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

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Chapter | 5 Proteins, Proteomics, and the Dysproteinemias
from the pancreas, and parathyroid hormone and calcitonin
from the parathyroid gland. Similarly, the adipose tissue
releases adiponectin and leptin, peptides affecting the
appetite and nutritional status, whereas the cytokines are
derived from cells of the immune system. The plasma
also contains tissue proteins lost during normal turnover
of cells, which can be valuable biomarkers of disease in
identifying when pathological events occur. For example,
enzymes determined in clinical enzymology for liver or
muscle damage and the troponins released from cardiac or
skeletal muscle are minor plasma proteins.
B . Functions of the Plasma Proteins
The functions of proteins in the body of animals are innumerable
and include forming the basis of structure of cells,
organs, and tissues; maintaining colloid osmotic pressure;
catalyzing biochemical reactions; and buffering acid-base
balance. The proteins are also multifunctional in plasma.
The functions of specific plasma proteins will be described
later in this chapter, but main functions of protein in blood
are in blood coagulation (fibrinogen), in host defenses
against pathogens (immunoglobulins, complement), in
transport of metabolites (transferrin, albumin), in regulation
of cellular metabolism (hormones), in prevention of
proteolysis ( α 1 -antitrypsin), in provision of nitrogen balance
for nutrition (albumin), and in maintaining osmotic
pressure (albumin). The biological activities of the plasma
proteins in these functions depend ultimately on their primary,
secondary, tertiary, and quaternary structures.
C . Factors Influencing the Plasma Proteins
1 . Age
At birth, plasma proteins of most species are low because
of minimal quantities of immunoglobulins . As the newborn
animal ingests colostrum, a rapid rise in immunoglobulins
occurs as a result of absorbed maternal immunoglobulins.
As the maternal immunoglobulins reduce in concentration
because of natural turnover, the neonate rapidly gains
immunocompetence and begins to synthesize its own
immunoglobulins. Upon reaching young adulthood, adult
levels of globulins are reached. In contrast to this general
trend, α 1 - acid glycoprotein is elevated in serum at birth in
piglets but declines over the first few months of life ( Itoh
et al. , 1993b ). With increasing age, the plasma protein concentration
increases as a result of a small decrease in albumin
and a progressive increase in globulins.
2 . Hormones, Pregnancy, and Lactation
In some species , the total plasma protein concentration in
the maternal blood decreases during gestation because of
a decline in albumin, even though there is a slight increase
in globulins. In a number of species pregnancy associated
proteins have been observed during pregnancy. The only
one in regular diagnostic use is equine chorionic gonadotropin
(formerly pregnant mare serum gonadotropin) used
to confirm pregnancy in mares ( Henderson et al. , 1998 ).
In pregnant bitches, an acute phase reaction occurs about
21 days after fertilization and acute phase proteins, especially
C-reactive protein, increase in the maternal serum
( Eckersall et al. , 1993 ; Vannucchi et al., 2002 ). In cattle
approaching term, there is a rise in γ -globulins and a corresponding
rise in total plasma protein, but at term a fall
occurs in this fraction because of transfer to colostrum
( Weaver et al. , 2000 ). The acute phase protein, serum amyloid
A (SAA), also increases in the maternal plasma around
parturition in cows ( Alsemgeest et al. , 1993 ). During lactation,
the total plasma protein decreases in some species
because of albumin decrease.
Some hormones (testosterone, estrogens, growth hormone)
promote an increase in total plasma protein because
of their anabolic effects, whereas others (thyroxine, cortisol)
tend to decrease the total plasma protein because of
their catabolic effects.
D . Handling and Identification of Proteins
Protein denaturation is the net effect of alterations in the
biological, chemical, and physical properties of the protein
by mild disruption of its structure. When blood samples
are taken for protein analysis, it is important that they are
handled correctly so that no artifacts are introduced that
could affect the investigation and its interpretation. If the
protein is allowed to even partially degrade, the assay will
not be accurate. Therefore, it is essential that denaturation
is avoided. The ability of plasma proteins to resist denaturation
in a blood sample taken for diagnostic analysis varies
between proteins; consequently, the sample should be handled
according to the analysis required. Fortunately, most
major plasma proteins are relatively resistant to denaturation
and can be assayed in samples that have been handled
carefully and have been kept away from elevated temperatures.
However, separation of plasma or serum from the
blood cells by centrifugation should be performed as early
as possible. Thereafter, many proteins are stable at 4°C
for several days and at –20°C for much longer (months to
years). Some proteins are less stable, with enzymes being
particularly susceptible to loss of activity with time, while
the stability of the peptide hormone ACTH is so low that
samples should be snap-frozen immediately to preserve the
intact peptide.
For identification and quantification of serum protein,
the protein component in serum must either be separated or
individual proteins must be measured independently. The
primary separation of the proteins in serum is between albumin
and the globulins. Albumin is a water-soluble, globular
protein that is usually identifiable as a single discrete