what is cholecalciferol?
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.
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.