succinate lyase is inhibited by an increase in the
level of aldosterone. Hormonal regulation of the
osmoregulatory process has also been shown for
Rana temporaria, but the exact mechanism is not
known.
The South African tree frog Chiromantis
xerampelina (Racophoridae) and the South
American species Phyllomedusa sauvagii (Hylidae) possess a uricotelic mechanism which protects against dehydration. Uric acid, as a proportion of the total nitrogen excreted, is 50-57 % in
C. xerampelina, about 80 % in P. sauvagii, but
only 24-25 % in the related species P. pailona,
P. iherengi and P. hypochondrialis; in other genera of the Hylidae, such as Agalychnia, Pachymeduse or Hyla, it is in fact always less than 3.5 %.
In the liver of the uricotelic species, the activities
of the enzymes of urea synthesis (arginase) and
uricolysis (uricase) are relatively low, and those of
the enzymes of uric acid synthesis (xanthine
dehydrogenase) are relatively high. Hyperolius
viridiflavus (Hyperoliidae) produces mainly guanine and hypoxanthine, which are deposited in
iridophores. Thus, the osmotic problems accompanying water loss are solved and, at the same
time, the high reflectance of these cells reduces
heat absorption [218]. H. nasutus, which is also
very resistant to dehydration, is, in contrast,
purely ureotelic.
Amongst the reptiles, the snakes and lizards
are all uricotelic, and the crocodile excretes
mainly urea but also ammonia. Turtles and tortoises may be ammoniotelic, ureotelic or uricotelic,
depending on their habitat. In addition to these
species-specific differences there is also variation
in the regulatory mechanisms. Aquatic and semiaquatic species excrete ammonia and urea in
about equal proportions; terrestrial species from
wet biotopes produce much more urea than
ammonia; and species from dry biotopes produce
about 60 % uric acid and 40 % urea. Transfer of
the turtle Trionyx spiniferus from freshwater to
brackish water with 0.7 % salt results in an accumulation of urea in the tissues.
The urine of birds consists almost entirely of
ammonium urate, i.e. it contains uric acid nitrogen and ammonia nitrogen in the ratio 4: 1. Needham (1931) in his pioneering work Chemical
Embryology described how, during chick development, ammonia is excreted initially (maximum
at day 3), then urea (days 5-9) and, finally, uric
acid; he cited this as an example of the biogenetic
rule also being valid in biochemistry. However,
already by 1935, he found that the urea in fact originated from preformed arginine. Today, it is
12.4 Nitrogen Metabolism of the Amino Acids
413
quite clear that in the chicken, as in other birds,
there is no functioning urea cycle.
Mammals are ureotelic. In addition to urea,
their urine always contains uric acid or allantoin,
as an end product of purine metabolism, and
many other nitrogenous compounds. Hippuric
acid, which is a product of the detoxification of
benzoic acid, is found at high concentration in the
urine of herbivores but only in trace amounts in
the carnivores and omnivores.
12.4 Nitrogen Metabolism
of the Amino Acids
The degradation of most amino acids begins with
the cleavage of the amino group and formation of
the corresponding keto acid. The L-amino acid
oxidases that may be present in tissues are not significantly involved in this process; the usual situation in animals is that deamination of amino acids
occurs by one of two more indirect pathways, one
via glutamate dehydrogenase (GDH) and the
other via the purine nucleotide cycle. In both
pathways, the amino groups of different amino
acids are initially transferred to a common intermediate (glutamic or aspartic acid), and only in a
second step, if at all, are they released as ammonia. The GDH pathway occurs typically in vertebrate liver. Here, the glutamic acid intermediate is oxidatively deaminated by mitochondrial
glutamate dehydrogenase. The fate of the ammonia released in the liver mitochondria differs
markedly in different species. In ureotelic animals, it is bound to carbamylphosphate synthetase I and then incorporated into citrulline, which
is released into the cytosol and there used for
urea synthesis. The second amino group required
comes from aspartic acid. In uricotelic animals,
the ammonia is converted to glutamine by mitochondrial glutamine synthase and is then used in
the cytosol for uric acid synthesis. The purine
nucleotide cycle (Fig. 12.3) is typical of vertebrate
muscle, which contains only low GDH activity.
The product of the transamination is aspartic acid
and its nitrogen then serves in the conversion of
IMP to AMP; ammonia is released from the AMP
by an AMP deaminase, giving again IMP.
Little is known of the relative importance of
these two pathways of amino acid deamination in
different invertebrate animals. Apart from in
birds and mammals, the complete enzyme system
of the purine nucleotide cycle has been found
only in the goldfish and in the hepatopancreas of
level of aldosterone. Hormonal regulation of the
osmoregulatory process has also been shown for
Rana temporaria, but the exact mechanism is not
known.
The South African tree frog Chiromantis
xerampelina (Racophoridae) and the South
American species Phyllomedusa sauvagii (Hylidae) possess a uricotelic mechanism which protects against dehydration. Uric acid, as a proportion of the total nitrogen excreted, is 50-57 % in
C. xerampelina, about 80 % in P. sauvagii, but
only 24-25 % in the related species P. pailona,
P. iherengi and P. hypochondrialis; in other genera of the Hylidae, such as Agalychnia, Pachymeduse or Hyla, it is in fact always less than 3.5 %.
In the liver of the uricotelic species, the activities
of the enzymes of urea synthesis (arginase) and
uricolysis (uricase) are relatively low, and those of
the enzymes of uric acid synthesis (xanthine
dehydrogenase) are relatively high. Hyperolius
viridiflavus (Hyperoliidae) produces mainly guanine and hypoxanthine, which are deposited in
iridophores. Thus, the osmotic problems accompanying water loss are solved and, at the same
time, the high reflectance of these cells reduces
heat absorption [218]. H. nasutus, which is also
very resistant to dehydration, is, in contrast,
purely ureotelic.
Amongst the reptiles, the snakes and lizards
are all uricotelic, and the crocodile excretes
mainly urea but also ammonia. Turtles and tortoises may be ammoniotelic, ureotelic or uricotelic,
depending on their habitat. In addition to these
species-specific differences there is also variation
in the regulatory mechanisms. Aquatic and semiaquatic species excrete ammonia and urea in
about equal proportions; terrestrial species from
wet biotopes produce much more urea than
ammonia; and species from dry biotopes produce
about 60 % uric acid and 40 % urea. Transfer of
the turtle Trionyx spiniferus from freshwater to
brackish water with 0.7 % salt results in an accumulation of urea in the tissues.
The urine of birds consists almost entirely of
ammonium urate, i.e. it contains uric acid nitrogen and ammonia nitrogen in the ratio 4: 1. Needham (1931) in his pioneering work Chemical
Embryology described how, during chick development, ammonia is excreted initially (maximum
at day 3), then urea (days 5-9) and, finally, uric
acid; he cited this as an example of the biogenetic
rule also being valid in biochemistry. However,
already by 1935, he found that the urea in fact originated from preformed arginine. Today, it is
12.4 Nitrogen Metabolism of the Amino Acids
413
quite clear that in the chicken, as in other birds,
there is no functioning urea cycle.
Mammals are ureotelic. In addition to urea,
their urine always contains uric acid or allantoin,
as an end product of purine metabolism, and
many other nitrogenous compounds. Hippuric
acid, which is a product of the detoxification of
benzoic acid, is found at high concentration in the
urine of herbivores but only in trace amounts in
the carnivores and omnivores.
12.4 Nitrogen Metabolism
of the Amino Acids
The degradation of most amino acids begins with
the cleavage of the amino group and formation of
the corresponding keto acid. The L-amino acid
oxidases that may be present in tissues are not significantly involved in this process; the usual situation in animals is that deamination of amino acids
occurs by one of two more indirect pathways, one
via glutamate dehydrogenase (GDH) and the
other via the purine nucleotide cycle. In both
pathways, the amino groups of different amino
acids are initially transferred to a common intermediate (glutamic or aspartic acid), and only in a
second step, if at all, are they released as ammonia. The GDH pathway occurs typically in vertebrate liver. Here, the glutamic acid intermediate is oxidatively deaminated by mitochondrial
glutamate dehydrogenase. The fate of the ammonia released in the liver mitochondria differs
markedly in different species. In ureotelic animals, it is bound to carbamylphosphate synthetase I and then incorporated into citrulline, which
is released into the cytosol and there used for
urea synthesis. The second amino group required
comes from aspartic acid. In uricotelic animals,
the ammonia is converted to glutamine by mitochondrial glutamine synthase and is then used in
the cytosol for uric acid synthesis. The purine
nucleotide cycle (Fig. 12.3) is typical of vertebrate
muscle, which contains only low GDH activity.
The product of the transamination is aspartic acid
and its nitrogen then serves in the conversion of
IMP to AMP; ammonia is released from the AMP
by an AMP deaminase, giving again IMP.
Little is known of the relative importance of
these two pathways of amino acid deamination in
different invertebrate animals. Apart from in
birds and mammals, the complete enzyme system
of the purine nucleotide cycle has been found
only in the goldfish and in the hepatopancreas of