Modelling Amino Acid Metabolism
Techniques for formulating feed for farm
animals are based mainly on whole-animal
studies, for the most part being designed to
describe input and output situations
developed to maximize productivity for
given levels of financial expenditure (essen-
tially equating to nutritive value of feed
ingredients). The majority of early models
of protein metabolism were simple in inter-
pretation and were readily applicable to the
feed industry. Early protein schemes were
empirical models used to predict nitrogen
requirements for growth and lactation (e.g.
ARC, 1980; AFRC, 1984) and were based on
the calculation of constants used to describe
specific events in the gut and peripheral
tissues. It is now recognized that these
models are oversimplistic, and the reliance
on average constants which do predict
changes in response to perturbations in
nutrient input is a major problem. Increased
knowledge about metabolism in individual
tissues has brought about a more integrated
approach to modelling, incorporating a
series of compartmentalized sub-models
which are combined to provide a mechanis-
tic integrated system which can still be
applied in the feed industry (Hanigan et al.,
1997). In order to describe amino acid
requirements, these models adopt different
approaches. The model described by
Baldwin et al. (1987) introduced the
concept of dynamic relationships integrated
over time for three tissue types: lean body,
adipose tissue and the viscera. This model
treats amino acids as a single pool but
allows competition between tissues for
amino acids and different rates of tissue
utilization. Gill et al. (1989) adopted a fully
integrated approach which takes account of
the effects of amino acid metabolism
through ten tissue beds supplied from a sin-
gle blood amino acid pool. The model does
not include mammary metabolism and is
necessarily complex using, as it does, data
for ten different tissues. The Cornell net
carbohydrate and protein system (CNCPS)
includes sub-models of post-absorptive
amino acid metabolism in which amino
acid metabolism is split into discrete func-
tions including needs for lactation, growth,
urine and metabolic faecal losses, and scurf
(O’Connor et al., 1993). The whole-animal
and whole-tissue models described above
are now supplemented by an increasing
number of models in which intermediary
metabolism of selected amino acids in some
organs is added to the flux through the
aggregated pool (Hanigan et al., 1997, 1998).Conclusions
It is apparent that amino acid flux in blood
represents the summation of a large number
of processes in individual tissues. In farm
livestock, the need to understand and
quantify these processes is essential in order
to optimize the use of dietary protein for
animal production and minimize the impact
of nitrogen loss from animal agriculture on
the environment. The relationship between
protein synthesis and degradation in indi-
vidual tissues and the magnitude of the flux
of amino acids between tissue beds and the
plasma pool in the young growing lamb has
been summarized (MacRae, 1993) and is
shown in Fig. 3.2. These data provide a clear
picture of the partition of amino acid meta-
bolism between anabolic and catabolic
processes in different tissues and the rela-
tionship between total protein synthesis
(280 g day^1 ) and protein accretion (39 g
day^1 ). In this respect, gut and skin repre-
sent 50% of total protein synthesis on a
daily basis but only contribute 10% towards
protein accretion, whereas for muscle the
values are 17.5 and 36%, respectively. The
partition between these processes differen-
tiates tissues in terms of their functions
within the animal and often represents
specific amino acid requirements; for
example mucin production by the gut and
export protein synthesis in the liver.
Changes in the overall requirements of an
individual tissue will inevitably have an
effect on the balance of amino acid supply to
others. In this respect, all tissues are ‘com-
peting’ within the same amino acid pool,
apart from the gut and liver both of which
receive an exogenous supply of amino acids
from the diet which supplements the58 C.J. Seal and D.S. Parker