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

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Chapter | 12 Diagnostic Enzymology of Domestic Animals
in liver than other organs led to the early conclusion that
increases of ALT activity in serum are specific for hepatocellular
injury, there is clear evidence that serum ALT
activity can also be increased as a result of injury to myocytes
as well. Dogs in a colony with canine X-linked muscular
dystrophy and ongoing muscle necrosis had increased
serum CK, AST, and up to a 25-fold increase in ALT activity
but a normal SDH activity, suggesting that myonecrosis
contributed to the increased serum ALT activity ( Valentine
et al. , 1988 ). This is consistent with the presence of some
ALT activity in cardiac and skeletal muscle of dogs. In a
case report of a cat with myokymia and neuromyotonia,
the CK activity was 28,380, whereas the ALT activity was
only 195 U/l; in a study of rhabdomyolysis in three dystrophin-deficient
cats, the CK activity ranged up to 2040
times the upper limit of the reference range, whereas the
ALT activity only increased to 19 times the upper limit of
the reference range, suggesting only a minimal increase of
serum ALT activity should be expected with muscle injury
in this species ( Galano et al. , 2005 ; Gaschen et al. , 1998 ).
Although at least one early study in dogs showed a
correlation between the magnitude of serum ALT activity
and histological evidence of necrosis, other studies
have reported little correlation (VanVleet and Albert,
1968 ) . Similarly, bile duct ligation of dogs led to a 25-fold
increase in serum ALT activity with minimal evidence of
hepatocellular necrosis. As discussed in the introduction,
the recognition of the formation of membrane blebs on
hepatocytes and the rupture of these blebs during various
conditions such as endotoxic shock, carbon tetrachlorideinduced
injury, cholestasis, and experimentally induced
hypoxia have led to the understanding that there can be
an increase of serum enzymes derived from the cytoplasm
of the cell in cases of reversible cellular injury. In summary,
the observation of increased serum ALT activity
indicates hepatocellular (or myocyte) injury, but it does not
necessarily imply irreversible injury and does not suggest a
specific cause.
B . Aspartate Aminotransferase
Aspartate aminotransferase (AST: EC 2.6.1.1) (formerly
glutamic oxaloacetic transaminase; GOT) catalyzes the
transamination of L-aspartate and 2-oxoglutatarate to oxaloacetate
and glutamate. As with ALT, pyridoxal-5 -phosphate
(PP) is required as a cofactor. Although serum ALT was
poorly saturated with PP in a study following exercise in
horses, 94% of the AST was saturated and present as the
holoenzyme ( Rej et al. , 1990 ). Providing PP in the assay
reagent may be less of a concern when determining serum
AST activity than when determining serum ALT activity.
AST activity is relatively high and in similar amounts
in liver and in skeletal and cardiac muscle, but it varies
between species ( Boyd, 1983 ; Keller, 1981 ). It is routinely
used in equine and food animal medicine as a screening
test for injury to both organs. Serum AST activity is readily
available on the biochemical profile, has a longer blood
half-life than sorbitol dehydrogenase and creatine kinase,
and is stable for days in serum at room temperature, refrigerated,
or frozen. AST is found in erythrocytes, and the
addition of erythrocyte lysate to serum increases the apparent
AST activity (unpublished data).
AST is located in the cytosol but is in higher concentrations
in mitochondria. There is only 48.1% amino acid
sequence homology between cytosolic AST (cAST) and
mitochondrial AST (mAST) from horse heart ( Doonan
et al. , 1986 ). Likewise, the nucleotide sequences of cDNA
of bovine mAST and cAST are also distinctly different
( Aurila et al. , 1993 ; Palmisano et al. , 1995 ). Although
there have been some efforts to show enhanced ability to
identify organ-specific injury by assays for mAST and
cAST, this has been shown to be of no diagnostic value
( Jones and Blackmore, 1982 ). It may be theoretically
possible to estimate the magnitude of reversible versus
irreversible cell injury by determining mAST and cAST
separately; however, this has not been studied empirically.
Although the half-life of AST has been reported to be
as long as 7 to 8 days in horses and as short as 163min
in dogs, neither of these seems reasonable based on data
obtained following carbon tetrachloride toxicity ( Fleisher
and Wakim, 1963 ; Zinkl et al. , 1971 ). Decreasing serum
AST activity in horses recovering from CCl 4 -induced hepatotoxicity,
as well as studies of equine myoglobinuria,
suggests a half-life of 3 to 4 days ( Bernard and Divers,
1989 ; Cardinet et al. , 1967 ; Noonan, 1981 ). In cattle with
mild CCl 4 -induced hepatotoxicity, serum AST activity during
recovery suggests a half-life of approximately 1 day
( Yonezawa et al. , 2005 ). Serum AST has a longer half-life
than creatine kinase, and therefore it would be expected to
have increased diagnostic sensitivity during recovery from
myocyte or hepatocyte injury.
Increased serum AST activity is observed with both
reversible and irreversible injury to hepatocytes and can be
seen following hepatocellular injury and cholestasis, similar
to serum ALT activity in dogs and cats. Likewise, serum
AST is increased following myocyte injury. In either case,
the definitive disease process cannot be identified, only
that cellular injury in muscle or liver has occurred. Because
serum AST activity cannot differentiate between hepatocellular
or myocyte injury, further testing is often required
using organ-specific enzymes such as sorbitol dehydrogenase
or creatine kinase. Markedly increased serum AST and
sorbitol dehydrogenase activity suggest acute or active
hepatocellular injury, and markedly increased serum AST
with modest to moderate sorbitol dehydrogenase activity
suggests chronic hepatic injury or recovery from acute
liver injury. Similar conclusions can be drawn using serum
AST and creatine kinase activity. As with other cytosolic
enzymes, serum AST activity cannot distinguish between