478
13 The Structure and Metabolism of Carbohydrates
As the result of a gene duplication, D. mulleri,
D. mojavensis and other species of the D. mulleri
subgroup have two loci, Adh-1 and -2, each with
only one promoter. The polypeptides encoded by
these genes form two homodimers and one heterodimer, which differ only slightly in their activities. Adh-1 is expressed only in the larvae of
most of these species, but in D. mojavensis and its
sibling species D. arizonensis it is also expressed
in the ovary of adult females; adh-2 is expressed
from larval stage 3 onwards [18, 286].
The ADHs of all Drosophila species are very
similar to the mammalian enzyme in their catalytic properties; they are strictly NAD specific
and have a broad substrate specificity, which
includes not only primary and secondary alcohols
but also, for example, terpenes. Secondary alcohols are metabolized faster than primary alcohols, and amongst the primary alcohols, shortchain forms up to butanol are metabolized more
rapidly than longer-chain forms [290, 297]. The
ADH of D. melanogaster, in contrast to the
enzyme of the sibling species D. simulans, is able
to oxidize acetaldehyde [191]. The alleloenzymes
Sand F of D. melanogaster show no significant
differences in specific activity, substrate specificity or affinity; however, the SS homodimer is significantly more heat stable than the FF dimer
(p. 145). The ADH activities in the genus Drosophila differ widely with the species, the organ and
the developmental stage. Drosophila species that
seek out fermenting fruits have relatively high
ADH levels; acetaldehyde is either used as a substrate in metabolism or is tolerated. In contrast,
species from rainforests and those that prefer
Hibiscus or feed on fungi have lower ADH activities; in these cases, acetaldehyde is not further
metabolized and is poorly tolerated. Compared
with other Drosophila species, D. melanogaster is
particularly ethanol tolerant and can make use of
low concentrations of ethanol, n-propanol or nbutanol as the carbon source; ethanol is found in
the habitats of these species and is chemoattractive to larvae and adults. The ADH in D. melanogaster is inducible by ethanol; the responsible
DNA region lies upstream of the distal promoter
between positions +604 and +634 [143]. Adults
of D. melanogaster that are homozygous for the S
allele have twice as much ADH activity and
immunologically detectable ADH protein as the
sibling species D. simulans, although the mRNA
concentrations are similar. Thus, the differences
in protein concentration involve differences in the
rate of translation or protein stability. Gene transfer experiments have shown that the expression
of the genes of the two species is very much
dependent upon the genetic background, and is
always greater in D. melanogaster than in D. simulans [168]. The ADH activity in D. melanogaster peaks in the third larval stage, and again in
freshly hatched adults; these maxima correspond
to variations in two mRNAs transcribed from the
same gene. The fat body, gut, Malpighian tubules
and male genital tract have the highest activities
in Drosophila, and the ADH organ distribution is
similar in other insects, e.g. in the house cricket
Acheta domestica.
The acetaldehyde produced in the ADH reaction is much more toxic than ethanol itself and
must be rapidly metabolized further. Four enzymes are involved in the metabolism of acetaldehyde: ADH itself, which can only oxidize acetaldehyde in some Drosophila species; the NADspecific aldehyde dehydrogenase (EC 1.2.1.3);
the flavin enzyme aldehyde oxidase (EC 1.2.3.1);
and catalase. The NAD-dependent aldehyde
dehydrogenases (ALDH) playa key role in ethanol metabolism in the mammals. In all mammals
they are organized into organ-specific systems of
isoenzymes. These isoenzymes may be localized
in the cytosol or in the mitochondria and may
have different substrate specificities, but they are
always homotetramers with subunits of about 500
amino acids. Not all ALDH isoenzymes have
been structurally or functionally characterized,
even in the case of human ALDH. ALDHl (cytosolic) and ALDHz (mitochondrial) of the liver
and y-aminobutyrate dehydrogenase (y-ABDH)
have high affinities for acetaldehyde; the ALDH3
of the stomach and lung, which preferentially
uses benzaldehyde as a substrate, and ALDH4,
which is a glutamic y-semialdehyde dehydrogenase, have a much lower affinity for acetaldehyde
[122]. Comparisons of the sequences of ALDHs
from the same cell compartment of different
mammalian species in each case show about 95 %
agreement, but exhibit only about 67 % agreement between cytoplasmic and mitochondrial
enzymes of the same species [103]. After some
initial difficulties, ALDH has also been detected
in Drosophila melanogaster. However, 75-90 %
of the acetaldehyde in tracer experiments is oxidized to acetic acid by alcohol dehydrogenase
[108]. Nevertheless, it is clear that ALDH, like
ADH, has a much higher activity in D. melanogaster than in the less ethanol-resistant sibling
species D. simulans. The aldehyde oxidase of
D. melanogaster has such a low activity for acetaldehyde (Km of about 10 mmol/l) that it must be
relatively unimportant for ethanol metabolism
13 The Structure and Metabolism of Carbohydrates
As the result of a gene duplication, D. mulleri,
D. mojavensis and other species of the D. mulleri
subgroup have two loci, Adh-1 and -2, each with
only one promoter. The polypeptides encoded by
these genes form two homodimers and one heterodimer, which differ only slightly in their activities. Adh-1 is expressed only in the larvae of
most of these species, but in D. mojavensis and its
sibling species D. arizonensis it is also expressed
in the ovary of adult females; adh-2 is expressed
from larval stage 3 onwards [18, 286].
The ADHs of all Drosophila species are very
similar to the mammalian enzyme in their catalytic properties; they are strictly NAD specific
and have a broad substrate specificity, which
includes not only primary and secondary alcohols
but also, for example, terpenes. Secondary alcohols are metabolized faster than primary alcohols, and amongst the primary alcohols, shortchain forms up to butanol are metabolized more
rapidly than longer-chain forms [290, 297]. The
ADH of D. melanogaster, in contrast to the
enzyme of the sibling species D. simulans, is able
to oxidize acetaldehyde [191]. The alleloenzymes
Sand F of D. melanogaster show no significant
differences in specific activity, substrate specificity or affinity; however, the SS homodimer is significantly more heat stable than the FF dimer
(p. 145). The ADH activities in the genus Drosophila differ widely with the species, the organ and
the developmental stage. Drosophila species that
seek out fermenting fruits have relatively high
ADH levels; acetaldehyde is either used as a substrate in metabolism or is tolerated. In contrast,
species from rainforests and those that prefer
Hibiscus or feed on fungi have lower ADH activities; in these cases, acetaldehyde is not further
metabolized and is poorly tolerated. Compared
with other Drosophila species, D. melanogaster is
particularly ethanol tolerant and can make use of
low concentrations of ethanol, n-propanol or nbutanol as the carbon source; ethanol is found in
the habitats of these species and is chemoattractive to larvae and adults. The ADH in D. melanogaster is inducible by ethanol; the responsible
DNA region lies upstream of the distal promoter
between positions +604 and +634 [143]. Adults
of D. melanogaster that are homozygous for the S
allele have twice as much ADH activity and
immunologically detectable ADH protein as the
sibling species D. simulans, although the mRNA
concentrations are similar. Thus, the differences
in protein concentration involve differences in the
rate of translation or protein stability. Gene transfer experiments have shown that the expression
of the genes of the two species is very much
dependent upon the genetic background, and is
always greater in D. melanogaster than in D. simulans [168]. The ADH activity in D. melanogaster peaks in the third larval stage, and again in
freshly hatched adults; these maxima correspond
to variations in two mRNAs transcribed from the
same gene. The fat body, gut, Malpighian tubules
and male genital tract have the highest activities
in Drosophila, and the ADH organ distribution is
similar in other insects, e.g. in the house cricket
Acheta domestica.
The acetaldehyde produced in the ADH reaction is much more toxic than ethanol itself and
must be rapidly metabolized further. Four enzymes are involved in the metabolism of acetaldehyde: ADH itself, which can only oxidize acetaldehyde in some Drosophila species; the NADspecific aldehyde dehydrogenase (EC 1.2.1.3);
the flavin enzyme aldehyde oxidase (EC 1.2.3.1);
and catalase. The NAD-dependent aldehyde
dehydrogenases (ALDH) playa key role in ethanol metabolism in the mammals. In all mammals
they are organized into organ-specific systems of
isoenzymes. These isoenzymes may be localized
in the cytosol or in the mitochondria and may
have different substrate specificities, but they are
always homotetramers with subunits of about 500
amino acids. Not all ALDH isoenzymes have
been structurally or functionally characterized,
even in the case of human ALDH. ALDHl (cytosolic) and ALDHz (mitochondrial) of the liver
and y-aminobutyrate dehydrogenase (y-ABDH)
have high affinities for acetaldehyde; the ALDH3
of the stomach and lung, which preferentially
uses benzaldehyde as a substrate, and ALDH4,
which is a glutamic y-semialdehyde dehydrogenase, have a much lower affinity for acetaldehyde
[122]. Comparisons of the sequences of ALDHs
from the same cell compartment of different
mammalian species in each case show about 95 %
agreement, but exhibit only about 67 % agreement between cytoplasmic and mitochondrial
enzymes of the same species [103]. After some
initial difficulties, ALDH has also been detected
in Drosophila melanogaster. However, 75-90 %
of the acetaldehyde in tracer experiments is oxidized to acetic acid by alcohol dehydrogenase
[108]. Nevertheless, it is clear that ALDH, like
ADH, has a much higher activity in D. melanogaster than in the less ethanol-resistant sibling
species D. simulans. The aldehyde oxidase of
D. melanogaster has such a low activity for acetaldehyde (Km of about 10 mmol/l) that it must be
relatively unimportant for ethanol metabolism