section
1 2 .3
Digestion and Absorption of Major Food Substances
215
TABLE 12-5
Specificities o f the Proteolytic Enzymes Involved in Gastrointestinal Protein Digestion
Enzyme*
Trypsin
Chymotrypsin
Carboxypeptidase A
Carboxypeptidase B
Elastase
Aminopeptidase
Pepsin
Dipeptidases
Tripeptidases
Rj, R2, R
3
= any amino
acid residue
Peptide bond
cleaved
H2N
O
O
II
N H -C H -C -N H -C H -C
I
Ri
r 2
Preferentially cleaves peptide bonds in which
Rj = Arg or Lys; R
2
= any amino acid residue
Rj = aromatic amino acid (Phe, Tyr, Trp); R
2
= any amino acid residue
Rj = any amino acid residue; R2 = any
C-Terminal residue
except Arg, Lys, or Pro
Rj = any amino acid residue; R2 = Arg or Lys
at the C-terminus
of a polypeptide
Rj = Neutral (uncharged) residues; R2=any amino acid residue
Rj = Most
N-terminal residues
of a polypeptide chain; R
2
= any residue except Pro
Rj = Trp, Phe, Tyr, Met, Leu; R
2
= any amino acid residue
H2N—CH—C
I
Ri
n h - c h - c ;
r 2
p
OH
Peptide bond
cleaved
O
II
H,N—CH—C + N H -C H —C4N H —ÇH—c;
R ■
R ■
R-.
o
OH
Choice of bond cleaved depends on specific enzyme. In a given molecule, only one
bond is cleaved by a tripeptidase.
*Trypsin, chymotrypsin, elastase, and pepsin are endopeptidases (trypsin is the most specific and pepsin is the least specific): carboxypeptidase A
and B and aminopeptidase are exopeptidases.
amino acids. Hydrolysis of oligopeptides by the brush-
border aminopeptidases releases amino acids. Leucine
aminopeptidase, a Zn2+-containing enzyme, is an integral
transmembrane glycoprotein with a carbohydrate-rich hy-
drophilic portion and active site protruding into the lumi-
nal side. It cannot degrade proline-containing dipeptides,
which are largely hydrolyzed intracellularly. Dipeptidases
and tripeptidases are associated with the brush-border
membranes, but their functions are not clearly understood.
The major products of intraluminal digestion of protein
are mixtures of amino acids (30-40%) and small pep-
tides (60-70%). Chymotryptic activity can be measured by
oral administration of N-benzoy 1
-L-ty rosy 1
- /j-am ino acid
(Ligure
12-12). On hydrolysis,
p-aminobenzoic acid
(PABA) and N-benzoy 1-L-tyrosine are released; PABA is
absorbed, conjugated to glycine by the liver, and excreted
in the urine to give an index of exocrine pancreatic func-
tion.
Absorption o f Amino Acids and Oligopeptides
Dipeptides and tripeptides that escape brush border
membrane peptidases are actively transported against a
concentration gradient by Na+-dependent mechanisms.
Inside the cell they are hydrolyzed.
Tree amino acids are transported into enterocytes by
four active, carrier-mediated, Na+-dependent transport
systems remarkably similar to the system for glucose.
These systems transport, respectively, neutral amino acids;
basic amino acids (Lys, Arg, His) and cystine; aspar-
tic and glutamic acids; and glycine and imino acids.
Some amino acids (e.g., glycine) have affinities for more
