 The third major class of lipids  The three cyclohexane rings (A, B, C) are connected to a cyclopentane ring (D).  Significantly different structure from other lipids  Not necessarily esters

 Cholesterol is the most important and most abundant steroid in the human body  Necessary for human life  Serves as a plasma membrane in all animal cells  Serves as raw material for the synthesis of other steroids › Such as sex and adrenocorticoid hormones  Exists in both free form and esterified with fatty acids.

 Cholesterol levels in the body are regulated by the liver.  In a dynamic state in the body, circulating in the blood  Cholesterol is hydrophobic, so it needs a water-soluble carrier to circulate in the blood.

 Lipoproteins transport cholesterol  Most contain a core of hydrophobic lipid molecules, surrounded by a shell of hydrophilic molecules. › Such as proteins and phospholipids

 High-density lipoprotein (HDL) › (“good cholesterol”) › Consists of about 33% protein and about 30% cholesterol  Low-density lipoprotein (LDL) › (“bad cholesterol”) › Consists of 25% protein and 50% cholesterol  Very-low-density lipoprotein (VLDL) › Mostly carries triglycerides (fats) synthesized by the liver  Chylomicrons › Carry dietary lipids synthesized in the intestines

 Transport of cholesterol from the liver starts out as a large VLDL particle.  The core of a VLDL contains triglycerides and cholesterol esters (largely cholesteryl linoeate. And is surrounded by polar phospholipids and proteins.  Fat is removed from the VLDL particles and the diameter decreases, and only cholesteryl esters are present.  Because fat is removed the density increases and the VLDL particle becomes a LDL particle.  LDL’s stay in plasma for 2.5 days.

 LDLs carry cholesterol to specific cells located in areas known as coated pits.  After the apoB-100 proteins binds specifically with the molecules in the coated pits, the LDL is taken inside the cell, where enzymes break down the lipoprotein, liberating free cholesterol.  LDL receptors must be sufficient in number, or cholesterol with build up in the blood; even with a low intake of dietary cholesterol.  Cholesterol levels are determined by both genetics and diet.

 HDL transports cholesterol peripheral tissues to the liver, transferring cholesterol to LDL.  Free cholesterols are converted to cholesteryl esters in HDL.  For the synthesis of bile acids and steroid hormones the esterified cholesterols are delivered to the liver.  Differing from LDL, HDL is bonded to the liver cell surface and transfers its cholesteryl esters.  The HDL will reenter the circulation due to its depleted lipid content.  High levels of HDL in the blood are desirable.

 Cholesterol, like all lipids, is insoluble in water.  Plaque-like deposits may form on the inner surfaces of the arteries, if cholesterol levels are elevated in the blood serum.  If the diameter of the blood vessels is decreased, blood flow will also decrease. This condition is known as atherosclerosis. This can lead to heart attacks, stroke, and organ failure.  Blockage may deprive cells of oxygen, ceasing function. The death of heart muscles is called myocardial infarction.

 Most cholesterol is transported by LDL  LDL concentration in the blood plasma drops, when LDL is removed from circulation.  When the concentration of cholesterol is high in the cells, the synthesis of the LDL receptor is suppressed, just as when the cholesterol concentration is low, the synthesis of LDL receptors increases.  Familial hypercholesterolemia is a disease contracted when there is a lack of LDL receptors.  High LDL content means high cholesterol content in the plasma.  High LDL content and low HDL content is a warning for atherosclerosis.

 The amount of cholesterol synthesized by the liver is controlled by the serum cholesterol level.  When serum cholesterol is high, synthesis is at a low and visa versa.  Low cholesterol diets can reduce serum levels but a number of drugs such as Lipitor and Zocor can inhibit levels of cholesterol in the liver.  These drugs block the synthesis of cholesterol inside the cells.

 It is desired to have high levels of HDL and low levels of LDL.  HDL lipoproteins carry cholesterol from plaques deposited in the arteries to the liver.  HDL levels can be increased by exercises and weight loss.