section
9.1
Classification
139
ferricyanide, cupric ion, or hydrogen peroxide) becomes
reduced with the simultaneous oxidation of the carbonyl
group of the sugar. This property is exploited in the estima-
tion of reducing sugars. The individual monosaccharides
are better quantitated by specific procedures, such as an
enzymatic procedure (e.g., for glucose, the glucose oxi-
dase method).
A reaction frequently used in the determination of
carbohydrate structure and for its identification in tis-
sue preparations is the
periodate reaction.
Periodate
(sodium periodate, NalO/t) oxidatively cleaves carbon-
carbon bonds bearing adjacent oxidizable functional
groups, e.g., vicinal glycols, a-hydroxyaldehydes, and
ketones. The vicinal glycols, upon periodate oxidation,
yield a dialdehyde; this reaction is quantitative. If the pe-
riodate oxidation is carried out with a compound con-
taining three consecutive carbon atoms, each of which
bears hydroxyl groups, the products are a dialdehyde
and formic acid. The latter is derived from the oxidation
of the central carbon atom. This observation is used to
determine whether a given sugar molecule exists in the
furanose or pyranose ring conformation. For example,
/J-D-methylfructofuranoside consumes only 1 mol of per-
iodate with no formic acid production, whereas
ft-D-
methylglucopyranoside yields
1
mol of formic acid with
the consumption of
2
mol of periodate.
Periodate cleavage of glycogen (a polyglucose; see
below) yields a polyaldehyde that can be coupled to
a visible dye reaction. This is referred to as
periodic
acid-Schiff
(PAS)
staining.
A tissue slice is treated with
periodate, followed by staining with Schiff’s reagent
(basic
fuchsin
bleached
with
sulfurous
acid).
The
polyaldehyde, if present, will combine with the NH
2
groups of the bleached dye to produce a magenta or purple
complex. The specificity of the reaction can be confirmed
by first treating another tissue slice with a-amylase (which
breaks down the glycogen to smaller fragments that are re-
moved by washing) and then staining it with PAS. Absence
of color confirms that the material is glycogen, whereas
the appearance of color suggests the presence of a nong-
lycogen carbohydrate, such as a glycoprotein.
Some Physiologically Important
Monosaccharide Derivatives
Sugar Alcohols
Sugar alcohols are polyhydric alcohols (or polyols)
formed when the carbonyl group of the monosaccharide is
reduced to a hydroxyl group (Figure 9-11). For example,
reduction of glyceraldehyde or dihydroxyacetone yields
glycerol, a component of triacylglycerols (fatty acid esters
of glycerol) and of phospholipids. D-Sorbitol (also known
as D-glucitol) is formed when D-glucose is reduced. Keto
sugars of more than three carbons can yield more than
one sugar alcohol. For example, the chemical reduction of
D-fructose yields a mixture of D-sorbitol and D-mannitol
(sugar alcohol of D-mannose) because of the creation of a
new asymmetrical center at C
2
when the ^C
=
0
group is
converted to a H-C-OH group.
Sorbitol, with about 35-60% of the sweetness of su-
crose, is used as a sweetener and flavoring agent. It is
hygroscopic and therefore is used as a humectant. Sor-
bitol (and other sugar alcohols) can accumulate in tissues
such as the lens, sciatic nerve, and renal papillae in certain
disorders (e.g.,
diabetes mellitus
and
galactosemia
) and
can lead to pathological changes. In these instances, the
intracellular accumulation of sugar alcohols is due to the
Щ О Н
CH .O H
C H ,O H
|
2
CH.O H
H C O H
|
1
H O CH
H C O H
і
O H
1
C H O H
H O CH
1
1
H O CH
H O CH
1
1
с н р и
H C OH
1
1
H C O H
H C O H
1
H C OH
1
1
H C O H
с н р н
hoY
______
с н р н
1
C H 2O H
C
G lycerol
O b tain ed from th e
reduction of eith er
D -glyceraldehyde or
d ih y d ro x y aceto n e.
D-Sorbitol
O b tain ed from th e
reduction of e ith e r th e
C , carbonyl g ro u p of
g lu co se o r th e C 2
carbonyl g ro u p of
fru cto se.
D-Mannitol
O b ta in e d from th e
red u ctio n of eith er th e
C 2 carbonyl g ro u p of
D -fructose o r th e C ,
carbonyl g ro u p of
D -m annose.
Xylitol
O b ta in e d from th e
red u ctio n of e ith e r
th e C , carb o n y l g ro u p
of D -x y lo s e o rth e C 2
carbonyl g ro u p of
D-Xylulose.
M yo-inositol
F IG U R E 9-11
Structures of selected sugar alcohols and myo-inositol.
