Vitamin B12 plays a unique role in mammalian metabolism not only because, as a coenzyme, it is involved in two completely different and unrelated biochemical pathways, - the synthesis of nucleic acid precursors and the catabolism of some fatty acids -, but even more because it gives an excellent example how different groups of living organisms work together and depend on each other for the supply of vital nutrients. Vitamin B12 is almost exclusively found in animal products. However, it is not synthesized by the animals themselves but, they are able in one or the other way to absorb vitamin B12 which is produced bv microorganisms. For instance in ruminants the bacteria in the rumen are the source of the vitamin, which is taken up by the gut, distributed over the tissues and which is subsequently consumed by man with the meat or with the milk. However, the quantity of vitamin B12 , which is available in the food. is so low, that it would be lost if not an elaborate system of carrier proteins and cellular receotor mechanisms selectively collected it from the food and delivered it to the tissues. Intrinsic factor, produced and secreted by the qastric mucosa, binds the vitamin, which enters the body with the food, and hands it over to the ileal mucosa cells, which carry specific receptors for this protein. When the vitamin enters the blood, the plasma transport proteins, the transcobalamins, take it up immediately and deliver it to the tissues.

B12 vitamines, liver, transcobalamins
J. Abels (Lyanne)
Erasmus University Rotterdam
hdl.handle.net/1765/25774
Erasmus MC: University Medical Center Rotterdam

Lindemans, J. (1979, May 23). Transcobalamin II-mediated uptake of vitamin B12 by rat liver cells. Erasmus University Rotterdam. Retrieved from http://hdl.handle.net/1765/25774