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

VI. Normal Plasma and Serum Proteins
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acidic and basic amino acid residues. Porcine albumin has
97 acidic and 83 basic amino acid residues, whereas feline
albumin has 100 acidic and 75 basic amino acid residues,
and these proportions result in a relatively high mobility for
feline albumin and low mobility for porcine albumin.
Albumin is synthesized in the cytoplasm of the hepatocyte,
being transferred from bound ribosomes to rough
membrane to cisterna, then via the smooth endoplasmic
reticulum to the Golgi complex, and through the membrane
to the sinusoid (Prinsen and de Sain-van der Velden,
2004). The rate of albumin synthesis is controlled by the
colloid osmotic pressure, although it can be influenced by
hormones such as insulin, thyroxine, and cortisol ( Evans,
2002 ). Only about 30% to 40% of albumin is in the blood;
the remainder is in the interstitial space. Once secreted into
the circulation, albumin is modified by covalent, irreversible,
but nonenzymic glycation of lysine residues such that
6% to 10% of albumin is in reality glycoalbumin following
conjugation to glucose or galactose. This can rise to
20% to 30% in (human) hyperglycemic patients ( Nakajou
et al. , 2003 ). Catabolism of albumin occurs in various tissues
where it enters cells by pinocytosis at a rate related to
atrial natriuretic peptide concentration ( Evans, 2002 ) and is
then degraded by protease action. Muscle, liver, and kidney
are the main contributors to albumin catabolism with 40%
to 60% of the total albumin being broken down in these
tissues (Prinsen and de Sain-van der Velden, 2004). The
turnover of albumin differs with species but is related to
the body size (see Table 5-2 ). The half-time for clearance
of albumin varies from 1.9 days in the mouse to 19.4 days
in the horse. Whereas originally radioactive isotopes such
as I 131 were required for studies on protein turnover, a new
generation of markers based on stable isotopes ( Preston
et al. , 1998 ; Prinsen and de Sain-van der Velden, 2004)
have been introduced, and it could be that diagnostic applications
of serum protein turnover rates will prove to be
valuable in the future.
2 . Function and Physiology
Maintaining the colloid osmotic pressure and the blood
volume is an important action for albumin and is a function
that is disproportionate to its serum concentration.
Although albumin is only about 50% of the total protein
mass in the circulation, it is responsible for 80% of the
colloid osmotic pressure. This is because it has a lower
molecular mass (67 kDa) than the mean of the globulins
(170 kDa) and also because of its contribution to the
Donnan effect from its high net negative charge (Prinsen
and de Sain-van der Velden, 2004). Another major function
of albumin is as a transport protein. A number of metabolites
circulate in blood bound to this protein. Binding
to albumin assists the transport of substances that are sparingly
soluble in aqueous media and also prevents loss
through the kidney of important small molecules. Thus,
fatty acids, cholesterol, bilirubin, nitric oxide, and metal
ions circulate bound to albumin ( Evans, 2002 ). As well as
metabolites, a variety of pharmacological compounds bind
to albumin and four discrete sites have been identified on
the molecule. Drugs such as phenytoin, digoxin, nonsteroidal
anti-inflammatories, and antibiotics interact via these
binding sites ( Evans, 2002 ). The exposed cysteine residue
of albumin, which does not form an internal disulfide
bond, has an important role in the action of albumin as an
antioxidant. The free cysteine is an avid scavenger of reactive
oxygen and peroxynitrite radicals such that albumin
may actually be the major and predominating antioxidant
in the circulation ( Anraku et al. , 2001 ). Albumin is a negative
acute phase protein and its concentration falls gradually
during infectious and inflammatory disease.
B . Acute Phase Proteins
Studies on individual serum proteins of domestic animals
have expanded greatly since the 1990s ( Ceron et al. , 2005 ;
Murata et al. , 2004 ; Paltrinieri, in press ; Petersen et al. ,
2004 ). This has largely been caused by the realization that
monitoring the levels of the acute phase proteins (APP)
can provide a means to assess the innate immune system’s
response to disease and in the ability of the APP to provide
a “ molecular thermometer. ” As these proteins change their
serum concentration by 25% in response to inflammation,
infection, and trauma, many conditions can cause their
elevation or decrease. Therefore, as quantitative markers
for disease they can be used for prognosis and monitoring
responses to therapy, for general health screening, as well
as for diagnosis of disease. The APP are highly sensitive
for the presence of pathological lesions while having a low
specificity for a particular disease. The APP are now recognized
as having an important role to play in the diagnosis of
disease in animals, but there are major differences between
species in the pathophysiological change in their concentrations
during an acute phase reaction. Furthermore, although
initial interest focused on proteins that increase in concentration
during this response (positive APP), a number of
serum proteins decrease in concentration and can be considered
to be negative APP.
In any one species, positive APP have been found that
have major, moderate, or minor responses. A major APP
has a low concentration in the serum of healthy animals,
often at 0.1 μ g/dl ( 1 μ g/liter) but with the concentration
increasing over 100- or 1000-fold on stimulation, reaching
a peak 24 to 48 hours after the insult and falling rapidly
during recovery. A moderate APP is present in the blood
of healthy animals, but on stimulation the concentration
will increase 5- to 10-fold, reach a peak concentration 2 to
3 days after stimulation, and decrease more slowly than
the major APP. A minor APP shows a gradual increase
and only increases in concentration by 50% to 100% of the