386
0. B. OOWEY AND J. R. SAROENT
General principles of nutrition and metabolism have been
enunciated over the years. From these principles general predictions
may be made about the nutrition of many species. But superimposed
on these generalities are the especial adaptations and modifications of
particular species. These may reflect mainly environmental adaptations,
which, in the case of fish, may possibly be greater than those occurring
amongst terrestrial animals. These diversities should be sought out
both because of their practical importance and in the establishment
of a well developed comparative nutrition.
II. PROTEINS
A . Nitrogen balance
Dietary protein is necessary for three main purposes (i) maintenance,
the making good of tissue wear and tear, (ii) the repletion of depleted
tissues and (iii) growth or the formation of new additional protein. The
utilization of dietary protein is mainly affected by its amino acid
pattern, by the level of protein intake, by the caloric content of the diet
and by the physiological state of the animal.
Assimilated amino acids may enter either anabolic pathways,
resulting in the de m o o biosynthesis of cellular protein, or theymay
enter catabolic pathways. I n the latter process tissue proteins are
broken down to amino acids that in turn are metabolized to simpler
products, the nitrogen moiety being removed and excreted and the
carbon skeleton being oxidized to carbon dioxide and water with the
release of energy. Tissue proteins are in a dynamic state with
anabolic and catabolic processes operating simultaneously so that an
open system is envisaged, with materials entering and leaving, and
the overall protein level is determined by kinetic factors. Some tissues
are more labile than others and have the capacity to hold varying
amounts of protein. Equally some proteins are less labile than others ;
in fact, the half life of certain proteins (e.g. collagen) is so long that
they can sensibly be regarded as inert. The continuing nature of protein metabolism (protein turnover) has led to the concept of a metabolic
pool of amino acids. This pool contributes to anabolic processes and is
also a recipient of protein catabolism. This pool has been viewed as the
" summation of protein metabolism in many centres integrated into the
dynamic state of the body as a whole " (Allison, 1957).
Another, rather longer standing concept established as a consequence of experiments on mammals is that of a protein reserve or
store. This developed long ago (Martin and Robison, 1922) from the
observation that when a subject in nitrogen equilibrium on a high
0. B. OOWEY AND J. R. SAROENT
General principles of nutrition and metabolism have been
enunciated over the years. From these principles general predictions
may be made about the nutrition of many species. But superimposed
on these generalities are the especial adaptations and modifications of
particular species. These may reflect mainly environmental adaptations,
which, in the case of fish, may possibly be greater than those occurring
amongst terrestrial animals. These diversities should be sought out
both because of their practical importance and in the establishment
of a well developed comparative nutrition.
II. PROTEINS
A . Nitrogen balance
Dietary protein is necessary for three main purposes (i) maintenance,
the making good of tissue wear and tear, (ii) the repletion of depleted
tissues and (iii) growth or the formation of new additional protein. The
utilization of dietary protein is mainly affected by its amino acid
pattern, by the level of protein intake, by the caloric content of the diet
and by the physiological state of the animal.
Assimilated amino acids may enter either anabolic pathways,
resulting in the de m o o biosynthesis of cellular protein, or theymay
enter catabolic pathways. I n the latter process tissue proteins are
broken down to amino acids that in turn are metabolized to simpler
products, the nitrogen moiety being removed and excreted and the
carbon skeleton being oxidized to carbon dioxide and water with the
release of energy. Tissue proteins are in a dynamic state with
anabolic and catabolic processes operating simultaneously so that an
open system is envisaged, with materials entering and leaving, and
the overall protein level is determined by kinetic factors. Some tissues
are more labile than others and have the capacity to hold varying
amounts of protein. Equally some proteins are less labile than others ;
in fact, the half life of certain proteins (e.g. collagen) is so long that
they can sensibly be regarded as inert. The continuing nature of protein metabolism (protein turnover) has led to the concept of a metabolic
pool of amino acids. This pool contributes to anabolic processes and is
also a recipient of protein catabolism. This pool has been viewed as the
" summation of protein metabolism in many centres integrated into the
dynamic state of the body as a whole " (Allison, 1957).
Another, rather longer standing concept established as a consequence of experiments on mammals is that of a protein reserve or
store. This developed long ago (Martin and Robison, 1922) from the
observation that when a subject in nitrogen equilibrium on a high