Editing Digestion (section)

==Non-destructive digestion==
Some nutrients are complex molecules (for example [[vitamin B12|vitamin B<sub>12</sub>]]) which would be destroyed if they were broken down into their [[functional groups]].  To digest vitamin B<sub>12</sub> non-destructively, [[haptocorrin]] in saliva strongly binds and protects the B<sub>12</sub> molecules from stomach acid as they enter the stomach and are cleaved from their protein complexes.<ref name="pmid21593496">{{cite journal |vauthors=Nexo E, Hoffmann-Lücke E | title = Holotranscobalamin, a marker of vitamin B-12 status: analytical aspects and clinical utility | journal = Am. J. Clin. Nutr. | volume = 94 | issue = 1 | pages = 359S–365S |date=July 2011 | pmid = 21593496 | pmc = 3127504 | doi = 10.3945/ajcn.111.013458 }}</ref>

After the B<sub>12</sub>-haptocorrin complexes pass from the stomach via the pylorus to the duodenum, pancreatic proteases cleave haptocorrin from the B<sub>12</sub> molecules which rebind to [[intrinsic factor]] (IF).  These B<sub>12</sub>-IF complexes travel to the ileum portion of the small intestine where [[cubilin]] receptors enable [[Assimilation (biology)|assimilation]] and circulation of B<sub>12</sub>-IF complexes in the blood.<ref name="pmid19627091">{{cite journal |vauthors=Viola-Villegas N, Rabideau AE, Bartholomä M, Zubieta J, Doyle RP | title = Targeting the cubilin receptor through the vitamin B(12) uptake pathway: cytotoxicity and mechanistic insight through fluorescent Re(I) delivery | journal = J. Med. Chem. | volume = 52 | issue = 16 | pages = 5253–5261 |date=August 2009 | pmid = 19627091 | doi = 10.1021/jm900777v }}</ref>