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

IV. Specific Enzymes
357
reversible and irreversible cell injury as the cAST can be
released by mechanisms involving nonlethal cell membrane
blebbing. However, because a major portion of AST
is of mitochondrial origin, the magnitude of increase during
reversible cell injury is expected to be less than in irreversible
injury, but this has yet to be clearly shown.
The diagnostic sensitivity of serum AST activity in
horses has been reported as 72% for hepatic necrosis and
100% for hepatic lipidosis ( West, 1989 ). The specificity of
serum AST activity was variable, and it decreased with primary
gastrointestinal and orthopedic conditions secondary
to affects on liver and skeletal muscle, respectively. In cattle,
the sensitivity is reported to be 94% for hepatic lipidosis,
100% for leptospirosis, but only 53% for hepatic abcessation,
and 46% for fascioliasis ( West, 1991 ). Specificity was
again variable depending on the primary condition.
In summary, serum AST determinations are still part of
many biochemical profiles because of their relatively high
sensitivity for detection of hepatocyte injury and myocyte
injury and stability in serum. However, serum AST activity
clearly lacks specificity when compared to tissue-specific
enzymes, such as sorbitol dehydrogenase and glutamate
dehydrogenase for the detection of hepatocyte injury and
creatine kinase for the detection of myocyte injury.
C . Sorbitol Dehydrogenase
Sorbitol dehydrogenase (SDH; EC 1.1.1.14), also known
as iditol dehydrogenase, catalyzes the following reaction:
sorbitol NAD ↔ fructose NADH
The active sites of SDH contain Zn 2 . Hence, when
EDTA blood collection tubes are used, SDH activity
is inhibited. Serum or heparinized plasma can be used
for analysis. Sample stability has also been of concern
for the use of SDH in diagnostic medicine with bovine
serum SDH activity stable for at least 5 h at room temperature,
24 h refrigerated, and 72 h frozen, whereas in equine
serum SDH remains stable for 5 h at room temperature,
5 h refrigerated, and 48 h frozen ( Horney et al. , 1993 ). In
another study, bovine SDH activity was stable for 1 month
at 20°C ( West, 1991 ).
SDH is not membrane bound and is located in the cytoplasm
of cells. The highest concentration of SDH activity is
in liver followed by kidney, but it is also found in most other
tissues at much lower amounts ( Boyd, 1983 ; Keller, 1981 ;
Nilkumang and Thornton, 1979). SDH activity is considered
liver specific in all species, and there have been no reports of
nephrotoxicity causing increased serum SDH activity.
The T 1/2 of SDH in blood is likely relatively short in
all species. Its reported T 1/2 , based on intravenous administration
of cat or dog liver extracts, is 3 to 4 h for cats and
5 h for dogs ( Nilkumhang and Thornton, 1979 ; Zinkl et al. ,
1971 ). The T 1/2 of SDH in swine is reported as 1.6 h. In dogs
and horses treated with CCl 4 , a rapid decrease in SDH
activity following peak activity, supports a T 1/2 of less than
12 h. The short circulatory half-life may be due in part to
the labile nature of the enzyme, similar to that observed in
serum samples in vitro. This short T 1/2 limits to some extent
the usefulness of the test, as it is easy to miss peak activity
following a hepatic insult, and serum SDH activity may be
within reference intervals in chronic hepatic disease.
Because of its short half-life and the labile nature of
SDH activity in serum, SDH activity is less favored for
detection of hepatic disease in dogs than serum ALT activity.
However, there are two occasions when SDH analysis
may be useful in dogs. First, in dogs with traumatic muscle
injury, where there is increased serum ALT and CK activity,
a determination of SDH activity will quickly rule out
whether there is concurrent hepatic injury. A second use of
SDH activity determination in dogs might be in conjunction
with ALT activity to determine if there is persistent hepatocellular
injury. If the ALT activity is markedly increased and
SDH activity is not, recovery is likely, but if both are markedly
elevated, an ongoing insult to the liver is likely present.
This sort of interpretation, however, is highly subjective and
would require repeated monitoring to be of value.
Serum SDH activity is of greater value than serum AST
activity in large animals because of its increased specificity
for hepatocellular injury. Marked increases of serum
SDH activity occur within hours of experimentally induced
hepatic necrosis in horses and cattle ( Noonan, 1981 ). Serum
SDH activity has been reported as a value for the detection of
hepatic lipidosis, hepatic necrosis, leptospirosis, fascioliasis,
and hepatic abscessation in cattle, and detection of hepatic
necrosis, lipidosis, and cirrhosis in horses ( Cebra et al. ,
1997 ; Lechtenberg and Nagaraja, 1991 ; West, 1989, 1991 ).
Whereas the specificity of serum SDH activity in both cattle
and horses with nonhepatic disease is 100%, the sensitivity
for detecting hepatic lipidosis, hepatic abscessation, and leptospirosis
in cattle was less than 50%; for detecting hepatic
cirrhosis and lipidosis in horses it was less than 50%; and for
detecting hepatic necrosis in horses it was 76% ( West, 1989,
1991 ). In essentially all conditions evaluated in these two
species, serum AST and glutamate dehydrogenase (GDH)
activity were more sensitive than SDH activity. However,
the specificity of serum AST and GDH was generally less
than the specificity of SDH activity. The lower sensitivity
may be in part due to the short half-life of the enzyme
in circulation; especially in chronic low-grade conditions
where large numbers of cells are not injured at any one
time, the SDH activity may not exceed the reference range.
D . Glutamate Dehydrogenase
Glutamate dehydrogenase (GDH) (EC 1.4.1.3) is a
mitochondrial enzyme that catalyzes the removal of
hydrogen from L-glutamate to form the corresponding ketimine
acid that then undergoes spontaneous hydrolysis to