Digestion and absorption
Amino acids chemically bound in proteins must be separated from the
parent protein unit, before they can pass from the lumen of the gut
across the intestinal wall (absorption) into the blood. This separation
occurs in the lumen of the gut with the help of proteolytic digestive
enzymes (proteases). The activity of the proteolytic enzymes is aided
by the secretion of dilute hydrochloric acid in the stomach. Presence
of the acid acidifies the ingested feed in the stomach which results
in denaturation of the protein. The process starts with denaturation
of the protein and continues with the cleavage into individual amino
acids or as pairs of amino acids (dipeptides), tripeptides and up to six
amino acid units in length (oligopeptides).
The break-down of the peptide chains is carried out by endopeptidases
(pepsin, trypsin, chymotrypsin) which cleave at the centre of a chain and
exopeptidases which cleave from the terminal ends. The amino acids
and oligopeptides are absorbed by mucosal cells which line the surface
of the intestine and finally enter the bloodstream as free amino acids.
Specific transport systems are responsible for the absorption of amino
acids. The absorbed amino acids are transported via the portal vein into
the liver, which is the principal organ for the metabolism of amino acids.
3.2. Protein metabolism and synthesis
The metabolism of protein is made up of two opposing processes
which run in parallel. The accretion of proteins (anabolism = synthesis)
and the breakdown of protein (catabolism = proteolysis) occur at
one and the same time. Synthesis predominates in young growing
animals and the protein is built into muscle whereas in mature animals
a balance is reached between synthesis and proteolysis with no
increase in the mass of the muscle but with continuous turnover.
The amino acid sequence of a protein is genetically predetermined
and all the required amino acids must be present at the same time
(synchronous synthesis). The organism is able to compensate for a
deficiency of non-essential amino acids within certain limits through
auto-synthesis. However, protein synthesis comes to a stop if one of
the essential amino acids is lacking because some amino acids (the
essential ones) cannot be synthesised by the organism (see 3.4 essential
amino acids). Amino acids which are not used to synthesise
protein or that are released from protein during degradation must be
broken down and excreted since the body has no mechanism to store
them. The carbon skeletons of amino acids are metabolised to supply
energy and the liberated ammonia which is derived from the nitrogenous
component must be “detoxified” and removed from the body.
This is achieved via the synthesis of urea in mammals, and uric acid
in poultry, which is a process with a very high energy requirement.
During periods of severe energy deficiency, protein may be catabolised
to supply energy for the upkeep of vital processes. However,
compared to the metabolism of fats and carbohydrates, efficiency of
the process is very low.
From the above it can be seen that:
• Protein and energy metabolism cannot be considered as unconnected.
In feed formulation this is taken into account by considering
the ratio of the limiting amino acids with respect to the metabolisable
or net energy content of the feed.
• Matching of amino acids provided for metabolism with the actual
requirement for metabolism must be as precise as possible
in terms of both quantity and composition (see chapter 3.6 Ideal
protein concept).
Apart from muscle growth only limited amounts of protein can be
stored. Some storage occurs in the liver. Otherwise, the degradation
of protein is relatively rapid and is expressed by the half-life. For example,
digestive enzymes which have a short half-life are particularly
affected and are thus highly susceptible to changing metabolic conditions
with respect to amino acid supply. Hence a temporary deficiency
in amino acids for the synthesis of the enzyme proteins can show up
as a loss in performance.
Great importance is therefore attached to the concept of the continuous
supply of free amino acids from the feed into the animal’s metabolism
(amino acid flux). This needs to be taken into account, when
supplementing amino acids to mixtures of feed. In modern practical
feeding systems, amino acid supplementation has been proven to be
an effective method to continuously balance the amino acid supply at
the site of protein synthesis.