Chemistry Reference
In-Depth Information
to an ion exchanger. After puriication-decolorization and following fractionation-crystallization,
xylose is obtained. In addition, xylitol is obtained by hydrogenation and following the fraction-
ation-crystallization process (Artik et al. 1993).
Production of xylose from xylan is a common process from which d-xylulose is an intermediate
product (Figure 3.6). This two-step process is realized by hydrolysis in the presence of H
2
O + acid
in the irst step and H
2
O + catalyst in the second step. Quality and purity of the raw product are
important to obtain a high amount of xylose (Artik et al. 1993).
Xylitol is produced by hydrogenation of xylose, which converts the sugar (an aldehyde) into a
primary alcohol (Gare 2003). Endogenous xylitol is produced in the liver from l-xylulose by an
nicotinamide adenine dinucleotide phosphate (NADP)-linked dehydrogenase, as a metabolite of
the glucuronate-xylulose pathway (Touster et al. 1956). The function of this cycle is obscure, but
production of glucuronic acid for synthetic processes and detoxiication reactions has been assumed
(Touster 1974; Sochor et al. 1979).
Ingested xylitol is absorbed by passive or facilitated diffusion from the intestine (Bässler 1969;
Lang 1969). The absorption rate is quite slow, which means that high oral doses may induce tran-
sient osmotic diarrhea. Unadapted persons can consume 30-60 g oral xylitol per day without side
effects. After adaptation, up to 400 g of xylitol have been taken daily without side effects (Mäkinen
and Scheinin 1975). Proposed adaption mechanisms involve induction of polyol dehydrogenase
activity in the liver (Bässler 1969) and selection of intestinal microlora (Krishnan et al. 1980).
Xylitol is apparently excreted by simple glomerular iltration (Wyngaarden et al. 1957). Although
there is no reabsorptive mechanism for xylitol (Lang 1969), very little is excreted in the urine, prob-
ably due to the fast diffusion from the blood to the tissues (Demetrakopoulos and Amos 1978). The
net xylitol utilization in humans is over 90% after moderate xylitol administration (Lang 1969).
Most of the exogenous xylitol is metabolized in the liver (Jakob et al. 1971; Wang and van Eys 1981),
although other tissues like the kidney, testes, adipose tissue, adrenal cortex, muscles, and erythrocytes
are also able to metabolize it (Lang 1969; Wang and Meng 1971). Xylitol is oxidized mainly to d-xylulose
by a nonspeciic nicotinamide adenine dinucleotide (NAD)-linked polyol dehydrogenase (Smith 1962;
Froesch and Jakob 1974), which then enters the pentose phosphate shunt via d-xylulose-5-phosphatase.
Another possible pathway of xylitol metabolism is oxidation to l-xylulose by a speciic NADP-linked
polyol dehydrogenase. In both these reactions, a reduced redox state is produced (the ratios of reduced
nicotinamide adenine dinucleotide (NADH)/NAD and reduced nicotinamide adenine dinucleotide phos-
phate (NADPH)/NADP increased), which has been regarded as a primary metabolic effect of xylitol.
The inal metabolic products of xylitol in the liver are glucose and glycogen (Froesch and Jakob 1974).
Most fruits including berries and plants contain xylitol. It is found in the ibers of many fruits
and vegetables, including different berries, corn husks, oats, and mushrooms (Gare 2003). The rich-
est natural sources of xylitol are plums, strawberries, raspberries, caulilower, and endives (Washüttl
et al. 1973). Xylitol is also an intermediate of mammalian carbohydrate metabolism. In the human
body, 5-15 g/day of xylitol is formed daily. It is roughly as sweet as sucrose with only two-thirds
the food energy (Hollman and Touster 1964).
Xylitol is used as a source of energy in intravenous nutrition because tissues can use xylitol
under postoperative and posttraumatic conditions, when considerable insulin resistance prevents the
effective utilization of glucose (Georgieff et al. 1985). Xylitol is also used as a sugar substitute due
Hydrolysis
Hydrolysis
Xylan
D
-Xylulose
D
-Xylose
(C
5
H
8
O
4
)
n
C
5
H
10
O
5
C
5
H
10
O
5
H
2
O + catalyst
Figure 3.6
Production of xylitol from xylan.
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