476
13 The Structure and Metabolism of Carbohydrates
splicing. GPDH-l is mainly found in adult flight
muscle, where it is involved in the glycerol
phosphate shuttle. GPDH-deficient mutants cannot fly. GPDH-2 and -3 are located in adults in
the abdomen, where GPDH-3 in particular produces the starting material for the synthesis of
phospholipids. GPDH-3 is encoded by exons 1-6
and has a length of 349 amino acids, GPDH-l has
3 additional amino acids encoded by exon 8, and
GPDH-2 has 10 more amino acids which are
encoded by exon 7. The GPDH-3 of Drosophila
melanogaster agrees with that of D. virilis at 98 %
of positions and with the mammalian enzyme at
65-67 % of positions [27, 141]. Duplications and
triplications of the GPDH locus have been discovered in Japanese populations of D. melanogaster
[262]. Otherwise, the GPDH of the Drosophila
genus shows unusually little polymorphism. Alleloenzymes of GPDH have been found in only
three out of 175 Drosophila species examined,
and in no case were differences in the in vitro
catalytic properties or temperature sensitivity
apparent [27].
13.3.4 Ethanol Metabolism
Three enzymes or enzyme systems may be
involved in the oxidation of ethanol to acetaldehyde in animals: NAD-specific alcohol dehydrogenase, catalase, and the microsomal ethanoloxidizing system. An NADP-dependent alcohol
dehydrogenase like that of bacteria has been
found in the animal kingdom only in the
dysentery amoeba Entamoeba histolytica [171].
Despite the extensive similarity of their catalytic
properties, the ADHs belong to two different
protein families. The "short" ADHs have only
about 250 amino acids and no zinc; these include
the Drosophila ADH, which is significantly
homologous to the protein P6 from the fat body,
the 17~-hydroxysteroid dehydrogenase from
human placenta, and the protein act III from the
fungus Streptomyces coelicolor [12, 221]. The
"long" ADHs have chains of about 350 amino
acids and two zinc atoms; representatives of this
family are known from such widely different
organisms as mammals, maize and baker's yeast.
Related to the long ADHs are the sorbitol dehydrogenases, threonine dehydrogenases and ~crystallin [140].
The alcohol dehydrogenase of the vertebrates
is a cytoplasmic enzyme with the highest activity
always in the liver and much lower activities in
the kidney, heart, lung and gut. ADH molecules
are homodimers of 40-kDa subunits, each chain
of which carries two zinc atoms bound to cysteine; at least one of these zincs is involved catalytically in the reaction. ADH is NAD specific and
catalyses the reversible conversion of numerous
substances with primary or secondary hydroxyl
groups to the corresponding aldehyde or ketone.
The activity ratio NAD+: NADP+ of liver ADH
in the horse has a value of 86.4, in the rat it is
17.5, and in an unidentified trout species it is 5.4
[271]. Isoenzymes with varying catalytic properties encoded at multiple loci have been found in
all mammals examined, including humans. The
numerous isoenzymes of human liver can be
divided into three classes (I-III) [76, 245].
Whereas ADH-I and ADH-II are restricted to
the liver, lung, kidney and alimentary canal,
ADH-III is found in nearly all cells. The three
ADH classes differ considerably in the structure
of their catalytic centre, substrate specificity,
affinity and inhibitor reactions. In the 11 amino
acids of the substrate-binding pocket there are
eight differences between I and II and ten differences between I and III [76]. Methanol is only
oxidized by I, and the affinity for ethanol decreases markedly in the order I > II > III. The classII and -III enzymes are particularly active with
long-chain substrates. ADH-I is more strongly
inhibited than ADH-II by 4-methylpyrazole
whereas ADH-III is almost completely insensitive to this inhibitor [82]. Class I, which is by far
the best investigated, includes homo- and heterodimers made up of the subunits a, ~ and y; these
are encoded by the three loci ADHb ADH2 and
ADH3, which show about 95 % sequence agreement in pairwise comparisons. Adult liver contains all three types of subunit, whereas the lung
contains only ~, and kidney and gut contain
mainly y. Embryonal liver expresses only a, and
~ appears after the 3rd month; y appears only
after birth.
Allelic variants exist for ADHz (~b ~z and ~Indianapolis) and ADH3 (Yb yz). ~1 and ~z differ
at only one position, and Y1 and Yz differ at two
positions. The amino acid substitution 47-Arg to
His between ~1 and ~z is especially interesting
[177]. Caucasians have predominantly ~b whilst
Asians have ~2' As the specific activity of the ~2
homodimer is considerably higher than that of ~b
the toxic acetaldehyde accumulates more easily in
the liver of many Asians, leading to a higher sensitivity to alcohol [82]. Class-IIADH enzymes are
homodimers of Jt chains encoded by the ADH4
locus, and class III consist of X subunits encoded
in the ADHs locus. The subunits of classes I, II
13 The Structure and Metabolism of Carbohydrates
splicing. GPDH-l is mainly found in adult flight
muscle, where it is involved in the glycerol
phosphate shuttle. GPDH-deficient mutants cannot fly. GPDH-2 and -3 are located in adults in
the abdomen, where GPDH-3 in particular produces the starting material for the synthesis of
phospholipids. GPDH-3 is encoded by exons 1-6
and has a length of 349 amino acids, GPDH-l has
3 additional amino acids encoded by exon 8, and
GPDH-2 has 10 more amino acids which are
encoded by exon 7. The GPDH-3 of Drosophila
melanogaster agrees with that of D. virilis at 98 %
of positions and with the mammalian enzyme at
65-67 % of positions [27, 141]. Duplications and
triplications of the GPDH locus have been discovered in Japanese populations of D. melanogaster
[262]. Otherwise, the GPDH of the Drosophila
genus shows unusually little polymorphism. Alleloenzymes of GPDH have been found in only
three out of 175 Drosophila species examined,
and in no case were differences in the in vitro
catalytic properties or temperature sensitivity
apparent [27].
13.3.4 Ethanol Metabolism
Three enzymes or enzyme systems may be
involved in the oxidation of ethanol to acetaldehyde in animals: NAD-specific alcohol dehydrogenase, catalase, and the microsomal ethanoloxidizing system. An NADP-dependent alcohol
dehydrogenase like that of bacteria has been
found in the animal kingdom only in the
dysentery amoeba Entamoeba histolytica [171].
Despite the extensive similarity of their catalytic
properties, the ADHs belong to two different
protein families. The "short" ADHs have only
about 250 amino acids and no zinc; these include
the Drosophila ADH, which is significantly
homologous to the protein P6 from the fat body,
the 17~-hydroxysteroid dehydrogenase from
human placenta, and the protein act III from the
fungus Streptomyces coelicolor [12, 221]. The
"long" ADHs have chains of about 350 amino
acids and two zinc atoms; representatives of this
family are known from such widely different
organisms as mammals, maize and baker's yeast.
Related to the long ADHs are the sorbitol dehydrogenases, threonine dehydrogenases and ~crystallin [140].
The alcohol dehydrogenase of the vertebrates
is a cytoplasmic enzyme with the highest activity
always in the liver and much lower activities in
the kidney, heart, lung and gut. ADH molecules
are homodimers of 40-kDa subunits, each chain
of which carries two zinc atoms bound to cysteine; at least one of these zincs is involved catalytically in the reaction. ADH is NAD specific and
catalyses the reversible conversion of numerous
substances with primary or secondary hydroxyl
groups to the corresponding aldehyde or ketone.
The activity ratio NAD+: NADP+ of liver ADH
in the horse has a value of 86.4, in the rat it is
17.5, and in an unidentified trout species it is 5.4
[271]. Isoenzymes with varying catalytic properties encoded at multiple loci have been found in
all mammals examined, including humans. The
numerous isoenzymes of human liver can be
divided into three classes (I-III) [76, 245].
Whereas ADH-I and ADH-II are restricted to
the liver, lung, kidney and alimentary canal,
ADH-III is found in nearly all cells. The three
ADH classes differ considerably in the structure
of their catalytic centre, substrate specificity,
affinity and inhibitor reactions. In the 11 amino
acids of the substrate-binding pocket there are
eight differences between I and II and ten differences between I and III [76]. Methanol is only
oxidized by I, and the affinity for ethanol decreases markedly in the order I > II > III. The classII and -III enzymes are particularly active with
long-chain substrates. ADH-I is more strongly
inhibited than ADH-II by 4-methylpyrazole
whereas ADH-III is almost completely insensitive to this inhibitor [82]. Class I, which is by far
the best investigated, includes homo- and heterodimers made up of the subunits a, ~ and y; these
are encoded by the three loci ADHb ADH2 and
ADH3, which show about 95 % sequence agreement in pairwise comparisons. Adult liver contains all three types of subunit, whereas the lung
contains only ~, and kidney and gut contain
mainly y. Embryonal liver expresses only a, and
~ appears after the 3rd month; y appears only
after birth.
Allelic variants exist for ADHz (~b ~z and ~Indianapolis) and ADH3 (Yb yz). ~1 and ~z differ
at only one position, and Y1 and Yz differ at two
positions. The amino acid substitution 47-Arg to
His between ~1 and ~z is especially interesting
[177]. Caucasians have predominantly ~b whilst
Asians have ~2' As the specific activity of the ~2
homodimer is considerably higher than that of ~b
the toxic acetaldehyde accumulates more easily in
the liver of many Asians, leading to a higher sensitivity to alcohol [82]. Class-IIADH enzymes are
homodimers of Jt chains encoded by the ADH4
locus, and class III consist of X subunits encoded
in the ADHs locus. The subunits of classes I, II