Page 277 - Color Atlas of Biochemistry

268 Tissues and organs
Digestive secretions
A. Digestive juices
Saliva. The salivary glands produce a slightly
alkaline secretion which—in addition to water
and salts—contains glycoproteins (mucins) as
lubricants, antibodies, and enzymes.
D–Amylase attacks polysaccharides, and a li-
pase hydrolyzes a small proportion of the
neutral fats. α–Amylase and lysozyme, amu-
rein-cleaving enzyme (see p. 40), probably
serveto regulatethe oral bacterial flora rather
than for digestion (see p. 340).
Gastric juice. In the stomach, the chyme is
mixed with gastric juice. Due to its hydro-
chloric acid content, this secretion of the gas- cleaving enzymes through micelle formation
tric mucosa is strongly acidic (pH 1–3; see
p. 270). It also contains mucus (mainly glyco-
proteins known as mucins), which protects
the mucosa from the hydrochloric acid, salts,
and pepsinogen—the proenzyme (“zymogen”)
of the aspartate proteinase pepsin (see
pp. 176, 270). In addition, the gastric mucosa
secretes what is known as “intrinsic factor”—a
glycoprotein needed for resorption of vitamin
B 12 ( “extrinsic factor”) in the bowel.
In the stomach, pepsin and related en-
zymes initiate the enzymatic digestion of pro-
teins, which takes 1–3 hours. The acidic gas-
tric contents are then released into the duo-
denum in batches, where they are neutralized
by alkaline pancreatic secretions and mixed
with cystic bile.
Pancreatic secretions. In the acinar cells,
the pancreas forms a secretion that is alkaline be affected.
–
due to its HCO 3 content, the buffer capacity
of which is suf cient to neutralize the stom-
ach’s hydrochloric acid. The pancreatic secre-
tion also contains many enzymes that catalyze
the hydrolysis of high–molecular-weight food
components. All of these enzymes are hydro-
lases with pH optimums in the neutral or
weakly alkaline range. Many of them are
formed and secreted as proenzymes and are
only activated in the bowel lumen (see
p. 270).
Trypsin, chymotrypsin, and elastase are en-
dopeptidases that belong to the group of ser-
ine proteinases (see p. 176). Trypsin hydro-
lyzesspecificpeptide bondson the Cside of
the basic amino acids Arg and Lys, while chy-
motrypsin prefers peptide bonds of the apolar
amino acids Tyr, Trp, Phe, and Leu (see p. 94).
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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Elastase mainly cleaves on the C side of the
aliphatic amino acids Gly, Ala, Val, and Ile.
Smaller peptides are attacked by carboxy-
peptidases, which as exopeptidases cleave in-
dividual amino acids from the C–terminal end
of thepeptides (seep. 176).
a–Amylase, the most important endoglyco-
sidase in the pancreas, catalyzes the hydroly-
sis of α1 4 bonds in the polymeric carbohy-
drates starch and glycogen. This releases mal-
tose, maltotriose, and a mixture of other oli-
gosaccharides.
Various pancreatic enzymes hydrolyze lip-
ids, including lipase with its auxiliary protein
colipase (see p. 270), phospholipase A 2 ,and
sterol esterase. Bile salts activate the lipid-
(see below).
Several hydrolases—particularly ribo-
nuclease (RNAse) and deoxyribonuclease
(DNAse)—break down the nucleic acids con-
tained in food.
Bile. The liver forms a thin secretion (bile)
that is stored in the gallbladder after water
and salts have been extracted from it. From
the gallbladder, it is released into the duode-
num. The most important constituents of bile
are water and inorganic salts, bile acids and
bile salts (see p. 314), phospholipids, bile pig-
ments, and cholesterol. Bile salts, together
with phospholipids, emulsify insoluble food
lipids and activate the lipases. Without bile,
fats would be inadequately cleaved, if at all,
resulting in “fatty stool” (steatorrhea). Re-
sorption of fat-soluble vitamins would also
Small-intestinal secretions. The glands of
thesmall intestine(theLieberkühn and Brun-
ner glands) secrete additional digestive en-
zymes into the bowel. Together with enzymes
on the microvilli of the intestinal epithelium
(peptidases, glycosidases, etc.), these en-
zymes ensurealmostcompletehydrolysis of
the food components previously broken
down by the endoenzymes.