section 12.3
Digestion and Absorption of Major Food Substances
211
Brush-Border Surface Hydrolysis
Products of o'-amylase digestion of starch and ingested
disaccharides are hydrolyzed by oligosaccharidases on en-
terocyte cell membranes to yield monosaccharides that are
transferred across the brush-border bilayer. The oligosac-
charidases are large glycoproteins (M.W. > 200,000) that
are integral constituents of the cell membrane (Chap-
ter 10). Their active sites project toward the luminal side.
They have pH optima at about
6
and
Km
values for sub-
strates in the range of 3-20 mmol/L. Some oligosaccha-
ridases are the following:
1.
Exo-l,4-a-D-glucosidase,
also called glucoamylase
or maltase, catalyzes sequential hydrolysis of terminal
glucosyl units linked in a (l —> 4) linkages from the
nonreducing ends of malto-oligosaccharides or
maltose.
2.
Sucrose
a-D
-glucohydrolase,
also called
sucrose-a-dextrinase or sucrase, catalyzes hydrolysis
of the a (l ->
2
) linkage of sucrose to release glucose
and fructose; of the a (l -* 4) linkage of maltose to
release two glucose units; and of the
a(l
—>
6
)
linkage in a-limit dextrins and isomaltose. It is a
single gene product that is posttranslationally cleaved
to form two distinct polypeptide chains, one chain
catalyzing the hydrolysis of an a (1 —>• 4) linkage and
the other of an
a{\
—>
6
) linkage.
3.
(i-
D-
Galactoside galactohydrolase,
also called
lactase,
catalyzes hydrolysis of the
f{ \
4) linkage
of lactose (or terminal nonreducing /3-D
-galactose
units in
(i-
n-galactosides)
into galactose and glucose.
4.
a,
a-Trehalose glucohydrolase,
also called
trehalase,
catalyzes the hydrolysis of trehalose into two glucose
units.
Hydrolysis of branched-chain oligosaccharides (a-limit
dextrins) to glucose thus requires sequential action of three
enzyme activities (Figure 12-8). Hydrolysis of oligosac-
charides is rapid and is not the rate-limiting step in their
absorption. However, accumulation of monosaccharides
in the lumen is limited by end-product inhibition of the
oligosaccharidases. The rate-determining step in absorp-
tion is the monosaccharide transport system, with the fol-
lowing exception. Mucosal lactase activity is the lowest
oligosaccharidase activity, and so hydrolysis rather than
absorption is rate limiting. Oligosaccharide digestion is
virtually complete by midjejunum.
Transport of Monosaccharides into the Enterocyte
Glucose and galactose compete for a common transport
system. This system is an active transport system; i.e.,
the monosaccharides are absorbed against a concentra-
tion gradient, it is saturable and obeys Michaelis-Menten
Surface
membrane
Serosal
FIGURE 12-9
Schematic representation of glucose (or galactose) transport by the
enterocyte. Glucose binds to the receptor, facilitated by the simultaneous
binding of two Na+ at separate sites. The glucose and Na+ are released
in the cytosol as the receptor affinity for them decreases. The Na+ are
actively extruded at the basolateral surface into the intercellular space by
Na+,K+-ATPase, which provides the energy for the overall transport.
Glucose is transported out of the cell into the intercellular space and
thence to portal capillaries, both by a serosal carrier and by diffusion.
(Reproduced with permission from G. M. Gray,
C a rb o h yd ra te A b so rp tio n
a n d M a la b so rp tio n in G a stro in testin a l P h ysio lo g y.
Raven Press,
New York, 1981.]
kinetics, and it is carrier-mediated and Na+-dependent.
Translocation of glucose (or galactose) is shown in
Figure
12-9. On the luminal side, one molecule of
glucose and two sodium ions bind to the membrane
carrier (presumably, Na+ binding to the carrier molecule
increases the affinity for glucose because of a conforma-
tional change). The carrier-bound Na+ and glucose are in-
ternalized along the electrochemical gradient that results
from a low intracellular Na+ concentration. Inside the cell,
the sodium ions are released from the carrier, and the di-
minished affinity of the carrier for glucose releases the
glucose. The sodium ions that enter in this manner are
transported into the lateral intercellular spaces against a
concentration gradient by the free energy of ATP hydrol-
ysis catalyzed by a Na+,K+-ATPase. Thus, glucose and
Na+ are transported by a common carrier, and energy is
provided by the transport of Na+ down the concentration
and electrical gradient. The low cytoplasmic Na+ con-
centration is maintained by the active transport of Na+
out of the cell coupled to K+ transport into the cell by
the Na+,K+-ATPase. Since the sugar transport does not
use ATP directly, it can be considered as secondary active
transport. Although this mode of glucose transport is the
most significant, passive diffusion along a concentration
gradient may also operate if the luminal concentration of
glucose exceeds the intracellular concentration.
The intracellular glucose is transferred to the portal cap-
illary blood by passive diffusion and by a carrier-mediated
system. Intracellular glucose can be converted to lactate
