Sterols

Sterols

Sterols are compounds containing a cyclic nucleus namely cyclopentanoperhydro phenanthrene (CPPP) and one or more hydroxyl groups. They are widely present in animal and plant tissue.

1. Cholesterol

Cholesterol is exclusively found in animals and is the most abundant animal sterols. It is widely distributed in all cells and is a major component of cell membrane and lipoproteins. In human beings, it is very important to control the normal level of cholesterol in blood.

Structure

Cholesterol is a C27(C₂₇H₄₆O) compound. It has one hydroxyl group at C3 and a double bond between C5 and C6. An aliphatic side chain is attached to C17. Cholesterol contains a total of 5 methyl groups (Fig. 6.1).

Cholesterol is the precursor of various physiologically important compounds such as bile acids, vitamin-D, steroid hormones etc.

Properties

They are white shining rhombic plate like crystals.

·           It is tasteless and odourless

·           It has a high melling point of 150°C.

·           It is insoluble in water and soluble in fat solvents.

·           It is a poor conductor of heat and electricity and serves as an insulator against electric charge. In brain, where it is present abundantly, it acts as an insulator against nerve impulse which are electrical in nature.

·           Cholesterol, when oxidised under suitable conditions, undergoes rapid oxidation to form a ketone-cholestenone.

·           The hydroxyl group of cholesterol readily forms ester with fatty acids, stearic acid etc.

·           It gives addition reactions such as hydrogenation and halogenation because of the presence of double bond.

Physiological importance of cholesterol

·           It is one of the essential constituents of cells.

·           It influences the permeability functions of the cell.

·           It controls the redcells from being easily hemolyzed.

·           It performs defensive action.

·           It assists the formation of bile acids and bile salts, 7- dehydrocholesterol, vitamin D3, corticosteroid hormones,androgens, estrogens and progesterone.

·           It acts as an antagonist to phospholipid.

2. Ergosterol

Ergosterol occur in plants. It is also found in yeast and fungi as the structural constituent of membranes. It is an important precursor for vitamin-D. When exposed to light, it is converted to ergocaliciferol, a compound containing vitamin-D activity.

Its structure is similar to that of cholesterol, but differs in the following aspects.

·           It has another double bond at C7-C8.

·           It has a double bond in the side chain.

·           It has an additional CH₃ group in the side chain (Fig 6.2).

3. Stigma sterol –

It is structurally similar to that of ergosterol except at C7 (Fig. 6.3) Stigmasterol and its derivatives sitosterols are probably the mostcommon sterol of plants. The important sources are soya bean and calabar beans.