Absorption of Ligands from the Gastrointestinal Tract

In general enteral absorption may occur throughout the whole length of the gastrointestinal tract but three areas are of special importance, depending on the formation constants of the particular metal—ligand complexes. These are the mouth (pH 7.4), the stomach (pH 1.6) and the small intestine (pH 6-6.5 in the duodenum and 6.5-7 in the jejunum). 

Specific carrier systems exist for the transport of some species across the intestinal wall (mucosa), e.g. glucose or amino acids. However, for other species the important properties for good absorption are the presence of a high proportion of a non-ionized form with a high lipid-water partition coefficient and a small atomic or molecular radius. It is generally assumed that ionized species cannot cross the mucosa whereas non-ionized forms equilibrate fairly freely, providing that they have molecular radii of less than about 3 nm, which corresponds to a molecular mass of 6000 Da. For metals the most important region for absorption is the small intestine, where the pH lies between about 6 and 7.4. 

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