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Vitamin B12 metabolism
Last reviewed: 23.04.2024
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Vitamin B 12 (cobalamin) - is a part of products of animal origin only: meat, liver, milk, eggs, cheese and others (the vitamin in the tissues of animals is a derivative of bacteria). Under the influence of culinary processing and proteolytic enzymes of the stomach, it is released and quickly binds to "R-binders" (trans-cobalamin I and III) - proteins with rapid electro-phoretic mobility, compared to the internal factor; to a lesser extent, vitamin B 12 is combined with an internal factor (WF, factor of Castle) - glycoprotein, produced by parietal cells of the fundus and the body of the stomach. Under the influence of proteases of pancreatic juice complex R-vitamin B, 2, vitamin B 12 is destroyed and released , which combines with the internal factor, forming a complex of vitamin B 12- WP. The latter is dnomerized and binds to specific receptors in the ileum. In the presence of calcium ions and at pH 7.0, this complex is cleaved and vitamin B 12 penetrates into the mitochondria of the cells of the intestinal mucosa. Hence, vitamin B 12 penetrates into the blood, where it combines with the transport protein transcobalamin II (TK II), which delivers the vitamin to the target tissues - hepatocytes, hematopoietic cells and others.
The release of vitamin B 12 from the TKP-B 12 complex in the cell occurs in 3 stages:
- binding of the complex to cell receptors;
- its endocytosis;
- lysosomal hydrolysis with the release of vitamin.
Part of the vitamin B 12 in the blood serum is associated with cobalophilins ("R-binders") - transcobalamin I and III. These B 12- binding glycoproteins release it only in the liver. In the presence of a large amount of vitamin B 12 about 1% of it can penetrate into the blood by passive diffusion.
The main depot of vitamin B 12 is the liver, in 1 g of which contains 1 μg of vitamin B 12. In healthy term infants reserves of vitamin B 12 in the liver up to 20-25 ug depot abruptly exhausted by the year. The daily requirement for vitamin B 12 in an infant is 0.1 μg, and in an adult it is 5-7 μg. In 100 ml of female milk contains 0.11 μg of vitamin B 12.
Vitamin B 12 is mainly secreted with bile, its loss also occurs with feces; a day, 0.1% of the total deposited vitamin is lost. The existence of the intestinal-hepatic circulation of vitamin B 12 - about 3/4 of the vitamin isolated with bile, is again reabsorbed. This explains the development of megaloblastic anemia in 1-3 years after receipt of the complete cessation of vitamin B 12 in the body. The physiological losses of the vitamin in the urine are very insignificant.
[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]