Properties of the transferrin associated with rat intestinal mucosa

The transferrin that is isolated from washed intestinal mucosal cell preparations consists partly of a fraction that has properties distinguishing it from serum transferrin. The serum transferrin contaminating mucosal preparations, even when fully saturated with iron and in the presence of proteinase inhibitors, also acquires the properties of the mucosal transferrin when the mucosa is homogenised. The mucosal transferrin is modified by a single cleavage of the polypeptide chain yielding a disulphide-linked peptide of 6550 daltons linked to the parent protein by a disulphide bridge. The amino-terminal sequence of the first 11 residues of this peptide could be aligned with both the known rat and human transferrin carboxy-terminal sequences. In both cases the sequence is preceded by a phenylalanine residue (residue 622 of human transferrin). This suggested that a mucosal chymotryptic enzyme was responsible even though rat transferrin is not susceptible to alpha-chymotrypsin if fully iron-saturated. Since transferrin mRNA is not found in the intestinal mucosa it must be imported from the serum. It remains uncertain whether the modified transferrin is present naturally and plays a role in iron absorption but these findings do indicate the eventual fate of any transferrin imported into an intestinal cell.