Cholecalciferol, also known as vitamin D3 , is the most important form of vitamin D , which mainly regulates calcium and phosphorus metabolism in the body.
Vitamin D is a fat-soluble vitamin that belongs to cholesterol , mainly in liver, milk and egg yolk, and is most abundant in cod liver oil . Vitamin D comes in two forms, vitamin D: calciferol or ergocalciferol ; and vitamin D3, cholecalciferol.
Vitamin D2 is converted from a plant alcohol, ergosterol (24-methyl-22-dehydro-7-deoxycholesterol). Under the action of ultraviolet light , ergosterol breaks its intramolecular B ring and converts it into vitamin D2.
Vitamin D3 is converted from 7-dehydrocholesterol in the skin by ultraviolet radiation. Vitamin D3 is the natural form of vitamin D, but not its biologically active form.
what is cholecalciferol used for
The role of vitamin D The symptoms of vitamin D deficiency in the body are osteomalacia and osteomalacia. The role of vitamin D is to increase blood calcium and phosphorus levels. Deficiency of vitamin D will lead to insufficient supply of calcium and phosphorus. Disorders causing bone mineralization. Its main functions are: ①Promote the absorption of calcium and phosphorus in the small intestine . It mainly stimulates the active transport of calcium on the surface of the microvilli of the small intestinal mucosa . It is generally believed that 1,25-(OH)2-D3 binds to receptor proteins in the cytosol of intestinal epithelial cells , and then this complex interacts with the nucleus to cause message transcription , forming a "calcium transporter " in cells . This transporter acts on the surface of microvilli to facilitate calcium absorption. In addition, vitamin D3 stimulates the production of a calcium-binding protein on the surface of the microvilli . The rate of Ca2+ uptake by intestinal epithelial cells is proportional to the number of calcium binding proteins . 1,25-(OH)2-D3 also promotes the absorption of phosphorus in the small intestine. ②1,25-(OH)2-D3 can promote the reabsorption of phosphorus and calcium in renal tubules . Vitamin D-sensitive calcium-binding proteins have been demonstrated in both chicken and rat kidneys. ③1,25-(OH)2-D3 plays an important role in bone calcium mobilization and bone salt deposition. It decalcifies the formed bone, which isPhysiological conditions require the presence of parathyroid hormone . Since the ribonucleic acid synthesis inhibitor , actinomycin D , can completely eliminate the role of vitamin D in mobilizing bone calcium, it can be considered that the role of vitamin D requires transcription to generate certain proteins that mobilize calcium .
25 -hydroxyvitamin D3 (25-OH-D3)
After vitamin D enters the blood circulation , it is mainly stored in the liver. In the liver, vitamin D3 is catalyzed by D3-25 -hydroxylase (requires the presence of NADPH, O2 and Mg2+) to generate 25-hydroxyvitamin D3 (25-OH-D3). The biological effect of this substance is 3 to 5 times greater than that of vitamin D3, but it has no physiological function at physiological doses, and it needs further metabolism . 25-OH-D3 enters the blood from the liver , binds to a specific globulin in the plasma , and is transported to the kidney . 1, 25 -dihydroxyvitamin D3, [1, 25-(OH) 2-D3] is produced by the action of enzyme , iron-sulfur protein , and cytochrome P-450 . 1,25-(OH)2-D3 is the most effective form of vitamin D3, and its biological effect is about 8 to 10 times that of vitamin D3. Animals without kidneys cannot produce 1,25-(OH)2-D3, so the kidney is the only place for 25-OH-D3 to undergo hydroxylation at the 1α position. Since 1,25-(OH)2-D3 is generated in the kidney and enters the blood circulation, and then exerts physiological effects in distant organs (such as the small intestine and stomach) , 1,25-(OH)2-D3 can also be converted into seen as a hormone .
Under certain conditions, 25-OH-D3 can be converted into 24,25-(OH)2-D3 by the action of 24-hydroxylase. 24-hydroxylase can also convert 1,25-(OH)2-D3 Hydroxylated to generate 1,24,25-(OH)3-D3. The physiological role of these 24-hydroxylated vitamin D3 is unclear, and they may be metabolic intermediates of inactivation of the vitamin D3 molecule. The final metabolite of vitamin D3 is excreted in bile . Mammalian and bird vitamin D2 is metabolized to 25-OH-D2. In rats and chickens, 25-OH-D2 was also found to become 1,25-(OH)2-D2. This substance has the same biological effect as its vitamin D3 counterpart in rats. In chickens, its activity is only equivalent to 1/10 of the corresponding vitamin D3.
negative feedback regulation
The production of 1,25-(OH)2-D3 regulates the production of the active form of vitamin D. The production of 1,25-(OH)2-D3 can be regulated by some negative feedback.
For example, 25-OH-D3 can inhibit the action of 25-hydroxylase and inhibit the 25-hydroxylation of D3; 1,25-(OH)2-D3 can inhibit 1α-hydroxylase and inhibit 1,25-(OH) ) 2-D3 generation. On the other hand, the metabolic process of vitamin D3 can also be modulated.
For example, when 1,25-(OH)2-D3 inhibits 1α-hydroxylase, 24-hydroxylase can be induced to produce 1,24,25-(OH)3-D3.
In addition, vitamin D3 and overall adaptation are regulated by the following factors: ① Parathyroid hormone is regarded as a vitamin D-stimulating hormone, which is the main factor regulating the production of 1,25-(OH)2-D3. ②In terms of diet, the production of 1,25-(OH)2-D3 was increased in animals deficient in vitamin D. A low calcium diet or hypocalcemia stimulates the production of 1,25-(OH)2-D3 by stimulating the level of 25-OH-D3-1-hydroxylase. Conversely, a high-calcium diet reduced the production of 1,25-(OH)2-D3 and increased the production of 24,25-(OH)2-D3. ③ Since 1,25-(OH)2-D3 can be regarded as a hormone that utilizes phosphorus, it can also stimulate and inhibit the production and inhibition of 1,25-(OH)2-D3 when phosphorus is deficient or the body's need for phosphorus increases. , 25-(OH) 2-D3 generation. Phosphorus levels in renal tubular epithelial cells are determinants of regulation of 1,25-(OH)2-D3 production. High phosphorus levels in renal tubular epithelial cells stimulate 24-hydroxylase; low phosphorus levels stimulate 1-hydroxylase to generate 1,25-(OH)2-D3.