Ingested cobalamins are released from the proteins/polypeptides to which they are linked in foods through the actions of pepsin and hydrochloric acid in the stomach. Free vitamin B12 binds to an R protein that is found in saliva and gastric juice. Within the alkaline environment of the small intestine, the R protein is hydrolyzed by pancreatic proteases, and free vitamin B12 is released and binds to intrinsic factor (IF). The vitamin B12-IF complex travels from the duodenum to the ileum, where it interacts with a protein receptor (called cubilin, IF receptor, or cubam). Cubilin then interacts with another protein, amnionless, which facilitates cubilin's attachment to the ileal cell's plasma membrane. Binding of the vitamin B12-IF complex to the receptor triggers active endocytotic internalization. Vitamin B12 is absorbed throughout the ileum, especially the distal third. Within the enterocyte, the vitamin is released from the IF complex. Next, in or before it is transported across the ileum's basolateral membrane, vitamin B12 binds to the protein transcobalamin II for transport in portal blood.