[The intestinal phase of peptide absorption]

Peptides and not amino acids are the prevailing degradation products of protein digestion which are formed in the intestinal lumen and are absorbed from the mucosa. These two families differ in absorption. The differences become manifest when the absorption of peptide mixtures is compared with that of equimolecular mixtures of free amino acids. The absorption of peptides occurs in two different ways: 1. Transport of intact peptides through the membrane into the mucosal cell and subsequent hydrolysis by intracellular peptide hydrolases. 2. Hydrolysis of the peptides by peptide hydrolases localized on the luminal side of the mucosal cell membrane and subsequent transport of the amino acids thus formed through the membrane. The two mechanisms of absorption do not exclude each other. The way by which energy is supplied for the transport is not yet elucidated. The transport of intact peptides is of nutritive importance only in case of dipeptides and tripeptides. It enables in particular the introduction of peptides that cannot be cleft by membrane-bound peptide hydrolases. The hydrolysis of peptides by membrane-bound peptide hydrolases and the subsequent transport of released amino acids is of importance for long-chain peptides. The difference in absorbing behaviour between the free amino acids released in the intestinal lumen and the amino acids released by peptide hydrolases at the mucosal membrane is discussed.