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chapter 12
Gastrointestinal Digestion and Absorption
Proteins
Protein is an essential nutrient for human growth, devel-
opment, and homeostasis. The nutritive value of dietary
proteins depends on its amino acid composition and di-
gestibility. Dietary proteins supply
essential amino acids,
which are not synthesized in the body. Nonessential amino
acids can be synthesized from appropriate precursor sub-
stances (Chapter 17). In human adults, essential amino
acids are valine, leucine, isoleucine, lysine, methionine,
phenylalanine, tryptophan, and threonine. Histidine (and
possibly arginine) appears to also be required for sup-
port of normal growth in children. In the absence from
the diet of an essential amino acid, cellular protein syn-
thesis does not occur. The diet must contain these amino
acids in the proper proportions. Thus, quality and quan-
tity of dietary protein consumption and adequate intake
of energy (carbohydrates and lipids) are essential. Pro-
tein constitutes about 10-15% of the average total energy
intake.
Animal proteins, with the exception of collagen (which
lacks tryptophan), provide all of the essential amino acids.
Vegetable proteins differ in their content of essential amino
acids (Table 12-4), but a mixture of these proteins will sat-
isfy the essential amino acid requirement. For example,
the lysine lacking in grains can be provided by legumes.
This combination also corrects for the methionine (which
is supplied in corn) deficiency of legumes. Such com-
binations (e.g., lentils and rice, chick-peas and sesame
seeds, spaghetti and beans, corn and beans) are widely
used in different cultures to provide for an optimal pro-
tein requirement. The recommended allowance for mixed
proteins in an adult in the U.S.A. is 0.85 g per kilo-
gram of body weight per day (Chapter 5). The allowances
TABLE 12-4
Limiting Essential Amino Acids in Plant Proteins
Source of Plant Proteins
Limiting Amino Acids
Grain
Maize or corn
Lysine, tryptophan
Millet, oats, wheat, or rice
Lysine, threonine
Legumes
Beans, immature
Methionine, isoleucine
Beans, mature
Methionine, valine
Peas
Methionine, tryptophan
Peanuts
Lysine, threonine
Nuts and oil seeds
Lysine, threonine
Coconut
Lysine, threonine
Vegetables
Methionine, isoleucine
are increased during childhood, pregnancy, and lactation
(Appendix IV).
Besides dietary protein, a large amount of endogenous
protein undergoes digestion and absorption. Endogenous
protein comes from three sources:
1. Enzymes, glycoproteins, and mucins secreted from
the salivary glands, stomach, intestine, biliary tract,
and pancreas, which together constitute about
20-30 g/day;
2. Rapid turnover of the gastrointestinal epithelium,
which contributes about 30 g/day; and
3. Plasma proteins that normally diffuse into the
intestinal tract at a rate of
1 - 2
g/day.
In several disorders of the GI tract (e.g., protein-losing
gastroenteropathy), loss of plasma proteins is considerable
and leads to hypoproteinemia.
Digestion
Protein digestion begins in the stomach, where protein
is denatured by the low pH and is exposed to the action
of pepsin. The low pH also provides the optimal H+
concentration for pepsin activity. The zymogen precursor
pepsinogen (M.W. 40,000) is secreted by the chief cells
and is converted to pepsin (M.W. 32,700) in the acid
medium by removal of a peptide consisting of 44 amino
acid residues. This endopeptidase hydrolyzes peptide
bonds that involve the carboxyl group of aromatic amino
acid residues, leucine, methionine, and acidic residues
(Table
12-5). The products consist of a mixture of
oligopeptides.
Chyme contains potent secretagogues for various en-
docrine cells in the intestinal mucosa. CCK and secretin
cause release of an alkaline pancreatic juice containing
trypsinogen, chymotrypsinogen, proelastase, and procar-
boxypeptidases A and B. Activation begins with that
of trypsinogen to trypsin by enteropeptidase (previously
called enterokinase) present in the brush-border mem-
branes of the duodenum.
Enteropeptidase cleaves between Lys
- 6
and Ile-7 to
release a hexapeptide from the N-terminus. Trypsin
autocatalytically activates trypsinogen and activates the
other zymogens. The importance of the initial activation
of trypsinogen to trypsin by enteropeptidase is manifested
by children with congenital enteropeptidase deficiency
who exhibit hypoproteinemia, anemia, failure to thrive,
vomiting, and diarrhea.
Trypsin, elastase, and chymotrypsin are
endopeptida-
ses.
Carboxypeptidases are
exopeptidases
(Table 12-5).
The combined action of these enzymes produces oligopep-
tides having two to six amino acid residues and free
