Lecture 3-Digestion and Absorption of Macronutrients: page 4

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Pancreatic secretions contain

Edopeptidases

Trypsin: cleaves peptide bonds on the carboxyl side of basic amino acids (lysine and arginine)

Chymotrypsin: cleaves peptide bonds on the carboxyl side of aromatic amino acids (tryosine, phenylalanine and tryptophan)

Elastase: cleaves peptide bonds on the carboxyl side of aliphatic amino acids (alanine, leucin, glycine, valine, isoleucine)

Exopeptidases

Carboxypeptidases A and B: zinc-containing metallo-enzymes that remove single amino acids from the carboxyl-terminal ends of proteins and peptides.

Carboxypeptidase A: polypeptides with free carboxyl groups are cleaved to lower peptides and aromatic amino acids

Carboxypeptidase B: polypeptides with free carboxyl groups are cleaved to lower peptides and dibasic amino acids

Figure 4
Intraduodenal sequential action of pancreatic endopeptidases and exopeptidases on dietary protein. Final products at right are substrates that enter the mucosal cell. Arginine (Arg), Cyseine (Cys), (From G.M. Gray and H.L. Cooper, Gastroenterology 61:535, 1971.)

Peptidases within the eterocyte brush-border membrane hydrolyze the oligopeptides generated by intraluminal digestion to free amino acids and di-and tripeptides. About 20 have been identified.

Classifications

Endopeptidases

Exopeptidases

Aminopeptidases

Carboxypeptidases

Intracellular peptidases digest absorbed di- and tripeptides. Ninety percent or more of the products of protein digestion appear in the portal blood as free amino acids.

Absorption of amino acids and peptides is developmentally regulated and is influenced by diet, hormones and growth factors.

A high protein diet results in upregulation of peptide and most amino acid transport. Short term fasting also increases absorption rates, but long term fasting decreases amino acid transport with little change in peptide absorption.

Absorbed amino acids (especially glutamine, glutamate, and aspartate) are the major respiratory fuels for the small intestine. About 10% of the absorbed amino acids are used for protein synthesis within the enterocyte.

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Lecture 1:Introduction to Nutrition in Western Civilization
Lecture 2: Dietary Macronutrients, Body Fat, and Blood Lipids
Lecture 3:Digestion and Absorption of Macronutrients
Lecture 4:Basic Principles of Nutrient Metabolism
Lecture 5:Obesity
Lecture 6:Fuel Utilization During Exercise
Lecture 7:Biochemistry of Oxidant Stress in Health and Disease Antioxidants
Lecture 8:Nutrition for the 21st Century

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