ビタミンB12

Vitamin B12 or cyanocobalamin obtained from food is initially bound by _haptocorrin_, a protein found in the saliva with high affinity for B12. This forms a _haptocorrin-B12_ complex. Cyanocobalamin passes through the stomach and is protected from acid degradation due to its binding to haptocorrin. In the duodenum, pancreatic _proteases_ release cobalamin from the _haptocorrin-B12 complex_ and from other proteins containing protein-bound B12 that have been ingested. Following this, the binding of cobalamin to a second glycoprotein, _intrinsic factor_, promotes its uptake by terminal ileum mucosal cells by a process called _cubilin_/AMN receptor-mediated endocytosis. After absorption into enterocytes, intrinsic factor is broken down in the lysosome, and cobalamin is then released into the bloodstream. The transporter ABCC1, found in the basolateral membrane of intestinal epithelial and other cells, exports cobalamin bound to transcobalamin out of the cell. Cyanocobalamin then passes through the portal vein in the liver, and then reaches the systemic circulation. The active forms of cyanocobalamin are _methylcobalamin_ and _adenosylcobalamin_,.

Vitamin B12 is quickly absorbed from intramuscular (IM) and subcutaneous (SC) sites of injection; with peak plasma concentrations achieved about 1 hour after IM injection. Orally administered vitamin B12 binds to intrinsic factor (IF) during its transport through the stomach. The separation of Vitamin B12 and IF occurs in the terminal ileum when calcium is present, and vitamin B12 is then absorbed into the gastrointestinal mucosal cells. It is then transported by transcobalamin binding proteins. Passive diffusion through the intestinal wall can occur, however, high doses of vitamin B12 are required in this case (i.e. >1 mg). After the administration of oral doses less than 3 mcg, peak plasma concentrations are not reached for 8 to 12 hours, because the vitamin is temporarily retained in the wall of the lower ileum.

This drug is partially excreted in the urine. According to a clinical study, approximately 3-8 mcg of vitamin B12 is secreted into the gastrointestinal tract daily via the bile. In patients with adequate levels of intrinsic factor, all except approximately 1 mcg is reabsorbed. When vitamin B12 is administered in higher doses that saturate the binding capacity of plasma proteins and the liver, the unbound vitamin B12 is eliminated rapidly in the urine. The body storage of vitamin B12 is dose-dependent.

Cobalamin is distributed to tissues and stored mainly in the liver and bone marrow.

During vitamin loading, the kidney accumulates large amounts of unbound vitamin B12. This drug is cleared partially by the kidney, however, multiligand receptor _megalin_ promotes the reuptake and reabsorption of vitamin B12 into the body,.