ism at the PEP branch point gradually switches
from PK to PEPCK and the metabolic rate decreases. The degree of phosphorylation of adenosine (energy charge) declines drastically during
the first 9 h and then becomes stable. It would
appear that a new stable metabolic state is
reached only with some difficulty. From 12 h, the
succinate and alanine concentrations in the animal show no further increase; only propionate
and acetate are produced and these are mainly
excreted into the surroundings. A very similar
metabolic process is also observed in the errant
polychaetes Nephthys hombergii and Anaitides
mucosa, and in the sedentary forms Scoloplos
armiger and Scololepis foliosa [225, 288].
Different metabolic reactions are found in the
various species of the polychaete genus Nereis;
during experimental anoxia, these produce Dlactate as well as succinate, propionate, acetate
and alanine, but no strombine. Nereis diversicolor, which as an inhabitant of the higher eulittoral
may also experience hypoxic conditions in nature
and is particularly anoxia resistant, on balance
produce more succinate and propionate than lactate during long-lasting anaerobiosis. In contrast,
lactate predominates in N. pelagica from the sublittoral, and this species tolerates only short periods of anoxia. N. virens, which extends from the
sublittoral to the lower eulittoral, exhibits intermediate behaviour [288]. Apart from the changes
in metabolic pathways and end products with
time during long-lasting habitat-dependent anaerobiosis in free-living animals, there are other
complications that render more difficult the scientific analysis of anaerobic energy metabolism.
Distinct seasonal differences have been observed
in the edible mussel Mytilus edulis, which in
nature is exposed to air during low tides. There
are, of course, also organ-specific differences in
enzyme complement and the end products of
anaerobic metabolism. For example, the bodywall muscles, the nervous system and the scales of
the polychaete Aphrodite aculeata contain an alanopine dehydrogenase of broad specificity,
whereas the gut has a highly specific strombine
dehydrogenase [242]. In Cardium edule, more Llactate than succinate is produced in the muscles
during habitat-dependent anaerobiosis, and the
reverse is true for the gills [155]. Organ-specific
differences in anaerobic end products have also
been reported in Mytilus edulis.
The far-reaching changes in energy-supplying
cell metabolism during habitat- and activitydependent anaerobiosis point to complex regulatory processes and interactions which are as yet
14.11 Pyruvate Reductases
541
only poorly understood. The targets of the regulatory mechanisms are probably mainly PFK,
hexokinase and PK, perhaps also PEPCK, and in
some animals lactate dehydrogenase. The most
important regulatory factors are the relative concentrations of ATP, ADP and AMP (and also the
energy charge) as well as arginine phosphate
[53, 68, 71, 84]. Octopine is thought to have
important regulatory functions in the cephalopod
Sepia, as has pH in the crustaceans and bivalves
[53, 224]. The activities of the various dehydrogenases depend very much, of course, on the
NADH:NAD+ ratio [71]. Information about
regulatory mechanisms is provided by the relationships between the kinetic-regulatory properties of the enzymes and changes in the metabolic
rate and metabolite concentrations during anaerobiosis; such data are available for various bivalves [68, 70, 71, 105, 155, 289], gastropods [51, 68,
190], cephalopods, annelids [103, 225, 288] and
crustaceans [71]. The mechanism contributing to
the reduction in metabolism during habitatdependent anaerobiosis is still something of a
mystery.
14.11 Pyruvate Reductases
The NADH produced by the GAPDH reaction
can be reoxidized in various reactions with pyruvate as the hydrogen acceptor (Fig. 14.5a). There
are several types of pyruvate reductase in the animal kingdom whose phylogenetic relationships
have not yet been defined. The lactate dehydrogenases (LD Hs) catalyse the reversible reduction
of pyruvate to lactate with NADH as the coenzyme. The LDHs are more-or-less ubiquitous;
they are absent from, for example, some protozoans of the genera Trypanosoma and Leishmania. They are especially active in muscles with
a marked capacity for activity-dependent anaerobiosis. The highest recorded LDH activity of
5000 U/g at 25°C was for the white musculature
of the tuna fish Euthynnus pelamis, which is one
of the fastest swimmers in the animal kingdom
[102]. Animals have either L- or D-specific LDH
according to their systematic positions (Table 14.6). The opine dehydrogenases catalyse the
reductive condensation of pyruvate or other keto
acids with the a-amino group of an amino acid,
using NADH as the coenzyme. Such enzymes
have been detected in various groups of marine
invertebrates.
from PK to PEPCK and the metabolic rate decreases. The degree of phosphorylation of adenosine (energy charge) declines drastically during
the first 9 h and then becomes stable. It would
appear that a new stable metabolic state is
reached only with some difficulty. From 12 h, the
succinate and alanine concentrations in the animal show no further increase; only propionate
and acetate are produced and these are mainly
excreted into the surroundings. A very similar
metabolic process is also observed in the errant
polychaetes Nephthys hombergii and Anaitides
mucosa, and in the sedentary forms Scoloplos
armiger and Scololepis foliosa [225, 288].
Different metabolic reactions are found in the
various species of the polychaete genus Nereis;
during experimental anoxia, these produce Dlactate as well as succinate, propionate, acetate
and alanine, but no strombine. Nereis diversicolor, which as an inhabitant of the higher eulittoral
may also experience hypoxic conditions in nature
and is particularly anoxia resistant, on balance
produce more succinate and propionate than lactate during long-lasting anaerobiosis. In contrast,
lactate predominates in N. pelagica from the sublittoral, and this species tolerates only short periods of anoxia. N. virens, which extends from the
sublittoral to the lower eulittoral, exhibits intermediate behaviour [288]. Apart from the changes
in metabolic pathways and end products with
time during long-lasting habitat-dependent anaerobiosis in free-living animals, there are other
complications that render more difficult the scientific analysis of anaerobic energy metabolism.
Distinct seasonal differences have been observed
in the edible mussel Mytilus edulis, which in
nature is exposed to air during low tides. There
are, of course, also organ-specific differences in
enzyme complement and the end products of
anaerobic metabolism. For example, the bodywall muscles, the nervous system and the scales of
the polychaete Aphrodite aculeata contain an alanopine dehydrogenase of broad specificity,
whereas the gut has a highly specific strombine
dehydrogenase [242]. In Cardium edule, more Llactate than succinate is produced in the muscles
during habitat-dependent anaerobiosis, and the
reverse is true for the gills [155]. Organ-specific
differences in anaerobic end products have also
been reported in Mytilus edulis.
The far-reaching changes in energy-supplying
cell metabolism during habitat- and activitydependent anaerobiosis point to complex regulatory processes and interactions which are as yet
14.11 Pyruvate Reductases
541
only poorly understood. The targets of the regulatory mechanisms are probably mainly PFK,
hexokinase and PK, perhaps also PEPCK, and in
some animals lactate dehydrogenase. The most
important regulatory factors are the relative concentrations of ATP, ADP and AMP (and also the
energy charge) as well as arginine phosphate
[53, 68, 71, 84]. Octopine is thought to have
important regulatory functions in the cephalopod
Sepia, as has pH in the crustaceans and bivalves
[53, 224]. The activities of the various dehydrogenases depend very much, of course, on the
NADH:NAD+ ratio [71]. Information about
regulatory mechanisms is provided by the relationships between the kinetic-regulatory properties of the enzymes and changes in the metabolic
rate and metabolite concentrations during anaerobiosis; such data are available for various bivalves [68, 70, 71, 105, 155, 289], gastropods [51, 68,
190], cephalopods, annelids [103, 225, 288] and
crustaceans [71]. The mechanism contributing to
the reduction in metabolism during habitatdependent anaerobiosis is still something of a
mystery.
14.11 Pyruvate Reductases
The NADH produced by the GAPDH reaction
can be reoxidized in various reactions with pyruvate as the hydrogen acceptor (Fig. 14.5a). There
are several types of pyruvate reductase in the animal kingdom whose phylogenetic relationships
have not yet been defined. The lactate dehydrogenases (LD Hs) catalyse the reversible reduction
of pyruvate to lactate with NADH as the coenzyme. The LDHs are more-or-less ubiquitous;
they are absent from, for example, some protozoans of the genera Trypanosoma and Leishmania. They are especially active in muscles with
a marked capacity for activity-dependent anaerobiosis. The highest recorded LDH activity of
5000 U/g at 25°C was for the white musculature
of the tuna fish Euthynnus pelamis, which is one
of the fastest swimmers in the animal kingdom
[102]. Animals have either L- or D-specific LDH
according to their systematic positions (Table 14.6). The opine dehydrogenases catalyse the
reductive condensation of pyruvate or other keto
acids with the a-amino group of an amino acid,
using NADH as the coenzyme. Such enzymes
have been detected in various groups of marine
invertebrates.