[Digestion and resorption of proteins]

A survey is given of the ultrastructure of the intestinal epithelial cell, especially of its lumenward membrane. Special attention is paid to the peptidases which are located in the ciliated border and within the cell. The authors deal with the purification of the membrane-bound aminopeptidase which is of importance in splitting dietary peptides and illustrate its specificity by the cleavage of casein. The amino acids which are liberated by peptide splitting have in part aminopeptidase-inhibiting properties. The possible digestion physiological consequences are discussed. In vitro experiments were performed to investigate the composition of the content of the distal part of the small intestine of the rat with regard to its possible dependence on the composition of various dietary proteins. The composition of the peptides of the intestinal content is essentially undependent of the amino-acid composition of the diet. There is no enrichment of certain amino acids. The importance of the resorption of the peptides is also evidenced by resorption studies in which enzymatic hydrolysates of proteins (i.e. peptide mixtures) were confronted with a free amino-acid mixture of the same over-all composition. Taking glycyl-glycine-glycine as a model, the authors demonstrate that, in determined ranges of concentration, tripeptides may in part be resorbed without degradation. Finally, the importance of peptide resorption is evaluated and conclusions are drawn as to further studies on the physiology and physiopathology of digestion.