section 12.3
Digestion and Absorption of Major Food Substances
209
'N
P
C H ,
M altotriose
M alto se
a-Lim it dextrin
F I G U R E 1 2 -7
Hydrolysis of starch by a-amylase.
include glycogen and the disaccharides sucrose and lac-
tose. The disaccharide present in mushrooms, trehalose, is
also digestible. The specific patterns of carbohydrate in-
take are influenced by cultural and economic factors. Many
plant carbohydrates are not digestible, and they constitute
the dietary fiber, which includes cellulose, hemicelluloses,
pectins, gums, and alginate (Chapter 9). The digestion of
starch (and glycogen) begins in the mouth during mastica-
tion and the mixing of food with salivary a-amylase, which
hydrolyzes starch to some extent. The digestive action of
salivary a-amylase on starch is terminated in the acidic
environment of the stomach. Starch digestion is resumed
in the duodenum by another a-amylase secreted by the
pancreas. Salivary and pancreatic a-amylases share some
properties and exist in several isoenzyme forms that are
separable by electrophoresis. In pancreatic disease, the
a-amylase level in the serum increases, and its mea-
surement and isoenzyme pattern are useful in diagnosis.
Carbohydrate digestion and absorption take place in a
well-defined sequence:
1. Intraluminal hydrolysis of starch and glycogen by
a-amylase to oligosaccharides of variable length and
structure;
2. Brush-border surface hydrolysis of oligosaccharides
and disaccharides (e.g., sucrose, lactose, and
trehalose) to their monomers by specific
oligosaccharidases that are integral to the cell
membrane of the enterocyte; and
3. Transport of monosaccharides (e.g., glucose,
galactose, and fructose) into enterocytes.
Digestion o f Starch
a-Amylase hydrolyzes starch into
a-limit dextrins
(branched
oligosaccharides
of five
to
nine
glucose
residues), maltotriose, and maltose (Figure 12-7). The
a-amylase is an endoglycosidase, and its action does not
yield free glucose. It cannot catalyze the hydrolysis of
a(l ->-
6
) linkages (the branch points). a-Amylase has
optimal activity at pH 7.1, an absolute requirement for the
presence of Cl- , and is stabilized by Ca2+.
