Complete Digestion of Peptides to Free Amino Acids Takes Place Both on the Enterocyte Surface and Within the Cells
Membranous-phase digestion of peptides is, in some respects, similar to that of carbohydrates; peptide-digesting enzymes, or peptidases, are present on the enterocyte surface membrane and extend into the glycocalyx.
These enzymes hydrolyze the peptide products of luminal-phase protein digestion, yielding free amino acids. Some of the longer-chain peptides are incompletely digested, yielding dipeptides and tripeptides. A large portion of dietary amino acids is absorbed directly in the form of dipeptides and tripeptides. This mode of absorption contrasts with that of carbohydrates, in which only monomeric, simple sugars may pass the apical membrane. Dipeptides and tripeptides that are absorbed intact are subsequently
FIGURE 30-11 ■ Membranous-phase digestion of peptides and carbohydrates. Note that tripeptides and dipeptides may be hydrolyzed to their constituent amino acids either on the apical membrane or within the enterocyte. In carbohydrate digestion, however, all disaccharide hydrolysis occurs at the apical membrane. Regardless of the site at which the final hydrolysis of peptides occurs, the product absorbed into the blood is free amino acid (see Figure 30-16).
hydrolyzed by the action of intracellular peptidases, which results in the formation of free amino acids that are then available for passage into the blood. Thus the final digestion of peptides to free amino acids may occur at either of two sites: on the surface membrane of the enterocyte or within the cell. In either case, the final product of protein digestion is free amino acid (Figure 30-11).
FIGURE 30-10 Luminal-phase and membranous-phase digestion of carbohydrate. Note that specific enzymes exist for each polysaccharide, and that a limited number of monomers are formed eventually from a relatively large number of starches and polysaccharides.