Intracellular hydrolysis of peptides.

Purification of the first dipeptidases, glycylleucine dipeptidase (EC 3.4.13.2) and proline dipeptidase (EC 3.4.13.9), from intestine, have shown them to be true dipeptidases, hydrolysing only dipeptides in their laevo form. Although they have quite different specificities they show great similarities in their chemical and physicochemical properties. Specific antibodies against the two dipeptidases have been raised and used in combination with the double-layer immunofluorescent staining technique to study their histological localization in the small intestine. The results conclusively demonstrated that both glycylleucine dipeptidase and proline dipeptidase are exclusively located in the cytosol of the enterocytes. This location fits well with the current idea of transport and hydrolysis of dipeptides in the intestine where the dipeptides are taken by a specific transport mechanism and are hydrolysed intracellularly. Although it is tempting to add a specific role for the dipeptidases of the enterocytes in the final digestion of exogenous proteins, studies so far have shown their identity with the corresponding dipeptidases of other tissues. It is therefore suggested that their role in the digestion process may be based entirely on their abundance in the intestine.