried to serum•Fat is removed, and density increases

•LDL carries cholesterol to the cells▫LDL receptors line the walls in

concentrated areas Coated pits

•Enzymes break down the now formed lipoprotein▫Freeing cholesterol from cholesteryl esters

•Without a sufficient number of LDL receptors, cholesterol would accumulate in the blood

*Transportation of cholesterol in HDL

•HDL transports cholesterol from peripheral tissues to the liver▫Transfers cholesterol to LDL

•Good to have a high level of HDL▫Removes cholesterol from blood stream

*Levels of HDL and LDL

•Insoluble in water•Most cholesterol is transported by LDL•High cholesterol inside cells cause the

synthesis of LDL receptor to reduce▫And vice versa

•High levels of LDL and low levels of HDL are symptoms of faulty transport▫Greater risk of atherosclerosis

Leads to heart attack, stroke, of kidney dysfunction

•Serum cholesterol levels control amount synthesized by liver▫High levels mean that synthesis is low▫Low levels mean that synthesis is high

•High levels of HDL are good, and low levels of LDL

•Atherosclerosis▫Plaque build-up in the arteries that

decrease blood flow▫My cause heart attack, stroke, of kidney

dysfunction