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Metabolism of vitamin B12
Last reviewed: 04.07.2025
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Vitamin B 12 (cobalamin) is found only in animal products: meat, liver, milk, eggs, cheese, etc. (the vitamin in animal tissues is a bacterial derivative). Under the influence of cooking and proteolytic enzymes in the stomach, it is released and quickly binds to "R-binders" (trans-cobalamins I and III) - proteins with rapid electrophoretic mobility, compared to the intrinsic factor; to a lesser extent, vitamin B 12 binds to the intrinsic factor (IF, Castle factor) - a glycoprotein produced by the parietal cells of the fundus and body of the stomach. Under the influence of pancreatic juice proteases, the R-vitamin B, 2 complex is destroyed and vitamin B 12 is released, which binds to the intrinsic factor, forming a vitamin B12 -IF complex. The latter is dimerized and binds to specific receptors in the ileum. In the presence of calcium ions and at pH 7.0, this complex is broken down and vitamin B 12 penetrates the mitochondria of the intestinal mucosa cells. From here, vitamin B 12 enters the blood, where it combines with the transport protein transcobalamin II (TC II), which delivers the vitamin to 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 release of vitamin.
Part of the vitamin B 12 in the blood serum is bound to cobalophilins ("R-binders") - transcobalamins I and III. These B 12 -binding glycoproteins release it only in the liver. In the presence of large amounts 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, 1 g of which contains 1 mcg of vitamin B 12. In healthy full-term children, the reserves of vitamin B 12 in the liver are 20-25 mcg, the depot is sharply depleted by the age of one year. The daily requirement for vitamin B 12 in an infant is 0.1 mcg, and in an adult - 5-7 mcg. 100 ml of breast milk contains 0.11 mcg of vitamin B 12.
Vitamin B 12 is mainly excreted with bile, and is also lost with feces; 0.1% of the total deposited vitamin is lost per day. The existence of an intestinal-hepatic circulation of vitamin B 12 has been proven - about 3/4 of the vitamin excreted with bile is reabsorbed. This explains the development of megaloblastic anemia 1-3 years after the complete cessation of vitamin B 12 intake into the body. Physiological losses of the vitamin with urine are extremely insignificant.
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