13.9 What are steroids?

1. 13.9 What are steroids? • Third major class of lipids. • It is a compound that contains three cyclohexane rings • Cholesterol • The most abundant steroid in the human body. • The most important steroid in the human body. • Serves as a plasma membrane component in all animal cells. • Red blood cells.

2. Cholesterol Continued • Second most important function is to serve as a raw material for the synthesis of other steroids. • Sex and adrenocorticoid hormones and bile salt • It exists in both free form and esterfied with fatty acids. • Gallstones • Cholesterol is necessary for human life. • It circulates constantly in the blood.

3. Since it is hydrophobic it needs a water soluble carrier to circulate in the aqueous medium of blood.

4. B. Lipoproteins carriers of cholesterol • Cholesterol and fat are transported by lipoproteins. • Most have a quart of hydrophobic lipid molecules surrounded by a shell of hydrophilic molecules. • Proteins and phospholipids

5. Lipoproteins • Four kinds • High-density lipoprotein (HDL) “Good Cholesterol”- 33% protein and 30% cholesterol • Low-density lipoprotein (LDL) “Bad Cholesterol”- 25% protein and 50% cholesterol • Very-low-density lipoprotein (VLDL)-Carries triglycerides synthesized by the liver. • Chyomicrons- Carry dietary lipids synthesized in the intestines.

6. C. Transport of Cholesterol in LDL • Transport of cholesterol from the liver starts out as large as VLDL particle • Core of VLDL contains triglycerides and cholesteryl esters • Mostly cholesteryllinoleate • Surrounded by a polar coat of phospholipids and proteins • VLDL is carried into the serum • Capillaries reach muscle or fat tissues triglycerides and all proteins are removed • Diameter shrinks and its core only contains cholestery esters • The removal of fat increases its density and it becomes LDL • Stays in plasma for 2.5 days

7. Transport of Cholesterol in LDL continued • LDL carries cholesterol to the cells • LDL- receptor molecules line the cell surface in coated pits • ApoB-100 protein binds to the LDL-receptor molecules in the coated pits • LDL- is taken inside the cell and enzymes breakdown the lipoprotein • Micheal Brown and Joseph Goldstein discovered if the LDL- receptors are not sufficient in number cholesterol accumulates in the blood

8. D. Transport of Cholesterol in HDL • High-density lipoprotein transport cholesterol from peripheral tissues to the liver and transfers cholesterol to LDL • In the serum free cholesterols in HDL are converted to cholesteryl esters • HDL binds to the liver cell surface and transfers its cholersteryl ester to the cell • Depleted HDL reenters the circulation

9. E. Levels of LDL and HDL • Cholesterol is insoluble in water • If its level is elevated in the blood serum plaque-like deposits may form on the inner surfaces of the arteries • Diameter of blood vessels decrease and blood flow decreases • Atherosclerosis • May lead to heart attack, stroke, kidney dysfunction

10. Continued • Most cholesterol is transported by low-density lipoproteins • When concentration of cholesterol molecules is high,less LDL is taken into the cells from the plasma • LDL concentration in the plasma rises • When cholesterol level inside the cell is low synthesis of the LDL receptor increases

11. When the cholesterol level inside the cells is low, the synthesis of the LDL receptor increases. • Familial Hypercholesterolemia • The cholesterol in the plasma may be as high as 680 mg/100mL , compared to 175mg/100mL • In general high LDL content means high cholesterol content in the plasma. • The serum cholesterol level controls the amount of cholesterol synthesized in the liver.

12. Diets in low cholesterol and saturated fatty acids usually reduce the serum cholesterol level. • Drugs can block the synthesis of cholesterol inside the cells and stimulate the synthesis of LDL- receptor proteins. • It is generally considered desirable to have high levels of HDL and low levels of LDL in the blood stream.