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

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Chapter | 12 Diagnostic Enzymology of Domestic Animals
2-osoglutarate. The liver has by far the highest concentration
of GDH activity ( Boyd, 1983 ; Keller, 1981 ). Lesser
amounts are found in the kidney and small intestine, where
the GDH activity is located in the proximal and distal
tubular epithelial cells and in the mucosal epithelial cells,
respectively. The GDH activity of nonhepatic tissues is relatively
small compared to that found in liver, where GDH
is concentrated in the central areas of the lobule. In all species,
increases in serum GDH activity are considered liver
specific. As a result, there has been little or no interest in
investigating isoenzymes of GDH in serum for diagnostic
purposes. GDH is a zinc-containing enzyme whose activity
can be inhibited by EDTA.
Bovine serum GDH activity reported as stable for
greater than 1 month at 20°C and was considered more
stable than SDH ( West, 1991 ). The in vivo half-life in six
cows was reported as 14 h ( Collis et al. , 1979 ). This value
is consistent with data from cattle recovering from hepatic
injury and suggests that the half-life of serum GDH is
greater than SDH but slightly less than the half-life of AST
( Braun et al. , 1995 ). The half-life of circulating GDH in
dogs is reported as 8 h, based on intravenous injection of
liver extract ( Zinkl et al. , 1971 ).
Serum GDH activity is used most commonly in food
animals and horses. Because of its location within mitochondria,
GDH should be released only with irreversible
cell injury. Following carbon tetrachloride–induced hepatic
necrosis in calves and sheep, GDH activity increases but
peaks approximately 1 day later than serum AST activity
( Boyd, 1962 ). This may be due to the intramitochondrial
location of GDH. Nevertheless, serum GDH activity
was shown to significantly increase in acute, subacute, and
chronic grass sickness in horses ( Marrs et al. , 2001 ).
However, serum GDH activity was found to be highly variable
in ponies exposed to pyrrolizidine alkaloids, suggesting
that GDH activity may only be diagnostically useful in the
acute stages of liver injury ( Craig et al. , 1991 ). This study
found that serum GDH activity was increased with zone
1 hepatocyte necrosis, but it returned to normal reference
intervals once all cells in this region were destroyed. These
findings are consistent with the reported hepatic location of
GDH in humans ( Burtis and Ashwood, 1994 ). Increases in
GDH activity of approximately 12-fold and AST activity
two-fold were observed 24h following halothane anesthesia
in horses, which also may reflect the centrolobular location
of GDH activity ( Durongphongtom et al. , 2006 ).
As suggested earlier, the sensitivity of GDH activity varies
depending on the nature of the disease. For example,
in a study of calves with hepatic disease, GDH activity
increased in only 60% of the animals ( Pearson et al. , 1995 ).
Similarly, in cattle, the sensitivity of GDH activity for the
detection of hepatic lipidosis, hepatic abscessation, leptospirosis,
and fascioliasis was only 28%, 53%, 71%, and 72%,
respectively ( West, 1991 ). However, with all categories of
hepatic disease described, the sensitivity of GDH activity
was higher than SDH activity. The specificity for GDH was
slightly less than that of SDH. In a similarly designed study
in horses, the sensitivity of GDH activity for detection of
hepatic necrosis, hepatic lipidosis, and hepatic cirrhosis was
78%, 86%, and 44%, respectively ( West, 1989 ). The sensitivity
was higher than that of SDH and comparable to that
observed with serum AST activity. The specificity of GDH
in this study was nearly 100%, which was comparable to
the specificity of SDH and superior to that of AST activity.
In a more recent study of the sensitivity of increased liver
enzymes for diagnosis of hepatic disease, GDH showed a
sensitivity of 63% ( Durham et al. , 2003 ). The determination
of GDH activity is best done in conjunction with the determination
of other hepatic enzymes and other indicators of
hepatic injury or disease.
Serum GDH determinations for diagnosis of hepatic
disease in domestic animals have received less attention in
the United States than in some other countries. However,
the increased stability of the enzyme, longer half-life, and
apparent greater sensitivity discussed earlier suggest it may
be a more useful test for horses and food-producing animals
than the determination of SDH activity.
E . Gamma Glutamyltransferase
Gamma glutamyltransferase (GGT) (EC 2.3.2.2) functions
in the gamma glutamyl cycle where it catalyzes the transfer
of gamma glutamyl groups from gamma glutamyl peptides
such as the tripeptide glutathione to other peptides, amino
acids, and water. In conjunction with a peptidase, GGT
plays a major role in regulation of intracellular glutathione
by hydrolysis of the tripeptide glutathione outside the cell
into its three components, which can readily be taken up by
the cells and be available for glutathione synthesis as needed
within the cell. GGT also functions in the GSH transferase/
GGT pathway that cleaves gamma glutamyl moieties from
GSH conjugates, which aids in the detoxification of xenobiotics
and carcinogens by rendering them more water soluble
and readily excreted ( Lieberman et al. , 1995 ). This pathway
also plays a role in metabolism of mediators such as leukotrienes,
hepoxillins, and prostaglandins.
The tissue distribution of GGT has been studied in
numerous domestic species with the highest concentration
found in kidney, pancreas, intestines, and the mammary
glands of dogs, cattle, goats, and sheep but at much lower
concentration in mammary gland of horses. Less GGT
activity is found in liver, spleen, intestine, lung, and seminal
vesicles. The GGT activity per gram of liver tissue is
consistently lower than in kidney but varies between species,
with the highest liver GGT activity in cattle, horses,
sheep, and goats. Serum GGT reference values are consequently
higher in those species than in dogs and cats
( Braun et al. , 1983, 1987 ; Milne and Doxey, 1985 ; Rico
et al. , 1977a, 1977b ; Shull and Hornbuckle, 1979 ).