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Dietary Protein Processing: Intraluminal and Enterocyte Surface Events

Gary M. Gray

10.1002/cphy.cp060418

Source: Supplement 19: Handbook of Physiology, The Gastrointestinal System, Intestinal Absorption and Secretion

Originally published: 1991

Published online: January 2011

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Sources of Protein for Digestion and Transport
2 Preliminary Attack on Protein by Pepsin in the Stomach
3 Intraintestinal Cleavage of Protein
3.1 Pancreatic Endopeptidases
3.2 Pancreatic Exopeptidases
4 Sequential and Simultaneous Action of Intraluminal Endopeptidases and Exopeptidases
5 Role of Enterocyte Brush Border Surface Membrane in Digestion and Transport
5.1 Interfacial Digestion of Oligopeptides
5.2 Transport of Intact Di‐ and Tripeptides
5.3 Relative Roles of Surface Hydrolysis and Intact Peptide Transport in Assimilation
5.4 Regulation of Protein Assimilation by Nutrients
Figure 1. Figure 1.

Outline of overall concerted actions of endoproteases (trypsin, chymotrypsin, and elastase) and exoproteases (carboxypeptidases A and B) on protein within intestinal lumen to yield free amino acids (30%) and small oligopeptides (2–6 residues, 70%).
Figure 2. Figure 2.

Sequential action of luminal pancreatic proteases on hypothetical protein with circled amino acid residues in protein chain (horizontal line). Initial attack at protein interior produces smaller fragments whose COOH‐terminal amino acid residue is then recognized and cleaved by the appropriate carboxypeptidase.
Figure 3. Figure 3.

Extension of peptide chain length enhances brush border aminooligopeptidase (AOP) activity. Effect of substrate chain length on affinity (Km) and maximal hydrolytic rate (Kcat, expressed as molecules of peptide hydrolyzed per molecule of enzyme per second). Subscripts refer to number of residues of particular amino acid component. In general, both affinity and rate are enhanced up to fivefold when peptide substrate is extended from a dipeptide to a tripeptide.

[From Kania et al. 19.]
Figure 4. Figure 4.

Elongation of side chain of NH2‐terminal amino acid increases affinity (lower Km, top; note logarithmic scale) and hydrolytic capacity (Kcat/Km, expressed as molecules cleaved per millimoles per liter substrate per second, bottom). Side‐chain lengths were estimated as described in Ref. 19. X indicates variable NH2‐terminal residue of peptide series; particular residue corresponding to side‐chain length is given on upper abscissa.

[From Kania et al. 19.]
Figure 5. Figure 5.

Diagram of catalytic site of intestinal aminooligopeptidase (AOP) based on kinetic analysis of oligopeptide series (see Table 4 and Fig. 4). Each subsite accommodates an amino acid residue beginning with Sn for NH2‐terminal residue.

[From Kania et al. 19.]
Figure 6. Figure 6.

Concerted action of intestinal surface peptidases on a theoretical oligopeptide in intestinal lumen. Cleavage proceeds sequentially from peptide's ends; aminooligopeptidase (AOP) and dipeptidyl aminopeptidase (DAP) recognize the NH2‐terminal residues, and carboxypeptidase (CBP) and dipeptidyl carboxypepdidase (DCP) act from the COOH‐terminal end of substrate. See Table 3 for detailed specificity data.
Figure 7. Figure 7.

Dipeptide with extended side chains has access to both intact transport and surface hydrolysis. Kinetic curves were generated from data from intact rat intestine over physiological range of substrate concentrations.

[From Rosen‐Levin et al. 30.]
Figure 8. Figure 8.

Abrupt augmentation of aminooligopeptidase (AOP) synthesis in intact rat intestine in response to 30‐min exposure of jejunal mucosa to tetrapeptide substrate, Gly‐Leu‐Gly‐Gly (GLGG). Control animals were perfused with equivalent Gly plus Leu amino acid mixture (G + L). After perfusion, [3H]leucine was placed in jejunal segment for 5 min and chased with tetrapeptide or amino acid solution for times given at top of figure. Intestines were then recovered, and total incorporation into AOP in endoplasmic reticulum (ER)‐Golgi and brush‐border compartments was determined, as detailed in Ref. 2. Arrows connect results for paired animals, and ratio of incorporation of GLGG to G + L is given over each pair. Overall, newly synthesized AOP doubled in response to short‐term exposure to its tetrapeptide substrate.

[From Reisenauer and Gray 29; copyright 1985 by the American Association for the Advancement of Science.]


Figure 1.

Outline of overall concerted actions of endoproteases (trypsin, chymotrypsin, and elastase) and exoproteases (carboxypeptidases A and B) on protein within intestinal lumen to yield free amino acids (30%) and small oligopeptides (2–6 residues, 70%).



Figure 2.

Sequential action of luminal pancreatic proteases on hypothetical protein with circled amino acid residues in protein chain (horizontal line). Initial attack at protein interior produces smaller fragments whose COOH‐terminal amino acid residue is then recognized and cleaved by the appropriate carboxypeptidase.



Figure 3.

Extension of peptide chain length enhances brush border aminooligopeptidase (AOP) activity. Effect of substrate chain length on affinity (Km) and maximal hydrolytic rate (Kcat, expressed as molecules of peptide hydrolyzed per molecule of enzyme per second). Subscripts refer to number of residues of particular amino acid component. In general, both affinity and rate are enhanced up to fivefold when peptide substrate is extended from a dipeptide to a tripeptide.

[From Kania et al. 19.]


Figure 4.

Elongation of side chain of NH2‐terminal amino acid increases affinity (lower Km, top; note logarithmic scale) and hydrolytic capacity (Kcat/Km, expressed as molecules cleaved per millimoles per liter substrate per second, bottom). Side‐chain lengths were estimated as described in Ref. 19. X indicates variable NH2‐terminal residue of peptide series; particular residue corresponding to side‐chain length is given on upper abscissa.

[From Kania et al. 19.]


Figure 5.

Diagram of catalytic site of intestinal aminooligopeptidase (AOP) based on kinetic analysis of oligopeptide series (see Table 4 and Fig. 4). Each subsite accommodates an amino acid residue beginning with Sn for NH2‐terminal residue.

[From Kania et al. 19.]


Figure 6.

Concerted action of intestinal surface peptidases on a theoretical oligopeptide in intestinal lumen. Cleavage proceeds sequentially from peptide's ends; aminooligopeptidase (AOP) and dipeptidyl aminopeptidase (DAP) recognize the NH2‐terminal residues, and carboxypeptidase (CBP) and dipeptidyl carboxypepdidase (DCP) act from the COOH‐terminal end of substrate. See Table 3 for detailed specificity data.



Figure 7.

Dipeptide with extended side chains has access to both intact transport and surface hydrolysis. Kinetic curves were generated from data from intact rat intestine over physiological range of substrate concentrations.

[From Rosen‐Levin et al. 30.]


Figure 8.

Abrupt augmentation of aminooligopeptidase (AOP) synthesis in intact rat intestine in response to 30‐min exposure of jejunal mucosa to tetrapeptide substrate, Gly‐Leu‐Gly‐Gly (GLGG). Control animals were perfused with equivalent Gly plus Leu amino acid mixture (G + L). After perfusion, [3H]leucine was placed in jejunal segment for 5 min and chased with tetrapeptide or amino acid solution for times given at top of figure. Intestines were then recovered, and total incorporation into AOP in endoplasmic reticulum (ER)‐Golgi and brush‐border compartments was determined, as detailed in Ref. 2. Arrows connect results for paired animals, and ratio of incorporation of GLGG to G + L is given over each pair. Overall, newly synthesized AOP doubled in response to short‐term exposure to its tetrapeptide substrate.

[From Reisenauer and Gray 29; copyright 1985 by the American Association for the Advancement of Science.]
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Article Title: Dietary Protein Processing: Intraluminal and Enterocyte Surface Events
 

How to Cite

Gary M. Gray. Dietary Protein Processing: Intraluminal and Enterocyte Surface Events. Compr Physiol 2011, Supplement 19: Handbook of Physiology, The Gastrointestinal System, Intestinal Absorption and Secretion: 411-420. First published in print 1991. doi: 10.1002/cphy.cp060418