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

IV. Specific Enzymes
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plays a role in amino acid catabolism and interorgan nitrogen
transport. Pyridoxal 5 -phosphate (PP) is the cofactor
of ALT, thus forming the active holoenzyme. PP is generally
present in serum in sufficient quantities to provide near
maximum activity of the ALT with only a reported 11%
and 7% inactive apoenzyme in dog and cat serum, respectively
( Stokol and Erb, 1998 ). There was no difference found
between the percentage of inactive apoenzyme in the serum
of normal animals and those with hepatic disease. However,
two dogs were identified with 14,225% and 336% greater
serum ALT activity when PP was added ( Mesher et al. ,
1998 ). Approximately half the ALT in serum from a group
of exercising Thoroughbred horses was in the inactive apoenzyme
form ( Rej et al. , 1990 ). Hence, because there are cases
in which PP seems to be limiting the measured ALT activity,
some, but not all, commercial assays for ALT now contain
added PP reagent.
ALT activity is found in several body organs, but the
magnitude of activity varies dramatically with species.
In dogs, the ALT activity per gram of liver is at least
four times greater than in other organs, although considerable
activity is found in both heart and skeletal muscle
( Clampitt and Hart, 1978 ; Keller, 1981 ; Zinkl et al. , 1971 ).
Similar findings are true for cats, but in horses, cattle, and
swine, the ALT activity per gram of tissue differs little in
liver when compared to muscle. Hence, based on tissue
concentrations of ALT, increased serum ALT activity is
somewhat specific for hepatic injury in dogs and cats but
offers no specificity for detection of liver injury in horses
and cattle.
ALT, found in the cytoplasm of hepatocytes, is also
found in mitochondria but generally at considerably lower
concentrations, depending on species and tissue. Although
it has been suggested that the mitochondrial enzyme may
be released into blood more slowly following hepatocellular
injury, this activity is still poorly understood and has
not been utilized as a diagnostic tool.
The half-life of ALT in blood is not clearly defined,
although the circulation time is obviously adequately
long to evaluate organ injury and release of ALT into
blood for hours to days after the event. In dogs, reports
have suggested half-lives of 3, 20, 45, and 60 h ( Fleisher
and Wakim, 1963 ; Reichard, 1959 ; Zinkl et al. , 1971 ).
Semilogarithmic plots of the decline in serum ALT activity
following peak activity induced by acute CCl 4 exposure
suggest a half-life of between 45 and 60h in dogs, although
this may be a slight overestimation, as injured tissue is still
likely present and contributing to the blood pool (unpublished
data). The half-life of ALT from feline liver extracts,
administered intravenously to cats, was estimated as 3 to
4h (Nilkumuhaug and Thornton, 1979). This is consistent
with the half-life of 6 h for ALP activity in the blood of cats
(Hoffman et al. , 1977).
Serum ALT has been recognized as a marker of hepatocellular
injury since the 1950s ( Chimsky et al. , 1956 ;
Cornelius, 1958 ) . The use of ALT as a diagnostic tool was
expedited by the development in the mid-1950s of a simple
coupled assay for ALT activity in serum that eliminated
the problem of product inhibition ( Reitman and Frankel,
1957 ). Numerous studies using carbon tetrachloride have
clearly shown the value of serum ALT as an indicator of
hepatocellular necrosis, especially in dogs and cats, but to
a much lesser extent in horses, cattle, swine, sheep, and
goats ( Cornelius et al. , 1958 ; Everett et al. , 1977 ; Noonan,
1981 ; Noonan and Meyer, 1979 ; Spano et al. , 1983 ; Turgut
et al. , 1997 ; Zinkl et al. , 1971 ). The length of time that
serum ALT activity is increased ranges from 9 to 23 days
in dogs, which suggests prolonged injury to the liver but
also supports the longer half-life suggested earlier ( Guelfi
et al. , 1982 ; Noonan, 1981 ; Turgut et al. , 1997 ). Relatively
mild increases in serum ALT activity occur in dogs and
cats with biliary obstructive diseases that cause serum
ALP activity to increase markedly ( Everett et al. , 1977 ;
Spano et al. , 1983 ). Hence, the ratio of serum ALT-to-
ALP activity is far greater in cases of hepatic necrosis than
with cholestasis, suggesting that very general interpretive
conclusions can be made by comparing the magnitude of
increase of serum activity of these two enzymes. Increased
serum ALT activity occurs with a wide range of other
disorders including hypoxia secondary to anemia, metabolic
diseases such as lipidosis, nutritional disorders such
as copper toxicosis, inflammatory or infectious diseases,
neoplastic diseases, and traumatic liver injury. Increased
serum ALT activity has also been associated with numerous
drugs; in many cases, these are likely idiosyncratic
reactions causing hepatocellular toxicity. Exposure to carbon
tetrachloride, mushroom alkaloids, or acetaminophen
is clearly a hepatotoxic event.
Mild to moderate increases in serum ALT activity
are also observed in dogs and cats with endocrine diseases
such as diabetes mellitus, hyperthyroidism, hyperadrenocorticism,
and hypothyroidism. For example, 163
(78%) dogs with diabetes mellitus have increased serum
ALT activity ( Hess et al. , 2000 ). Cats with diabetic ketoacidosis
commonly have increased serum ALT activity
( Bruskiewicz et al. , 1997 ). Increased serum ALT activity
is common in dogs with hyperadrenocorticism or dogs
treated with glucocorticoids ( DeNova and Prasse, 1983 ;
Dillon et al. , 1980 ; Solter et al. , 1994 ). It has been shown
in rats that ALT synthesis may be induced by glucocorticoids
in order to increase function of the gluconeogenic
pathways. However, experimental treatment of healthy
dogs with glucocorticoids did not result in an increase in
the concentration of hepatic tissue ALT activity, suggesting
that increased hepatic mass plays a larger role than
increased hepatocellular enzyme induction for an observed
increased serum ALT activity ( Solter et al. , 1994 ).
Although early studies of increased serum ALT activity
following experimentally induced hepatocellular injury
and the studies demonstrating much higher ALT activity