Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 5 Proteins, Proteomics, and the Dysproteinemias
genetic code in nuclear DNA directs the primary sequence
of amino acids during protein synthesis. Formation of
the peptide bonds between the amino acids of the protein
is followed by folding of the protein into its natural conformation.
With up to 20 different amino acids in protein
chains of 100 residues or more, several million structural
arrangements are feasible for any one protein. It is essential
for protein function that they form the correct native
conformation, and protein folding is an essential process
following synthesis. Folding is the responsibility of chaperones,
which guide the growing protein chain to produce
the single structure that will ensure its full activity ( Walsh,
2002 ).
TABLE 5-2 Albumin Turnover in Animals
Species T 1/2 (days) Reference
Mouse 1.9 (Allison, 1960)
Rat 2.5 (Allison, 1960)
Dog 8.2 (Dixon et al. , 1953)
Sheep 14.3 (Campbell et al. , 1961)
Cow 16.5 (Cornelius et al. , 1962)
Human 19.0 (Putnam, 1975)
Horse 19.4 (Mattheeuws et al. , 1966)
C . Catabolism of Proteins
1 . Turnover of Proteins
Throughout an animal’s body, proteins are continually
being synthesized and broken down, resulting in a continuous
turnover of protein. In a healthy animal, the rates of
synthesis and degradation are in equilibrium, but during
disease these can alter. Plasma proteins are subject to the
same process, and a function of albumin, the most abundant
plasma protein, is to provide amino acid for the natural
turnover of protein in peripheral tissues. Albumin is
taken up by pinocytosis into tissues where lysosomal proteases
hydrolyze the protein, releasing the amino acids for
utilization by the cells for synthesis of their own proteins
( Evans, 2002 ). There is no storage capability in the body
for protein. As a result, amino acid in excess of requirement
for cellular protein synthesis is utilized for the central
pathways of metabolism. The carbon skeleton of amino
acids can be used for provision of energy via the tricarboxylic
acid cycle and oxidative phosphorylation or may
be converted to glucose or lipid and stored for later use.
Carnivores derive as much as 40% to 50% of their energy
from dietary protein, whereas omnivores and herbivores
derive from 10% to about 20% from this source.
The rate of degradation of the plasma proteins is
expressed as their turnover, fractional clearance, or as their
half-life, which is the time taken for their concentration to
fall by 50%. Plasma half-lives were originally determined
by measurement of the rate of disappearance of radioisotope
labeled protein. More recently, proteins labeled with
stable isotopes and measured by mass spectrometry have
been used for this purpose ( Preston et al. , 1998 ; Prinsen
and de Sain-van der Velden, 2004). Clearance half-lives
for cellular proteins range from a few hours (enzymes) to
as long as 160 days for hemoglobin in bovine red cells.
The clearance half-life for plasma protein can be as long as
3 weeks. Plasma albumin in humans has a normal half-life
of 19 days, α 1 -acid glycoprotein has a half-life of 5.5 days
( Putnam, 1975 ), and γ -globulins have a half-life of 7 days
( Andersen et al. , 1963 ). The plasma half-life of albumin
shows considerable variation between species ( Table 5-2 ).
It is associated with the size of the species with murine
albumin having a T 1/2 of 1.9 days, whereas equine albumin
has a T 1/2 of 19.4 days.
2 . Urea Cycle
During digestion, protein is not only broken down to amino
acid, but gut bacteria can degrade the amino acids releasing
ammonia, which is absorbed along with the amino acids.
This is an important consideration in the management of
liver disease, as sterilization of the gut by antibiotics can
reduce the ammonia load on the liver. Once absorbed,
amino acids, along with ammonia, are transported in the
portal vein to the liver and then to other tissues where the
amino acids are utilized for protein synthesis.
The liver is the central processing organ for nitrogen
metabolism, and approximately 75% of the amino acid
(and ammonia) absorbed from the intestine is transported
into the hepatocytes. Transaminase reactions facilitate the
transfer of amino groups to appropriate α -ketoacids in the
formation of the required balance of nonessential amino
acids. If not required for protein synthesis, amino acids
undergo deamidation by mitochondrial enzymes including
glutamate dehydrogenase and glutaminase. Amino groups
are also transferred to oxaloacetate, with the formation of
aspartate, which along with an ammonium ion and a carbonate
group are the precursors of urea. In this way, amino
groups from excess amino acids are transferred into urea
for safe excretion.
In the urea cycle ( Fig. 5-1 ), which takes place in hepatocytes,
the initial step is the formation of carbamoyl
phosphate in the mitochondria from an ammonium ion,
a carbonate ion, and ATP. This step, which is under metabolic
control and is activated by an increase in the cellular
arginine concentration, occurs when there is an excess of
amino acids in the hepatocyte. The carbamoyl phosphate
combines with ornithine to form citrulline. This metabolite