Vitamin B12 absorption and cellular uptake

Vitamin B12 from food is released in the stomach and binds to salivary haptocorrin. Vitamin B12 is released from haptocorrin in the small intestine by pancreatic proteases and is then able to bind intrinsic factor (IF). The IF-vitamin B12 complex binds to the cubam receptor (Cubilin [Cub]–amnionless [Amn]) in the terminal ileum, followed by internalization and release into the circulation where it is bound by transcobalamins (TC). TC delivers vitamin B12 to the cellular TC receptor (also called CD320). Intracellularly, vitamin B12 is reduced and converted to adenosylcobalamin (in mitochondria) and methylcobalamin (in the cytosol). Refer to UpToDate for additional information about causes of vitamin B12 deficiency and the aspects of absorption with which they interfere.