It ?-hydroxy ? methyl glutaryl CoA which in

It is insolu­ble substance and along with other substances, tends to precipitate in and along the lining of the blood vessels, thereby restricting the flow.

Ingested cholesterol is absorbed along with other lipids. It is normally present in blood to the extent of 150 to 250 mg per 100 ml, being equally distributed between the cells and the plasma. In the cells cholesterol occurs in free form, while in the plasma about 75% is found in the form of cholesterol esters.

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Cholesterol is synthesized in the body from two-carbon units in the form of acetyl CoA formed either from fatty acids or from the metabolism of the carbohydrate through pyruvate.

Two molecules of acetyl CoA condense to form acetoacetyi CoA which reacts with third molecule of acetyl CoA to form ?-hydroxy ? methyl glutaryl CoA which in turn, gives rise to the important intermediate compound called mevalonic acid which is activated by two molecules of ATP to yield 5-dlphospho-mevalonic acid (mevalonic acld-5-pyrophosphate).

The 5-diphosphomevalonic acid in the presence of ATP loses CO2 and water to form isopentenyl pyrophosphate which can also exist in an isomeric form 3, 3-dimethylellyl pyrophosphate.

These compounds are said to, be the forerunners of many important biological compounds including carotenoid pigments and cholesterol.

One molecule of 3, 3 dimethylellyl pyrophosphate now reacts with one of isopentenyl pyro­phosphate to yield geranyl pyrophosphate which with another mole­cule of isopentenyl pyrophosphate forms farnesyl pyrophosphate,with the removal of inorganic pyrophosphate at each stage.

The two mole­cules of farnesyl pyrophosphate finally condense to form the hydro­carbon squalene which by ring closure and loss of methyl groups is readily converted into cholesterol by enzymes present in the liver.

It has been estimated that 15 to 20 gm cholesterol is synthesized daily in the body of human.

The excess of cholesterol is eliminated from the body chiefly in the faeces. In the liver, cholesterol is reduced to dihydrocholesterol and coprostenol which pass into the intestine via the bile duct and are not reabsorbed.