ticle bound and is also present in other molluscs
like Sepia and Aplysia. The Cardium type is
soluble and specifically activated by magnesium
ions. In the freshwater snail Lymnaea stagnalis, a
soluble amino acid is found not only in the hepatopancreas, but also in the foot tissues. This
enzyme is specific for basic amino acids [82].
12.4.2 Aminotransferases (Transaminases)
For most amino acids, transfer of the amino
group is both the first step in degradation and the
last step in de novo synthesis. Usually the 2oxoglutarate/glutamate pair plays the role of
amino group acceptor/donor. When amino transfer to 2-oxoglutarate is tested with a large number
of amino acids, aspartic acid and alanine, and
often also leucine and tyrosine, are found to be by
far the most active donors. The ratio of aspartate
transaminase to alanine transaminase differs with
the species and the tissue; in most cases, the
aspartate transaminase has the higher activity and
has special responsibilities for transport across
the mitochondrial membrane [48]. Alanine,
aspartate, tyrosine and histidine transaminases
are all homologous, but agree pairwise in only
16-25 % of the 400-500 positions [115, 151, 205].
Of all the aminotransferases, the aspartate
transaminase (AspAT or GOT) is the best
known. It has been studied in detail in vertebrates, protozoans, fungi and bacteria. All vertebrates possess both mitochondrial and cytoplasmic
aspartate transaminases (m-AspAT and cAspAT), which differ in all species in about 50 %
of their sequences, but have identical tertiary
structures. Together with mitochondrial and cytoplasmic malate dehydrogenases, they make up
the most important mechanism for introducing
reduction equivalents into the mitochondria; this
mechanism is the malate-aspartate shuttle. The
two isoenzymes of aspartate transaminase are
dimers with 45-kDa subunits of about 400 amino
acids. They have been sequenced via cDNAs in
various mammals, the chicken and E. coli
[65, 186]. According to these data, the mitochondrial and cytoplasmic forms had already
separated about 1000 million years ago and
should, therefore, also be widespread in the
invertebrates. The rate of evolution of c-AspAT
appears to be somewhat higher than that of mAspAT [68]. More detailed information on the
structure and intracellular localization of the
AspATs of invertebrates does not appear to be
available. The enzyme of the parasitic flagellate
12.4.3 Glutamate Dehydrogenases
415
Trichomonas vaginalis is unusual in that, in contrast to the mammalian enzyme, it is not highly
specific for the pairs glutamate/2-oxoglutarate
and aspartate/oxaloacetate; it also shows high
activity with the aromatic amino acids phenylalanine, tyrosine and tryptophan [141].
Cytoplasmic and mitochoncrial isoenzymes are
also known for vertebrate alanine transaminase
(AlaAT or GPT). In the mammals, the cytoplasmic form predominates in carbohydrate catabolizing tissues, such as muscle and heart, and the
mitochondrial form predominates in organs with
intensive gluconeogenesis, such as the liver and
kidney. It may be concluded from this that in vivo
the mitochondrial form catalyses the reaction alanine to pyruvate, and the cytoplasmic form does
the reverse. The only alanine transaminase to
have been isolated and characterized from an
invertebrate is that from Drosophila nigromelanica [46]. Here, the enzyme is a homodimer of
113 kDa and, apart from its approximately fivefold higher affinity for alanine (Km = 5.2 mmoll
1), corresponds in its catalytic properties to the
mammalian enzymes of the rat liver, and porcine
and bovine heart. The enzyme is involved in
energy extraction from proline in the flight muscles; the high activities in the fat body aid proline
synthesis, in that the alanine transported from the
muscle to the fat body provides nitrogen for the
synthesis of glutamic acid and proline from 2oxoglutarate (Fig. 12.6). In the flight muscles of
Schistocerca gregaria, the cytoplasmic alanine
transferase is monomeric and dimeric with subunits of 63 kDa, and the mitochondrial form is
dimeric and tetrameric [61].
12.4.3 Glutamate Dehydrogenases
Organs with central metabolic functions, such as
the liver, kidney, hepatopancreas and fat body,
have high activities of GDH, whereas the activities are lower in most muscles. High activities of
GDH are found in some muscles in association
with the use of proline in energy-yielding metabolism, e.g. in the flight muscles of various
insects and in the mantle muscles of the cephalopod Loligo pealeii (see Table 15.7a, p. 582). The
typical animal GDH, as isolated and characterized from the liver and kidneys of mammals,
birds, amphibians and fish, and from the fat
bodies or flight muscles of insects [244], is a molecule of 310-340 kDa with six identical subunits
of 50-57 kDa and is localized in the mitochondrial matrix. The sequences of the approximately
like Sepia and Aplysia. The Cardium type is
soluble and specifically activated by magnesium
ions. In the freshwater snail Lymnaea stagnalis, a
soluble amino acid is found not only in the hepatopancreas, but also in the foot tissues. This
enzyme is specific for basic amino acids [82].
12.4.2 Aminotransferases (Transaminases)
For most amino acids, transfer of the amino
group is both the first step in degradation and the
last step in de novo synthesis. Usually the 2oxoglutarate/glutamate pair plays the role of
amino group acceptor/donor. When amino transfer to 2-oxoglutarate is tested with a large number
of amino acids, aspartic acid and alanine, and
often also leucine and tyrosine, are found to be by
far the most active donors. The ratio of aspartate
transaminase to alanine transaminase differs with
the species and the tissue; in most cases, the
aspartate transaminase has the higher activity and
has special responsibilities for transport across
the mitochondrial membrane [48]. Alanine,
aspartate, tyrosine and histidine transaminases
are all homologous, but agree pairwise in only
16-25 % of the 400-500 positions [115, 151, 205].
Of all the aminotransferases, the aspartate
transaminase (AspAT or GOT) is the best
known. It has been studied in detail in vertebrates, protozoans, fungi and bacteria. All vertebrates possess both mitochondrial and cytoplasmic
aspartate transaminases (m-AspAT and cAspAT), which differ in all species in about 50 %
of their sequences, but have identical tertiary
structures. Together with mitochondrial and cytoplasmic malate dehydrogenases, they make up
the most important mechanism for introducing
reduction equivalents into the mitochondria; this
mechanism is the malate-aspartate shuttle. The
two isoenzymes of aspartate transaminase are
dimers with 45-kDa subunits of about 400 amino
acids. They have been sequenced via cDNAs in
various mammals, the chicken and E. coli
[65, 186]. According to these data, the mitochondrial and cytoplasmic forms had already
separated about 1000 million years ago and
should, therefore, also be widespread in the
invertebrates. The rate of evolution of c-AspAT
appears to be somewhat higher than that of mAspAT [68]. More detailed information on the
structure and intracellular localization of the
AspATs of invertebrates does not appear to be
available. The enzyme of the parasitic flagellate
12.4.3 Glutamate Dehydrogenases
415
Trichomonas vaginalis is unusual in that, in contrast to the mammalian enzyme, it is not highly
specific for the pairs glutamate/2-oxoglutarate
and aspartate/oxaloacetate; it also shows high
activity with the aromatic amino acids phenylalanine, tyrosine and tryptophan [141].
Cytoplasmic and mitochoncrial isoenzymes are
also known for vertebrate alanine transaminase
(AlaAT or GPT). In the mammals, the cytoplasmic form predominates in carbohydrate catabolizing tissues, such as muscle and heart, and the
mitochondrial form predominates in organs with
intensive gluconeogenesis, such as the liver and
kidney. It may be concluded from this that in vivo
the mitochondrial form catalyses the reaction alanine to pyruvate, and the cytoplasmic form does
the reverse. The only alanine transaminase to
have been isolated and characterized from an
invertebrate is that from Drosophila nigromelanica [46]. Here, the enzyme is a homodimer of
113 kDa and, apart from its approximately fivefold higher affinity for alanine (Km = 5.2 mmoll
1), corresponds in its catalytic properties to the
mammalian enzymes of the rat liver, and porcine
and bovine heart. The enzyme is involved in
energy extraction from proline in the flight muscles; the high activities in the fat body aid proline
synthesis, in that the alanine transported from the
muscle to the fat body provides nitrogen for the
synthesis of glutamic acid and proline from 2oxoglutarate (Fig. 12.6). In the flight muscles of
Schistocerca gregaria, the cytoplasmic alanine
transferase is monomeric and dimeric with subunits of 63 kDa, and the mitochondrial form is
dimeric and tetrameric [61].
12.4.3 Glutamate Dehydrogenases
Organs with central metabolic functions, such as
the liver, kidney, hepatopancreas and fat body,
have high activities of GDH, whereas the activities are lower in most muscles. High activities of
GDH are found in some muscles in association
with the use of proline in energy-yielding metabolism, e.g. in the flight muscles of various
insects and in the mantle muscles of the cephalopod Loligo pealeii (see Table 15.7a, p. 582). The
typical animal GDH, as isolated and characterized from the liver and kidneys of mammals,
birds, amphibians and fish, and from the fat
bodies or flight muscles of insects [244], is a molecule of 310-340 kDa with six identical subunits
of 50-57 kDa and is localized in the mitochondrial matrix. The sequences of the approximately