420
12 Small Nitrogenous Compounds
ris. In the teleosts, CPS III has been detected in
the liver of Micropterus salmoides and at lower
activities in several other species [8]. In parallel
with the CPS activity, the activity of glutamine
synthase is much higher in the liver of marine
elasmobranchs than in the freshwater rays and the
teleosts. Attempts have been made to classify
invertebrate CPSs according to substrate,
dependence upon N-acetylglutamic acid, and
intracellular localization, but this is not always
possible. In the absence of sufficient sequence
data, very little can be said about the phylogenetic relationships of the different CPSs. The
earthworm Lumbricus terrestris and the turbellarian Bipalium kewense possess CPSs with the
properties of types I and III, but part of the activity in the earthworm is cytosolic. CPS III-like
enzymes have also been detected in various terrestrial gastropods, and CPS II-like activity has
been detected in the parasitic worms Ascaris and
Schistosoma as well as in some insects.
Arginases are widely distributed in the animal
kingdom, although they may have grossly different activities in different species. As expected,
they have the highest activity in organs that synthesize urea, but are also found at high activity in
different tissues of ureotelic and non-ureotelic
animals, e.g. in the kidneys of vertebrates of all
classes, in the gut and epidermis of some vertebrates, in the liver of several bird species, and in
some invertebrates. Arginases have many other
functions in addition to their role in the urea
cycle: the introduction of arginine into the citric
acid cycle via ornithine, glutamic acid and 2oxoglutarate; the furnishing of ornithine for the
biosynthesis of proline and polyamines; and
osmoregulatory urea formation [110]. Thus, the
enzyme in the fat bodies of the Indian silk moth
Hyalophora cecropia serves mainly in the formation of proline from arginine, whilst that in the
flight muscles is involved in energy provision.
There is a corresponding 20- to 30-fold increase in
arginase activity during metamorphosis of this
species, compared with the non-flying cockroach
Blaberus cranifera, where the activity remains
relatively low.
The liver arginases of ureotelic animals are
cytosolic; in contrast, the arginases of other
mammalian tissues and all the arginases of uricotelic and ammoniotelic animals are localized in
the mitochondria, e.g. in the livers of birds, reptiles and teleosts [42], and in the fat body of the fly
Aldrichina grahami. The liver arginase of the
elasmobranchs is also mitochondrial, despite the
fact that in these animals the synthesis of arginine
from citrulline takes place in the cytoplasm. It
may have something to do with the osmoregulatory role of urea in the elasmobranchs that the
localization of their liver enzymes is intermediate, i.e. like the ureotelic vertebrates they have
mitochondrial carbamylphosphate synthases, but
like the non-ureotelic vertebrates their arginases
and glutamine synthases are also mitochondrial
[44]. The arginases of mammalian liver have
molecular masses of 110-115 kDa with subunits
of 35 kDa and probably a trimeric structure. The
sequence of the human liver arginase agrees in
87 % of its 322 amino acids with the liver enzyme
of the rat, and 41 % with yeast arginase. Human
kidney, small intestine and lactating milk glands
contain isoenzymes which differ structurally to
those of the liver [100, 133]. Various avian and
reptilian arginases are reported to have molecular
masses of 280 kDa, and that of the liver enzyme
of the clawed frog Xenopus laevis is 76 kDa with
18-kDa subunits. The enzymes of the earthworm
Pheretima communissima and the sea snail
Concholepas concholepas are monomers of
25-27.5 kDa [43, 114]. It was assumed for a long
time that liver arginases occurred only in the
livers of ureotelic vertebrates ("Clementis rule",
1937). After the discovery of liver arginases in the
birds and lizards, Mora proposed in 1965 that a
distinction be drawn between ureotelic and uricotelic liver arginases, of which the ureotelic are
lower in molecular weight, higher in substrate
affinities and have higher activities against the
arginine analogue L-canavanine (Fig. 12.2). However, this concept is not completely justified. It is
true that the liver arginases of ureotelic and uricotelic animals show no immunological crossreactivity, but the enzymes isolated from the liver
and kidney of the chicken and from the liver,
kidney and brain of the lizard Calotes versicolor
agree in many of their properties with the arginases of ureotelic animals. In particular, it is not
true that uricotelic arginases, with Km values of
100-200 mmol/l, always have lower substrate
affinities than the ureotelic variety (Km
10-20 mmol/l); in fact, there are also arginases
with relatively low Km values in non-ureotelic animals, e.g. in the lizard Calotes, the frog Xenopus,
the teleost Genypterus, the molluscs Chiton and
Concholepas, and the beetle Tribolium [42, 43].
Most arginases are activated by Mn2+, Ni2+ or
C0 2 + and inhibited by Mg2+, Cd 2 + or Zn2+
[43, 114]. The pH optimum in the presence of
Mn2+ ions is always about 9-10, but is only pH 7
in the presence of C0 2 + or Ni2+ . Almost all arginases are inhibited by their substrates as well as by
12 Small Nitrogenous Compounds
ris. In the teleosts, CPS III has been detected in
the liver of Micropterus salmoides and at lower
activities in several other species [8]. In parallel
with the CPS activity, the activity of glutamine
synthase is much higher in the liver of marine
elasmobranchs than in the freshwater rays and the
teleosts. Attempts have been made to classify
invertebrate CPSs according to substrate,
dependence upon N-acetylglutamic acid, and
intracellular localization, but this is not always
possible. In the absence of sufficient sequence
data, very little can be said about the phylogenetic relationships of the different CPSs. The
earthworm Lumbricus terrestris and the turbellarian Bipalium kewense possess CPSs with the
properties of types I and III, but part of the activity in the earthworm is cytosolic. CPS III-like
enzymes have also been detected in various terrestrial gastropods, and CPS II-like activity has
been detected in the parasitic worms Ascaris and
Schistosoma as well as in some insects.
Arginases are widely distributed in the animal
kingdom, although they may have grossly different activities in different species. As expected,
they have the highest activity in organs that synthesize urea, but are also found at high activity in
different tissues of ureotelic and non-ureotelic
animals, e.g. in the kidneys of vertebrates of all
classes, in the gut and epidermis of some vertebrates, in the liver of several bird species, and in
some invertebrates. Arginases have many other
functions in addition to their role in the urea
cycle: the introduction of arginine into the citric
acid cycle via ornithine, glutamic acid and 2oxoglutarate; the furnishing of ornithine for the
biosynthesis of proline and polyamines; and
osmoregulatory urea formation [110]. Thus, the
enzyme in the fat bodies of the Indian silk moth
Hyalophora cecropia serves mainly in the formation of proline from arginine, whilst that in the
flight muscles is involved in energy provision.
There is a corresponding 20- to 30-fold increase in
arginase activity during metamorphosis of this
species, compared with the non-flying cockroach
Blaberus cranifera, where the activity remains
relatively low.
The liver arginases of ureotelic animals are
cytosolic; in contrast, the arginases of other
mammalian tissues and all the arginases of uricotelic and ammoniotelic animals are localized in
the mitochondria, e.g. in the livers of birds, reptiles and teleosts [42], and in the fat body of the fly
Aldrichina grahami. The liver arginase of the
elasmobranchs is also mitochondrial, despite the
fact that in these animals the synthesis of arginine
from citrulline takes place in the cytoplasm. It
may have something to do with the osmoregulatory role of urea in the elasmobranchs that the
localization of their liver enzymes is intermediate, i.e. like the ureotelic vertebrates they have
mitochondrial carbamylphosphate synthases, but
like the non-ureotelic vertebrates their arginases
and glutamine synthases are also mitochondrial
[44]. The arginases of mammalian liver have
molecular masses of 110-115 kDa with subunits
of 35 kDa and probably a trimeric structure. The
sequence of the human liver arginase agrees in
87 % of its 322 amino acids with the liver enzyme
of the rat, and 41 % with yeast arginase. Human
kidney, small intestine and lactating milk glands
contain isoenzymes which differ structurally to
those of the liver [100, 133]. Various avian and
reptilian arginases are reported to have molecular
masses of 280 kDa, and that of the liver enzyme
of the clawed frog Xenopus laevis is 76 kDa with
18-kDa subunits. The enzymes of the earthworm
Pheretima communissima and the sea snail
Concholepas concholepas are monomers of
25-27.5 kDa [43, 114]. It was assumed for a long
time that liver arginases occurred only in the
livers of ureotelic vertebrates ("Clementis rule",
1937). After the discovery of liver arginases in the
birds and lizards, Mora proposed in 1965 that a
distinction be drawn between ureotelic and uricotelic liver arginases, of which the ureotelic are
lower in molecular weight, higher in substrate
affinities and have higher activities against the
arginine analogue L-canavanine (Fig. 12.2). However, this concept is not completely justified. It is
true that the liver arginases of ureotelic and uricotelic animals show no immunological crossreactivity, but the enzymes isolated from the liver
and kidney of the chicken and from the liver,
kidney and brain of the lizard Calotes versicolor
agree in many of their properties with the arginases of ureotelic animals. In particular, it is not
true that uricotelic arginases, with Km values of
100-200 mmol/l, always have lower substrate
affinities than the ureotelic variety (Km
10-20 mmol/l); in fact, there are also arginases
with relatively low Km values in non-ureotelic animals, e.g. in the lizard Calotes, the frog Xenopus,
the teleost Genypterus, the molluscs Chiton and
Concholepas, and the beetle Tribolium [42, 43].
Most arginases are activated by Mn2+, Ni2+ or
C0 2 + and inhibited by Mg2+, Cd 2 + or Zn2+
[43, 114]. The pH optimum in the presence of
Mn2+ ions is always about 9-10, but is only pH 7
in the presence of C0 2 + or Ni2+ . Almost all arginases are inhibited by their substrates as well as by