13.3.3 Biosynthesis of Fructose, Sugar Alcohols and Glycerol
475
broad substrate specificity for carbonyl compounds [99, 237]. Sorbitol dehydrogenase (SDH)
is found in the same organs as aldose reductase,
although in fact the highest activities are found in
the liver, in which there is no aldose reductase
activity. Here again, the enzymes of many mammalian species have very similar properties: they
contain Zn2+, are strongly NAD specific, but can
produce ketoses from a range of different sugar
alcohols of five to seven C atoms. The native
enzymes are homotetramers of about 140 kDa;
the subunits from human liver and those from
many other mammals show up to 25 % sequence
agreement with the alcohol dehydrogenases
(AD H) [146]. Comparison of the SDH sequence
with the three-dimensional model of horse-liver
ADH explains why the SDH is inactive against
ethanol and the ADH is inactive against sorbitol:
the zinc ligand in the SDH is a glutamate but in
the ADH it is a cysteine [75]. Erythrocytes also
contain SDH and human erythrocytes can therefore convert 3 % of the glucose taken up into sorbitol. The SDH activity of the blood cells varies in
all vertebrate classes between 0.01 and 0.17 U/g
haemoglobin [17].
NADP- and NAD-specific "sorbitol dehydrogenases" are also found in various insect but none
has yet been isolated. Sorbitol is important
mainly as a cryoprotectant in the insects, e.g. during diapause. From the fluctuations in activity of
the two enzymes in Bombyx mori, it may be concluded that the sorbitol required at the beginning
of diapause is synthesized from glucose or glycogen by the NADPH-specific aldose reductase,
and that the NAD-specific sorbitol dehydrogenase is responsible for the opposite reaction, the
resynthesis of glycogen at the conclusion of diapause. In fact, the NADPH-dependent activity is
more-or-less constant at 2-3 mU/mg protein, and
the enzyme activities of the NADPH-yielding
pentose phosphate cycle increase considerably at
the start of diapause. The NAD-specific sorbitol
dehydrogenase is almost undetectable in diapause eggs but increases at the completion of diapause to values of over 6 mU/mg protein [83].
The sugar molecule mannitol, which as a storage
product of fungi, algae and higher plants can
reach concentrations of up to 10 % of the dry
weight, is exploited by herbivorous animals, as
shown for example by nutritional experiments
with locusts. Initially, the NAD-specific sorbitol
dehydrogenase was thought to be responsible.
However, an alternative route is apparently taken
in the pulmonate snail Helix aspersa. Here, the
hepatopancreas and the gut wall contain an
NAD(P)-dependent, H20z-producing mannitol
oxidase at high activities; this appears to be localized in the endoplasmic reticulum and not, as first
assumed, in the mitochondria or peroxisomes
[173].
The dihydroxyacetone phosphate produced
during glycolysis can be reduced to sn-glycerol-3phosphate by the cytoplasmic NAD-specific
glycerol-3-phosphate dehydrogenase (a-glycerophosphate dehydrogenase, GPDH). Hydrolytic
cleavage of the phosphate residue leads to glycerol. As a product of anaerobic energy-yielding
metabolism, glycerol is not important for the
Metazoa as its formation is energetically inefficient. Glycerol phosphate, however, is of great
importance to all animals as the starting point for
the synthesis of glycerol lipids. In particular, in
insect flight muscles it is exploited as a transport
vehicle for reducing equivalents across the mitochondrial membrane (glycerol phosphate shuttle). In this process, the glycerol phosphate produced in the cytoplasm is reoxidized to dihydroxyacetone phosphate by glycerol-3-phosphate: coenzyme Q-oxidoreductase (glycerol
phosphate oxidase), located on the outer surface
of the inner mitochondrial membrane, and the
electrons are passed on to the respiratory chain.
Free glycerol serves as a cryoprotectant in some
insects and anurans (p. 466). Exogenous glycerol
can be phosphorylated by the mitochondrial glycerol kinase in mammals, bacteria and some
insects [220].
Comparative biochemical data are available
only for the GPDH of higher vertebrates and
insects. The rabbit enzyme agrees in more than
90 % of its 348 amino acids with the mouse
enzyme, and in almost 89 % of positions with that
of the chicken. This gives a relatively low substitution rate of 0.19 . 10- 9 per nucleotide per
year, similar to the rate for cytochrome c
[14, 125]. Isoenzymes of GPDH have been found
in all eukaryotes examined in detail. The mouse
has two GPDH genes, of which the gdc-110cus on
chromosome 15 is expressed only in the adult animal, and locus gdc-2 on chromosome 9 only in the
embryo. The genes consist of eight exons, whereby intron 2 lies on the border between the
adenine- and nicotinamide-binding domains and
intron 3 is on the border between the NADbinding and catalytic domains. There would also
appear to be two GPDH loci in man, the rat, rabbit and chicken [125].
Drosophila has only one GPDH gene with
eight exons, from which three tissue- and stagespecific isoenzymes are produced by alternative
475
broad substrate specificity for carbonyl compounds [99, 237]. Sorbitol dehydrogenase (SDH)
is found in the same organs as aldose reductase,
although in fact the highest activities are found in
the liver, in which there is no aldose reductase
activity. Here again, the enzymes of many mammalian species have very similar properties: they
contain Zn2+, are strongly NAD specific, but can
produce ketoses from a range of different sugar
alcohols of five to seven C atoms. The native
enzymes are homotetramers of about 140 kDa;
the subunits from human liver and those from
many other mammals show up to 25 % sequence
agreement with the alcohol dehydrogenases
(AD H) [146]. Comparison of the SDH sequence
with the three-dimensional model of horse-liver
ADH explains why the SDH is inactive against
ethanol and the ADH is inactive against sorbitol:
the zinc ligand in the SDH is a glutamate but in
the ADH it is a cysteine [75]. Erythrocytes also
contain SDH and human erythrocytes can therefore convert 3 % of the glucose taken up into sorbitol. The SDH activity of the blood cells varies in
all vertebrate classes between 0.01 and 0.17 U/g
haemoglobin [17].
NADP- and NAD-specific "sorbitol dehydrogenases" are also found in various insect but none
has yet been isolated. Sorbitol is important
mainly as a cryoprotectant in the insects, e.g. during diapause. From the fluctuations in activity of
the two enzymes in Bombyx mori, it may be concluded that the sorbitol required at the beginning
of diapause is synthesized from glucose or glycogen by the NADPH-specific aldose reductase,
and that the NAD-specific sorbitol dehydrogenase is responsible for the opposite reaction, the
resynthesis of glycogen at the conclusion of diapause. In fact, the NADPH-dependent activity is
more-or-less constant at 2-3 mU/mg protein, and
the enzyme activities of the NADPH-yielding
pentose phosphate cycle increase considerably at
the start of diapause. The NAD-specific sorbitol
dehydrogenase is almost undetectable in diapause eggs but increases at the completion of diapause to values of over 6 mU/mg protein [83].
The sugar molecule mannitol, which as a storage
product of fungi, algae and higher plants can
reach concentrations of up to 10 % of the dry
weight, is exploited by herbivorous animals, as
shown for example by nutritional experiments
with locusts. Initially, the NAD-specific sorbitol
dehydrogenase was thought to be responsible.
However, an alternative route is apparently taken
in the pulmonate snail Helix aspersa. Here, the
hepatopancreas and the gut wall contain an
NAD(P)-dependent, H20z-producing mannitol
oxidase at high activities; this appears to be localized in the endoplasmic reticulum and not, as first
assumed, in the mitochondria or peroxisomes
[173].
The dihydroxyacetone phosphate produced
during glycolysis can be reduced to sn-glycerol-3phosphate by the cytoplasmic NAD-specific
glycerol-3-phosphate dehydrogenase (a-glycerophosphate dehydrogenase, GPDH). Hydrolytic
cleavage of the phosphate residue leads to glycerol. As a product of anaerobic energy-yielding
metabolism, glycerol is not important for the
Metazoa as its formation is energetically inefficient. Glycerol phosphate, however, is of great
importance to all animals as the starting point for
the synthesis of glycerol lipids. In particular, in
insect flight muscles it is exploited as a transport
vehicle for reducing equivalents across the mitochondrial membrane (glycerol phosphate shuttle). In this process, the glycerol phosphate produced in the cytoplasm is reoxidized to dihydroxyacetone phosphate by glycerol-3-phosphate: coenzyme Q-oxidoreductase (glycerol
phosphate oxidase), located on the outer surface
of the inner mitochondrial membrane, and the
electrons are passed on to the respiratory chain.
Free glycerol serves as a cryoprotectant in some
insects and anurans (p. 466). Exogenous glycerol
can be phosphorylated by the mitochondrial glycerol kinase in mammals, bacteria and some
insects [220].
Comparative biochemical data are available
only for the GPDH of higher vertebrates and
insects. The rabbit enzyme agrees in more than
90 % of its 348 amino acids with the mouse
enzyme, and in almost 89 % of positions with that
of the chicken. This gives a relatively low substitution rate of 0.19 . 10- 9 per nucleotide per
year, similar to the rate for cytochrome c
[14, 125]. Isoenzymes of GPDH have been found
in all eukaryotes examined in detail. The mouse
has two GPDH genes, of which the gdc-110cus on
chromosome 15 is expressed only in the adult animal, and locus gdc-2 on chromosome 9 only in the
embryo. The genes consist of eight exons, whereby intron 2 lies on the border between the
adenine- and nicotinamide-binding domains and
intron 3 is on the border between the NADbinding and catalytic domains. There would also
appear to be two GPDH loci in man, the rat, rabbit and chicken [125].
Drosophila has only one GPDH gene with
eight exons, from which three tissue- and stagespecific isoenzymes are produced by alternative