6. Nitrogen Metabolism and Excretion
The major nitrogenous compounds excreted by vertebrates are ammonia, urea, uric
acid, trimethylamine oxide and small quantities of nitrogen containing compounds
including free amino acids. Trimethylamine oxide is present in certain marine fishes
but its status as a true metabolite is in doubt; it probably is obtained exogenously
from th e diet (BALDWIN, 1963). Ammonia, urea and uric acid may be derived from
the deamination of amino acids, a process preparatory to their subsequent metabolism or con version to glucose or fat. Uric acid, along with other nitrogenous
compounds, including allantoin and urea, may also be formed during the metabolism of nucleic acids (purine metabolism) but the quantities are usually minor compared to those from deamination.
The nitrogenous end products from deamination of amino acids vary in different species, and are closely related to the amounts of water that are normally available. Ammonia is the primary end-product of deamination, but owing to its high
toxicity cannot be accumulated in the body. An ammonia concentration of 0.03
mM in the blood is fatal in rabbits. Adequate amounts of water must be available
for ammonia excretion, so that it only predominates as a major end-product of
amino acid metabolism in certain aquatic species, which are said to be ammoniatelic. Ammoniotelic species include teleost fishes, certain aquatic Amphibia, like
the mudpuppy, Necturus macolosus, and the toad, Xenopus laevis, as well as some
crocodilian and chelonian reptiles (see BALDWIN, 1963). Ammonia is lost by diffusion across the gills of fishes and the skin of Necturus (FANELLI and GOLDSTEIN,
1964) while in Xenopus and ammoniotelic reptiles it is excreted in a copious urine.
'The conversion of ammonia to other products is an indispensable adaptation
to limitation of the availability of water' (BALDWIN, 1963). It has been calculated
(H. SMITH, 1951) that 1 g of nitrogen requires 300 to 500 ml of water for its excretion
by an ammoniotelic species like the alligator. In birds and reptiles, which convert
the ammonia to uric acid, only about 10 ml of water would be required for such
excretion, while if transformed to urea 50 ml would be needed in man and 10 ml
in the desert rat, Dipodomys. Aquatic amphibians may be ammoniotelic, but terrestrial species form urea. Reptiles vary in this respect; some aquatic species are
ammoniotelic while terrestrial species excrete urea, uric acid or both. Such categor ies are not mutually exclusive; mammals and birds, for instance, all excrete some
ammonia.
Apart from reflecting the need for reduced urinary water loss, the formation
of urea has a more positive aspect in some species, in which it can be utilized as
a solute that contributes to the animals' osmotic equilibrium with its environment.
In the marine Chondrichthyes (H. SMITH, 1936), the coelacanth, Latimeria (PICKFORD and GRANT, 1967) and the marine frog, Rana cancrivora (GORDON,
SCHMIDT-NIELSEN, and KELLY, 1961), urea may normally be present in the body
fluids at concentrations approaching 500 m-Osmole/l. This contributes to the osmotic constitution of the body fluids, and helps to maintain them at a level that
is hyperosmotic to sea-water. The African lungfish, Protopterus aethiopicus, aestivates in mud during periods of drought, and accumulates urea in concentrations
as high as 500 m-Osmole/l (H. SMITH, 1930a). This is excreted when water becomes
available. The desert toad, Scaphiopus couchi, also aestivates in the earth during
34
The major nitrogenous compounds excreted by vertebrates are ammonia, urea, uric
acid, trimethylamine oxide and small quantities of nitrogen containing compounds
including free amino acids. Trimethylamine oxide is present in certain marine fishes
but its status as a true metabolite is in doubt; it probably is obtained exogenously
from th e diet (BALDWIN, 1963). Ammonia, urea and uric acid may be derived from
the deamination of amino acids, a process preparatory to their subsequent metabolism or con version to glucose or fat. Uric acid, along with other nitrogenous
compounds, including allantoin and urea, may also be formed during the metabolism of nucleic acids (purine metabolism) but the quantities are usually minor compared to those from deamination.
The nitrogenous end products from deamination of amino acids vary in different species, and are closely related to the amounts of water that are normally available. Ammonia is the primary end-product of deamination, but owing to its high
toxicity cannot be accumulated in the body. An ammonia concentration of 0.03
mM in the blood is fatal in rabbits. Adequate amounts of water must be available
for ammonia excretion, so that it only predominates as a major end-product of
amino acid metabolism in certain aquatic species, which are said to be ammoniatelic. Ammoniotelic species include teleost fishes, certain aquatic Amphibia, like
the mudpuppy, Necturus macolosus, and the toad, Xenopus laevis, as well as some
crocodilian and chelonian reptiles (see BALDWIN, 1963). Ammonia is lost by diffusion across the gills of fishes and the skin of Necturus (FANELLI and GOLDSTEIN,
1964) while in Xenopus and ammoniotelic reptiles it is excreted in a copious urine.
'The conversion of ammonia to other products is an indispensable adaptation
to limitation of the availability of water' (BALDWIN, 1963). It has been calculated
(H. SMITH, 1951) that 1 g of nitrogen requires 300 to 500 ml of water for its excretion
by an ammoniotelic species like the alligator. In birds and reptiles, which convert
the ammonia to uric acid, only about 10 ml of water would be required for such
excretion, while if transformed to urea 50 ml would be needed in man and 10 ml
in the desert rat, Dipodomys. Aquatic amphibians may be ammoniotelic, but terrestrial species form urea. Reptiles vary in this respect; some aquatic species are
ammoniotelic while terrestrial species excrete urea, uric acid or both. Such categor ies are not mutually exclusive; mammals and birds, for instance, all excrete some
ammonia.
Apart from reflecting the need for reduced urinary water loss, the formation
of urea has a more positive aspect in some species, in which it can be utilized as
a solute that contributes to the animals' osmotic equilibrium with its environment.
In the marine Chondrichthyes (H. SMITH, 1936), the coelacanth, Latimeria (PICKFORD and GRANT, 1967) and the marine frog, Rana cancrivora (GORDON,
SCHMIDT-NIELSEN, and KELLY, 1961), urea may normally be present in the body
fluids at concentrations approaching 500 m-Osmole/l. This contributes to the osmotic constitution of the body fluids, and helps to maintain them at a level that
is hyperosmotic to sea-water. The African lungfish, Protopterus aethiopicus, aestivates in mud during periods of drought, and accumulates urea in concentrations
as high as 500 m-Osmole/l (H. SMITH, 1930a). This is excreted when water becomes
available. The desert toad, Scaphiopus couchi, also aestivates in the earth during
34