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

IV. Specific Enzymes
365
Serum CALP activity is a good screening test for
hyperadrenocorticism because of its high sensitivity for
detecting increased cortisol secretion over time. It is not a
diagnostic test because of its low specificity for the diagnosis
of hyperadrenocorticism ( Solter et al. , 1993 ; Teske
et al. , 1989 ). Increased serum CALP activity observed in
animals with disease processes other than hyperadrenocorticism
is consistent with a presumed increase in cortisol
secretion as indicated by reportedly abnormal low-dose
dexamethasone suppression tests or ACTH stimulation
tests in dogs with nonadrenal disease ( Kaplan et al. , 1995 ).
Hypophosphatasemia is uncommon in domestic species,
although it is observed in humans with the genetic
mutations to the TNS ALP gene. Similar genetic mutations
have not been reported in domestic species to our knowledge,
but reduced alkaline phosphatase activity has been
associated with zinc deficiency in cattle ( Machen et al. ,
1996 ; Sharma and Joshi, 2005 ).
Significant amounts of ALP activity have been found in
seminal plasma from numerous species. Low seminal ALP
activity has been proposed as a means of differentiating
partial or complete blockage of the epididymal and deferent
ducts from testicular azoospermia and oligospermia
( Stornelli et al. , 2003 ; Turner and McDonnell, 2003 ).
The serum activity of several diagnostic enzymes
appears to be induced by various pharmaceutical agents.
Anecdotally, serum ALP is the enzyme most often
increased in drug safety trials. There are likely many drugs
that stimulate some increase in liver ALP activity; however,
an incomplete understanding of the factors regulating ALP
synthesis in many cases makes it difficult to determine if
increased ALP activity is due to primary induction of synthesis,
a secondary response of the liver to drug-mediated
cytokines, or drug-induced hepatic injury.
Glucocorticoids are well-recognized inducers of ALP
activity in dogs. The initial response to treatment of dogs
with high doses of glucocorticoids is an increase of LALP
activity in the liver and serum ( Solter et al. , 1994 ). This is
followed after approximately 5 to 7 days with the appearance
of the CALP isoenzyme. Interestingly, the magnitude
of the increase in CALP activity in the experimental treatment
of normal dogs has never been shown to reach the
magnitude of CALP activity in many clinical patients. This
suggests that the induction of CALP synthesis may result
from glucocorticoids acting in a synergistic manner with
other cytokines that are increased in various pathological
conditions. Glucocorticoids do not have the same ability to
induce ALP activity in cats and horses ( Ellison and Jacobs,
1990 ; Hoffmann et al. , 1978 ).
Markedly increased serum CALP activity has also been
observed in vacuolar hepatopathy of Scottish terriers ( Twedt,
2004 ). Although the hepatopathy is similar to steroid hepatopathy,
and all of the dogs have the increased CALP
activity, they have normal serum GGT activity, ALT activity,
and bilirubin concentration, along with normal ACTH
stimulation and low-dose dexamethasone suppression
tests. Preliminary evidence indicates that these dogs have
increased serum 17-hydroxyprogesterone or progesterone
concentrations, suggesting that other adrenal hormones may
result in increased CALP activity and vacuolar hepatopathy.
Phenobarbital is another drug routinely associated with
increased serum ALP activity. It is not known whether this
occurs as a result of enzyme induction or hepatotoxicity.
However, increased serum ALP activity is considered a frequent
observation in epileptic dogs treated with phenobarbital,
whereas hepatotoxicity is an infrequent observation
( Foster et al. , 2000 ; Muller et al. , 2000 ). In a retrospective
study of 78 dogs treated with phenobarbital for epilepsy,
19 had serum ALP activity greater than two-fold the upper
limit of the reference interval, with 11 having predominantly
CALP activity and 7 having predominantly LALP
activity ( Gaskill et al. , 2004 ). However, pretreatment serum
ALP activity was not reported. In a prospective study of 23
otherwise healthy epileptic dogs treated with phenobarbital,
8 had ALP activity greater than the reference range and
3 had ALP activity two-fold greater than the upper limit
of the reference range ( Gaskill et al. , 2004 ). Two of these
3 dogs had predominantly the CALP isoenzyme, and 1 the
LALP isoenzyme. In a subsequent study of 11 phenobarbitaltreated
dogs with increased serum ALP activity, the predominant
isoenzyme in 6 of the dogs was CALP and in 5
was LALP ( Gaskill et al. , 2005 ). Histopathological evaluation
of liver biopsies from these 11 dogs revealed more
severe and frequent abnormalities than in controls but
no increase of ALP activity in liver tissue. Focal areas of
injury may have enhanced the release of ALP from the
membranes and an increase of serum ALP. These studies,
however, do not explain why CALP is present in a few
phenobarbital-treated dogs but not others, with or without
evidence of hepatic disease.
G . Lipase
The lipase (EC 3.1.1.3) that is of interest in the diagnosis
of pancreatic disease is a low-molecular-weight protein of
approximately 42 kDa that hydrolyzes triglycerides at the
1 and 3 positions, leaving a monoglyceride. Pancreatic lipase
binds at the lipid-water interface emulsified in the presence
of bile salts, colipase, and calcium. The original pancreatic
lipase assay described used an incubation medium consisting
of an emulsion of long chain triglycerides in a buffer
containing glycocholic acid and colipase. Such assays
minimize nonpancreatic lipase and esterase activity but do
not completely inhibit it, which tends to broaden the diagnostic
reference intervals. This decreases the sensitivity of
total serum lipase activity to detect pancreatic disease and
decreases specificity because of an increased false-positive
rate. To further illustrate the phenomenon of nonpancreatic
lipase in serum, serum lipase activity has been determined
in two conditions in which serum lipase activity would be