140
chapter
9
Simple Carbohydrates
1
C O O "
I
2
H C OH
I
3 H OCH
I
*
H C OH
I
5
H C O H
I
6
снрн
D -G luconate
C H O H
C H O
I
H C O H
I
H O C H
H C O H
I
H C O H
I
_
c o o
o -G lu co n o -1 ,5 -lacto n e
D -G lu cu ro n ate
A gluconic acid in
A cyclic e s te r of th e
w hich th e a ld e h y d e
o -gluconic acid.
functional g ro u p
is rep laced by
th e carboxylic
acid group.
A u ran ic acid,
U ranic acid c a n form
fo rm ed w h en
a cyclic h em iacetal
th e h ig h est
ring. H aw orth
n u m b e re d c arb o n projection form ula
b e a rs a carboxyl
of o n e of th e a n o m e rs
g ro u p .
ofD -glupuronate.
A b iosynthetic derivative
of o -g lu cu ro n ate in plants
an d in a n im als o th e r th an
p rim ates a n d g u in e a pigs.
T h u s, for h u m a n s,
it is a vitam in a n d
is req u ired in th e sy n th esis
of co llag en .
FIGURE 9-12
Sugar acids and their derivatives.
high levels of precursor sugars in the plasma, which are
converted enzymatically to the respective sugar alcohols
in the cytoplasm. Sugar alcohols are not metabolized as
rapidly as their precursors, and reconversion to the precur-
sor is also slow.
Mannitol is widely distributed in nature and occurs in
the exudates of many plants. It has about half the sweet-
ness of sucrose. Clinically, mannitol is administered intra-
venously as an osmotic diuretic in patients with acute renal
failure. It is not metabolized appreciably, is filtered by the
glomerulus, and is not reabsorbed by the tubules; hence,
it is excreted in urine (Chapter 39). The nonreabsorbable
solute holds water, limits back-diffusion, and thus main-
tains urine volume in the presence of decreased glomerular
function. Intravenous mannitol is also used to relieve an
increase in pressure and in volume of cerebrospinal fluid.
Xylitol, a five-carbon polyol, is widely distributed in the
plant kingdom. It has about twice the sweetness of sucrose.
A potential benefit of xylitol and other sugar alcohols used
as sucrose substitutes may be the prevention of
dental
caries.
The beneficial effect may be due to the inability
of the oral microorganisms to utilize the sugar alcohols.
Sugar alcohols, though derived from reducing monosac-
charides, are not reducing agents and exist only in the
straight-chain form, since they cannot form cyclic hemi-
acetal or hemiketal linkages. However, cyclic compounds
related to sugar alcohols do exist, e.g., mw-inositol, a com-
ponent of some phospholipids (Figure 9-11).
Sugar Acids
The sugar acids are obtained when a carbonyl group or
a hydroxyl group is oxidized to a carboxylic acid group
(Figure 9-12). The physiologically important sugar acids
are
aldonic
and
uronic acids.
An aldonic acid is obtained
when the aldehyde group in an aldo sugar is oxidized;
thus, oxidation of D-glucose at Ci yields D-gluconic acid.
a-D -g lu co sam in e
(2-deoxy-2-amino- a- D-glucopyranose)
H
NH,
a-D-galactosamine
(2-deoxy-2-amino- a- D-galactopyranose)
F IG U R E 9-13
Structures of two amino sugars.
