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

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Chapter | 12 Diagnostic Enzymology of Domestic Animals
marketed by Technicon in 1964 ( Skeggs, 2000 ). The autoanalyzer
increased the availability of reduced cost serum
enzyme analyses, which ultimately led to their routine use in
both human and veterinary diagnostic medicine.
The advancement of clinical enzymology included the
development and evaluation of enzyme assays for use in
nonhuman animal species, some of which have been found
useful, whereas others have been dropped for various reasons.
In some cases, the decision to investigate an enzyme
for diagnostic use may have related not only to its potential
clinical relevance but also to the efficiency of offering
the test. In spite of recognition of species differences, veterinary
medicine has often followed human medicine in its
choice of diagnostic tests. The bias toward enzyme assays
used in human medicine is due in part to their availability on
automated analyzers, making all tests low-cost, on-demand
assays with a high degree of precision and accuracy.
The automation of enzyme assays, and the popularity
of the serum chemistry profile in veterinary medicine,
has allowed retrospective studies to be conducted and has
given the veterinarian an opportunity to critically evaluate
the diagnostic function of the common assays in a
large number of animals on a regular basis, as well as gain
a “ feel ” for the results, thereby allowing for more subtle
clinical interpretations. It is likely that diagnostic tests that
are not automated are less understood and interpreted in a
more rigid manner with less appreciation for nuances and
significance of the test result. Interestingly, during the first
approximately 30 years of serum enzyme testing in veterinary
medicine, these tests were often viewed as “ diagnostic
” tests, whereas in the past approximately 20 years their
variable and often somewhat limited degree of diagnostic
specificity has been appreciated and they are now most
often recognized as “ screening ” tests.
Although serum enzyme activity is reported as part of
numerous studies published in the literature, the number of
studies directed primarily at answering specific questions
regarding the enzymes appears to have decreased since the
late 1990s from what might be considered the heyday of
clinical enzymology in the 1960s to mid-1990s.
III . FACTORS AFFECTING SERUM ENZYME
ACTIVITY
As the field of clinical enzymology has developed, so has
our understanding of the physiological factors responsible
for the alterations in serum enzyme activity that occur with
disease, although several unanswered questions remain.
Organ specificity, subcellular location of the enzyme, the
mechanism of enzyme release from cells, the clearance
from blood, and the rate of induction of enzyme synthesis
all affect to a lesser or greater extent the diagnostic accuracy
of the various enzyme assays (Hoffmann and Solter, 1989 ;
Solter, 2005 ). This section discusses the physiological,
biochemical, and anatomical factors that affect changes to
serum enzyme activity.
A . Organ Mass and Enzyme Tissue
Concentration
The roles that enzyme tissue concentration and organ mass
play on the magnitude of blood enzyme activity are relatively
straightforward. Organs with a high concentration of
an enzyme have the potential to cause a greater increase in
serum enzyme activity with disease. For example, the intracellular
to extracellular concentration gradient of hepatocellular
alanine aminotransferase (ALT) is 100,000:1.
Injury to hepatocytes, therefore, has the potential of causing
markedly increased serum ALT activity. The higher
the concentration gradient of the enzyme or protein marker
between the cell and the interstitial space, the faster is the
translocation of significant quantities of the enzyme to the
interstitial space and ultimately the blood ( Mair, 1999 ).
Likewise, the liver has a large mass, thereby adding to the
potential increase in serum ALT activity.
B . Cell Location
The location of cellular enzymes relative to the blood,
urine, or other fluids is an especially significant determinant
of whether an increase in enzyme activity will occur
with enzyme release and in which fluid it will be found. A
well-known example is renal tubular gamma glutamyltransferase
(GGT) located on the luminal surface of renal tubular
epithelial cells. Injury to these cells results in release
of the gamma GT into urine but not into blood. Similarly,
alkaline phosphatase (ALP) located on the luminal surface
of enterocytes is lost into the gut lumen rather than blood
with enterocyte injury. Hepatocellular ALP, however, with
activity over both bile canalicular and sinusoidal surfaces,
can be increased in both bile and blood.
C . Mechanisms of Release of Cytoplasmic
Enzymes or Other Protein Biomarkers from
Cells to Blood
Cytoplasmic enzymes are contained within cell membranes.
Healthy plasma membranes are thought to be impermeable
to macromolecules such as enzymes. Therefore, alteration
in the cell membrane is necessary to allow cytoplasmic
enzymes to gain access to the blood. In the event of cell
necrosis, perforations and tears of the cell membrane allow
the release of cytosolic contents in a relatively straightforward
process. However, increases of serum enzymes do not
always correlate to the degree of histological evidence of cell
necrosis. Hence, it is has been long postulated that under certain
circumstances, cytoplasmic enzymes may “ leak ” from