What is the class function of steroid?
Steroid: widely distributed in the biological world, characterized by a β-hydroxy group on the third carbon atom and a side chain on the 17th position. Some sterols also have a Δ5 double bond. Animal embryonic tissue and skin sterols as well as plant sterols such as sterol have a double bond on the side chain. The sterol is present in the living body in free (non-esterified form) or in the form of a fatty acid ester.
Sterol is a colorless waxy solid, soluble in organic solvents such as ether, chloroform, benzene and hot ethanol. It is insoluble in water and cannot be saponified (see table). Some important sterols and their sources are listed in the table.
1 animal sterol
Cholesterol, also known as cholesterol. An important component of vertebrate cells, with the highest content in the nervous tissues and adrenal glands (percent wet weight), the lowest content in the heart, and a considerable amount in the liver, kidneys and skin. The gallstones in the human body are mainly composed of cholesterol.
Normal human plasma cholesterol content is 150-250 mg per 100 ml, and the ratio of free to esterification is about 1:3. The main esterified fatty acids are linoleic acid, followed by oleic acid, followed by saturated acids (mainly palmitic acid) and arachidonic acid.
The cholesterol content in human red blood cells is about 100 mg per 100 ml of packed cells, almost all free cholesterol; it is in dynamic equilibrium with plasma free cholesterol. The nerve tissue mainly contains free cholesterol and is not exchanged with other cholesterol stores in the body. The adrenal glands and gonads are rich in esterified forms of cholesterol.
Mammalian cells (except mature red blood cells) can synthesize cholesterol, but the rate of synthesis varies widely. The liver and small intestine are the main sites of synthesis. Animals can also take cholesterol from food. Its physiological function is not fully understood.
Cholesterol is a component of the cell membrane and organelle membrane. The plasma membranes of red blood cells, liver cells and myelin contain a considerable amount of cholesterol; it regulates the physical state of biofilm lipids and maintains the fluidity of the membrane. The nerve myelin contains a lot of cholesterol. It has a relationship with the formation of myelin, maintaining stability and function. In addition, cholesterol is a precursor to the synthesis of bile acids and steroid hormones. 7-dehydrocholesterol is present under the skin of animals and can be converted to vitamin D3 under ultraviolet light.
2 phytosterols
Plant sterols are important components of plant cells and cannot be absorbed and utilized by passive objects. Both sterol and sitosterol are derivatives of myristal, two more carbon atoms than cholesterol and one ethyl group on C-24.
3 yeast sterol
Ergosterol is the major sterol component of yeast and ergot. A steroid compound containing 28 carbon atoms. It is converted into vitamin D2 by ultraviolet light irradiation. Vitamin D plays a key role in the homeostasis of calcium and phosphorus in animals.
Bile acid bile alcohol
Bile acids and bile alcohols: Degradation products of cholesterol in mammals and other vertebrates. Higher vertebrates (mammals, birds, snakes) contain C24 bile acids in the bile, forming a conjugate bile salt with taurine or glycine. The bile of the lower original vertebrates contain C27 bile acids and/or C26 or C27 bile alcohols, some form esters with sulfuric acid, and some form bile salts with taurine. From the perspective of biological evolution, Haslwood proposed that animal evolution has a certain relationship with the molecular evolution of bile alcohol and bile acids. The molecular evolution process is as follows: C27 (and C26) alcohol (sulfate) - → C27 acid (and taurine-bound compound) - → C24 acid (and taurine-bound compound) - → C24 acid (and glycine-binding compound) The physiological function of bile salts is to secrete into the intestine to promote the emulsification and absorption of lipids. Certain liver diseases and obstructive jaundice cause a decrease in bile secretion that affects the digestion and absorption of lipids.
