In this article we will discuss about the Cholesterol Metabolism:- 1. Occurrence of Cholesterol Metabolism 2. Chemistry of Cholesterol Metabolism 3. Absorption 4. Normal Concentration in Blood 5. Physiological Importance in the Body 6. Factors affecting Level in Blood 7. Excretion 8. Transport 9. Biosynthesis 10. Role of HMG-CoA Reductase 11. Affect of Drugs 12. Test.
Contents:
- Occurrence of Cholesterol Metabolism
- Chemistry of Cholesterol Metabolism
- Absorption of Cholesterol Metabolism
- Normal Concentration of Cholesterol in Blood
- Physiological Importance of Cholesterol in the Body
- Factors affecting Cholesterol Level in Blood
- Excretion of Cholesterol
- Transport of Cholesterol
- Biosynthesis of Cholesterol
- Role of HMG-CoA Reductase in Cholesterol Metabolism
- Affect of Drugs on Cholesterol
- Test for Cholesterol
1. Occurrence of Cholesterol Metabolism:
Human body contains large quantities of cholesterol which are found in brain and nervous tissues. Other tissues such as liver, kidney, spleen and skin also contain fairly good amounts of cholesterol. The total amount of cholesterol is about 140 grams in the body of a man weighing 70 kg.
The greater part of the cholesterol of the body is synthesized (about 1 gram per day) whereas about 0.3 gram per day are provided by the average diet. It is not synthesized in plants. Dietary cholesterol is obtained only from animal sources like meat, liver, brain and egg yolk (a particularly rich source).
2. Chemistry of Cholesterol Metabolism:
a. Cholesterol is a white, waxy, solid found associated with fats but chemically different from them.
b. It has a parent nucleus which is said to be cyclopentanoperhydrophenanthrene nucleus.
c. It has a hydroxyl group at C3, an unsaturated bond at C5-C6, two methyl groups at C10 and C13 and 8 carbon paraffin side- chain attached to C17.
d. It is an alcohol.
e. It occurs free and combined with fatty acids by ester linkage at the hydroxyl group.
f. Cholesterol in ester form is often referred to as ‘bound’ cholesterol esters. These are normally rich in linoleic acid.
g. Steroids closely related to cholesterol include 7-dehydrocholesterol which occurs in skin and can be converted by ultraviolet radiation to vitamin D3, di-hydro-cholesterol, bile acids, hormones of the adrenal cortex and of the sex glands.
3. Absorption of Cholesterol Metabolism:
a. A great part of the ingested cholesterol is absorbed. Plant sterols are not absorbed.
b. On a high cholesterol diet, man absorbs a maximum of only 15 mg of cholesterol per kg body weight per day.
c. The cholesterol absorbed regulates the endogenous production of cholesterol which is about 14 mg per kg. body weight per day.
d. Bile is essential for its absorption.
e. Pancreatic juice plays an important role because cholesterol esterase present in it hydrolyzes the esters of cholesterol present in the diet. After absorption it is again esterified in the cell and then released for transport via the lymphatic to the thoracic duct.
4. Normal Concentration of Cholesterol in Blood:
140-220 mg per 100 ml of blood. It increases with ages and also during pregnancy.
5. Physiological Importance of Cholesterol in the Body:
a. It is the essential constituent of cells.
b. It aids in the permeability of the cells.
c. It controls the red cells from being easily hemolyzed.
d. It functions as the defensive action.
e. It transports fat to liver in the form of cholesterol ester for oxidation.
f. It assists the formation of bile acids and bile salts, 7-dehydrocholesterol and vitamin D3, corticosteroid hormones, androgens (male sex hormones), estrogens and progesterone (female sex hormones).
g. It helps the granulation of cell division.
h. It acts as an antagonist to phospholipid.
6. Factors affecting Cholesterol Level in Blood:
a. Dietary Fat:
Fats (butter fat, hydrogenated fat) containing higher saturated fatty acids cause increased serum cholesterol. But fats (corn oil, sunflower seed oil, safflower seed oil, fish oil, cotton seed oil) rich in polyunsaturated fatty acids cause marked reduction in serum cholesterol level.
b. Dietary Cholesterol:
It tends to increase the serum cholesterol level.
c. Dietary Carbohydrates:
Consumption of excessive amounts of sucrose causes an increase in serum cholesterol level.
d. Heredity:
Persons who are prone to become obese have a high level. The level becomes slightly higher in persons belonging to blood group A and AB.
e. Caloric Intake:
Intake of excess calories causes a significant increase in plasma cholesterol level.
f. Proteins:
Increase in protein intake does not change the plasma cholesterol level but low protein intake causes reduction in plasma cholesterol level.
g. Vitamin B-complex:
Nicotinic acids in large amounts causes lowering of plasma cholesterol level; whereas pyridoxine deficiency causes increased blood cholesterol level.
h. Minerals:
Magnesium salts do not bring about any change. The conversion of acetate to cholesterol is depressed by iron salts and increased by manganese salt.
i. Physical Exercise:
Physical exercise brings about a lowering in the serum cholesterol level.
j. Fibre:
Increasing the fibre content of the diet, causes the excretion of cholesterol and bile acids in the feces and brings about a significant reduction in serum cholesterol level on a high fat-high cholesterol diet.
7. Excretion of Cholesterol:
a. Half of the cholesterol eliminated from the body is excreted in the feces after conversion to bile salts.
b. Coprostanol is the principal sterol in the feces which is formed from cholesterol in the lower intestine by the bacterial flora.
c. A large portion of the biliary excretion of bile salts undergoes enterohepatic circulation. The bile salts not reabsorbed or their derivatives are excreted in the feces.
8. Transport of Cholesterol:
a. Cholesterol in the diet after absorption from the intestine is incorporated into chylomicrons and VLDL being accompanied by other lipids.
b. The greater part of cholesterol is found in the esterified form. It is transported as lipoprotein in the plasma. The highest proportion is found in the LDL.
c. Some plasma cholesteryl ester may be formed in HDL as a result of the trans-esterification reaction in plasma by lecithin: Cholesterol acyltransferase (LCAT).
d. Patients with parenchymal liver disease show a decrease of lecithin: Cholesterol acyl transferase activity and abnormalities in the serum lipids and lipoproteins.
9. Biosynthesis of Cholesterol:
Liver is the principal organ for its synthesis. Other tissues such as adrenal cortex, intestine, skin, ovary, kidney, testis also can synthesize cholesterol. The microsomal and eytosol fraction of the cell is responsible for cholesterol synthesis. It is interesting to note that the brain of the newborn can synthesize cholesterol while the adult brain cannot synthesize cholesterol.
The biosynthesis of cholesterol is divided into five stages:
a. Mevalonate, a six carbon compound, is synthesized from acetyl- CoA.
b. Isoprenoid units are formed from mevalonate by loss of CO2.
c. Six isoprenoid units condense to form squalene (C30H50).
d. Squalene cyclizes to form lanosterol, the parent steroid. |
e. Cholesterol (C27H46O) is formed from lanosterol after several steps with the loss of three methyl groups:
(1) Cholesterol synthesis is extra-mitochondrial and follows two pathways:
(i) Two molecules of acetyl-CoA condense to form acetoacetyl-CoA by cytosolie thiolase enzyme,
(ii) In liver, acetoacetate formed in the mitochondria diffuses into the eytosol and is activated to acetoacetyl-CoA by acetoacetyl-CoA synthase requiring ATP and CoA.
Acetoacetyl-CoA then combines with a molecule of acetyl-CoA to form HMG-CoA by HMG-CgA synthase.
(2) HMG-CoA is converted to mevalonate in a two stage reduction by NADPH catalyzed by a microsomal enzyme HMG-CoA reductase which catalyzes then rate-limiting steps in the pathway of cholesterol synthesis and is the site of action of the most effective class of cholesterol lowering drugs, the HMG-CoA reductase inhibitors (Statins).
(3) Mevalonate is phosphorylated by ATP to form several intermediates which by decarboxylation produces isopentenyl pyrophosphate.
(4) Farnesyl pyrophosphate is formed by the condensation of three molecules of isopentenyl pyrophosphate.
(5) Geranyl pyrophosphate is formed by the condensation of dimethylallyl pyrophosphate with another molecule of isopentenyl pyrophosphate. This, with a further condensation with isopentenyl pyrophosphate, forms farnesyl pyrophosphate. Two molecules of farnesyl pyrophosphate condense to form squalene by squalene synthetase with the help of NADPH+ H+.
(6) Squalene is then converted to squalene epoxide by squalene epoxidase. Squalene epoxide is converted to lanosterol by oxidosqualene: lanosterol cyclase.
(7) The methyl group is oxidized to CO2 from 14-des-methyl lanosterol and two more methyl groups are likely removed to produce zymosterol which on isomerisation forms Δ7,24-Cho- lestadienol. This Δ7,24-cholestadienol by the help of NADPH and O2 yields demosterol which, on reduction, produces cholesterol.
(9) The intermediates from squalene to cholesterol are attached to a special carrier protein known as the squalene and sterol carrier protein. It seems likely that it is in the form of cholesterol sterol carrier protein that cholesterol is converted to steroid hormones and bile acids and participates in the formation of membranes and of lipoproteins.
(9) Farnesyl pyrophosphate is the branch point for the synthesis of the other polyisoprenoids, dolichol and ubiquinone. The activity of HMG-CoA reductase is decreased during fasting. The activity is not reduced in diabetes mellitus. But the activity of HMG-CoA reductase is inhibited by cholesterol feeding.
Administration of insulin or thyroid hormone increases the activity of HMG-CoA reductase; whereas the administration of glucagon and glucocorticoids reduces the activity of the enzyme.
More recent experiments have shown that cholesterol synthesis is inhibited by cAMP, indicating that one or more reactions in the synthetic pathway may be controlled by a cAMP-dependent protein kinase. Plasma cholesterol in humans is made lower by reducing the amount of cholesterol in the diet. An increase of 100 mg in dietary cholesterol causes a rise of 5 mg cholesterol per 100 ml of serum.
Hypercholesterolemia has been observed in uncontrolled diabetes mellitus, impairment of liver, obstructive jaundice, glomerulonephritis, hypothyroidism and nephrosis. Hypocholesterolemia has been found in anemia, hyperthyroidism, hepatic diseases, infection, carcinoma and acute pancreatitis.
Polyunsaturated fatty acids can lower cholesterol level. Because, the cholesteryl esters of polyunsaturated fatty acids are more rapidly metabolized by the liver and other tissues which may enhance their rate of turnover and excretion.
Recently, it has been found that saturated fatty acids cause higher rates of secretion of VLDL by the perfused liver than do unsaturated free fatty acids. Cholesterol level is also decreased on the minimum intake of animal fat.
The cholesterol level in tissues is increased due to uptake of cholesterol-containing lipoproteins by receptors, e.g., the LDL receptor, uptake of free cholesterol from cholesterol-rich lipoproteins to the cell membrane, cholesterol synthesis, and hydrolysis of cholesteryl esters by the enzyme cholesteryl ester hydrolase.
The level is also decreased due to efflux of cholesterol from the membrane to lipoproteins, particularly to HDL, or nascent HDL, esterification of cholesterol by ACAT (acyl- CoA: cholesterol acyltransferase), and utilization of cholesterol for synthesis of other steroids such as hormones or bile acids in the liver.
10. Role of HMG-CoA Reductase in Cholesterol Metabolism:
a. Marked decrease in the activity of HMG-CoA reductase has been observed in fasting animals which shows the reduced synthesis of cholesterol during fasting.
b. HMG-CoA reductase in liver is inhibited by mevalonate and by cholesterol.
c. Cholesterol is considered to act by repression of transcription of the HMG-CoA reductasegene, cholesterol synthesis is also inhibited by LDL-cholesterol taken up via LDL receptors. A diurnal variation takes place in both cholesterol synthesis and reductase activity.
d. HMG-CoA reductase activity is increased by the administration of insulin or thyroid hormone, whereas its activity is decreased by glucagon or glucocorticoids.
e. HMG-CoA reductase exists in both active and inactive forms.
11. Affect of Drugs on Cholesterol:
a. Sitosterol is hypocholesterolemic agent which blocks the esterification of cholesterol in the gastrointestinal tract and thus reduces cholesterol absorption.
b. Drugs like choloxin and neomycin cause the increased fecal excretion of cholesterol and bile acids.
c. Clofibrate acts by inhibiting the secretion of VLDL by the liver or by inhibiting hepatic cholesterol synthesis.
d. The hypocholesterolemic drugs include nicotinic acid and estrogens.
e. Significant reductions of plasma cholesterol can be effected by the use of cholestyramine resin.
f. The fungal inhibitors of HMG-CoA reductase, mevastatin and lovastatin, reduce LDL cholesterol levels by up-regulation of the LDL receptors.
g. Probucol causes to increase LDL catabolism, but its antioxidant properties are more important in preventing accumulation of oxidized LDL in arterial walls. Oxidized LDL is a prime cause of atherosclerosis.
12. Test for Cholesterol:
a. Salkowski’s Test:
A little cholesterol is dissolved in 2 ml of chloroform. An equal volume of conc. H2SO4 added to it. Shaked gently, upper layer of chloroform turns red and the sulphuric acid layer assumes a yellow colour with green fluorescence.
b. Libermann-Burchard Reaction:
A crystal of cholesterol is dissolved in 2 ml of chloroform in a dry test tube. 10 drops of acetic anhydride and 2 drops conc. H2SO4 are added. Mixed well. A red rose colour develops which quickly changes through blue to green.