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) Chylomicrons 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 Section D

*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