Protein-bound vitamin B12 from foods binds to R-protein/haptocorrin/transcobalamin I in the stomach, free food-cobalamin in the saliva. The cobalamin-haptocorrin complex gets transported to the upper small intestine, where the complex is cleaved and B12 binds to a glycoprotein called ‘Intrinsic Factor’. In a part of the lower small intestine, the cobalamin-intrinsic factor complex can pass into the intestinal mucosa cells, by binding to a receptor, which allows the complex to pass the membrane. This transport mechanism is calcium-dependent. Inside the mucosa-cell ATPases dissociate the cobalamin from the intrinsic factor.
Cobalamin binds to the transportprotein transcobalamin II (TCII), and is released into the bloodstream. The cobalamin-TCII complex is the active part of vitamin B12 in the plasma, it supplies cells with vitamin B12. About 6-20% of plasma vitamin B12 are bound to transcobalamin II. The bigger part, 80-90% are transported by transcobalamin I. This protein binds excess cobalamin and brings it to the liver.
The body can store 2-5mg of vitamin B12, 6o% of it in the liver, and most of the rest in the skeletal muscles. About 3-8mcg vitamin B12 daily are excreted by the liver into the bile acids. This cobalamin can partly be resorbed in the gut. This recycling process of vitamin B12 from liver to gut is called ‘enterohepatic circulation‘. (1,2)
Maybe I missed something, but I couldn’t find the transportprotein that transports vitamin B12 from the liver back into other body parts. I wonder, wether the body can freely access liver cobalamin stores or if it depends partly on the enterohepatic circulation for vitamin B12 supply. This would make the access of the body to liver-vitamin B12 dependent on bile acids and the intrinsic factor and calcium.
Edit: Folate can also be recycled via enterohepatic circulation (3).