section
9.1
Classification
145
a-o -G lu co p y ran o sy l-(1 — 4 )-a-D -glucopyranoside
M altose
a-o -G lu co p y ran o sy l-(1 -*-2)-p -D -foictofuranoside
S u c ro s e
p-D -G alactopyranosyl-(1 —*4)- p-D -glucopyranoside
L a c to se
H
O H
H
H
a
-D -G Iucopyranosyl-(1— -1 )-a-D -glucopyranoside
T re h a lo se
p-D -G lucopyranosyl-(1—*4)-p-D -glucopyranoside
C ello b io se
p-D -G aIactopyranosyl-(1 —*4)-p -D -fructofuranoside
L actu lo se
FIGURE 9-20
Structures of some disaccharides.'Anomeric carbon atoms. All structures are Haworth projections.
tract, principally in the colon by microbial action, is trans-
ported to the liver via the portal circulation and inactivated
by conversion to urea (Chapter 17). Oral administration of
lactulose relieves hyperammonemia by microfloral con-
version in the colon to a variety of organic acids (e.g., lac-
tate) that acidify the colonic contents. Lactulose is neither
broken down nor absorbed in the small intestine. Reduc-
tion of the colonic luminal pH favors conversion of ammo-
nia (NH3) to ammonium ion (NHj), which is not easily
absorbed, and thus its absorption is decreased. Reduction
of luminal pH may additionally promote a microflora that
causes a decrease in the production of ammonia as well as
an increase in its utilization. The osmotic activity of the
disaccharide and its metabolites causes an osmotic diar-
rhea, which is useful in eliminating toxic waste products.
Another
nonabsorbable
disaccharide
used
in
the
treatment
of
hepatic
encephalopathy
is
lactitol
(/3-galactosidosorbitol).
Compared
to
lactose,
lacti-
tol has the advantage of higher palatability and fewer
side effects (e.g., flatulence). Ammonia production in
the colonic lumen by urease-producing bacteria can be
reduced by administering antibiotics such as neomycin
or metronidazole. The therapeutic effect of the combined
use of a nonabsorbable disaccharide and an antibiotic
may result from the metabolism of the disaccharide by
antibiotic-resistant bacteria.
Sucrose, a widely occurring disaccharide found in many
plants (cane sugar and beet sugar), consists of glucose and
fructose moieties linked together through C[ of glucose
and C
2
of fructose. Sucrose is not a reducing sugar and
does not mutarotate.
Because of its sweet taste sucrose is consumed in large
amounts. The perception of sweetness is mediated by taste
buds submerged in the tongue and oral mucous mem-
branes. The taste bud, a pear-like organ, consists of sensory
cells (taste cells) interwoven with a branching network of
nerve fibers. The taste bud contains two additional cell
types: basal and supporting cells. Sensory cells have a
short life span of about
1 0
days, and new cells are derived
from basal cells that continually undergo mitosis. Sensory
