416
12 Small Nitrogenous Compounds
500-amino-acid chains have been determined, for
example, for the liver enzymes of humans, several other mammals and the chicken, and for the
human brain enzyme [3, 150, 163, 191]. In the
frog liver, an equilibrium between forms of 250
and 500 kDa is shifted by NADH in favour of the
smaller form and by NAD+ in the opposite direction. The enzyme of bovine liver differs from that
in, e.g., rat liver in being organized into aggregates of 2 to 3 MDa.
Most animal GDHs are active with both coenzymes, although the rate of reaction with NADH
is always higher than with NADPH. The activity
ratio NADH:NADPH differs widely with the
origin of the enzyme, pH and other parameters:
it is less than 2 for the bovine and chicken liver
enzymes, 6-8 for the two isoenzymes of the
beetle Popillia japonica [144], and 15 for the liver
enzyme of the tuna fish. Reductive amination is,
as a rule, more rapid than oxidative deamination,
with the activity ratio NADH:NAD+ varying
from 2.5-6.0 for GDH from various invertebrates
and the gills and kidneys of fish to over 20 for the
liver GDH of various vertebrates. GDHs are not
absolutely specific for glutamic acid. Whereas the
bovine enzyme has only minor activity against
other L-amino acids, like glutamine, norvaline
and 2-aminobutyrate, the frog-liver GDH is much
less specific. The enzyme from both tadpoles and
adults of the bullfrog has, for example, with glutamine 38 % of the activity shown with glutamic
acid. The pH optima of animal GDHs are all very
similar: pH 7-8 for glutamate synthesis, and
somewhat higher at pH 8-9 for glutamate catabolism.
The typical GDH is regulated allosterically,
with ADP and AMP as activators, GTP in particular as an inhibitor, and ATP with negligible
effect. NADH also inhibits under certain conditions; its effect is relieved by ADP. There are
many strong interactions between allosteric effectors. One would expect that cells suffering an
energy shortage would catabolize glutamate, and
synthesize it during periods of surplus energy. In
fact, GDH is stimulated during an energy shortage by low GTP:ADP ratios; situations of surplus
energy, however, are characterized by high ratios,
and GDH should be inhibited. Thus, glutamate
synthesis is apparently regulated more by substrate supply than by allosteric effects [144]. In
some cases, tissue-specific GDH isoenzymes with
varying kinetic-regulatory properties have been
described. Thus, in the Amazon fish Arapaima
gigas and Osteoglossum bicirrhosum, ATP activates GDH in the kidney but inhibits the enzyme in
the gills; the activation by ADP is much greater
for the kidney GDH than for the gill enzyme. The
biological significance of these differences is not
clear. In contrast, the differences in kineticregulatory properties of the GDH isoenzymes
from the flight muscle and fat body of the beetle
Popillia japonica are clearly interpretable. Proline is used as an energy source in the flight muscles of this species. The glutamic acid formed in
the process is deaminated by the muscle GDH,
giving free ammonia; in the flight muscles of the
tsetse fly Glossina morsitans and the potato
beetle Leptinotarsa decemlineata, on the other
hand, alanine is produced by transamination
(Fig. 12.6). The 2-oxoglutarate produced from the
glutamate enters the citrate cycle as malate and
pyruvate, and is released as water and CO2• The
2-oxoglutarate produced from carbohydrate in
the fat body of Popilla is aminated by GDH to
glutamate, which is then used in the de novo synthesis of proline. The muscle GDH, with a relatively low substrate affinity and marked nucleotide regulation, is specifically adapted to glutamate oxidation, and is strictly regulated by the
energy status of the flight muscle. In contrast, the
fat-body GDH is almost independent of the nucleotide status of the cell and is specialized for glutamate synthesis by its high affinity for 2oxoglutarate in the presence of NADPH [144].
Some animals have GDH enzymes which deviate from the normal pattern in their location or in
being specific for only one coenzyme. The GDHs
from the hepatopancreas of the mussels Mytilus
edulis and Modiolus demissus and from the
mantle muscle of the cephalopod Loligo pealeii
are absolutely specific for NADHlNAD+ [212].
The Loligo enzyme, in contrast to all other
GDHs, shows the same activity in both reaction
directions. The Cnidaria and Ctenophora have
GDHs that are only NADPHlNADP+ active and
show no nucleotide regulation but are strongly
influenced by the salt concentration [70]. 1\vo
very different GDHs are found in the coral Acropora latistella: an NADPH-specific enzyme localized in the cytoplasm, and a mitochondrial
enzyme that differs from the GDH of higher animals by its high specificity for NADHlNAD+ and
the absence of allosteric effects with ADP and
GTP [70]. Trypanosoma cruzi contains two completely different GDHs. One is absolutely specific for NADPHlNADP+ and is not affected by
adenine nucleotide; it is similar in all characters
to the enzyme from Plasmodium chabaudi and
corresponds to the NADP-specific GDH of bacteria, fungi and higher plants. The second Trypa-
12 Small Nitrogenous Compounds
500-amino-acid chains have been determined, for
example, for the liver enzymes of humans, several other mammals and the chicken, and for the
human brain enzyme [3, 150, 163, 191]. In the
frog liver, an equilibrium between forms of 250
and 500 kDa is shifted by NADH in favour of the
smaller form and by NAD+ in the opposite direction. The enzyme of bovine liver differs from that
in, e.g., rat liver in being organized into aggregates of 2 to 3 MDa.
Most animal GDHs are active with both coenzymes, although the rate of reaction with NADH
is always higher than with NADPH. The activity
ratio NADH:NADPH differs widely with the
origin of the enzyme, pH and other parameters:
it is less than 2 for the bovine and chicken liver
enzymes, 6-8 for the two isoenzymes of the
beetle Popillia japonica [144], and 15 for the liver
enzyme of the tuna fish. Reductive amination is,
as a rule, more rapid than oxidative deamination,
with the activity ratio NADH:NAD+ varying
from 2.5-6.0 for GDH from various invertebrates
and the gills and kidneys of fish to over 20 for the
liver GDH of various vertebrates. GDHs are not
absolutely specific for glutamic acid. Whereas the
bovine enzyme has only minor activity against
other L-amino acids, like glutamine, norvaline
and 2-aminobutyrate, the frog-liver GDH is much
less specific. The enzyme from both tadpoles and
adults of the bullfrog has, for example, with glutamine 38 % of the activity shown with glutamic
acid. The pH optima of animal GDHs are all very
similar: pH 7-8 for glutamate synthesis, and
somewhat higher at pH 8-9 for glutamate catabolism.
The typical GDH is regulated allosterically,
with ADP and AMP as activators, GTP in particular as an inhibitor, and ATP with negligible
effect. NADH also inhibits under certain conditions; its effect is relieved by ADP. There are
many strong interactions between allosteric effectors. One would expect that cells suffering an
energy shortage would catabolize glutamate, and
synthesize it during periods of surplus energy. In
fact, GDH is stimulated during an energy shortage by low GTP:ADP ratios; situations of surplus
energy, however, are characterized by high ratios,
and GDH should be inhibited. Thus, glutamate
synthesis is apparently regulated more by substrate supply than by allosteric effects [144]. In
some cases, tissue-specific GDH isoenzymes with
varying kinetic-regulatory properties have been
described. Thus, in the Amazon fish Arapaima
gigas and Osteoglossum bicirrhosum, ATP activates GDH in the kidney but inhibits the enzyme in
the gills; the activation by ADP is much greater
for the kidney GDH than for the gill enzyme. The
biological significance of these differences is not
clear. In contrast, the differences in kineticregulatory properties of the GDH isoenzymes
from the flight muscle and fat body of the beetle
Popillia japonica are clearly interpretable. Proline is used as an energy source in the flight muscles of this species. The glutamic acid formed in
the process is deaminated by the muscle GDH,
giving free ammonia; in the flight muscles of the
tsetse fly Glossina morsitans and the potato
beetle Leptinotarsa decemlineata, on the other
hand, alanine is produced by transamination
(Fig. 12.6). The 2-oxoglutarate produced from the
glutamate enters the citrate cycle as malate and
pyruvate, and is released as water and CO2• The
2-oxoglutarate produced from carbohydrate in
the fat body of Popilla is aminated by GDH to
glutamate, which is then used in the de novo synthesis of proline. The muscle GDH, with a relatively low substrate affinity and marked nucleotide regulation, is specifically adapted to glutamate oxidation, and is strictly regulated by the
energy status of the flight muscle. In contrast, the
fat-body GDH is almost independent of the nucleotide status of the cell and is specialized for glutamate synthesis by its high affinity for 2oxoglutarate in the presence of NADPH [144].
Some animals have GDH enzymes which deviate from the normal pattern in their location or in
being specific for only one coenzyme. The GDHs
from the hepatopancreas of the mussels Mytilus
edulis and Modiolus demissus and from the
mantle muscle of the cephalopod Loligo pealeii
are absolutely specific for NADHlNAD+ [212].
The Loligo enzyme, in contrast to all other
GDHs, shows the same activity in both reaction
directions. The Cnidaria and Ctenophora have
GDHs that are only NADPHlNADP+ active and
show no nucleotide regulation but are strongly
influenced by the salt concentration [70]. 1\vo
very different GDHs are found in the coral Acropora latistella: an NADPH-specific enzyme localized in the cytoplasm, and a mitochondrial
enzyme that differs from the GDH of higher animals by its high specificity for NADHlNAD+ and
the absence of allosteric effects with ADP and
GTP [70]. Trypanosoma cruzi contains two completely different GDHs. One is absolutely specific for NADPHlNADP+ and is not affected by
adenine nucleotide; it is similar in all characters
to the enzyme from Plasmodium chabaudi and
corresponds to the NADP-specific GDH of bacteria, fungi and higher plants. The second Trypa-