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IF is very important for IF-dependent B12 absorption in the terminal ileum, the mechanism by which most of dietary B12 is absorbed. The B12-IF complex bind to IF receptors on enterocytes, but does the complex in its entirety get absorbed, or does B12 alone get absorbed and the IF is left outside?
The latter mechanism I came across in source 1 (see below), which stated the following:
IF-B12 complex binds to specific receptors. Following this, the vitamin B12 is absorbed into the cytosol of the enterocyte by endocytosis, leaving behind the IF at the brush border.
However, several other books and articles (like 2-4) rather vaguely state that the IF-B12 complex is absorbed into the enterocyte. I've found two articles that discuss the existence of these two viewpoints, (5 and 6), but they are kind of old (from 1965 and 1989 respectively) and I don't know if the "mystery's been solved" as it were.
- Concise Textbook of Human Physiology (this specific page)
- The Mechanism of Absorption of Vitamin B12 (cobalamin) in the GI Tract
- Marks' Essential Medical Biochemistry (this specific page)
- Textbook of Pharmacology (this page)
- Intrinsic Factor
- Vitamin B12 absorption and malabsorption
After reversible attachment of intrinsic factor-cobalamin complex to its ileal surface receptor, an energy-dependent process prevents removal of the complex from the cell surface by EDTA or acid; cobalamin dissociates from intrinsic factor and, as suggested by previous workers, binds to a molecule antigenically similar to transcobalamin II; and intrinsic factor is slowly(after some hours)degraded and forms breakdown products that are detectable in ileal extracts.
References Textbook of Pathology Page 318
One article Intrinsic factor mediated absorption of cobalamin
Recent articles- Physiology of absorption of vitamin b12